props and revs

[krazyfisher]

so which is better to be over reving or under reving I have just changed props from 17 pitch to a 15 pitch with the 17 it would rev to 5100 full trim now with the 15 it will rev to 5900 maybe more the tacho doesnt go any higher, so which is the better.
rev range: 5200-5800
motor: 140hp tohatsu
boat: 540 coxcraft reefrunner
thanks

[Spaniard_King]

Your engine will be way better off with the 15.

You should have improved hole shot too.

With the 15 you have a lot less load on the engine as it can reach full RPM

Garry

[mark221263]

Hey there. I always prefer to run a prop that allows my engine to run to the top of the WOT range or even a little more.

The benifits are that your not lugging your motor and gain much quicker responce. If you load the boat up you can can get onto the plane easier, if its rough the boat will hold a slower plane without the engine lugging down and falling off the plane. This is especially important if it gets rough or you fish outside or cross bars.

For the outboards benifit the lower gearbox unit and the engine is under much less stress because the finer pitch prop is giving you the mechanical advantage similar to using a lower gear in your car.

My 150 mariner has a wot range of 5000~5500 and I can get 5800 wot with a 17" prop, 2 people and 1/2 fuel. I keep a pretty througher log book of fuel used etc and the change from my earlier 19" prop that would only run to 5300 has not effected ecconomy even though I now sit 200rpm higher for the same cruise speed of 26 knots. Top speed has dropped 2 knots.

For me I'd try to run at least at the top end of wot recommended range. Try and find a prop dealer like solas or one of the better equiped merc outlets to try different types of props as well as pitch changes prior to selecting the best one for your needs.

Cheers Mark