No, your assumption is not valid. Fuel consumption for all boats is a
product of more than the brake mean specific fuel consumption (BSFC) of the
engine. This curve alone will deliver your gallons per hour query. (These
specs can be found with the engine manufacturer), but also the efficiency of
the hull, reduction ratio of the marine gear, sea state and the propeller.
Ultimately, you as the owner have to work these numbers out yourself. I
believe the answer you are really looking for is fuel consumption per sea
mile and that will be a compromise of cost vs. time. However, knowing the
real performance of your boat is never a bad thing, as you may find that
subtle changes in prop, gear ratio weight balance and mode of operation can
yield big benefits in economy.
Steve

"NoComplaints" wrote in message
...
I have a 7.4 L MercCruiser Engine on my 29 ft Bayliner. While on
plane, I know the fuel usage for cruising RPM and max speed RPM.
SInce I do not have any gear changes, I have been assuming fuel usage
(gal/hour) is linear with RPM and I can extrapolate to lower RPM than
crusing. Is this a good approximating assumption?