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"Japanese Torpedo Modifications at Pearl Harbor" Topic


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3,073 hits since 21 Jun 2010
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GuruDave21 Jun 2010 10:31 a.m. PST

OK, I am hoping that someone can help me understand this.

In several recent shows that attempt to explain the "mysteries" of the Japanese attack on Pearl Harbor, the modifications to the air-dropped torpedoes are described. Usually something similar to the following is stated: "The Japanese had to figure out how to drop torpedoes into the shallow harbor without them striking the bottom before leveling out to their nominal running depth." The solution is often described as "break-away" wooden fins fitted to the aft end of the torpedoes.

However, this doesn't make much sense to me. Fins generally don't do anything other than serve to move the center of drag aft to keep the torpedo pointed in the right direction. Larger fins would induce more drag on the torpedo, which would give it a shorter range but also serve to slow the dive, which is what need to be done for the torpedoes used at Pearl Harbor. However, I don't know why they would be considered "break-away." If the fins broke off on contact with the water, they wouldn't do much good in preventing the torpedo from diving deeply. Also, they might de-stabilize the torpedo if they didn't completely detach. I suspect the "break-away" aspect is a misconception born from the fact that the fins are made of wood. Why wood? Well, probably because it is cheap and plentiful. The bouyant properties of wood would actually be a disadvantage since they would tend to lift the aft end of the torpedo and cause it to dive, but I suspect the small amount of bouyancy wasn't much compared to the weight of the torpedo.

One show even goes so far as to build a 1/5 scale model of a Japanese torpedo, including the wooden fins, and showed some very poor high speed footage of the model dropped into a water tank at scale speed. There wasn't much to see, and you couldn't tell whether the fins broke off or not on impact with the water. I suspect that was intentional since the producers of the show were laboring under the "break-away" misconception and couldn't explain why the fins seemed to work even though they didn't break away.

So, my conclusion is that the wooden fins attached to the aft end of the torpedoes dropped at Pearl Harbor imparted just enough added drag to the torpedo to keep it from diving too deeply. The decreased range wasn't a problem because of the short distances involved. The fins did NOT break off when the torpedo hit the water or any other time prior to impacting the target, and the fact that they are made of wood was only an expedient and cost savings measure.

Personal logo John the OFM Supporting Member of TMP21 Jun 2010 10:39 a.m. PST

Whatever the idea was behind the fins, it seemed to have worked.

jdpintex21 Jun 2010 11:30 a.m. PST

I've heard the wood thing, but I guess I just missed the "break away" issue. They look pretty substantial in the pictures/re-enactments/whatever, that I've seen.

Besides, if they broke away, wouldn't you have an issue with debris messing up the props of the torpedo?

GuruDave21 Jun 2010 11:31 a.m. PST

Well, good enough, yes, but not perfectly:

"Japanese estimated that, out of the 40 drops planned for Pearl Harbor, 27 torpedoes would hit home. (In the actual attack, there were 36 successful drops, of which 25 scored
hits; 11 missed, malfunctioned, or bore into the muck; four met unknown fates.)"

PDF link

Personal logo John the OFM Supporting Member of TMP21 Jun 2010 11:46 a.m. PST

Is that from the Japanese AAR? grin

GuruDave21 Jun 2010 11:48 a.m. PST

Here is another theory that, while not explained, is hinted at in a few articles.

The wooden fins, by increasing the aerodynamic stability of the torpedoes, help to keep the nose of the torpedo up while it drops from the aircraft into the water. Since the torpedo enters the water at a more shallow angle, it doesn't tend to dive as deeply before settling out. The added fins are intended to stabilize the torpedo in the air, and have no function in the water. Whether they "break-away" or not probably doesn't matter. It would be ideal if they broke off once the torpedo entered the water, but I doubt the impact on speed or range would matter within the confines of Pearl Harbor. On the other hand, as jdpintex points out, having them partly break away would be a real problem. So, I think if I had designed them, I'd make the fins strong enough to withstand impact with the water and live with the added drag.

This explanation makes even more sense than my original theory.

Here is the article:

link

SgtPain21 Jun 2010 12:00 p.m. PST

My understand that the fins were for aerodynamics purposes. They keep the torpedo level as it enters the water, without the fins the torpedo would enter the water at a steeper angel and then dive to about 45 feet where it level off and then came back up to it running depth. Since the fin is only to keep torpedo level as at falls from the airplane, it breaks away when it hits the water.

Happy Little Trees21 Jun 2010 12:16 p.m. PST

I always thought the fins caused the torps to level out faster after they hit the water, so they didn't dive as deep. Also thought the ones they showed getting mounted in PEARL HARBOR looked legit. It's not that bad a movie if you only watch the attack and ignore the rest.

Ed Mohrmann Supporting Member of TMP21 Jun 2010 1:04 p.m. PST

Yes, the nose attitude of the torpedo is the critical
issue, since Pearl Harbor really isn't very deep.
Nose up/level = more shallow running torp.

I never thought about it before, but we used to train
quarterbacks to up/level/down the nose of a football
when throwing it, depending upon how far they expected
the ball to go. Not that I was a QB coach, of course.

Same difference, I guess, a semi-pointy object moving
at velocity through a fluid medium.

Lion in the Stars Supporting Member of TMP21 Jun 2010 1:19 p.m. PST

Pearl Harbor is *very* shallow: my subs would pull mud and debris off the bottom every time we went in. I don't think we were scraping bottom, but we might as well have been. Worse, there are only a couple spots that you can tie up a ship with a 30+ foot draft, like carriers, battleships, and big subs.

troopwo Supporting Member of TMP21 Jun 2010 3:57 p.m. PST

Angle the fins, or fit them part way up the body might help.

Tom Bryant Supporting Member of TMP21 Jun 2010 11:38 p.m. PST

If you can find it there was an issue of the Naval Institutes Naval History magazine from 1991 that went into some detail on the subject. IIRC it had drawings of the torpedo configurations. It also had salvers drawings for the USS Oklahoma and West Virginia. Scary stuff to look at.

bsrlee22 Jun 2010 7:45 a.m. PST

I remember reading somewhere (Nat Geo?) that the 'fins' were along the sides of the torpedoes and secured by bands front & rear rivetted to the fins. When they hit the water the fins took a lot of the initial shock, stopping the torpedo from sinking as deep with the initial impact but not changing its orientation.

Ideally this impact also caused the fins to crack up and break away, loosening the rivets & allowing the torpedo to swim out of the now loose bands.

I don't think any of the subsequently recoverd torpedoes still had any sign of the add ons they found one in the 1980's or 90's in a mud bank, then contacted the pilot of the plane that dropped it.

Cuchulainn23 Jun 2010 7:24 a.m. PST

I found this article on how the Japanese modified their torpedoes, and how the modifications worked.

PDF link

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