Prop advice for 25hp tohatsu

[henryfreston]

25hp 2 stroke tohatsu on the back of a honwave t40 sib with an aluminium floor. Would be looking for mainly top speed performance. Usually only has a relatively light load of around 100-130kg including me and fishing gear.

At WOT I am getting 23.5 knots. This weekend coming I am uprating it to a 30hp by changing the restricter between the carb and the block, the gasket only alows for around 70% air flow into the head. So I am hoping to gain 2-2.5 knots from that.

Don't know how much performance I can get from changing the prop. I don't even know if I need a smaller or larger prop

[mattvolvo1]

You will have to wait and see how it performs with the boat obviously if you are meeting the top end of the RPM range just now then you will find you will need to go up a pitch on the prop to bring this back down when you change it to a 30 but wait to see how it performs first.

[joey_1987]

You really need to know what rpm's you're doing @ WOT. 30hp with a light load would mean you could probably go up a pitch (but before you do provide the details of the current prop). I have that engine on a 3.95m alloy open boat and I went from the 11" pitch stock alloy prop (24-25 knots top end calm water) to a 12" pitch SOLAS Saturn 3 SS and have less noise, less vibration, marginally less rpm's @ WOT but now can hit 28 knots and cruise at 20 and the engine sounds much happier. Also check if your engine has the rubber grommet in the bottom area of the cowling, if not then get this as it a) protects from salt spray and b) reduces noise, mine is a 2006 and it didn't come standard.

[henryfreston]

Originally Posted by joey_1987

You really need to know what rpm's you're doing @ WOT. 30hp with a light load would mean you could probably go up a pitch (but before you do provide the details of the current prop). I have that engine on a 3.95m alloy open boat and I went from the 11" pitch stock alloy prop (24-25 knots top end calm water) to a 12" pitch SOLAS Saturn 3 SS and have less noise, less vibration, marginally less rpm's @ WOT but now can hit 28 knots and cruise at 20 and the engine sounds much happier. Also check if your engine has the rubber grommet in the bottom area of the cowling, if not then get this as it a) protects from salt spray and b) reduces noise, mine is a 2006 and it didn't come standard.

thanks that was a lot of help

I will get a tachometer ordered then, are the £10 ones with the wire you wrap around the HT leads ok?
That is an impressive improvement! And yes it comes with the rubber grommet

[joey_1987]

As for the tachometer, I have no experience with that type I have used a laser type digital one which works fairly well. Yeah definitely a good change and only going to keep the alloy prop as a spare. What surprised me most was the reduction in vibration through the tiller and getting on the plane earlier (the Solas SS prop has a greater blade surface area than the alloy and is also much thinner I am sure this works for efficiency). Good about the grommet, should've been standard back when I bought mine but no biggie. Also not sure what SS props go for in the UK but being in AUS I bought mine from USA when the AUD was strong was about $180AUD landed.

[Downhilldai]

My mate runs a Twatsoo 25 on an Avon 340 rib and gets 35mph (around 30kts) No rev limiter on those motors.