Gas engines burn 1 gallon per hour for each 10 HP generated (not rated engine
power, but the amount of power you are actually using at the time).
Once fully on a plane, running at low angle of attack (about 3 degrees), planing boats have surprisingly level resistance curves: the faster you go, the less hull
is in the water
. Therefore, the best economy once fully planing is simply the speed you are going when the engine
is turning the RPMs where it makes maximum torque (also, the RPMs where the engine is the most efficient at converting fuel
to HP). For typical gasoline V8 engines, that's 3500 RPMs.
28 feet with twin small block V8s: you will probably be able to fully plane, so your best cruise speed will probably be at 3500 RPMs.
Let's say your boat goes 30 knots flat out, engines turning 4500 RPMs (They should turn about 5000, but its not a new boat, right? so its probably full of gear
, lots of things have absorbed water
so weight is higher, the bottom and props are not so smooth, etc). Your engines will be burning something like 25 gallons per hour each, so you'll be getting something like 30 knots/50 gallons per hour = 3/5 of a nautical mile per gallon.
You will burn close to this at any planing speed. But near 3500 RPMs, you will get about 10 to 20% better efficiency, so you should approach 4/5 of a mile per gallon. This is very typical for planing power boats.