Propping for max HP?
 

Looking at the dyno sheets on my Pfaff 700's, I see that they make max HP at 5700rpm. I can only turn the 32B1's that came with the boat 5200rpm. Any chance the boat would pick up any speed with some 30's? That should put me very close to the max HP. I have a LH 30 and can probably borrow a RH to try. Running low 90's now, always looking for more.

They make 677@5200, 700@5700

In my experience, the best way to prop comparison test is in calm water with a stop watch. This gives you both top speed and acceleration. Do you have access to a river?

Hard to say, most likely won't go any faster. But it would sure accelerate harder.

Are your current props labbed? If not that would make a big difference.

depends on how much HP you are giving up at 500 rpm below max HP

Only talking 25 hp diff. As Marcbam said, same top speed, higher rpm. It would all depend on how well the boat does everything, docking, planing, cruise, fuel burn as to which prop would be preferrable.

I have no issues with planing or cruise speed. Current props are not labbed. I probably just need to do some back to back testing and decide from there.

I am also turning the props out as well. Might try turning them in and see what happens.

If you aren't making peak HP then you are likely suffering on top speed. In addition acceleration will be improved by propping down from your current set.

The best analogy I can give is look at cars and their top speeds. Many, if not most, pull the highest MPH in something other than top gear (i.e top OD gear). Usually 4th gear for a 5 speed tranny or 5th gear for a 6 spd tranny.

I went from labbed 30's to labbed 26's. In the end top speed was the same even though there was close to a 700 rpm difference. Planning was good with both props, the big change was in acceleration. As you can expect the 26's were way better.

You could either lab your current props and gain 200-250 rpms or have them de-pitched and labbed and get exactly where you want to be. If it were me, I would have your 32's made to work with your current set up. Won't be the cheapest, but in the end you will be happier.

You simply need to test. Once we built bigger motors in a 382 I ended up with 4 sets of props. In the end:

28P 15 5/8 maximus cruised the best, 60GPS @ 3,500 with a WOT of 83 @ 5,250
32P Labbed Bravos cruised 58 @ 3,500GPS and WOT of 82 @ 5,350
30P Labbed Bravos cruised 53 @ 3,500GPS and WOT of 87 @5,600

So, what do you want? I wanted cruise speed and stayed with the Maximus.

@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. .