Originally Posted by SailingHarmonie
You can usually come pretty close on fuel burn rates by assuming 0.35 lb of diesel/HP/hr, and diesel
weighs about 8 lb/gallon. A 4-154 is rated at 62HP at full rpm, and then power
needed falls off with the CUBE of the boat speed below that (pretty close…).
So if full throttle 62HP plushies you at 7.5 knots, to go 5.5 knots only takes 24 HP, hence a huge drop in fuel needed.
A. Diesel weighs 7lb/gal or perhaps slightly less depending on temp.
B. For engines following a propellor curve, fuel efficiency drops as RPM goes down. It doesn't go down as fast as the demand drops, so fuel economy will improve as you slow, but gal/hr will not follow the cube rule
that power demand follows.
C. Currently the very best tractor motors get somewhere near 18hp-hr/gal or so, only one on the market has exceeded 20. These are diesel motors near 500hp. I cite tractor motors because there is very good data for these motors and it is available.
At a max power of 62hp the engine should be using 3.4-3.7gal/hr (17-18hp-hr/gal)
At a power demand of 20hp, the motor
should be producing about 12hp-hr/gal. so near 1.7gal/hr fuel usage.
At 10hp it'll be 9-10hp-hr/gal.
At 5-6hp efficiency will be near 7hp-hr/gal. For an Irwin 46 I would expect you to be using about 0.7-0.8gal/hr. That will be at a speed of about 5.5kt or so.