additional data.... by examining the BSFC vs. the torque curve, you can
easily identify the optimum engine speed for max power at the best fuel
consumption. Then you must adjust prop and gear to take best advantage of
that RPM for the best boat speed. I am assuming the boat is proped for the
best max speed, not economy. I need to caution you on a little understood
fact though. The performance curves previously mentioned are a product of
test runs on an engine brake (dynometer). These runs were performed under
full throttle conditions, where the engine speed was controlled through the
application of load, not throttle change. This is important because unlike a
diesel engine, power output is controlled by limiting both air and fuel (a
diesel is controlled by fuel alone). The resriction of air changes the
thermal efficiency of the engine because the restriction of air will impede
the speed of flame propagation across the cumbustion chamber. (Spark advance
is the band-aid that tries to compensate for that) Consequently, the diesel
is more efficient under partial throttle conditions of operation in pounds
per HP/hour than that a gasoline engine. The point that I am making is that
fact is not shown in those performance curves, so you must understand that
those curves are only a starting point when a gas engine is being described.
This is especially a problem with your engine because of the large
combustion chamber and short stroke. (The optimum spark advance for your
motor is in excess of 40 total degrees as opposed to say a Harley at 35
degrees) This is probably a lot more info than you needed to know, but I
think this is interesting never-the-less.
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?