Quote:
Originally Posted by difran25
now when you say 1p
= 200 rpm, you mean wot i should be 4200?
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1P = 200 RPM Change
So if your boat was WOT @ 4200 RPMs with a 21P, it would increase to 4600 RPMs if you
dropped to a 19P. (2
drops in pitch = 400 RPM gain). Inversely if you stepped up to a 23P you would
LOSE 400 RPMs and be at 3800 RPMs.
Keep in mind this is how it should work mathematically, but is all depends on the motor / gear ratio. Your motor has to have the enough horespower / torque to move the weight of your boat at any given speed. If I put a 23P on my 19' Blue Water with its V6, it might not have enough power to get up on plane to ever get to 3800 RPM's. (It would be like starting your car from a dead stop in 4th gear.)
Also cupping of the prop and diameter plays a big part in this. A higher diameter prop has more surface area which = more friction = less slip = more bite. So a 21P x
13.75 might give you 4400 RPMs @ WOT and a 21P x
14.75 might only give you 4200 RPMs @ WOT AND cause porpoising (bow bounce) because it has too much bite. See isn't this fun how much is involed in this??
For 350's usually sicking in the 14.25 range is pretty good and I HIGHLY recommend getting SS props unless you are always in the shallows snagging things. SS is so stiff you can plane quicker, cruise at a lower RPM, get more top end all while SAVING GAS. They pretty much pay for themselves.
I did a side by side test on my dad's 20' Maxum with a v6. Swapped between a 21P x 14.25 SS and a21P x 14.25 aluminum. The results? The boat used to over rev by 500 RPMs and could hit about 50MPH, just by switching to SS it sucked the RPMs down by 450 and ADDED 6 MPH on top end. Better hole shot and cruising speeds. All those RPMs were just wasted $$$$$'s.
Re: Cruising RPM???
