doctor_ratz

I have a 96, 38 foot Formula, 382 SR1 with 500 HP's and bravo's. I am replacing the 500's with 540's this winter. The question is in the drive ratio. The boat came with 1.35:1 drive ratios, so the props turn very fast. I was running a 22 pitch 4 blade at about 5000 RPM to about 65 MPH on GPS. It seems like most other guys with newer boats tend to run faster with similar or less power and a lot higher pitch prop. The main difference seems to be drive ratio. Most have a 1.5:1. The question is, performance wise, would it be better to have a higher pitch prop at a slower prop RPM, or a lower pitch prop at a higher RPM if the engine power was the same. Also, strength wise, is there an advantage to a lower or higher gear ratio drive in terms of how well it will hold up to increased power.

Panther

I would think you would like the 1.50 and bigger props better.

Carder

I agree with the bigger props and 1:50 or 1:65

t500hps

1.35 seem better suited for those running out of pitch in props....odd that yours has 1.35. I've always heard that 30P were the "most efficient" prop but can't tell you why, just what I've heard. Assuming your 540's make 600ish HP and 1.5 bravos your probably going to need 26P.

blue thunder

I think a higher prop speed yields the best performance. It lessens the prop efficiency issue due to the faster impact with prop to water. Slower just kind of pushes thru the water. Lots of friction just pushing thru the water.

jeffswav

Like was said before the 1.35 ratio is normally used when you run out of prop. Here is a interesting link for you, have fun with it. http://www.rbbi.com/folders/prop/propcalc.htm

Panther

rule of thumb is highest ratio and biggest prop until you get to the 34-36" bravo 1's with a boat that size and weight. the heavier boats can't overcome the increased torional load the lower gear ratio gives with a heavy v bottom... it's like driving uphill in overdrive. I lost 2 mph going from a 1.50 down to a 1.30 and had to give up 6" in pitch. You'll probably burn less fuel as well. I burned more with the lower ratio.

doctor_ratz

Again, thanks for the input.
I suspected that at some point a higher pitch and a lower gear ratio (1.5 - 1.65) would be more efficient simply because drag is a function of relative surface speed. Just as it is on a hull in water, so will it be on the surface of the prop blade.
Not sure I can financially make all the changes this year, but with the added HP of the 540's, I would like to go to an XR drive. When I do so, I want to get the best ratio for the application rather than just replacing with what I had.
Thanks

bajabob

doctor ratz i like the 136 i have in my boat 5500rpm i am over 80mph my buddy has same boat one 136 drive the other 150 drive both runnig 23 pitch mirage prop 150drive did 40mph at 3600rpm the 136 did 40mph at 3000rpm!! gps now thats on a 24 baja i am runnig a 540 dart . dart pro i heads dart single plane intake with spacer and 1000cfm holley carb . very happy with this setup . the main thing is to prop the boat at the rigth rpm ! my case was 5500. my boat run the best ;any lower it a dog ! not as fast. now on other boats i am not shure this would be the same because of the hull and weigth good luck

Panther

If you have a 1.36 in your boat and can turn the same size prop as the other baja with the 1.50 than you have more power than he does. I'm not trying to be rude but it's not apples to apples.

Also, smaller boats sometimes like the lower gear ratio (1.36) because they are small enough and light enough to overcome the increased torque load of the lower gear ratio!