krj wrote:
OK Jeff,
What do you estimate my usage should be with 4-108, 30' LWL, 20,000
displacement, two blade 16 x 14 prop.

To drive a boat at a Speed-Length ratio of 1.2 requires about 1 HP for
every 700 pounds. So to push 20,000 at 6.6 knots requires 28 HP,
which should burn about 1.5 gallons per hour. It would seem that the
4-108 is overkill for this boat, but you have plenty of reserve. If
you push to hull speed, the requirement goes up to 1 HP for every 500
pounds, or 40 HP or around 2.2 gals/hours. If you back off to under 6
knots, then you only need one HP per 1000 pounds, or 1.1 gals/hour.

So how close did I come to your experience?

A prop calculation will cost you an extra 50 cents, but your goal
there should be to have a large enough prop to handle the power, and
the engine reving at a sweet part of its power curve. My rough guess
(and admit this is one area I never really understood) is that the
blade area your prop is small for the horse power you may be putting
through it - this would cause cavitation.