HP vs Prop/ RPM
 

If everything stays the same except added HP, how many RPM would be gained from 100hp gain? For conversation sake the peak HP is in the same relative rpm range or slightly higher.

GENERALLY speaking, about 500 rpm...

If you are looking to see how much more prop you might need I would first run the new motor with the original/existing prop. Yes you may need more prop but sometimes not. You just turn the existing prop faster. Get a solid baseline first. THEN prop shop if needed.

I’ve always propped for close to peak hp + - If slightly higher I didn’t care due to trim/tab water conditions. No need to load the engines more than you have to. I’m in the same boat only I started out of the gates with all new everything except props that came with boat. I need to down size as I’m only pulling 5,300 max. Still pull extremely hard in mid range for an old 40 straight bottom but giving up top end speed. I’m searching for 2 pitch sizes smaller trying to achieve 5,900-6,000

Where you want to cruise is also a factor. My plans for my Formula 233 project, are a 21p prop. 30 mph slow cruise around 2600rpm. Fast cruise at 40 mph, 3500 rpm. Max rpm 5600 ish, at 65mph. 13% slip.

This way my slow cruise has the BBC turn relatively slow, and still lots of vacuum at the 40 mph fast cruise. All without lugging the engine, but keeping rpm in check.

23p would put it at 2375 rpm at 30 mph. Probably too low, certainly with any weight in the hull. 30mph in rough water needs a good easy prop to push feathering the throttle.

I added about 60 ish hp to a 454 mag in a 24 Outlaw and gained 400rpms.

It really will depend on your HP starting point also. If you add a 100 hp to an 800HP engine, your rpm gains will be less than if you add it to 400hp engine

Starting with 330. Ending between 550 and 600. Dart block, 4.25 stroke, etc. Prolly do a 565. Massive torque is the goal. Which naturally leads to decent HP.

Why are you asking the question? As said above 400-500 rpm. Dont sell the prop you have now with only 100 more HP you will probably just spin it faster if not, you could send it somewhere and get more pitch or cup and be happy...

The previous owner of my 12 m had blown 468 b&m 250’s. He only turned 4,400. My NA 565’s turn them 5,300. I didn’t think I’d even see that and was on a rampage to get other props. Although I do need I’m glad I just stayed with what I had to gauge what I need. And I’m not done chasing power either.

IMO… as others just ck it out as is. I hate lazy boats even if tip end speed is the same.

Currently a stock 454 making 330hp and 375-400ft/lbs of torque. Most likely going to a 565 with 600hp and 700+ft/lbs of torque. So I was wondering what kind of rpm gain per 100hp to expect. I'm going to build it it for max torque and keep the rpm down. 5500 rpm max. Probably less. I want it to live. I'll have enough to worry about keeping the outdrive alive. Fortunately it only weighs in about 3500lbs with fuel, gear and two people.
Going by the 500rpm /100hp I'd be picking up 1300 rpm. I really don't plan on ever spinning it to 6k. Not that it wouldn't be capable with all forged internals. Goes back to wanting it to live a long time.

try your prop and will likely go up 2 pitch sizes based on what your end goal is. Seems like so many people are afraid to turn some RPM
‘s. They love it.

Smitty,

Are you starting from a bare block and building all new? Kinda sounds like it. I'd say you could pretty easily top 800HP with a 565, and still be reliable, depending on how you dress it out; carbs/EFI, heads, cam, intake and such. With a 3500lb boat, you're going to have a beast. And I'm guessing you have a Bravo drive, given your starting point and expressed concern about the drive. You're definitely going to find the limits of that drive with a 565 of nearly any build.

Thanks. Brad.
(937)545-8991

I actually have two blocks to choose from. One will give me 519cui, the other 565. Both are Dart blocks retired from drag cars. Crank is 4.250. I've never built a "low HP 565, or 519 for that matter. 1000hp and up is the norm. But dropping compression, and a good set of oval ports I'm going to try to keep it around 600hp. Will probably still have 700ft/lbs. The 565 will be easier to get pistons for. The oval ports (probably FloTek) should limit the power available and have awesome velocity, which is good for a nice even torque curve. And ya, the Bravo is the weak link. Being respectful accelerating and a fairly light rig I think it'll do better than expected. It's also an early Bravo which are supposed to be more durable. But at that power level none of them are bullet proof. Nothing is set in stone yet. But I need to talk it thru and get others opinions to be sure I don't make a grave mistake somewhere along the line.

Back OP just an example when I went from 330’s to 480 hp’s. Basically the 330 on roids type deal. I went from 23p to 25p mirage props and gained 1,000 plus rpm. Just throwing that out there. 28’ challenger straight bottom V all the way to the transom.

WOW, that's a lot. What'd you pick up, 10-12mph?

was a solid 10 plus went from a boring 66 sluggish boat to a very responsive 77-78. Lightening headers. John sr had identical boat with 525 sc’s and we ran neck and neck.

This is back in early 90’s when running 80 was respectable on the water. 😂.