This thread is probably long dead, but never mind. Stumbled across it and felt I may have something useful to offer.
Whenever a weak point is found, designers try and put in a fix.
E.g. if a US or UK submarine accidentally bumps into an iceberg (especially a black one with a red star painted on, that was playing ‘chicken’) there is a possibility that the propeller of one may get bust (one Brit sub is supposed to have found bits of bronze propeller in its casing on getting home!). So a secondary motor - 'outboard (US) or ‘eggbeater’ (UK) is fitted.
I believe one electrical fitter was trying to remove the slip rings at the top of the eggbeater mounting column one day, so he took off the big nut that was in the way. He was rewarded with a 13" wide fountain, as the motor dropped out. Someone slammed the Motor Room door on him and turned on the salvage air blow to pressurize the compartment, and left the silly bugger in the compartment half full of water until divers could cover the hole, and they could blow out the water, release the pressure and let him out. All good fun. I don’t suppose a 24" prop shaft could ‘fall out’, and you’d need to be very quick slamming the motor room door, but in principle that is how you’d deal with it. Mind you, if not on the surface, you’d be sunk before you got the ballast tanks blown and enough air in the motor room to stop you going down! British subs had a lot of subdivision, so in theory collision damage to one end compartment would limit the flooding to recoverable amounts.
Shaft seals wear out, so they had 2 of standard ones, and a super-duper different sort, and an inflatable one for last-ditch (UK Polaris boats). Hull openings are minimised, as any hole in the pressure vessel is a source of increased stress, and hull valves are made VERY strong. Hopefully all the stick-out bendy bits (fin, planes, periscopes etc. are made so that they will bend and collapse before affecting the pressure hull, if hit.
So the question is impossible to answer - the problems and solutions are always changing and developing, as designers strive to sort out the worst.
Looking back on driving the reactor of an SSBN, nearly 40 years ago, I am struck by how accurate the Simpsons portrayal of nuclear safety is. (I have a nice little nodding bird in front of me as I write this!) I made a tremendous contribution to nuclear safety, 35 years ago, by resigning to gett jobs at which I was competent, or at least harmless!) So the manpower aspect is very important - in the UK we got paid about 30% extra, not for danger money (What could possibly go wrong?) but for being responsible and 'fessing up ’ at once if any mistake was made - no witch hunts, just get it put right! There are too many ways of making simple - but potentially lethal - mistakes. The ‘fix’ was to have very, very thorough training and selection. (I obviously slipped through.)
Re. the string trick, we used string across the dining hall, tight up top, with a key hanging in the middle - it sagged to the floor as we went down. Another aspect of this was to make sure you were not in a cabin with the door shut on diving! (You might have to wait 6 weeks to open it again!)
Subs are designed not to collapse until 2 or 3 times their maximum permitted diving depth (which is reassuring) but most accidents happen because of combinations of many, little errors. In training we were told how a submarine had sunk alongside at HMS DOLPHIN (home of UK subs) one evening, with 3 men onboard. They identified about 13 different mistakes that led to it - hoses and wires through hatches, filling tanks unsupervised, doors left open when they should be closed, draught marks not checked frequently enough - all sorts of little, everyday faults that just happened to combine. (All three guys ran up the boat trying to shut doors behind them and finally got to the escape compartment at the front and got that door shut. They were saved by the trot sentry (quartermaster) who stood with his foot over the (obstructed) engine room hatch until the water was up his nose, to slow down the rate of flooding. All 3 were floated out alive 16 hours later, I believe.)
Nukes can sit on the mud - HMS REPULSE demonstrated that very successfully when CND protesters delayed its launch by half an hour and there wasn’t time to get it out of the muddy channel! Vickers Sub Association have the embarrassing picture on line! But normally they don’t dare, because the reactor is like a coal fire - it goes on burning after you stop feeding it; so you MUST always have good cooling water supply, and you don’t mess about risking blocking a hull opening by sitting in mud. Diesel subs are therefore better for sneaky inshore silent stuff, while nukes are kept for the open water - which is best, a bike or a car? Depends where you want to use it!
Personally, I always thought the weakest point of a nuke was the single prop - too many ropes, nets, icebergs, rocky bits etc. about. Coming back from Murmansk harbour or the Caspian Sea, or wherever it was we lurked, at 2 knots on the eggbeater, never struck me as a viable proposition.
Ballast tanks were pretty robust - mostly outside the boat, and if one fell off - well, we had a dozen.
Hope that helps
Capn. Scuttle
I wouldn’t like to be in a submarine with me onboard
