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?

Nocomplaints:

While on plane, you are generally correct that fuel consumption is linear
with rpm. Once you fall off of plane there is a peak as the boat plows a big
bow wave. It then drops exponentially towards idling rpm.

Gas engine manufacturers rarely give you prop horsepower and fuel
consumption vs rpm curves. To see the curves for a diesel go to the Yanmar
or Cummins marine engine sites.

David

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

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?

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?