It all depends. What''s the weight of the boat, beam, prop pitch, etc. I have a 4-108 in a 40'' Jeanneau which displaces 16,000 lbs, two blade folding prop, 12''8" beam and I burn a bit less than a gallon an hour at 2,000 rpm (about 6 knots). Hope this helps.
I agree with Dave,
I have 4-108 in my Morgan 382 1979, displacement of 17000 lbs, three blade prop. I burn .75 gallons per hour @ 2400rpm which pushes me along nicely at 6.5 knots in flat water.
Fred
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