Hull difference....german, american

[jazman]

Originally Posted by Sailor Steve

Is that right? The pressure per square inch will be the same, but I don't know enough about it to know if the size makes a difference overall. Just asking.

Size has nothing to do with the pressure found at a depth. You put a Gato or a Type VII at 300 feet, they face the same pressure.

[cmdrk]

Drawing from my previous readings, which hasn't been recent, it is my understanding that the Type VII's pressure hull was exposed more to direct contact with the ocean. The ballast tanks were saddled onto the pressure hull's exterior.

While the US boat's pressure hull had ballast tanks wrapped around it with an outer hull encasing the whole. The pressure hull's steel was thinner and maybe less strong than German hulls. But, the outer hull and ballast tanks should have provided some outer protection - kind of like surface ships torpedo blisters.

A lot would depend on the strength and placement of the charge. Also, it has been debated on what protective effects a greater depth offered against a well placed charge. Would the greater sea pressure already have the hull nearer to failure or would the greater sea pressure contain the explosive force in a way forcing most of the energy up where the water is less compressed?

The greater depth gave a sub more time to manuever out from under an attack and in general it was better to have the charges going off above you rather than underneath.

Just some thoughts - who knows:hmm:

[Sailor Steve]

Originally Posted by cmdrk

Drawing from my previous readings, which hasn't been recent, it is my understanding that the Type VII's pressure hull was exposed more to direct contact with the ocean. The ballast tanks were saddled onto the pressure hull's exterior.

While the US boat's pressure hull had ballast tanks wrapped around it with an outer hull encasing the whole. The pressure hull's steel was thinner and maybe less strong than German hulls. But, the outer hull and ballast tanks should have provided some outer protection - kind of like surface ships torpedo blisters.

I'm pretty sure the outer hull is for hydrodynamic purposes only, and has a lot of vent holes in it. The 'pressure' hull is just that; it takes the pressure directly.

As to the debate whether the depth reduces the effective radius of the charge or makes the hull more susceptible to damage, the only answer I can think of is "Yes". Sorry, doesn't help much, but both seem to me to be true.

[ETR3(SS)]

Originally Posted by cmdrk

Drawing from my previous readings, which hasn't been recent, it is my understanding that the Type VII's pressure hull was exposed more to direct contact with the ocean. The ballast tanks were saddled onto the pressure hull's exterior.

While the US boat's pressure hull had ballast tanks wrapped around it with an outer hull encasing the whole. The pressure hull's steel was thinner and maybe less strong than German hulls. But, the outer hull and ballast tanks should have provided some outer protection - kind of like surface ships torpedo blisters.

A lot would depend on the strength and placement of the charge. Also, it has been debated on what protective effects a greater depth offered against a well placed charge. Would the greater sea pressure already have the hull nearer to failure or would the greater sea pressure contain the explosive force in a way forcing most of the energy up where the water is less compressed?

The greater depth gave a sub more time to manuever out from under an attack and in general it was better to have the charges going off above you rather than underneath.

Just some thoughts - who knows:hmm:

Well drawing from my firsthand knowledge on how submarines work, the ballast tanks wouldn't provide any sort of protection from sea pressure. Reason being is the pressure of the water in the ballast tanks is equal to that of sea pressure, no matter what depth. Now the reason for that is the bottom of the ballast tanks have grates to allow sea water to come in easily when the vents are opened on the top of the ballast tank. I know thats how it worked on my boat and id assume that that part of submarine design hasnt changed.

[Penelope_Grey]

Originally Posted by jazman

Size has nothing to do with the pressure found at a depth. You put a Gato or a Type VII at 300 feet, they face the same pressure.

Semi-True, however the Gato has a larger surface area, therefore the total area of pressure is greater than that of a U-Boat. as in.... Pressure per square inch is the same, but a Gato has a lot more inches than a U-Boat does. So will naturally not be able to go as deep due to being larger.

[AVGWarhawk]

Where in the heck is an engineer when you need one.

[seafarer]

I found this at www.submarineresearch.com

Test depth can be thought of as an engineering estimate of what pressure will be required on one side of a hull to breach the hull, taking into account such factors of hull strength as hull diameter, hull thickness, framing, and intrusions. Naval engineers tend to be conservative in their estimates and the varied factors tend to render an estimate as just that, an estimate. The engineers back into the problem by first estimating the crush depth of a hull, then creating the theoretical test depth by a applying a decimal factor to the crush depth. Different national navies apply varying factors. The United States Navy has used a factor of 1.5, but this has changed many times. Of course, computers are able to make such estimates much more trust-worthy, however, the accounts described "Steep Angles and Deep Dives" are, for the most part, in hulls designed before the advent of the computer.

In the American Navy, hull designers depend on the experience of submarines to verify their estimates. Buships requires a submarine captain to immediately notify both Buships and the Chief of Naval Operations in writing when a boat under his command exceeds test depth. The captain's professional career may be jeopardized by a zealous attention to recording a dive that went wrong. Only in wartime can a captain reasonably explain the need to exceed test depth. For this reason submarines exceeding test depth sometimes fail to make note of the dive in their deck logs.

The simplest application of determining hull strength is the hull thickness. The thicker the hull metal the stronger the hull and the deeper the test depth, assuming all other factors are constant. Prior to the Balao class U.S. submarine, hulls were built of mild steel (MS) which had a maximum tensile strength of 60,000 pounds per square inch and a yield strength of 45,000 psi with 23 percent elongation. The thickness of hull plating until about 1943 was specified in terms of the weight of a square foot of plate rather than the actual thickness, and this was gradually increased from 20 pound plate (approximately one half inch) to twenty seven and a half pounds per square inch in the Salmon (SS-182).

Another change in the Balao class was the change in material used for hulls. High tensile steel was a chromium-vanadium alloy with a maximum tensile strength of 50,000 psi with 20 percent elongation. When the composition was changed to titanium-manganese alloy, because of wartime shortages, the strength dropped to 45,000 psi. The Salmon's hull was about seven eighths of an inch thick giving her a test depth of 250 feet. Conning tower shells were thicker as protection against surface guns.

The thick-skinned boats came along in 1942 with a test depth of 412 feet. These boats had the same seven eighths inch thick hull as Salmon, but the quality of hull steel ie., high tensile strength steel had significantly improved. The crush depth of these boats was estimated to be around 450 feet. Fleet type submarines built during the Second World War were to last through much of the cold war. These boats have careers that have lasted over fifty years with many still being used by foreign navies.

Anyone know any similar figures for u-boat hull steel?

P.S. www.uboat.net mentions that type ViiB/C hulls were made from steel approx. 0.73 inches thick, but I cannot find anything about the type of steel used.

P.P.S. from www.uboatarchive.net in the REPORT ON "U-570" (H.M.S. "GRAPH") (U-570, a VIIC was captured by the British in 1941)

http://www.uboatarchive.net/U-570BritishReport.htm

page 37

Hull
The hull is of all welded construction, the only riveting being on the engine room cover plate and the flanges of the dished ends of No. 3 main ballast tank.
2. The frames are of bulb section bars with no flanges. There are 82 frames.
3. The thickness of the pressure hull plating is 0.88 in., decreasing to 0.63 in. towards the ends.
4. The keel is 1.8 ft. high and 3.6 ft. wide. It is free flooding and iron ballast is carried at the ends.
5. The thickness of the pressure plating of the conning tower is 1.26 in.
6. The endings of the pressure hull are formed with dished plates which carry the housings of the torpedo tubes. The thickness of these endings is 1.378 in.

[seafarer]

Found this in a 2003 forums post on uboat.net, but no source was given for the information (post was made by forum member "Scott" under Technologies and Operations)

Hull Structural for the Type IXC, IXC40, IXD2, XB and XXI.

The cylindrical pressure hull had a diameter of 4400mm (14.42') and was made of 18mm (,73")
steel with inside bulb tee frames 200X11 (7.88" web depth X .43" web thickness) on 700mm (27.56")
centers. The plating is gradually reduced throughout the tapered section fore and aft to 16mm (.63"),
nd the frames are correspondingly reduced to 130 X 9 (5.12" X .35"). Frame spacing remains
unchanged in the tapered sections.

The specification for the pressure hull plating and frames calls for a tensile strength of 74,000 psi
with a yoeld point of 51,300 psi. The steel was known as No. 52; the specification for plates is
KM 9104, and for the frames is KM 9103

[jazman]

Originally Posted by Penelope_Grey

Semi-True, however the Gato has a larger surface area, therefore the total area of pressure is greater than that of a U-Boat. as in.... Pressure per square inch is the same, but a Gato has a lot more inches than a U-Boat does. So will naturally not be able to go as deep due to being larger.

Well, you're almost right.

Take a square inch of plate, set in a nice solid frame. The net pressure on that plate is outside pressure - inside pressure. At 350 feet, that's pretty high. Say that the plate is strong enough--for whatever reason (thickness, material, bracing). And the forces on that plate, if you do a Free Body Diagram, are evenly distributed on the outside, with an equal but opposite force spread around the edges of that plate (because it's sitting in a frame). You can intuit that the middle is going to dimple / collapse first--there are some strange forces applied in there to the material.

Say the pressure is 300 psi. On that plate, you have three hundred pounds. Around the edge of that plate, the 300 pounds are over 4 inches of the mounting frame, so it's 75 pounds/inch.

So, you make that plate a piece 9 inches square (3 x 3). The total force on the outside is now 2700 pounds, and the frame has 12 inches, so it's 225 pounds /inch. Hmm, that's a lot higher. The forces working on the material in the plate are higher throught (just do a FBD on an infinitesimally small edge piece), it will, if you slowly increase the pressure, collapse before the small plate.

But suppose you build a grid frame to hold that larger plate, where it's basically a 3x3 frame to match. Then it's all the same. It comes down to what you design it to do. The total German design--thickness, bracing, etc., was designed to operate at a deeper depth. The bracing gets a little easier if you keep the ship smaller. More bracing: Money and Construction Time, and increased complexity (with increased possibility of poor build quality), etc etc etc.

But what you get is a smaller ship, and we Yanks need our Ice Cream machines.

BTW, a long long time ago, in a Galaxy far away, I was offered a job at Electric Boat. I turned it down for warmer pastures.

[Bulkhead]

Ok ok ok.... so the pressure is the same, nothing to do with the pressure because its the same for each boat at the same depth, and its not about the size either, or surface for the pressure to work on. But its about the supporting structure inside all of the surface plating. Lets call it "the sub sceleton".... The gato needs a stronger and alot heavier sceleton to face the pressure than a type-VII. If the size is twise it may need twise the number of framework to stand the same depth i dont know.
The basic question was whats the difference between the two subs, witch after all the reading ive done in this thread (BTW, thanks for all the input) i guess it narrows down to structure, construction and building techniqs.

Please continue with all stuff you can find.

Thanks

[Palidian]

However if the hull is close to being round, size should not matter. Try smashing an egg with your hand.

http://www.wonderquest.com/egg-pressure.htm

Originally Posted by Penelope_Grey

Semi-True, however the Gato has a larger surface area, therefore the total area of pressure is greater than that of a U-Boat. as in.... Pressure per square inch is the same, but a Gato has a lot more inches than a U-Boat does. So will naturally not be able to go as deep due to being larger.

[cmdrk]

Originally Posted by Sailor Steve

I'm pretty sure the outer hull is for hydrodynamic purposes only, and has a lot of vent holes in it. The 'pressure' hull is just that; it takes the pressure directly.

As to the debate whether the depth reduces the effective radius of the charge or makes the hull more susceptible to damage, the only answer I can think of is "Yes". Sorry, doesn't help much, but both seem to me to be true.

I understand the pressure hull is in no way sheltered from sea pressure. The ballast tanks would have equalized pressure inside and out and thus the pressure hull would have to resist that pressure.

What I was thinking about was, what protective effect would the US boat design have against a depth charge? Wouldn't the explosive force be disipated by the outer hull and water in the ballast tanks?

My understanding of physics is limited but I thought torpedo blisters were suppose to spread the force some so the force per sq. in would be lower. And, yes it would cause detonation away from the main hull. (shrugs)

[Penelope_Grey]

@Jazman, what you have posted there I really do not understand at all, and I have an A-level in physics(not enough I think). It might be the wording, but I cannot understand your explanation, not sure if the others did, but I don't.

Simplyfying it is what I was trying to do, in simplistic terms the bigger an object is the more area there is for the pressure to exert therefore the overall sum of the pressure is higher than on a smaller object generally, this assumes that the structures/materials of said object are similar or identical.

Build quality is something that I think both the Americans and Germans did well. The Americans wanted their boats for speed and fleet purposes (speed) while the germans wanted them to operate alone and be able to go beyond the Aliied ASDIC capabilities (dive deep) and in early war the Germans did succeed at this, a U-Boat could go deeper than the allied ASDIC could.

Further, taking German U-Boat's as a main example, obviously the Kriegsmarine had deep diving in mind, now, a type IX with a slightly thicker hull, still couldn't go as deep as Type VII could. And I don't mean the VIIC/41 I mean the ordinary VIIC. From what I learned in college, build can only stave off so much.

[scrag]

The size of the ship has very little to do with it at the depths we are talking about in regards to Sea Preasure - the same argument can be stated with the overall water depth - the Pacific is deeper so the water mass is different or the salinity content is different and that will disapate the blast effect differently. While the size of the ship may contribute to how much hull is exposed to the blast - water preasuure (44 ib per sq inches for every 100 feet) treats the preasure hull the same equal way. There is a lot more to sub construction - bulkhead thickness, water tight compartment integrity, framing arrangement (external or internal frames), fixtures exposed to sea preasure i.e. trim and drain system and the pumps of the system, main induction valve and diesel exhaust - you get the idea. One thing the game leaves out (seemingly) is the improved Gato depth capabilities seen in Mid WWII to the end allowing US boats to operate 500 plus feet. The Germans also had better depth capabilities too but faced the same issues. Very often, failure of a valve or other system than the preasure hull resulted in a loss of the boat at deep depths.
To answer the question about ballast tanks water dispating a blast - nope - water does not compress so it will (practically) transmit any force equally, so the effect on a submerged U Boat would transmit the effect directly to the preassure hull. Yes you would get rippling and distortion of the metal structure but the presure hull had to restrain the additional effect of the preasure wave moving through the water. Remember the string going across the engine compartment being taught on the surface but having slack at depth - that is a practical example of the effect.

[jazman]

Originally Posted by Penelope_Grey

@Jazman, what you have posted there I really do not understand at all, and I have an A-level in physics(not enough I think). It might be the wording, but I cannot understand your explanation, not sure if the others did, but I don't.

It wasn't physics, it was engineering. I guess at some point it starts looking like Magic. Ah, the joys of Technocracy!