So many variables and finding the sweetspot has to do with how she was set up unless you have a variable pitch
prop. As stated in a different language
, diesels burn about .45 lbs per hp/hr (varying some) used. Fixed propellers can only give max power and max speed for one rpm
. for the given application. However, as pointed out above, the faster you go, the more friction is being faught by engine and boat. Then there are other efficiencies having to do with rpms that the engine was designed for. And of course limitations on hull
speeds, speed/length, wave period. In the example above, jrd22 is using about 15 hp.(at .45lbs/hp and a 1gph burn) Acording to my book and if he is set up efficiently, he is about 35 foot waterline at a little over 10 T. If that's the case, at 20 hp used he will get a little over 7 kts and only 8 kts at 35 hp used and I would imagine the numbers would be lower because he probably doesn't have a variable pitch
prop of course...
But, the basic idea for fuel economy that you have some real control over is speed. The slower you go, the better you'll do. Just keep her in the operating range specified by the engine manufacturer for the pitch prop you have for the boat. Unloaded and low engine speed can cause "slobber" and premature cyl wear as well as carbon buildup.