[Tyhm]

Hm. Okay, how about this:

Each ship has an invisible field around the front of it, so:
Ship: =>, Field: )

=>)

If this field passes boat B, it does the following three things: Hits boat B for speed/2, hits boat A for speed/3, reduces boat A's speed by 50%. For as long as the field remains over boat B and boat A has speed, it continues do damage.

So:

=>)
=>) No damage

=>)(<= damages each, faster boat deals more damage than slower boat

=>) - this boat does no damage
A - this boat hurts that boat (and itself)

Since boats have no turn radius and no reverse, they're not gonna hit with anything but the bow...so this seems safe. If you keep gunning it into the other boat, eventually the other boat will break (at great cost to your own boat). If two boats play chicken, the one that flinches first may take more damage for taking a side collision (or they may be ready with their cannons)...

RAMMING SPEED!