Most
boat owners will report x gph at y kts, and they usually say that that was from fill to fill
fuel measurements. But that is always misleading. You spend a significant amount of time idling and getting in and out of
marinas, dropping the
anchor, etc that never gets accounted for.
So I have concluded that the only realistic way, unless someone reports Flowscan, real time data, is to look at it somewhat theoretically. There are two approaches to do this.
One is to use a pretty good
rule of thumb that it takes 15 hp per 10,000 lbs for a true, round bottom
displacement boat, and 20-25 hp per 10,000 lbs for a semi-displacement
hull like downeaster to push the boat to displacement speed- defined as 1.34*sqrt(lwl).
That would mean that your boat would take 68-85 hp. High output engines at lower rpms typically make 15 hp per gph of
diesel burn. So your boat would take 4.5-5.7 gph to go 9.1 kts. That is on the low end of what you reported.
The other way is to
plug your boat's parameters into boatdiesel's
power required calculator and then use 18 hp per gph for the higher hp loadings. If I
plug your data into the calculator, I get 350 hp at 15 kts or 19.3 gph, within 10% of what you reported.
So, collect all of the data that respondents give you, but take it with a big grain of
salt.
BTW, downeaster hulls are pretty inefficient, but handle heavy
weather better than almost all other hull types. I have a 34' downeaster and it burns a lot of
fuel at 15 kts.
David