prpoeller selection

[Herman Melville]

Hi,

I know there are propeller threass on here but I have never chosen a propeller before and coudl use some help.

I have a 35hp evinrude short shaft and it's pushing along a 4m rib. I suppose the rib weighs 100kg? maybe less.

I am interested in speed - no surpise.

I have looked up on the parts database and they reccommend several propellers for it....from 11.25 * 7 through to 10 * 13 and several in between.

Any suggestions er welcome. I don't want to exceed 6k rpm and I doubt (although I don't know) whether my old motor has a rev limiter...

Any help? I assume that the 10 * 13 will make it go the fastest ?

[Fenlander]

I have no experience of 35hp on a 4m RIB but when prop refining you need to start with three known factors... existing prop size... max speed seen... engine revs at that max speed.

[Limecc]

Use the calculator here as a rough guide
https://goodcalculators.com/boat-prop-calculator/

Leave the speed field blank and assume a prop slip of 10% if you don't know the current slip from speed and rpm testing.

This will give you the theoretical peak speed for your max rpm. Whether the engine will be able to reach max rpm is another matter but at least you will know the speed increments between props. This is independent of diameter. You need to know the gear ratio though.

- Large dia prop does the same speed as a small dia if the pitch is the same.
- Larger diameter prop has more blade area and will have less slip for the same pitch
- Blade area needs to increase the boat heavier the boat gets, 25% slip is quite bad, 5% is very good. High slip wastes engine power.
- 1" of pitch equals approximately 200rpm
- A four blade prop has approx the same area as a three blade one pitch lower if the diameter is the same

If I've got anything wrong above the more experienced guys will correct.

[jeffstevens763@g]

Fenlander is right you need base info first you can do the theory but it seldom equates to real life you also need to know the power band of the engine I'e say 5500-6300 for example and as always it's a compromise running two or three props gives versatilatiy to some extent but most accept the drop in performance to weight, sea conditions etc.