best way to prop out f225

[NiVani 1]

We run a twin 225hp on our landing craft.
What the best way to prop/pitch this vessel, on light load or on max load??

We carry 2.5 ton on a 9.7m alloy landing craft.
we were run with 3x16x21.5 props, then fitted 151/4x20 prop.
we run average of 4500rpm @22knt - 25knt.

boat weighs around 2.5tone

any thoughs?

cheers

[boats.net]

With that much of a load probably would need to use smallest pitch prop available to get rpm's within the 5000-6000 rpm range.

[Harper]

Boats.net's reply assumes that you mean that you are running 4500 RPM at full throttle. I'm not so sure that their assumption is valid.

So please explain what you mean by "we run average of 4500rpm @22knt - 25knt." Because if that's where you usually cruise, and it's at part throttle, then their assumption is all wet, and you are likely propped pretty well.

[NiVani 1]

thanks boat.net, and yes harper, we are only at cruising speed, part throttle.

i don't wont to try push to 5000rpm due to the stress the engine are already under.

so stick to the 4 blades or lower pitch to keep stress as low as possible for the gear box.

cheers

[Moonlighter]

Nivani, in order to determine whether you are propped correctly, you will need to know the max revs your boat can achieve at full throttle with its normal load, using the current propellors.

The basic test of "correct prop for boat and weight" is that the engine must be able to achieve something close to its max recommended rpm at full throttle. This is the only way to know if you are propped correctly. There is no other way.

You need to tell us the max achievable revs on your boat, otherwise no-one can help you. It should be close to 6000rpm, and at least 5500rpm. It will not stress your gearboxes to do this test.

If you cannot achieve at least 5500rpm with your current props, you will be putting excess load on your engines and it WILL have an adverse effect on your engine's lifespan, as well as use more fuel than necessary. It is like driving a car around in 6th gear all the time!

[NiVani 1]

Thanks Moonlighter, once i have rebuilt the gear box, i will do a test and will report the max RPM i can achieve with the current props.

cheers