So far I have found the following:
Shooting sails to zero has no effect on combat speed or manoeuvrability. Where can you buy the "outboard motors"?
So far (2 play-throughs) boarding has only yielded 3 cannon.
Disabling crew yields pretty pictures of dead crew on deck. This can reduce the number of (Fire giant/vikings), but I have not noticed a reduction in other combats.
Ship speed is erratic (but first time you dial in full-speed you will only achieve about half speed). The enemy appears to speed up at once.