Diving Depths - U-Boats and FleetBoats

[csargemg]

Is there a reported reason why the U-Boat diving capabilities were so much greater than those of the American submarines, particularly in consideration of diving ability vs. boat length? The American subs can barely dive deeper than their own lengths, while Type VIIs are reported (as I'm sure you know by Erich Topp) to have gone down as far as 250 meters.

Is the cruising range and the massive amounts of fuel storage needed on the fleet boats a factor, or what?

[Sailor Steve]

When the British captured their first type VII, they were astonished to find that the Germans were building pressure hulls out of 25mm steel, which is not quite 1 inch thick; as opposed to their own, which was only 1/2" (12.7mm). the Americans were using 5/8" (14mm) until the Balao class, which not only had 7/8" thickness, but was also made from STS (Special Tensile Steel), otherwise known as armor plate.

That said, some US boats did indeed reach depths they were told not to go to, with little or no problem.

[csargemg]

Even still, the test depth of the Type VII was 230 meters, and the test depth of the Balao was 122 meters. Perhaps the small size of the Type VII pressure hull magnifies its greater thickness?

[madmax]

There is an element of design involved.... The weakest link etc and shape of the pressure hull.

The Germans were some years ahead of everyone in both submarine /rocket and airplane design. Probably a few germans lying around in both East and West sub development programs.:arrgh!:

[-Pv-]

The problem of how deep to go is relative though. In the Pacific the Japanese had bad intel on US sub diving depth and habitually se their DC too shallow (not having good sonar made this worse early in the war) so really all that in necessary is that you can go deep enough to play the game successfully and this is certainly the case (in an unmodded game.)
-Pv-

[AVGWarhawk]

Balao class submarines were an improved version of the previous Gato class. They were designed to dive to a depth of 400 feet as opposed to the 300 feet for Gato class boats


Apparently this is all they were designed to do, 400 feet. Many went past this mark so I guess the designer were keeping it conservative.

[U-Bones]

A hull that is almost twice as thick and tested to go almost twice as deep? Sounds pretty predictable to me

[SteamWake]

Balao class submarines were an improved version of the previous Gato class. They were designed to dive to a depth of 400 feet as opposed to the 300 feet for Gato class boats


Apparently this is all they were designed to do, 400 feet. Many went past this mark so I guess the designer were keeping it conservative.

Imagine that american engineers being conservative ?! :know:

Some went past that depth and never returned.

[Ducimus]

Thickness and grade of steel used on the pressure hull most certainly effected crush depth.

Without having design planes in front of me, im guessing that size of the boat, (larger the boat, the more water it displaces, the greater the pressure per square inch on the hull as a whole? ) and the structural arrangement/placment of the ribs and supportiing skeletal members also would make a difference.

[LZ_Baker]

The pressure per square inch is the same no matter what the area. Larger area = Larger force = Same PSI. The thickness of the hull matters, but the strength of the material matters even more. Normal steel has a yeild of 32,000 psi. High strength steel is 51,000 psi. HY-80 has a 80,000 psi yield. So 1/2" of HY-80 would be stronger than 1" of normal steel. As stated above, the US hulls were thinner but were made of a stronger material (I would assume, especially late war when Germany was running short of speciallty materials). Also, every engineering design has factors of safety. You rate something for a force when in reality it could take 5 times that force without yielding (example - any type of rigging equipment) The US subs were rated for 400' but one managed to go down to 1000' and survive (don't remember what boat, its posted around here somewhere).

[Storabrun]

I would assume that the thickness is actually more important than the strength of the material in this case. I'm no expert in submarine design, but if we want to simplify this it's probably more related to bending of a bar than to pulling it apart. 80,000 psi is better than 32,000 of the double thickness when it comes to pulling a rod apart, but not when trying to bend it since strength against bending is proportional to the square of the thickness. The better quality steel would have an edge of 25% for pulling, but the thicker normal steel would be 60% stronger against bending.

[LZ_Baker]

I would assume that the thickness is actually more important than the strength of the material in this case. I'm no expert in submarine design, but if we want to simplify this it's probably more related to bending of a bar than to pulling it apart. 80,000 psi is better than 32,000 of the double thickness when it comes to pulling a rod apart, but not when trying to bend it since strength against bending is proportional to the square of the thickness. The better quality steel would have an edge of 25% for pulling, but the thicker normal steel would be 60% stronger against bending.

Touche :up:

[Chock]

Not sure how scientific this theory is, but I assume that a smaller boat would flex less; i.e. it's easy to bend a very long steel bar with your bare hands but a short piece of one is going to be tricky as you wouldn't have the leverage. So I assume a shorter boat would have better torsional strength, which might be a factor when the water pressure begins to push down on things at diferent positions on the hull.

With US boats being initially designed for fleet actions in support of surface ships - as opposed to German ones designed to act independently from the outset - they were considerably longer, and presumably would flex more.

Ever since WW1, Germany was noted for being well in advance of other countries in techniques for metal fabrication (notably welding). This is remarked upon in many allied documents with regard to captured WW1 German equipment, so maybe this is also a factor.

[NEON DEON]

What U-boat had a 25 mm hull?
VII C: 18.5 mmVII/41 21 mm

[Storabrun]

Yes it's much easier to build a small, deep diving boat than a large one. It's not very practical to use supporting rods (or walls) everywhere inside a sub, so shorter distances help (less torque). Lenth of the boat is probably less of a problem than width and height (or diameter if it was completely cylindrical), since the walls between compartments work as a bulkhead.

[Crosseye76]

And don't forget guys, your not just talking about the pressure hull here.

A Submarine is a ship, not a an unbroken tube of steel. A lot of these dive limits were not based only on the yield strength of the pressure hull.

They were also based on all the many other systems that had to stand up to sea pressure. Shaft seals, Periscope and other mast glands, depth sensing inputs, Etc.

I can't dig up the source now, but I seem to remember a comment about the U.S. pumps. Based on a WW 1 design, they could only overcome sea pressure down to a certain depth, so they could not pump overboard below 300 - 400 feet or so.

Every time we went below a certain depth on the 643 boat, there was a "Deep Submergence" bill. This closed some un-needed sea pressure exposed systems (The Depth input line to the Hovering system for instance) and gave instructions on systems to monitor and keep an eye on.

We never worried about the Pressure hull giving up the ghost, it was FAR more likely that a problem would arise from all the various holes in that hull than from the hull itself.

[Sailor Steve]

What U-boat had a 25 mm hull?
VII C: 18.5 mmVII/41 21 mm
The quote was from Padfields War Beneath The Sea. I assume he was generalizing. Still, it's a lot more than 1/2".

[Bilge_Rat]

The US subs were rated for 400' but one managed to go down to 1000' and survive (don't remember what boat, its posted around here somewhere).


That was the USS Chopper, a Balao class, but the hull was permanently damaged, so the safe operating depth would probably be somewhere between 400 and 1,000.

In Silent Victory, I also read about an earlier sub, P class I think, which accidently dove down to 600 feet while under attack in the summer of 1942. The hull was damaged, but in the text, its not clear if its from the dive or the depth charging.

So I would think a Balao class could go down to 600 feet without breaking a sweat and if it was in perfect condition, could probably edge down to 800 feet without too much risk.

[AhhhFresh]

Ever since WW1, Germany was noted for being well in advance of other countries in techniques for metal fabrication (notably welding). This is remarked upon in many allied documents with regard to captured WW1 German equipment, so maybe this is also a factor.
I think this may be a big part of it... almost all of the US ships pre-Pearl were riveted instead of welded. The demand for fast ship construction changed that pretty quickly, but I don't imagine they were experts from the start. The S-boats had completely riveted hull constructions, and it wasn't until the Gatos that they started using fully welded hulls.

I wouldn't be surprised to find that the Germans had been using fully welded hulls since WWI in their U-boats.

EDIT: Whoops! Missed the important part of the quote!

[7Enigma]

Very intesting thread everyone. Keep up the chatter!:up:

[jetthelooter]

i faintly remember reading an article on the differences between US submarine construction theory and practice and the german same. if i am remembering correctly the germans used a much smaller determination of crush depth than the amercians did meaning that their doctrine called for much deeper diving before reaching crush depth. this allowed them a smaller margin for error when approaching the rated crush depth.

the american estimations were much more conservative making for a much shallower listed crush drepth as the concern was for safety more than anything. in reality both the german and american subs realistically had very similar depths they could dive without damage or implosion. it seemed to me from that article it was more a matter of naval doctrine than actual depth at which the sub would implode. if i remember right the american method for estimating max dive depth was about half of crush depth which is what is usually listed as that sub types max depth.

from what i have read about american sub skippers they would exceed the listed max safe depth by up to half and tend not to record said depth in the log.

so if you consider that the german and american subs had similar dive capabilities and depths. one navy was just doctrinely more conservative than the other.