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HP vs Prop/ RPM

Old 09-18-2022, 10:25 AM
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Default 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.
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Old 09-18-2022, 11:31 AM
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GENERALLY speaking, about 500 rpm...
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Old 09-18-2022, 05:13 PM
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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.
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Old 09-18-2022, 05:34 PM
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Iíve always propped for close to peak hp + - If slightly higher I didnít care due to trim/tab water conditions. No need to load the engines more than you have to. Iím 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 Iím only pulling 5,300 max. Still pull extremely hard in mid range for an old 40 straight bottom but giving up top end speed. Iím searching for 2 pitch sizes smaller trying to achieve 5,900-6,000
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Old 09-18-2022, 11:37 PM
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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.
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Old 09-19-2022, 12:10 AM
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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
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Old 09-19-2022, 04:49 PM
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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.
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Old 09-19-2022, 05:02 PM
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Originally Posted by Smitty275 View Post
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.
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...
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Old 09-19-2022, 08:08 PM
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The previous owner of my 12 m had blown 468 b&m 250ís. He only turned 4,400. My NA 565ís turn them 5,300. I didnít think Iíd even see that and was on a rampage to get other props. Although I do need Iím glad I just stayed with what I had to gauge what I need. And Iím not done chasing power either.

IMOÖ as others just ck it out as is. I hate lazy boats even if tip end speed is the same.
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Old 09-19-2022, 08:35 PM
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Originally Posted by Keith Atlanta View Post
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...
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.
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