Attack Analysis:
The goal of the Abele brothers has always been to determine what happened to the Grunion. Although some may disagree we feel that the evidence is strong enough that we have determined its location to at least make the next step in finding out what happened.

Below is the information we have about the attack. In light of our readership, particularly submariners, hopefully someone can help develop one or more viable hypothesis as to the demise of the Grunion.

It should be pointed out that there are some comments about the crew (shoddy construction, forgot to pull the firing pin) that should be taken with a grain of salt. There were major operational problems in the beginning of the war both with the torpedoes and with the subs themselves. To leave those comments out, in my opinion, would be a disservice to finding the truth.

There are three witnesses (diagrams and dialog) and a number of other bits of ancillary info.

Aiura: (Aiura was the military commander on the Kana Maru) Yutaka translating:

“The escort subchaser also came in and out the fog, at last we lost contact and the night came. KOANO MARU again went on alone in the foggy Bering sea toward KISKA. At July 30 08:00 we reached at the north of Kiska Is. about 20 nautical miles apart. But the dense fog prevented us to approach. We were forced to drifting and waiting the fog getting thin. The fog was getting thin for a moment and back to dense. We could do nothing but waiting. This day whole day the fog was dense and we were impatient. 15:30 A friendly seaplane fighter[Pete] ditched near us and we picked up. The plane and a pilot were both safe. The pilots were so young as just graduated his boy air school, similar age of my[S.AIURA] son. I was impressed with his figures that do his duty in the Northern sky. It was a happening that our ship saved him but I was so glad that we could help him. At the evening the fog still dense, we were alerting the enemy submarine attack. 17:15 We turned outside the open ocean, under 15 knots speed maneuvering zigzag course, decided to enter the KISKA harbor next morning.

July 31 Next early morning we again reached off KISKA harbor but the fog was still dense. We are drifting and waited. 04:40 The fog became a little thin and we can get our position by astronomical observation. We confirmed the altitude and latitude fixed the course toward the KISKA.

Enemy Torpedo Struck Home

05:15 We were avoiding the “MacArthur’s reef’ and at 158 degree 12 nautical miles distance of Segula Is. changed course toward 255 degree.

[*2] 05:47 “Torpedo! starboard fore!” Suddenly the signal master WAKISAKA first sergeant shouted. I saw two torpedo wake overlapped at I 00m starboard fore in the course crossing 45 degree with us, rapidly approaching. I at once ordered “Full turn starboard”. As KANO MARU went fast and the rudder turned much degree, the ship change the course rapidly toward right. I prayed the god and got tense for few second that was the most intolerable moment in my life. One torpedo wake passed after the stem we could avoid it, but other one hit the machinery room starboard side where was my back below and large explosion and sound occurred that was like a rumbling of the hell ground.

At the same time the main engine instead of its usual rhythmical sounds, but made two or three times of inertial dull movement and stopped. I was shocked by the force from the bottom and seized unconsciously the handrail and the base of compass. In a moment the machinery room was flooded whole, main engine lost power. We could not go anymore. Also generator, radio communication equipment and other auxiliary machines absolutely stopped. It was my disappointing but I had nothing to do.

The crews and soldiers seemed they did not feel fear at all and was vigorously preparing anti submarine combat or launch work of seaplane.

Lucky Dud Torpedo

This time we found a periscope of the submarine at very near right fore. Immediately 8em gun and 13mm machine gun started shooting. 8cm gun has less possibility to hit the submarine, but we thought the sounds of the gun was the only way to tell the KANO MARU’s crisis to the KISKA base. And also the 13mm machine gun fire were useless against the sub under the water but the splash aids the aim of 8cm gun crews. Further more the 8cm gun on the aft poop deck was malfunction by the heavy shock of torpedo explosion.

The periscope that had been right fore, gradually moved to right aft. 05:57 From the right 157 degree 300m distance the sub shot second salvo. One torpedo wake line from right aft passed below the ship bottom at about the bridge. It was no harm. How lucky we were I To transmit our crisis to the KISKA base, we planned as the last way to use the seaplane that we had picked up yesterday. The plane was still hooked under the derrick. The whole crews set the plane on the sea surface by only man power. The pilot, NMRASAWA the second class air soldier, tried much efforts but the engine never started so we had no way but accept the fate. The periscope sometimes appeared and moved from the stem to the portside. 06:07 From the left 135 degree very near the sub shot third salvo. Three torpedoes wake came toward us. Perhaps the sub shoot whole the rest torpedoes in the tubes and intended to finish us. I gave up whole, thinking the torpedoes must finish KANO NLARU and hard to breathe in more than ten seconds. Two torpedoes hit! but nothing happened! One torpedo struck the bridge fore, No.2 cargo hold. But unexpectedly it didn’t explode, lost its head and the rest body floated on the water tail down and about 0.5m part dry. Next one torpedo struck amidships portside, but also dud. Last one torpedo went away passing near ship stem. How lucky we are! I thank to the God protection.

06:10 Then we find the periscope at left 13 5 degree about 400m distance. Our forecastle 8cm gun and 13mm machine gun again started the fire. The sub kept the periscope up and moved calmly ignoring damaged KANO MARU. We clenched our fists but had no way to do. [*3] Then the sub seemed to begin to surface. The conning tower made ripple on the surface and the wave began washing the conning tower. I think the sub was unable to sink KANO MARU by the torpedo ( reload the stocked torpedo to the tube needs much minutes ) so finish KANO MARU by its gun or the sub thought KANO MARU could do no harm any more. Just then a 8cm gun shot hit the washing wave, made water column and dull water explosion sound. Also we saw the swell of heavy oil. All crews shout ‘BANZAI!’

Aiura: second source:

This day’s antisubmarine combat action aimed only the periscope so continued intermittently about 20 minutes. At last we found the periscope at left 135 degree about 400m distance, immediately start firing. The fourth shot that was the 84th counting from the first shot got directly the target.

About this submarine sink, later coming mine layer ISHIZAKI and other ships observed much oil spouting, piece of lifeguard buoy, chips that seemed to be the material of submarine decks, and other many things. They confirmed the sinking and radioed to the Fifth fleet and combined fleet chief of general staff through the fifth guard command. [*4]

More important info is about the torpedo. They, IJN Kiska base soldiers, took the rest of the torpedo which lost the head and floating near Kano Maru. They towed the torpedo body by boat to the Base, and observed. Aiura reported, apparently the head and body connection was irregular work. Because it seemed more than 30 bolts hole exists around the head-body connection part, but only three bolts were used and the rest was partial 10cm length silver-wax work not by welding.

Aiura said it is apparently not regular navy-yard work.

Aiura also reported, he thought the maneuver of the sub was strange. The sub seemed to have approach to the aft behind of Kano Maru shadowed from the forecastle deck gun, but is was too near to keep safe from the gun fire. Forepeak deck gun could at least bow or aft-end of the submarine. And Aiura said also it was strange 5 in 6 torpedoes were dud. 2 of them hit but not exploded. Aiura said the torpedo men must forgot the normal procedure to unlock safety pin.

AIURA’s Torpedo Analysis

Second Witness: RIKIMARU NAKAGAWA, ARMY Medical Sublieutenant

While I was looking aft on boat deck(port) in front of my cabin, I saw bubble running on the surface figuring half circle just 2 or 3 hundreds meters apart from shipside. I shouted ‘Must be Submarine!’ Then just at the head-end of bubble running, big black-brown water arose, something oily afloat on the surface, after a while a black thin bar appeared on the surface then fell down and submerged. I in spite of myself cried out ‘Good, You got what you deserved!’, and I am convinced the submarine was sunk by the shell. ”

Rikimaru Nakagawa.jpg

Third Witness: was a “contract newspaper man” who visited the site after the attack.


I had got on a motor launch and left KANO MARU from portboard.

The boat ran over the stem and went to the starboard side of the ship.

Then I saw at left side of the boat, about 200m apart from my boat, that was 500m from KANO MARU’s stem, something was afloating on the surface. That were two cylinder, like oil drum, upright about 0.5m over the surface.

I found that objects but couldn’t approach so I didn’t understand what they were.

When my boat went on to the starboard and aft side of the KANO MARU, I saw the same two objects at the front of my boat, 50m distance, that was about 400-500m from the KANO MARU’s stern.

Again I didn’t understand what they were.

Later I saw a torpedo at land, that was without explosive warhead, picked up by the same(my) DAIHATSU boat.

Then I heard from Navy men (about the torpedo), and I understand what I had seen were torpedoes that lost its warhead and afloating upright.

SYOWA 17(1942) August 3rd PM 03:30
Army contract newspaperman

Kenji provided a sketch. Two copies are provided each with slightly different annotation


Kenji Hamada -1.jpg

Few submariners think that a 3 in shell that hits the wake caused by the conning tower would sink a Gato class sub. However we must consider the proximity of the 84th shot and “dull water explosion” Whatever happened was in some way amplified by the shot. Most submariners are aware of the poor quality of the MK 14 torpedoes. They pre-exploded, would run low, did not detonate upon contact and occasionally would circle around. In addition the submarine batteries were subject to explosions and fires. The exterior torpedo doors were closed manually and such things as a displaced gasket would be devastating.

It also should be noted that the sub did not implode / explode which would have happened if the compartments had been closed off. This tends to imply that what ever happened was rather sudden.

On the slide path there appears to be some debris, some fairly large, and about 25 ft of the bow of the sub appears to be buried or blown off.

133 Comments for 'The Attack Analysis'

    January 10, 2008 | 11:37 am

    Hi Ed! Nice to see you back.

    Risk Analysis…You won’t get any flak from me. I used a Threat weapons severity/probability Matrix, and sent Bruce the crayola version using flow charts. Same difference, same results.

    Torps 1/1 and 1/2; This revolves around the transcription error with the range the torps were spotted out. At 1 klick there’s the chance KANO got her butt around. Shorter range, it’s unlikely, and torp 1/1 (aimed forward of bridge) missed astern.

    “Why a Port Turn as opposed to Starboard Turn?”; Because a starboard turn would have taken GRUNION right under the KANO’s stern.

    “if Grunion Sonar knew the #6 Torpedo was still running”; Unlikely IMO. At 0607 when GRUNION fired the last salvo, torp 3/3 would have been completing the loop astern and lost in GRUNION’s prop noise. After making the 180 course reversal, she’d have left a big knuckle in the water, would have just been clearing it at 0610, and was undoubtedly making alot of noise adjusting trim.

    Any way to SWAG the KANO’s rate of deceleration and drift? Or figure an approximate residual forward speed for KANO at the various time hacks? This is also key to clearing up the attack plot…

    January 22, 2008 | 2:20 pm

    Attack Analysis Methodology

    Some of the analyses that are going on are problematic because there is not clear agreement on the methodology of determining a credible hypothesis. I think everybody’s time will be used more efficiently if we spend a few minutes thinking about that methodology

    Our objective is to find a credible hypothesis for the loss of the Grunion.

    I would start with there is no complete or absolute credibility. Credibility comes only from a stream of consistency. The longer the stream (with out inconsistencies) the greater the credibility.

    Consistency is learned associations. There is nothing magical or “true” about those associations. A child that over and over watches Dumbo the elephant fly will hypothesize that elephants can fly. An engineer who has learned that the buoyancy on an object is consistent (equal to) with the weight of the displaced fluid will consider that a credible hypothesis. Winning is consistent with the Patriots.

    Credibility is not a state of the real world but a state of the mind. What is credible to one individual may not be credible to another. Likewise consistency is not a state of the real world but a state of the mind.

    Therefore I think this methodology requires that we decide whose minds have to be convinced. I would argue that it should be a group of individuals with at least the following characteristics:

    o Submariners, or those with excellent knowledge of subs

    o Independent thinkers, not prone to go along with the crowd

    o With enough interest, enthusiasm and energy to thoroughly study the situation

    o Knowledge of submarine physics.

    I would also argue that selection should be natural, ie those who want to be part of and will contribute significantly to the process. No dictators for the selection.


    There are dangers in starting the analysis with hypotheses that are all encompassing or hypothesis of complex transformations. Because of that complexity it is very easy to miss inconsistencies. In addition it is also easy to get attached to one’s hypotheses and then ignore or skip over the inconsistencies. That process of ignoring inconsistencies I label “neurotic censorship”. It is very common.

    For these reasons I think we should start as much as possible with elemental hypotheses, hypothesis of simple transitions or patterns. Then we should generate an organized list of those hypothesis that have enough credibility that investigating them is no longer worthwhile. In the vernacular we call those hypothesis with high credibility “facts” meaning only that we will not spend more time investigating them, and that we “believe”, that we “know”. We are saying that (for us) the stream of consistency for those is long enough to have passed the threshold of believability.

    Here are some elemental hypotheses where the credibility (for me) is high enough to be labeled “facts”

    It is the Grunion
    There is hydrostatic damage to the hull
    There are several cracks in the upper part of the hull
    The superstructure is almost entirely gone.
    The AB Hatch was blown open
    There is crimping around the bow. The bow is gone.
    ET is a holder for insulators
    There is hydrostatic damage to the conning tower.
    The loss of the sub was the result of a very complex transformation
    The 84th shot had something to do with loss of depth control.
    The sub was close to the surface, or was surfacing, because of operational problems.

    Once we have developed this list of “facts” we can get into the all-encompassing-hypothesis
    mode submitting them to our “heavy hitters” to see if discrepancies can be found.

    That process (using the heavy hitters) actually will provide a measure of credibility. Unanimous agreement indicates high credibility. 50% indicates pretty low credibility

    The methodology being described here is, of course, itself a hypothesis and will only be credible for those individuals who have checked (thought about it) and found with it a stream of consistency.

    We are fortunate in this situation because we are not locked to any administratively determined methodology for determining credibility, such as a jury system, a board of inquiry, or peer review as used in academia and in the medical field.

    We can choose the methodology that will come up with a hypothesis that over time is most likely to extend that stream of consistency. That is what the methodology of science is all about.

    It is an exciting opportunity.

    Terry Terrass
    January 28, 2008 | 6:30 am

    27 Jan 2008

    Hypotheses for Loss of Bow

    One theory advanced for the loss of the bow is the explosion of a circular run torpedo. In my opinion past postings from a number of sources provide a variety of reasons to reject this theory.

    In lieu of this theory, based primarily upon the location of the break as shown in the sketches by Jim Christley, I am now inclined to believe that the bow was lost as a result of the dynamic forces experienced during the descent to the bottom. The portion of the pressure hull near the break had a number of penetrations. The largest, and the ones I believe most significant, were those to port and starboard for the shafts to tilt the bow planes. As best I can determine the next largest penetrations are the two extending upward into the superstructure, one for bow plane rigging and the other to drive the anchor windlass and capatan. In addition there are several other smaller penetrations with shafts for controlling windlass/capstan (and perhaps other) functions from topside. I believe that the combined effect of these penetrations resulted in the hull being less able to resist stresses caused by depth increases in this area than in portions of the pressure hull without these discontinuities.

    Even more significant than the stresses resulting from increasing depths, in my opinion, are those which were very likely to have been caused as a result of the bow planes being rigged out. The large surface area of the planes, when combined with a sinking velocity much higher than designed service speeds, could produce a total force much larger than what would be experienced when conditions were normal. On the assumption that the ship was trying to surface immediately before it was lost, it seems most likely that the bow planes were on “full rise” while the ship was sinking and, if so, the dynamic forces would have caused the hull to start rupturing on its’ under side as a result of tension. It is felt that the possibilities of rupturimg in the oppositte direction or with an axial twist would be much less likely.

    Between my poor skill in anayzing the photos and only limited time for trying to do so, I leave it to others to see if there is any indication of the validity of this hypothesis.

    The after portion of the Forward Torpedo Room could be another area particularly prone to stress failure because of its configuration with both the torpedo loading hatch and the transitions with the Forward Battery compartment bulkhead and No. 1 Main Ballast Tank. Perhaps this is the reason for the “major structural break” indicated by Item #3 on the Christley sketch. Here, without the forces caused by the bow planes, there was a break but no separation.

    Terry Terrass

    February 3, 2008 | 10:41 pm


    If the bow section was in its place during the sinking, certainly the bow planes would have behaved as you suggest. But, the links of those to the rest of the hull structure were just steel rods, designed to sustain the stress at, probably 12 to 15knots (ths sub´s max submerged speed was 10), so I believe that they would have been torn away before they could have caused any damage to the pressure hull.

    You mention the cracks in the forward section, and the hull rupturing from below. I don´t think this would happen: the pressure hull was the same thickness both up and down, so I don´t see why it would break near the keel first (of course it had to break somewhere first) but, precisely, the keel should have acted as a reinforcement to the structure in that place. In the upper portion, however, there were no longitudinal strength members (someone correct me if I´m wrong, this is important), so the weakest point should be in that area.

    BUT, carrying on with your analysis, and taking your observacions a little further: there are cracks both fore and aft. Do they look the same. NO. Cracks fore have there edges bent and the hull seems disaligned. Cracks aft have prefectly flat edges, and the plates have only separated, no misaligment here. My conclusion is that the cracks towards the bow were caused be compression of the upper pressure hull and the ones aft by traction forces.

    How could this have happened? If Grunion hit the bottom bow first (with of without the FTR still attached) the front end would have been bent upwards, causing cracks near the deck. When the aft portion of the boat hit the bottom it must have suffered a “whip” effect (remember that crush depth had been passed long before so probably these compartments where also flooded) and produced the cracks with clean edges, just tearing the plates appart.

    Further clues (don´t dare to call them evidence) to a bow-on impact:

    Absence of damage to stern (not even bent prop blades)
    Periscope shears bent forward (if attack periscope was still up it must have “whipped” too, the force being so strong that the mast broke appart and now lays across the deck)
    Gun pointing upwards. It has been suggested that the whole pedestal in the wrong position, as if laying on deck, but if you look at pictures of similar guns you will notice: gun barrel and layer´s seat should be parallel (they are not) and trunnion support structure should be perpendicular to barrel (they are in the same line). Besides, the small elevation gear that worked with the large one under the barrel is missing. So the barrel is free in elevation. This damage could have been caused by the large gun´s inertia when hitting the bottom.

    So, you see, we have both looked at the cracks, just got different interpretations.

    Terry Terrass
    February 9, 2008 | 9:11 am


    Guido –

    Here are some comments relative to your response to my “bow plane hypothesis” posting.

    There are several references below to John Alden’s book “The Fleet Submarine in th U.S. Navy”. They are not .specifically applicable to the GATO class but are probably applicable to at least some extent.

    I mentioned the shafts for the bow plane rigging and the windlass/capstan not because of whatever effect they may have had by themselves but because of the contribution to pressure hull stresses which may have been induced by their hull penetrations. Reference page 17 states ”…deficiencies were discovered in the longitudinal strength of the F-4 and F-5 hulls and there were local structural weaknesses around the escape trunk and windlass room above the forward torpedo room. The upper part of the pressure hull had to be increased from ¾ inch to 1 inch plating in some places, along with other design changes … “. There was no mention of keel strength.

    I have several problems with the way you characterize the pressure hull in the torpedo room. First, the hull in the torpedo rooms is “single hull”, not double hull like the other compartments. Secondly, and perhaps more important, besides being single hull the torpedo rooms do not have the circular cross-section of the other compartments. The bottom of the FTR consists of large flat horizontal plates. The center portion of these plates form the bottom of the lower reload torpedo pit while the higher horizontal plates extend outward from the top of the pit to the pressure hull. This arrangement is pictured on page 215 along with sketches showing the differences in the Electric Boat and Portsmouth designs for framing in the non-circular hull sections. The lower/outer side of these plates is subject to full sea pressure in the underlying No.1 Main Ballast Tank.

    In the discussion of hull strengths and the need for circularity the following quotes from pages 16-18 may have some applicability to an explanation for the loss of GRUNION’s bow:

    “ … the greatest hazard is local structural failure at a so-called “stress raiser” – a place where a hard spot, discontinuity, or flaw in the structure causes the stress to be concentrated. As their understanding of these factors grew, submarine designers and builders tried to avoid abrupt changes in the shape of the hull and to maintain perfect circularity of the cross-section wherever physically possible”.

    On page 215 there is a note on the sketch of the Electric Boat design for the framing of non-circular single hull design which says, in part “The larger bending moment gives a stress to be added to the P/A stress. By trial and error scantlings are increased as necessary to reduce stress to an acceptable level.” (I assume that P/A means ‘pressure/area’)

    As an attempt at a “bottom line”, even though the forward torpedo room is designed to withstand the same submergence pressures as the rest of the ship, I doubt that it has the same inherent safety margin as the other compartments with their circular hull sections. I do not know how to quantify this but think that the typical slight “dishing” of the pressure hull between the frames in the circular hull sections illustrate my contention that hull strength is less in the torpedo rooms than elsewhere. Likewise, because of the different configuration, my intuition tells me that there could be greater stress concentrations in and around the torpedo rooms, whether caused by submergence pressure or dynamic forces, than those in the circular hull sections. Beyond that, if the basic design process required trial and error, it is unlikely that there would have been much, if any, analysis of abnormal contingency situations which might have been experienced by GRUNION.

    Given all the variables of hull strengths and stress concentrations I will not hazard a guess as to just what caused the various indications of damage which have been observed.

    Likewise I would not hazard a guess as to what happened to the deck gun although it must have been quite severe.

    The situation is somewhat different for the periscope shears. Based upon pictures I have seen and stories I have heard about boats getting hit by surface ships it appears that it does not take all that much force to bend the shears over, especially when one considers that they represent a long lever arm. Perhaps, as a contrary view, is the experience of USS IREX (SS 482), a Fleet Snorkel boat, after getting a new fiberglass sail. In February 1960, after a big ASW exercise in the very rough seas of the Greenland-Iceland-UK gap my boat passed IREX which was then enroute to a port visit in Belfast, Ireland. IREX looked weird – the rough seas had knocked off the entire fiberglass sail so that all we saw was the periscope shears but they all looked upright and intact. Back in port she got a new sail but I never learned any details about what had happened although I am surprised that the periscope shears would not have suffered extensive damage. I think that I did hear, much later, that larger and stronger fasteners were used for the replacement sail.

    Post Script
    After preparing the above I went back and reread your comments, and specifically those which stated “….the keel should have acted as reinforcement to the structure in that place.” When I had first read those words I was inclined to agree but then decided that I had better learn something about the keel. When I could find nothing about keels in my reference books, I went on the Web. I could find nothing definitive there either.

    Failing in that attempt, and at the risk of being very wrong, the views which follow seem to be consistent with what I can remember.

    Rather than the keel in a GATO class sub being a longitudnal strength member as you imply, I now think that the term “keel” refers to a location at the bottom of the ship, mostly in the bottom of a tank, rather than a part of the hull.. I am not sure that there is a keel structural member as such but if there is one at all I do not believe it is a major strength member. For the compartments between the torpedo rooms I believe that the double hull construction provides the strength which comes from the keel in surface ships. There are vertical plates in the bottoms of the ballast tanks which are located where I would expect to find a keel. I would not consider them as serving the function of a keel since they seem to be relatively thin and intended primarily to hold the outer hull to the pressure hull. If this is correct they would not need to be thick and strong since the outer hull does not have to withstand full submergence pressure.

    Although my view may seem inconsistent with the mention of a “keel” in the following quote from the on-line version of “The Fleet Type Submarine” I think that my view is basically consistent with the actual construction features and the relative lack of hull rigidity.
    (Start of quote)
    The construction of the submarine, therefore, is on the basis of the fabrication of a series of watertight containers into one large watertight cylinder by means of watertight joints. However, since the submarine must operate at times at great depths, these watertight containers must be strong enough to withstand the pressure head of sea water at that depth. Therefore, the watertight containers must be pressure vessels, that is, watertight containers or cylinders capable of withstanding great pressure. The fabrication of these containers into the hull of the vessel is illustrated in Figure 1-11.
    Pressure vessels, while capable of withstanding great pressure, do not in themselves possess great rigidity. Being subject to mechanical action (leverage), they must be secured to each other by one common strength member (the keel), as well as by watertight connections (bulkheads). The submarine with its keel, pressure hull, and watertight bulkheads is shown in Figure 1-12.
    In the double-hull type of submarine, the pressure hull is inside the outer hull; between the two hulls are the water and the fuel oil tanks. The double-hull construction extends from the after bulkhead of the forward torpedo room to the forward bulkhead of the after torpedo room. (End of quote)

    In my view the keel, whatever it is, plays a minor role in holding the compartments together; primarily this is done by the watertight connections (bulkheads). Although a substantial keel might have prevented the loss of GRUNION’s bow, the cumulative experience of the GATO class would seem to indicate that it was not necessary.

    Terry Terrass

    Travis Mullan
    February 11, 2008 | 4:56 pm

    Further to the discussion on the effect of excessive pressure on diving planes please see the following:

    April 5, 1945 at 1525 the bow planes on the USS Spadefish were unintentionally rigged out while at flank speed in rough seas.

    The stbd rigging gear was carried away immediately. Also, the universal joint was fractured.

    The collar on the horizontal shaft sheared off at the drift pin.

    There was no damage to the port side diving plane.

    There was no damage to the hull in way of the shafts for rigging or actuating the planes.

    The crew was able to make temporary repairs although they failed later in the patrol.

    Permanent repairs were made at Guam at the end of the patrol.

    For ballast tank arrangements see ——-
    Line drawing of the General Arrangement of the Gato (SS-212) class, main and fuel ballast cut out.

    You will see that the Number 1 main ballast tank is in the after portion of the Forward Torpedo Room. The tank top serves as the deck in this area. The bow of the Grunion broke off in the “pit” area a few feet forward of the number 1 tank.

    February 14, 2008 | 5:18 pm


    Something as obvious as the MBT1 forming the deck of the FTR I completely missed. The only sources I have are cutaway diagramas and I simply undestood that the ballast tank form I could see didn`t represent its complete form, and that it was round. Certainly I would have never thought of leaving a flat surface exposed to sea pressure. A terrible mistake on my side.

    Taking that into account I have also some comments, for which I hope you´ll be able to send some feedback.

    Regarding the safety margin, I believe it is called the “safety factor”. That means that the crushing depth must be equal to the max diving depth times safety factor. (A safety factor of 2 in a design for 300ft diving depth implies a crush depth of 600ft). It is also spplied to the whole submarine and not to individual compartments, as it is meant that none of them will implode at any shalower depth. After that, one compartment may be stronger than others, but that`s a plus.

    The flat top of MBT1 puzzles me a little. You mentioned “stress raisers”, and certainly the anlges around the pit worked as such. But the tank top is not even distorted, and the implosion occurred around the pressue hull (very close, I think, to the penetratrions you mention for the capstain/windlass). Even more, the top of a small tank that was immediately fore of MBT1 is still there, completely flat, but the tank is gone. What kind of tank was this?

    About the keel: I`m not that proficient in English to know the word for the lower part of a tank. But I do concur with the quote from “The Fleet type submarine”. However, the keel is not just a piece of steel In many cases it is a structure, the objective of this is to achieve greater strength using relatively thin plates. In many cases it will take the shape of a long box. Many (not all) of the side tanks were divided in port and starboard, so I suppose that a solid piece of metal separated both at the bottom, that would be the keel. Also remember that subs were intended to “bottom” and a substancial structure should keep the lower portion of the outer hull to crush, as well as during dry docking.

    Your posts made me realize that me thesis that a dud had hit Grunion and that the “black water column” was a product of the torpedo`s air flask explosion is wrong. The forward-most fuel tank is still further aft of MBT1, well clear from the missing bow, so it could have not been damaged by a dud torpedo.

    But I`m still against a direct hit, as this should have hit exactly at the same frame where the reload torpedoes warhead would have been. This wuold have set a chain reaction and the reloado torpedoes would have exploded as well, but the damage to the submarine would have been much more extensive.

    Furthermore, alll imploded compartments did so along there top side, but the FTR shows signs all around the hull, except at the bottom, which you would expect to be the weakest point especially after the design around the hull penetrations had been modified. Did the FTR implode above (shallower) crush depth? That would be a clue to that it was damaged before getting that deep.

    Travis`post makes me believe that the bow planes didn`t do any damage to the pressure hull, in the worst of cases they were torn away or distorted.

    But then, what caused the sinking? Did the shell hit the FTR? If so, why did it implode? It should have been flooded.

    I think I´m more or less where I started, but as interested and intrigued too!

    Terry Terrass
    February 16, 2008 | 7:42 am


    Travis Mullan –

    Here are some comments relative to your information about SPADEFISH and her unintended bow plane rigging incident.

    I question what inferences applicable to GRUNION can be made from the SPADEFISH experience given the following.

    SPADEFISH was a “thick skin” BALAO class boat whereas GRUNION was a “thin skin” GATO.

    Presumably the SPADEFISH bow planes were in the “15 degree dive” position normal for surface running rather than “full rise” which was probable for GRUNION given its circumstances. This would reverse the direction of the forces acting on the hull. (Note: Rigging in the planes at 15 degrees dive, rather than at zero, to decrease diving time, was a modification phased in as the war progressed but was undoubtedly incorporated into SPADEFISH during construction.)

    Depending upon how far the bow planes had rigged out by the time of their failure an analysis by resolution of forces would indicate that the magnitude of the forces tending to move the bow vertically would be less than if they were fully rigged out.

    The force exerted on the hull by the planes, whatever their position, would be affected by the speed through the water. Even with a flank bell rough seas might keep velocity less than would be expected normally. Beyond that, while I do not really know, I think that conventional wisdom contends that velocities reached while sinking to the bottom can be quite high.

    I have not been able to find enough information about the rigging mechanisms to form any opinion as to just what SPADEFISH parts may have failed or the effects of those failures. I am surprised that the crew would have been able to effect any repairs, even temporary ones. Likewise I have been surprised by the number of times that bow plane rigging problems have been mentioned in my various readings. These mentions have not included any elaboration beyond indicating that the problem was repaired.

    Predicting just where and why rupture would have occurred in any single hull, non-circular hull.section is too difficult to be meaningful because of the number and complexities of the design, construction, and environmental variables which might be meaningful. This seems to be particularly applicable to #1 MBT and adjacent structures.

    Where Is the Bow?
    To the best of my knowledge this is still a mystery. As a far-out speculation I will mention a possibility I saw on a TV program with a variation on the UFO theme. It was about unidentified submersible objects. I don’t remember its title but think it was on the History Channel. The part that caught my interest was a search for a light plane which had crashed some years before in the channel between Catalina Island and the Southern California coast. Notwithstanding what was considered to be reliable evidence of where the crash had occurred, divers were unable to locate the wreck. One theory advanced as to where to continue the search was that sea action, over time, would probably carry a relatively light object such as the plane further down even a fairly gentle slope. Given the characteristically rough Bearing Sea conditions and the depth at which the GRUNION hull has been located, is it possible that the bow lies further down slope? To what extent was this area explored during the searches made to date?

    Terry Terrass

    Terry Terrass
    February 18, 2008 | 8:47 am

    17 February 2008

    Guido –
    Several comments relative to your posting of February 14, 2008.

    The cutout drawings are useful for many purposes but fall short when trying to answer certain questions. This is especially the case for trying to understand the Forward Torpedo Room.

    Relative to the “safety factor”, at least as I think it was used for the design of GATO era submarines, it was not used as a design objective but rather represented a minimum acceptable criteria for the whole submarine and all of its component compartments and structures. Whether by choice or by circumstances, many submarines took advantage of the fact that the actual design was much more conservative than called for by the “safety factor”.

    Maybe it is just semantics but I do not believe that the FTR “imploded”. I am more inclined to believe that the bow broke off due to the differences in forces acting on it as compared with the forces acting upon the hull further aft with the location of the break being determined by where the stresses were greatest.

    The small tank forward of #1 MBT is the WRT (“Water Round Torpedo”) Tank. I think that it was designed to withstand full submergence pressure but would normally be vented to the FTR except when being used to flood the torpedo tubes. I do not understand what you are saying about the top being there but the tank being gone.

    The vertical plates at the bottom of the hull sections come closer to serving the function of a keel than of being boundaries between the port and starboard side tanks. For the MBTs the port and starboard tanks are effectively interconnected by having open flood ports. For the fuel tanks the port and starboard tanks are interconnected by openings in the vertical plates so that only one compensating water line is needed. The distinction between the port and starboard sides comes from what occurs at the upper end of the tanks. For the MBTs the port and starboard tanks need to have separate piping to vent the air to submerge the ship. The fuel tanks need to have separate fuel filling and transfer line connections to the tops of the port and starboard tanks since the fuel rises to the top of each tank.

    The strength of the keel, whatever the keel is, is much more important for drydocking than for bottoming the ship because in drydock it has to support the full deadweight of the ship. For bottoming it only has to support the negative buoyancy which is minimized for a variety of reasons.

    Please see my posting responding to the posting by Travis Mullan relative to the role the bow planes may have had in the loss of GRUNION.

    As I have stated before, and continue to believe, GRUNION sank as a result of the cumulative effect of flooding in several locations, most significantly in the Forward Torpedo Room through one or more torpedo tubes and in the Main and Engine Air Induction piping penetrated by KANO MARU shell hits. As I have stated before I believe that many of the 84 shots fired by KANO MARU landed in the water and continued on with enough velocity to cause significant damage by their impact even though they did not hit with enough force to cause detonations which could have been seen by observers on the maru.

    Terry Terrass

    Travis Mullan
    February 19, 2008 | 7:04 pm

    The question raised by Guido of why the ballast tanks were not imploded has only two possible answers. The first answer being that they were damaged and became flooded preventing implosion. This is highly unlikely since all would have had to have been damaged individually. The second answer is more interesting in that they were most likely flooded for the initial dive and equal to the surrounding sea pressure while the Grunion was submerged. This would mean that the Grunion was not surfacing when the Kano Maru made the final shot from her 3” gun. An earlier posting stated that Commander Abele was probably encouraging his diving officer to counter a broach and get the boat back down. This is most likely correct. Remember that the Kano Maru had been firing steadily at the Grunion during the attack and the Commander must have been aware of this through his periscope observations and reports from his sound man. I can not think of a reason why he would voluntarily surface his boat under the gun of an enemy. Evidence of the ballast tanks being flooded is found in the intact appearance of the #1 ballast tank in the FTR and in other areas of the hull. The outer portions of the sub, visible to the observer, are the ballast tanks. The pressure hull lies between the crescent shaped tanks and serves as the inner side of the tanks. On the Grunion today, the tanks around the control room and crews mess room seem to be intact along with the pressure hull. In areas of the hull where the pressure hull imploded the tanks have been distorted only at the top. There are cracks in some of the tanks which are likely due to implosion induced stress or impact with the bottom. This would indicate that these tanks were flooded.

    Another item observed in the photos and video is that the main induction piping does not seem to have imploded. There are many breaks at the flanges, however, I was not able to see any distortion of the piping. It is possible that the main induction valve, housed in the after portion of the conning tower fairwater, was damaged by shell fire. The starboard side of the conning tower seems to have been damaged significantly more than the port side. The starboard side of the Grunion was presented to the Kano Maru over the majority of the attack while the port side only at the end. There are many other accounts of American subs being fired upon while at periscope depth but reporting no damage……

    Lastly—the reference to the Spadefish was only as a reference with the differences in construction well understood. There are few reports available outlining the damage caused by inadvertent out-rigging of the forward planes at speed. Another 2 cents worth.

    Terry Terrass
    February 25, 2008 | 9:00 am


    Travis Mullan –

    Here are some comments relative to your posting of 2/19/2008 re imploding of ballast tanks.

    I assume that the term “imploding” implies that the tank ruptures as a result of excessive differential pressure on the tank boundary rather than being subjected to external forces such as pressure hull failures.

    Given that the ballast tanks have flood port openings close to the keel there is no way they can be subjected to static differential pressures exceeding the difference in sea pressures between their upper and lower volumes, about what they experience when they have been blown for surface running. Transiently they could experience higher differential pressures while there is flow through the flood ports but this would be for a very brief period and with a relatively small pressure difference.

    The statement is made that “Grunion was not surfacing” because the tanks were undoubtedly flooded from the initial dive and were equal to the surrounding sea pressure. Had the GRUNION been trying to surface they would have blown the tanks with their normal ballast tank blow system and the tank pressures would still equal the surrounding sea pressure.

    I think that it is quite likely that the water column which was mentioned in the KANO MARU report resulted from the hit by the 84th shot releasing whatever air was in a ballast tank, either by penetrating the outer skin of the tank or, less likely, by rupturing the vent piping between the tank and its vent valve. The decking would tend to break up the air column if it came from the vent piping.

    The point has been made previously, by both me and others, that GRUNION was probably trying to surface, not because it wanted to but because it seemed necessary in order to keep from sinking.

    Although I am confident that the ballast tanks did not implode because of differential pressures I think that it is quite likely that they could have been damaged and/or distorted by the implosion of nearby pressure hull compartments.

    Terry Terrass

    February 26, 2008 | 7:41 pm

    Hi Terry, sorry for the delay.

    Thanks for the explanation about the “split” fuel tanks. Openings in the keel structure could be made for different puroposes without lessening its strength. Of course it would have to bear more weight when drydocking, but in that case the weight distribution would be more or less even, as the wooden blocks under the hull must be at the same height. However, I think that when bottoming, the bottom´s shape or just a rock may result in an uneven surface and thus greater local stress than in drydocking.

    About the Water Around Torpedos tank. As far as I can see from the schematics on navsource (check Travis Mullan`s post regarding this source) the floor of the FTR was formed, from stern to bow, by the top of MBT #1, Water Round Torpedo tank and Trim Tank#1. In the photos of the “pinched bow” you can see the front end of MBT#1 and a horizontal deckplate forward from it. This would be the top of the WRT, and you can see the beams underneath it. But the tank´s sides and front are missing completey. There`s no sign, that I can see, of the Trim Tank.

    I`m not sure either tha the FTR imploded, but I can`t dismiss it completely. All other (visible) comparments imploded along their upper side, but the FTR didn`t. Actually the remaining upper side between the fore Battery bulkhead and the escape trunk is intact. If there was an implosion, it was a radial one, pinching the round pressure hull but not its flat bottom.

    The flooding theory doesn`t convince me yet, as it would reduce the differential pressure and the risk of implosion. The flow of water through either a torpedo tube or a shell hole can be easily calculated, and would result in a complete flooding after a few seconds in the first case and maybe a minute in the second. There have been some posts regarding floodings along the discussion.

    I would like to ask you a couple of questions regarding the induction and exhaust pipes, for your observations are very interesting:

    I`ve seen a picture of the main induction valve with a piece of metal sticking out of it. This valve should have been shut, can you see if it is in the open position?

    Also, if the pipes had been damaged, there would still be the hull exhaust and supply valves, also in the shut position. The aft compartments are imploded so they were not flooded; what would have been the efect of the flooding of the pipes only? Stern heavy, up bubble, broaching? Could this have left a trace of bubbles on the surface, looking like a torpedo`s wake?

    Even answers to this wouldn`t solve the mistery, but at least would explain a couple of minutes of the story.
    Travis Mullan navsource

    Travis Mullan
    February 29, 2008 | 7:29 pm


    Check the following link for construction details of Gato boats:

    The following boats list construction, etc in their image links:

    SS 283 Tinosa construction
    SS 266 Ray construction
    SS 270 Raton construction and shows a fleet boat surfacing. Looks like the bow was up before the shears.
    SS 266 Pogy construction
    SS 258 Hoe under deck repair—good exhaust and induction shot.
    SS 239 Whale keel
    SS 237 Trigger keel
    SS 236 Silversides main induction
    SS 226 Corvina torpedo hit
    SS 259 Jack torpedo hit

    Hope this helps.

    March 19, 2008 | 1:00 pm

    The report from the 2/27th New London meeting can be seen at in the Grunion folder. Select Hypothesis Report.pdf.

    Christley, Thompson and Galler have provided a very objective analysis of what we know today about the loss of the sub. It is worth studying carefully.

    Tom Rabil
    March 21, 2008 | 7:05 pm

    Having seen the Jonas Ingram be torpedoed, I would expect the explosion to have been significantly greater.
    If the sub was preparing to surface, it would have gotten the diesel ready and opened the exhaust and induction valves. If anything happened in that critical transition period, the sub would have had little speed capability and been much more vulnerable to flooding from a smaller shell hit. Also, if a ballast tank was breeched, there’s that much less buoyancy available for recovery.
    If the sub went down nose first fast, taht could account for the damaged bow.

    March 29, 2008 | 1:12 pm

    I have reviewed the Sinking Hypothesis Report from the Christley, Thompson ,Galler meeting of 2/27/2008. I agree with their report in many respects, most significantly that it is highly improbable that GRUNION’s own torpedo caused her loss.

    I do have a number of comments that I believe warrant consideration. Their objectivity stems from several factors:
    1. My 18+ years of submarine experience on seven different submarines, five being Diesel. including USS CAVALLA (SS 244). GRUNION and CAVALLA were built at Electric part of the 1941 Construction Program with commissionings in April 1942 and February 1944 respectively. Although both boats were GATO’s CAVALLA incorporated some improvements resulting from wartime experience.
    2. Pasr personal contact with a number of WWII submarine commanding officers.
    3. Research based upon several detailed submarine books

    In my opinion the most notable omission from the Report is the highly probable flooding of the Forward Torpedo Room, a hypothesis I have tried to advance for some time. Mike McMahon in his E-mail of 3/22/2008 described the event as follows:
    “I think that a possible flooding in the Forward Torpedo Room during reload should be
    added as a possible cause. It is extremely likely that, after the
    expenditure of all 6 torpedoes loaded in the forward tubes, a reload
    of some of the remaining 4 torpedoes was started. We know from
    message traffic that there were no torpedoes remaining aft, and that
    Grunion had been depth charged the day before. We also know that Tang
    had a flooding casualty in a torpedo room later on in the war caused
    by depth charging. The inner and outer doors are interlocked but that
    does not prevent flooding as Tang demonstrates. If there were a
    severe forward flooding, that would explain an attempt to surface.
    Flooding in the Forward Torpedo Room would result in a heavy overall,
    heavy forward condition leading to a sinking by the bow. Finally the
    bow is broken off from the collision with the bottom.”

    Although not mentioned in the Hypothesis Report or by Mike, I believe that the bubble path observed by the Kano Maru after the 6th torpedo was fired is a very significant clue as to the Torpedo Room flooding, even though it has led some to erroneously hypothesize a circular run.. I believe that the bubbles came from air leaking past a torpedo tube muzzle door which was kept from closing tightly by the deformed door gasket while the tube was being blown down preparatory to reloading.
    Although blowing down the tube does not necessarily indicate that a reload was being initiated, there are reasons to believe that such was the case – (a) reloading promptly would be the standard practice, (b) having a torpedo ready might be useful for self-defense in case of need. It might be that the actual reload would have been deferred for some specific reason such as avoiding an up or down angle while actually moving the torpedo. If it were decided to surface for a gun action, the surfacing could be expected to produce an angle.

    The Hypothesis Report assumes that if GRUNION were surfacing it would be for a gun action. Not addressed was the possibility that it was desired to surface as a last resort in order to cope with flooding. The Kano Maru mentions seeing only the periscope and the “ripples around what looked like the “control tower” but did not report seeing the deck area. For a normal surfacing, and even more for a “battle stations gun action” surfacing, .given their proximity the Kano Maru should have had no trouble seeing the GRUNION deck area. This indicates that GRUNION was heavy from having already experienced at least some amount of flooding by the time it broke the surface..

    Since there were no more shots fired by the Kano Maru after its 84th (the one observed to hit), it is reasonable to assume that GRUNION sank before another shot could be fired, i.e. in less than 32 seconds. (If 84 shots were fired in the 23 minutes between 0547 and 0610, the average firing interval was 16.42 seconds).
    Assuming that Captain Abele ordered, or concurred with, surfacing it can be assumed that he hoped that GRUNION would be able to remain on the surface while the flooding casualty was corrected.and be able to escape. And, thinking ahead, he might even have considered the possibility of using the deck gun, either to finish off Kano Maru or, at least, to facilitate making his escape. To this end he may have alerted the gun crew to standby although it is debatable whether this could have been done concurrently with the efforts being made to cope with the flooding emergency..

    The Hypothesis Report infers that the shell damaged the main engine and/or ships ventilation supply valves. Penetration of the piping aft from these valves would have had the same effect. The Hypothesis Report indicates that this would result in adding 16-20 tons of weight. One way or another I think it highly likely that this piping did flood.

    Elsewhere in The Hypothesis Report this flooding was described as”a significant loss of buoyancy, a problem but probably not a serious one”..
    This compares with the following approximate sea water capacities:
    Conning Tower 24.32 tons
    Safety Tank 23.23 tons
    Negative Tank 7.51 tons
    Bow Buoyancy 31.69 tons
    The Hypothesis Report further states “Subsequent attempts to compensate via safety tank would have failed due to inability to pressurize tank.”. This was due to damage to the Safety Tank Vent port riser from the shell.

    The Hypothesis Report does not mention the debris including pieces of submarine decks and other flotsam seen by the mine layer Ishizaki nor the extensive topside damage which can be seen on the GRUNION photos. Although the Hypothesis Report statement “Okun has stated that the shell likely would not penetrate any great distance into the water” may be correct it appears to me that many of the shells must have traveled far enough to cause considerable impact damage even though slowed enough by the water to prevent their detonation which could have been seen from the Kano Maru and reported..

    The Hypothesis Report infers that GRUNION’s 90 degree starboard turn due to gyro angle considerations was ill-advised. I do not understand the description and logic of this inference but do not think that it is warranted given that both torpedoess #4 and #5 hit as intended. The fact that they did not explode was a torpedo exploder problem, one not recognized as such by the Navy for over a year into the war, and certainly not the result of any GRUNION shortcoming

    David Hardy
    October 3, 2008 | 6:11 pm

    It was well documented (after the problem was finally admitted) that American torps were running lower than their depth setting, and had major problems with the detonator, esp. with the classic broadside shot. Some of the detonators’ components were too fragile, and if hitting at near 90 degrees would break rather than initiate the detonation.

    Of course the Japanese commander would not know this, and when he observes torps passing under his ship, or hitting it without effect, he’d conclude that someone set the depth too low or forgot to activate the detonator. It’d be a natural error.

    David Hardy
    October 5, 2008 | 4:35 am

    Another consideration: what happened to the shell after detonation? I approximate, but a 15-20 lb shell (75-88mm range) with 1.5 lb of explosive, leaves a lot of steel mass there. The hull fragments would have gone sideways, knocking the control room crew out of action (with the force of 4-8 hand grenades) and making the boat unmanageable. But the nose would have continued on, doing whatever damage it did (and at 400 yards, on a flat trajectory, roaming the length of the boat and puncturing many compartments) before possibly inflicting a second puncture in the pressure hull. The effect of such a hit cannot be written off.

    Conversely, I suspect that the effect of a torp warhead detonation would have been more dramatic than is reported and than is evident on the images.

    One must also account for freaks of bad luck. A shell detonation after passing thru water and just before reaching the sail would have resulted in a major water hammer effect breach in the sail, followed by the remainder of the shell doing the above damage.

    The missing fore portion might be explained by the impact of the boat with the ocean floor. After falling a mile through the sea, it could have built up enough velocity to shear the forward area off at first impact.

    John Kiracofe
    October 5, 2008 | 5:31 am

    My apologies in advance if my comment seems to be “condescending” or in any way insulting, as this is certainly not my intention. With regard to the question of where is the Forward Torpedo Room or more simply where is the front part of this vessel, my thought is that if this vessel sank with the front part pointing (more or less) towards the floor of the ocean and the back part pointing (more or less) towards the surface, then perhaps the front part of the vessel is actually embedded in the ocean floor. Could it have actually been impacted into the seabed? And then the seabed held the front portion like a vice and the weight of the back end caused the back of the vessel to shear off at the point where it was sticking out of the surface of the ocean floor? Also could the angle of the bent metal at the remaining front portion of the vessel (in the photos) be somehow used to indicate the angle of the impact upon the ocean floor? Not a hypothesis, just a thought.

    God Bless the souls of those lost in this incident and all who are helping to resolve the issues associated with it.

    David Hardy
    October 25, 2008 | 7:42 pm

    I think Ed Walson’s theory of unintentional broaching beats the alternate of surfacing to engage with the gun.

    Target is known to have several heavy machine guns (13mm is close to our Browning .50) and a cannon, since they were all being fired at the periscope (and the sound of exploding shells would have been audible). Range is about 400 yards, an easy shot for a heavy MG.

    If you surface, the MGs will cut down your gun crew long before they can get to the gun, let alone ready it for firing. Even if they survived, the ship’s gun will get off several shots before they can fire. After that, a vulnerable sub would be trading shots on an equal basis with a ship with the ability to absorb a fair number of hits.

    Surfacing for gun action at that range would be unthinkable. On the other hand, if they broached, commands were issued to dive, and at that moment a shell wipes out the men who could have countermanded it (and at least a 3″ hole left to flood) that might explain things. I’m not sure of the layout of the Gato, but a detonation of that type (probably a 12-14 lb projectile) inside the hull might well have taken out the officers, the helmsman, etc., killed or stunned everyone for quite a distance, wrecked the controls, started fires, etc.

    John Patchen
    November 17, 2008 | 7:54 pm

    As a former Army tank commander and gunner I believe that the likeyhood of the 8cm (actually 76.2mm) gun causing a catastrophic rupture of the hull unlikely. Chances are that the 8cm gun in question was the Mark 3 Naval Gun which was intended as a medium duty anti-aircraft gun. Ballistically speaking I don’t think the odds are even remotely close. Let’s take a look at some of the facts:

    -The Type 3 was a naval AA gun, I’ve researched many guns, but this one seems to be hard to find any data on, however I have never found any indication that they made a Armor Piercing round for this gun as it was designed to shoot aircraft, had it been a DP gun that would be a different story. Based on the data for the gun and the muzzle velocities indicated I think it would be safe to assume that she was firing HE-Frag or HE rounds, both would have proximity fuzes.

    -The 76.2mm guns all had a very flat, shallow trajectory. The Type three had a muzzle velocity of 670m/s or 2200 fps. It fired a 6kg projectile at a direct max range of 7600m, the effective range was 4000m direct fire, 5600 meters in the AA role.

    -The HE-Frag and HE projectiles both had a stated armor penetration ability at 500 yds of between 75-61mm Rolled Homogenous Armor (RHA) at a 90 degree incedence. This value drops to 61-40mm when angle of incedence is 60 degrees.

    -Grunion’s pressure hull was made of high quality steel equilvalent to about 111% integrity of the RHA standard steel of MIL-A-12560. So, until I can find some more solid data on the steel type used on Grunion I will use the properties of MIL-A-12560 for my hypothesis.

    Now here is my hypothesis/arguement against the catastrophic 3″/76.2mm shell hit.

    1) Most witnesses agree that the shell hit the breaking water of at the conning tower. Here is the first and most significant detractor against a catastrophic hit because:

    a. The round would be traveling at a velocity of about 2200fps on a virtually flat trajectory–but fired at a negative angle from the bow AA gun. Assuming that the gun was mounted in pretty much the standard fashion you are looking at a depression angle of -6.5 degrees. Even if the boat was traveling on perfectly calm seas, it is most likely that the round would have had a angle of incedence with the WATER of about 21.5-22.45 degrees. That is assuming ideal circumstances. It is well documented that the seas in the area of the attack are choppy at best. I would hypothesize that the angle of incedence with the water was somewhere around 52-64 degrees.
    Now you are talking about a round striking the water before it strikes the Grunion-we have no idea of how “short” the round was, but based on known ballistic data it is safe to say the the round lost 45-60% of its penetrating capacity right away, and 5-10% per inch traveled in the water. It is highly unlikely that the round reached Grunions hull with the ability to penetrate the hull in scenario. Further more, upon contact with the water the proximity fuse would have been activated. Therefore it is most likely that the round had already begun its detenation cycle outside of Grunion’s hull. Even if the round reached the Grunions hull intact, it only had enough energy to penetrate around 17-20mm of RHA, we know that the steel on Grunion was of higher quality that RHA and was of an approximatel thickness of 5/8-11/16″ or 17.39mm when adjusted for the encreased hardness of the steel, it computes to a corresponding 25-28mm equivalent RHA factor–simply put, the round was out of energy.

    The only possible way for the round to have sank Grunion was if it scored a clean virtually square hit on the conning tower…and most of the witnesses state that the round struck the “wave” or “water” at the base.

    Travis Mullan
    November 26, 2008 | 4:32 pm


    Have you seen this analysis?


    Travis Mullan
    December 1, 2008 | 6:55 pm

    Further to John’s excellent comment above:
    If the original Gato class conning tower design, with the mod to the after end, is reviewed it will be noted that the pressure vessel of the conning tower lies roughly 6+ feet from the top of the conning tower fairwater. This plan also shows that the lower portion, almost half, of the conning tower pressure vessel lies under the main deck superstructure. In the Grunion’s case, with the periscope shears just breaking the surface, it is highly unlikely that a 3″ round would be able to inflict much if any damage to the actual watertight portion of the conning tower. It may be that a round penetrated the fairwater in the periscope or Quartermaster’s area which could explain some of the damage to the light metal of the fairwater.

    Haines Brown
    December 8, 2008 | 3:26 pm

    Terry, for further information and photos of damage to the Irex sail, refer to my site:

    Haines Brown, ET1 (SS)

    January 2, 2009 | 1:46 am

    facinating read might i start off with a lot of theories. might i ask a couple of questions (im not a mariner or engineer) 1) what was the weather like at the time? says foggy but was there roling seas and would this have made a differance to the track of the torpedoes? 2) the ship was running for the harbour -was the attack made as the ship was accelerating if so wouldnt the torpedoes timing be off as its calculted on a fixed speed? 3) the ship mentioned it was avoiding a reef could the sub have struck something while manovering?
    lastly as a wartime area would there have not been minefields nearby and could a possible mine be the culpret. ok some have this might have been covered but these are just thought i would like to add if noone has covered them.
    last is there no sign that in panic or even organised that the crew tried to abandon ship?
    may they rest in peace.

    January 2, 2009 | 2:16 am

    just noticed something else in the photos the hatch covers hve locing bars do they not that are extended when the handle is turned if the haches blew open wouldnt they be visible and bent? from the photos they look like they are retracted which would mean the hatch was manually opened or was open when the sub went down. surfacing to use the deck gun dosnt sound right as they knew they were close to port and the escort may be near.
    noone seems to know if the sub happened upon the ship by chance or had been stalking it if so would it be possible they had run too long without surfacing or extending their snorkel and were low on air. they were engaged in battle previously did they take damage they were unaware of? i believe water on the batteries causes leathal gasses. also knowing they could be possible target for escorts me personally would have had my sonar guy listening hard for their screws surley he would have heard the torpedoe coming if it circled?
    lol sorry guys im just a novice but i like to think outside the box i hope im not sounding to crazy.

    EJ Johnson
    January 2, 2009 | 4:39 am

    I’m not a submariner but want to thank those who served and those who care enough to blog here and help to find answers to this very interesting story. We all come to a better understanding of this grat country with this. God bless and goos luck.

    January 4, 2009 | 6:41 am

    the circle runner is a very big Possibility…It happened to the USS Tang and is suspected in the sinking of other fleet boats….If the USS could be photographed, we could compare the damage from her torpedo strike with the possible possible torpedo damage on the grunion.

    Travis Mullan
    January 5, 2009 | 8:16 pm

    See link below for picture of a torpedo test on a deactivated fleet sub:

    G Wilson
    April 20, 2009 | 5:47 am

    I have studied the images on this site and read through most of the comments on the condition of Grunion as she lies on the bottom. The images, drawings, plans and research are extraordinarily well done. In my opinion the catastrophic damage to the bottom half of Grunion’s hull is most probably due to heavy impact with the bottom after falling 500 fathoms while full of water. Topside damage could have been the result of rolling over and sliding down the slope- although my first thought was that she had been run over by a surface ship.

    Images most certainly show a large impact explosion- of whatever cause- which started Grunion on her last dive. The huge amount of damage topside was caused by that set of impacts as shown in the images and exhaustively discussed.

    Regarding the conning tower hatch- bent into a “U” shape and the missing bow, I offer another possible explanation: Explosive combustion (dieseling) inside the hull caused by extremely rapid compression of air in the hull mixed with oil (hydraulic, fuel oil, lube oil…) as water enters the compartment quickly.

    When a sub goes down it retains air in each compartment if it is intact . As the water rushes in at at phenomenal rate, the hydrocarbons and air in the compartment compress and explode. The resultant explosions blow out each bulkhead one by one as the sub sinks. In the bow torpedo room the explosion might either blow the bow off by itself or trigger secondary explosions (torpedo warheads principally) which blow the bowcap (the rounded end of the pressure hull) off the submarine. This phenomenon could also be the cause of the conning tower hatch damage and probably the battery hatch (it appears the dogs on the hatch have been torn off leaving the hatch looking normal in its locked up position.)

    The bow of the I-52 has a similar missing portion-bowcap and outer hull- probably due to a dieseling explosion in the forward torpedo compartment.

    Matt Turner
    March 30, 2010 | 2:04 am

    i do have a question? what was the final moments of the Uss grunion i mean it sounds like she was deph charged more then a couple times for this much damage to the sub and also did anyone on the dives find the torpedo hatches open?

    Travis Mullan
    October 8, 2010 | 7:03 pm

    I have been reviewing the photographs, CDs and discussion for the umteenth time in an attempt to come up with a reasonable explanation for the missing bow section and the battery room, control room failure to implode. Beginning with the midship area I would like to propose the following:
    In re-examining the conning tower hatch and surrounding area I have found it reasonable that the damage to the area may not have been due to physical contact, with the sea bottom for instance, but due to the beginnings of implosion. The conning tower pressure hull is supported at its forward end by the vertical bulkhead which does not appear to have much distortion. The next supporting structure, with the exception of the strengthening rings, is in the area of the periscope shears. Located between the periscopes and the forward bulkhead is the access hatch to the bridge which is located forward on the starboard side. Looking at this area on the Grunion photos there is a great deal of downward distortion of the upper conning tower pressure hull mainly in the center. This distortion has caused the periscopes to be drawn forward and down. In looking at the hatch it can be seen that the hatch trunk has been drawn downward on the port side as the conning tower distorted downward. This progressed until the hatch separated from the hatch trunk alowing water ingress to the hull. Once the separation occurred the inflow of water stopped further distortion of the conning tower.
    As the sea water entered through the distorted hatch it ran into the conningtower and then down into the control room and pooled against the forward bulkhead and in the pump room below. The inflow of water began to compress the air within the sealed control room and after battery heating it as the pressure increased. If the battery had been spilling acid due to the extreme down bubble the acid would mix with the sea water and generate gas. As the air temperature continued to increase a flash fire would have swept these compartments and possibly touched off a battery explosion. If the explosion were of sufficient force it could account for the open hatch behind the conning tower fairwater.
    I hope that this will generate some thought about this area and possibly some other ideas as to what happened here.

    Regarding the 25 feet of missing bow area:

    Several Gato and Balao boats experienced uncontrolled dives before, during and after WWII. Those that survived these traumatic occurrences have documented the cases well. The reasons range from jammed hydroplanes to errors in ballast and boyancy tank control. For whatever reason, if the Grunion found herself in a similar situation she would probably have been traveling forward with an increasing bow down attitude. Blowing ballast first and then reversing the motors have been documented as successful in some of the survivor cases. In the case of the Grunion let’s say that they were unable to recover in time. With the bow being the lowest point on the boat it would be subjected to the greatest amount of sea pressure and would be the first compartment to implode. The loss of boyancy and increased weight at the bow would increase the forces already working to sink the boat.
    In reviewing the photos and video of the wreck it is clear that the colapsed compartments did so from the overhead in a downward direction. This may be due to the structural strength provided by the internal decks. The bulkheads seem to have withstood the forces well and maintained their shape. The forward area of the forward battery room overhead looks quite similar to the torpedo room overhead area on the other side of the bulkhead. Let’s say that the torpedo room colapsed from the overhead downward which would deform the metal downwards and inwards resulting in the beginnings of the deformation observed on the wreck as she was found. As the boat continued toward the sea floor the forward battery would have been the next compartment to colapse. This would result in additional loss of boyancy and encreased weight on the forward end of the sub. Finally with no boyancy remaining the Grunion would have impacted the sea bed with considerable velocity. We know that she struck bottom on the slope of a sea mount which would have resulted in an impact at an angle of somewhat less than 90 degrees. Picture the forward torpedo room with the overhead colapsed to the deck level, as in evidence in the case of the other compartments, and with a raised area to the torpedo room/battery room bulkhead. The forward area of the bow may very well have imbedded itself in the sea bottom and sheared off in front of the forward ballast tank, the pit area. If the bow was bent downward and was overrun by the sub what remained of the bow may bave impacted the bottom a second time before the stern settled and she slid the remainder of the way to the bottom. This would account for the way the hull is deformed at the bow as she sits on the bottom now.

    Please share your thoughts and ideas.

    Mark Westmoreland
    May 20, 2013 | 4:34 pm

    I try to put myself in the shoes of the captain. Without personal knowledge of his demeanor I could only say what I might have done. There could have been a calamity of errors here not simply one lone reasoning for the sinking.

    If I had just fired off all those worthless fish, and as some captains knew of their unreliability, I may have become emotional compromised in my anger towards their makers and those who ordered their use. Fish that have not only ran deeper than they were expected but now the damn things have hit the target and broke like some child’s play toy.

    I’m not a very happy man at the moment. My fastest action in taking the enemy ship out is now the only reliable item I have, my deck gun. Knowing that the enemy ship has surface weapons I wish to put distance, speed and my gun to use. All ahead Flank, blow main ballast, order the gun crew to stations, Left full rudder. The cavitation begins to create a wake of bubbles seen from the enemy ship but I want my gun on her as quick as possible. As the boat rises a lucky shot from the enemies gun passes under water then back up ripping a hole in the submarine causing the buckling of steel and hatch in the conning tower. There is a high pressure surge of water entering the submarine causing the air to compress and perhaps blowing outwards the already loosened mess hatch.

    I might have order a dive in hopes of repairing the damage and getting farther away to resurface and use the deck gun but the damage is more severe and with the opened hatch the submarine floods quickly and sinks. If the boat had rolled over and struck the sea bed bow first upside down it is possible for it to have rolled over while settling on the bottom at the angle it hit causing the metal to have been bent downwards as some post indicate.

    It is also possible that a circling torpedo could have been to blame for the bubbles seen on the surface and while the submarine begin to rise the malfunction fish hit the bow or the conning tower but did not detonate and only began to cause flooding while the errant shell from the surface vessel struck the conning tower and then the mess hatch flew up under pressure giving three point of flooding not simply one or two. The bow damage could still then have been caused by striking the sea bottom.

    The, “Big black-brown water arose”, could have been a mixture of leaking fuel mixed with cavitated water as the submarine angled downwards at flank speed or as the submarine dove because of a failure in equipment which had been reported on other submarines with uncontrollable dives.

    I would love to find more links to any video or photos of the grunion after the ROV dives.

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