Bob,

I'm not directly knowledgeable about your boat or motor, so
my answers are based on my experience with possibly very
different boats & motors.


How much water would an open valve dump into the bilge.

I believe it could be anywhere from quite a bit to just a drip. On
our engine we have a block-drain valve that can be from full open
to full closed. If we ever closed it but not tightly enough it might
just drip.



Could this be a hull problem even though there is no water ingress when
the boat is docked? Could my movement be forcing water through a crack
in the hull, but remain watertight at dock?


YES. On my fiberglass boat I once hit a sandy bottom. I checked the bilge
and
saw no water. While running to home-dock a LOT of water entered the bilge.
I didn't notice until I slowed down to come into the dock and lots of water
ran forward. (Scarry!). Then I increased speed trying to get back to
dock before we sank. Turns out I should have gone slowly to dock because
it was the fast speed that was "pumping" the water into the crack. (End
of story: We didn't sink.)


Good Luck.
Gary