Hiding under ships

[Sailor Steve]

That would of course depend on how clear the water was and how deep the periscope can be raised without the seals leaking.

I don't know the answer to either one, and both may be variable, so the only real objection I would have is - are there any reports of it actually being done. I'm certain that hiding that way was done, but was it by periscope or by hydrophone alone? I don't know.

[I'm goin' down]

at Truk I was being depth charged by one dd and four patrol boats after I had sunk three stationary merchants near the southwest entry channel. I ended up fleeing to the nearest Jap port and moored, submerged, in a boat slip in an attempt to escape their weaponry. The boat slip quay walls blocked their depth charge explosions. The dd docked alongside the slip for a few hours, like a polar bear waiting for a seal to come up for air. But I never did. Eventually the dd departed, as did the patrol boats which had been swarming about laying down depth charges. I got away after dark , noting the Japs had sunk one of their own merchants while they were firing at my boat. My boat: no damage with one wounded. Kill totals: three merchants, plus one merchant the Japs sunk gunning for my boat.

Originally Posted by I'm goin' down

at Truk I was being depth charged by one dd and four patrol boats after I had sunk three stationary merchants near the southwest entry channel...

Hmm...southwest channel, huh?! I just got orders last night to head to Truk and see what I can see...

Thanks for the intel! Maybe I'll get lucky! I poked my nose into Rabaul last night before heading back to Pearl. No one was home. Ended up with a pretty lousy patrol. Only bagged two merchants and a light cruiser.

[DaveyJ576]

Originally Posted by Sailor Steve

That would of course depend on how clear the water was and how deep the periscope can be raised without the seals leaking.

I don't know the answer to either one, and both may be variable, so the only real objection I would have is - are there any reports of it actually being done. I'm certain that hiding that way was done, but was it by periscope or by hydrophone alone? I don't know.

Actually, this technique is called an "under hull". It was a fairly common, but extraordinarily dangerous maneuver during the Cold War. You would come up behind the target, raising the scope very carefully and very briefly. The conning officer, usually the CO, would basically visually guide the boat in under the ship and use the scope to gather visual intelligence about the ship's rudder and propeller configuration, machinery cooling intakes, sonar dome arrangement, etc. The sonar techs are at the same time gathering reams of data on machinery noise. The boat would be at battle stations and rigged for ultra-quiet. Obviously, since this was primarily a visual evolution, it had to be conducted during the day and in very calm waters. You had to be able to see and you had to be able to precisely control the boat. A properly maintained hydraulically raised periscope can be raised as deep as 200 feet, but it will go up very slowly because it is pushing against a lot of water pressure. The older style cable raised scopes would probably be less than that, I would estimate about 140-150 feet.

The water had to be deep enough to allow for a quick escape should you be detected. You would go deep, make a sharp turn, and go flank to clear the zone. Even still, if you were detected it could be a very bad day. You were within ship mounted ASW weapons range and they would have them in the water before you could blink.

While it was possible to do this with a diesel boat, the target would have to be going fairly slow. Anything more than about 8 knots and the boat just would not be able to close the range or keep up while underneath. A nuclear boat has a much higher performance margin so it was more common with them. Remember though, the faster you go the more your sonar performance is degraded so you still wanted to chose a target that was going fairly slow.

As far as WWII goes, I am not aware of any occasions where this occured. However, I can not yet state that definitively. As far as sneaking into harbors under a ship (as in the movie Destination Tokyo), consider this: an average size merchie will draw about 15-20 feet of water to the keel. From the keel to the tip of a raised periscope of a fleet boat is about 71 feet. You would want at least 20 feet between the tip of the scope and the keel of your target. For safety purposes, you would want at least 50 feet between the keel of your boat and the bottom. Add all these numbers up and you get approximately 160 feet. Contrary to popular belief, most harbors of the world are not that deep, even in the main channel. You would be doing well to get enough water to submerge the boat to periscope depth, much less sail the boat under a ship. While a very entertaining movie, Destination Tokyo was very wrong in this regard.