Prop diameters and height

[robbo1]

Hi guys, I am hoping for a simple answer but have found out in this fe business it is not always that simple.

Question is:

If I have a catamaran and running a 47mm diameter prop with strut height at 3mm above sponsons, then decide to run a 42mm prop (same brand and pitch) would I need to lower the strut 2.5mm? That would put the strut at 0.5mm above sponsons.

My rationale to this is that the 47mm prop is 5mm larger than the 42mm. Divide that 5mm by two and it gives you 2.5mm, hence that is where I have come up with the 2.5mm lower strut. Hope that makes sense, or have I totally gone off the garden path?

[Jesse J]

Probably better to think about the prop area change (which is a spurred of diameter) rather than a linear distance.

But, as always, what are your objectives? If you want to go slower, maybe you don’t have to move the strut, as the boat will be sitting lower in the water. YMMV.

[Speed3]

Don't move the strut.

[robbo1]

Don't move the strut.

and the reason to not move the strut?

[Speed3]

Would you change the height of the strut every time you change the diameter of the prop?

When the strut is in a lower position on the transom it lifts the back of the boat and makes it run more wet.

So if you want to lift the back of the boat and have it run more wet go right ahead.

In many cases the handling improves but the speed does not increase. The ampdraw may also increase.

But if you are studying speed. I would leave it and adjust strut angles to accomplish bow lifting or stern lifting.

And change props to increase speeds.

[robbo1]

Would you change the height of the strut every time you change the diameter of the prop?

When the strut is in a lower position on the transom it lifts the back of the boat and makes it run more wet.

So if you want to lift the back of the boat and have it run more wet go right ahead.

In many cases the handling improves but the speed does not increase. The ampdraw may also increase.

But if you are studying speed. I would leave it and adjust strut angles to accomplish bow lifting or stern lifting.

And change props to increase speeds.

Setup is something I still struggle to get my head around.

I lowered the strut on the weekend by 1.5mm without changing anything else and got a speed increase of 5mph… go figure....

[rol243]

well thats easy to figure. lower strut by 1.5 mm gives 1.5 mm more prop in the water equals more speed.

[robbo1]

Contradicting answers rol243. I’m no expert by any means but from speed3 answer, if I’m understanding correctly the lower strut will give less speed. I am certain you are both correct in the strange realm of setup. I guess it’s just a matter of getting more test days and experiment more to see what works and what doesn’t.

[rol243]

well it only makes sence to see the results . maybe the prop needed that little more water bight .

[robbo1]

That makes sense