rpm of props
 

Is it better to spin a smaller diameter smaller pitch prop faster by gear ratio or a larger diameter larger pitch prop slower again by changing gear ratio.This is on surface drives. Any thoughts or ideas.

slower rpms and bigger props , now don´t you wonder why surface drives work so good with diesels....

Bruce, how r-ya ? ck.the Eliminator Forums look for the thread called New Cummins/Mercruiser Project ck. it all out great info. by some smart cookies. they talk about the stuff I think you are interested in. let me know what ya think. thanks Jay 33' Sutphen w/Kaama drives. Mikey u-ck.too :)

You have to be careful, because larger isn't better! It depends also on the characteristic of the engine. The problem of surface drives are that you need much torque to spin the full submerged prop if you are not planing. Here the bigger diameter the harder to spin.

Here are some facts for consideration:

• Diesel engines have a max. rpm of 4,000 w/ big torque
• Gas engines rate at 5,200 w/ medium torque
• Tuned gas engines spin up to 6,000 w/ big torque (like supercharged)

Dustin Whipple told me for my supercharged engines not to change the prop and the gears because of the gain in rpm and torque I would have to benefits:

• Higher top speed by higher rpm
• Shorter planning time by higher torque and same prop diameter / pitch

My considerations depand on a matching prop!

Hope my thoughts will help you.

-Gerhard-

Nice to hear from an ol' Kaama guy from way across the pond, as they say. Also larger diameter props also produce higher leverage against shafts, housings, and bearings etc.

Many, many variables in each set up. Each set up varies per the boat CG, X dimention, etc. and the desired goals. Most often I like low gear ratios and big pitches with the largest diameter the boat will allow you to use.

Discussing this set up "verbally" with a few key people is the best idea.