"Wayne.B" wrote in message
...
On 24 Jul 2006 07:28:58 -0700, "Varis" wrote:

You may have noticed boats usually have a cruising speed listed. I'm
not sure how exactly that is determined (and I doubt there is a solid
common standard) but I'd think it takes into account speed, fuel
efficiency and what's best for the engine.

It's not written in stone, but there actually is a commonly used
standard for cruising speed. For gas engines it is usually 70% of
max RPM at wide open throttle, and for diesel engines 85%. This
assumes that the engines have been correctly propped so that max WOT
RPMs occur at the peak of the horsepower curve.

Doing a little more math yields the result that gas engines operating
at 70% of WOT are developing about 50% of their maximum horsepower,
and for diesels at 85% of WOT, about 72% of max hp. Those numbers are
useful for estimating fuel consumption at cruising speed. For
example, a 260 hp gas engine at cruising speed is developing about 130
actual hp, which in turn translates to 13 gph using the commonly
accepted estimate of 1 gph for every 10 hp. For a diesel it is about
1 gph for every 20 hp actually developed.


OK, so that theoretically gives me 9 gph at cruising speed. What does
cruising speed mean? Is it the most engine friendly speed (longevity) or is
it the most fuel efficient speed (gph)?