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.