[SubConscious]

I'm using the stock version of SH3 with the 1.4 patch and I've found it to be pretty easy to sink most ships with the deck gun. For Coastal and Small Merchants, I have been able to dispatch them with as few as three well placed rounds at the waterline under the smokestack - not all in exactly the same place, but along a line. If this doesn't do it, an additional 1-2 rounds on the waterline in the fore and aft storage compartments will usually do the trick.

I take out Tug Boats, Small Coastal Vessels and Trawlers with the 2cm gun. Blast the engine, then just forward of the engine to catch the fuel.

T2 and T3 tankers require more effort, but they usually blow up if I can first hit them several times in the engine room, then several times just aft of the forward superstructure. I think I'm using about 15-20 rounds on these.

I have found C3s will sink more readily than C2s. For C3s, I try to nail the engine room, under the front of the superstructure (fuel?) and along the stern in very rapid succession, as they'll settle quickly. I have noted that if I hit the stern just a few feet behind the superstructure that it will ignite a fire at that location.

C2s are the pain in the rump. In my experience, you have to hit them in very rapid succession in the engine room and along the stern to get them to sink. This could be superstitious behavior, but I'm of the belief that if I leave them for any length of time, they manage to stop the flooding and can list heavily for days. C2s usually take me 15+ rounds to sink.

Note that if you leave the area or submerge to allow faster time compression that the ship might not be there when you get back (even when it is at zero knots). If your watch crew doesn't give you the "ship sighted" message, then you may not receive credit for sinking the crippled vessel.

Oh - one last thing: I have found that having a gunner on deck significantly improves the deck gun reloading time.

Good luck and good hunting!

<edit> For what it's worth, I think the deck gun is too powerful in the stock game. From what I've read, it should take a multitude of rounds to sink a large cargo vessel.