Diving Depths - U-Boats and FleetBoats
 

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?

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.

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?

Dive Depths
 

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!:

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-

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.

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

Imagine that american engineers being conservative ?! :know:

Some went past that depth and never returned.

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.

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).

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:

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.

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

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.