Prop rpm's how do you figure them.

[Red Rider]

First I need to make sure that you understand I am not asking how to select a prop for a motor. Its a little more complicated than that. <br /><br />I have a prop, prop shaft, rudder, shaft packing, shaft support (all the underwater running gear) etc. The boat is tunnel drive. I have an engine that I know will definitely provide enough power for the boat/prop combination as it is a larger hp/cubic inch version of the original engine. <br /><br />What I don't have is the original transmission. And I don’t know what the original gear ratio was. I have a very good idea on how to measure pitch etc. At least good enough to get the basic pitch and diameter etc.<br /><br />What I can't figure out is how to determine the rpm's to turn the prop. If I know what rpm's to turn the prop at WOT, then it is easy to figure the gear ratio I need. It’s got to be simple. With transmissions and gears not being cheap, I just can’t go swapping around until I find a combination that works.<br /><br />Is this a case of not being able to see the forest because the trees are in the way? I have racked my brain trying to figure this out.

 

[Forktail]

Re: Prop rpm's how do you figure them.

Sounds like an inboard. <br /><br />Are you trying to find out what transmission you need?<br /><br />If you know the rpm the engine runs at, and know the pitch of the prop, you should be able to have a good marine transmission shop set you up with the right transmission gearing.<br /><br />Maybe the manufacture of the boat can help. You said it was a tunnel drive...is it a Penn Yan?

 

[huanca]

Re: Prop rpm's how do you figure them.

ok a big project in your hands. i suggest you but dave gerr´s propeller handbook (i fot it for $20 at powells.com) how much hp do you have and what is your boat and engine weight fully loaded? how fast do you want to go? i/o and o/b usually have a gear ratio of approximately 2:1. what rpms is wot on your engine? the props max diameter is fixed by your hull and shaft and needed clearance. get the largest prop you can turn and still get 15% of diameter clearance between blade tips and hull (to avoid noise mostly). o/b and i/b props usually turn between 2000 and 3000 rpms. if you know your engine rpm you can figure the gear ratio to run the prop between 1000 and 2000 rpms at wot. it will take you about a week of reading but well worth it. good luck