In this case (and in my proof of concept testing), the engines are destroyed from naval gunfire in my test mission. Alternately, I think it is possible to destroy the engines from air dropped bombs or air dropped depth charges.
The naval gunfire of course demonstrates an entirely different problem associated with the external damage model and graphic for the submarine - one I've tried to tackle in the past but without good results. Basically, once the pressure hull has been holed, you should not be able to submerge without causing flooding. I haven't yet figured out a way to solve this problem - I'm not even sure it's possible.
Related to tater's comment about destroying a diesel from vibrations, I've seen video footage from Los Angeles' engine room during sea trials from a simulated depth charge attack (may have been simulated RBUs, but I can't recall). It was an eye opening and impressive sight to say the least, especially the damage to hull mounted, or hard mounted lockers. While I'd say it's not really possible to "destroy" a diesel from a depth charge attack without also losing the boat, it doesn't mean the diesel has to be physically ruined (e.g. broken cam, rod, or piston head). It's quite possible to get a mission "kill" due to the loss of a critical support system (e.g. seawater cooling suction valve(s) inoperable).
Link to the proof of concept:
http://rapidshare.de/files/39064306/..._v0.01.7z.html
Notes:
-This proof of concept is for Stock SH4 v1.5 only.
-This version is not a silver bullet - there are still some problems such as max spped at a flank bell with 2 diesels destroyed. In this case I still managed 21 knots with 2 diesels destroyed.
-All permutations and combinations of diesel engine damage have not yet been tested
-This proof of concept is only for the diesels, and has not yet been implemented for the electric motors
-This only effects the damage model, and does not change or otherwise modify the hard coding of the submarine propulsion system, therefore it is not possible to choose the number of diesel engines to have online.
-This proof of concept is only for the Gato.
Testing
A reliable "test" mission needs to be created. I've been using a warship, but this has a number of problems mostly associated with the reproducibility and repeatability of results. I suggest lowering the zone HPs for engines to test to 1 therefore minimizing the damage the ship takes.
If anyone wants to create a reliable test mission this would be appreciated. I would suggest two possibilities:
1. Use an underpowered naval mine to damage the boat. The trick here is to not have to move the boat to get the mine to detonate (e.g. it would cause the damage on mission start).
2. Use an aircraft with underpowered bombs to damage the boat. The aircraft should leave after dropping the bombs.
Either case is preferable to my current warship method and would be necessary to test all permutations and combinations of diesel engine configurations, as well as to identify limitations in this approach (e.g. surface speed as mentioned above).