[Crannogman]

If they're making 15kts in a storm, they're probably warships. Predict their course as best you can go to periscope depth about 500y parallel, go to action stations and silent running. Open all tubes and set all fish to high speed. When you spot the first ship, fire a spread from your front tubes and then turn away when the fish hit, go to flank speed and decks awash (or fully surfaced). If the survivor gives chase, it's a destroyer - fire a spread from your aft tubes. If the survivor does not pursue, it's a heavy; go on ahead and set up another ambush.

You could try to shell the survivor and set him alight - most likely he'll be zig-zagging for a while anyway. If it's an escort, he will most likely run to where your torpedoes emanated and start drifting, pinging and listening. If so, you can either make good your escape on the surface, or else try to give him a spread while he's sitting there (but it may be hard to hear him).

Of course, if you ID the first one as a friendly, most likely he's being chased by a carrier and you should fire a blind spread at the second contact :p