On Thu, 2 Oct 2008 15:47:10 -0700 (PDT), NoComplaints
wrote:

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?

I think this depends on what you mean by "lower RPM.

I think that there should be a significant step in fuel use as you go
from displacement speed (~6 kn for your boat) to planing speed.

--
Peter Bennett, VE7CEI
peterbb4 (at) interchange.ubc.ca
GPS and NMEA info: http://vancouver-webpages.com/peter
Vancouver Power Squadron: http://vancouver.powersquadron.ca