Under propped advice please

[Bateser]

Hi guys im running a mariner 40 elpto 2 cylinder 2 stroke and on the leg it says 5200 rpm but i am hitting 6000 the prop is 10 3/8 13 pitch can you advise me on the prop size im looking for.

[Pikey Dave]

Originally Posted by Bateser

Hi guys im running a mariner 40 elpto 2 cylinder 2 stroke and on the leg it says 5200 rpm but i am hitting 6000 the prop is 10 3/8 13 pitch can you advise me on the prop size im looking for.

5200 sounds low for a 2 stroke. 6000 sounds more like it. The max RPM isn’t normally shown on the leg. Got a photo?

[Bateser]

Just checked it says max rpm 5250

[Pikey Dave]

Originally Posted by Bateser

Just checked it says max rpm 5250

Every day’s a skool day[emoji848]

Is the current prop the correct diameter? If so, you can reckon on 200rpm/1” of pitch as a ball park figure. So to lose 800rpm you theoretically want to go up 4” in pitch. If you can go up in diameter, you can lose RPM that way too.

[Bateser]

Cheers Dave. I have been looking at the manual for the 3 cylinder model and they quote a 10x16, 10x17 and 10x19 in their list and an estimated speed.

[Pikey Dave]

Originally Posted by Bateser

Cheers Dave. I have been looking at the manual for the 3 cylinder model and they quote a 10x16, 10x17 and 10x19 in their list and an estimated speed.

So you’re dropping diameter, in that case I think based on your current setup, 10x17 should be in the ballpark. I prefer to be towards the under propped end of the scale.

[paintman]

Changing a standard prop by 1" will give an increase or decrease of 150/200 rpm.

Assuming all figures are accurate, the boat/engine are correctly trimmed & you are running with your normal load then you need to drop around 750 rpm.
Prop pitches are normally in whole numbers & I don't know if props for yours change in 1" or 2" increments.
Something around 17 might be worth a try.

ETA Whilst I was doing mental arithmetic it seems you & PD have had the same thought process - just a bit quicker

[Bateser]

This is the prop sizes i was looking at but unsure how to use the info as the boats get heavier as the list goes down. Many thanks for your help bud.

[beamishken]

If its rated to 5200rpm but the tach says its turning 6000rpm I'd check the tach is telling the truth with an optical tach. If its rated 5200max I'd expect there to be a rev limiter that would stop it going that much over rated maximum. Its an easy cheap check before you start spending money on propellers

Sent from my SM-G950F using RIB Net mobile app

[Bateser]

Thanks bud will check that out.