@t500hps,
With which prop is best gpm by 3.500 rpm?

Ive tried 29`s 30`s 31`s and 33 pitched props . My Best speed was closest to peak hp and higher rpm plus the engines weren`t bogged down at cruise.
Id rather be making 2 psi at peak tq then 5 trying to swing a huge pitched prop.

I didn't have fuel flow meters so I assumed the fastest speed at 3,500 rpms gave the best gpm???. Best I could tell the boat was burning 35-40 g/h at 3,500rpms (could have been slightly more?)
.

I dont see how overpropping nets fuel efficiency. Reducing prop slip, I can see , but overpropping by pitch? Just because an engine rev's less, doesnt mean its using less fuel. If it takes 50% throttle opening, to turn a 30P prop at 60mph, or 40% to turn a 26P prop at 60mph, I'd think that the 26p would net better fuel economy. Generally speaking, most boats require a certain HP number to go XX mph.

We have single speed transmissions basically. One of the benefits of having an engine propped where it can go to 6000rpm, vs 5000rpm, is better performance from idle to wot. Hence why a diesel engine that makes big torque at 1500-2500rpm, but is dead at 3000, doesnt work well in most high performance boats. The torque number is there, but the powerband isnt. Throw a multspeed tranny in there, and its a different ball game..

Today I was fooling around with my new challenger. Cruising at 70mph, the engine was making 24hp and 73ftlbs of torque. Keyword, cruising. Now, what kind of power you think it takes to get a 38 Cigarette to cruise at 70mph? About 600hp per engine, or 1200hp total. Think about that next time you wanna tune your boat for a "lean cruise" . :)

My vote, is if the engine has the components to withstand your peak power number, prop it for that. Most likely you'll have a boat that planes better, accelerates better, and is easier on the drivetrain. .