CAPTAIN IN A BOX™
transom a half-inch to make it even. What effect would it have on
speed, turns, and bow attitude? The boat is a 20-foot bowrider.
With 56 gallons of fuel in the stern, it gets heavy. I’d like more
speed, and less porpoising. Would I risk getting ventilation on
turns by raising the ventilation plate?
JOHN ADEY: There really is no reference to the location of
the anti-ventilation plate to the bottom of the keel. If you take
a look at any of the installation manuals online, you will find
that the drive template location is found by determining the
driveshaft center, based on transom height and angle, and then
there’s a small allowance for performance. We have to assume
that the builder performed testing to determine the acceptable
performance. Moving the drive up could contribute to ventilation
and cooling problems from interrupting the flow of water to the
raw-water intakes. I’d hate to see you change the height and re-glass the transom only to have complicated the issue. You might
see excellent straight-line performance but then the prop loses its
bite going into a turn.
You didn’t mention any experimentation with prop pitch; this
will be the single largest improvement you can make instead of taking a shot in the dark at drive height that you may seriously regret
later. Many boats have the stern-weight issue you’re experiencing.
For starters, put 20 feet of anchor chain in the bow, see where that
gets you, then experiment with your propeller pitch. I’d trust the
boatbuilder on this one, and keep the height the same.
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