Anti cavitation plate - when to move the engine?

[family affair]

After consulting the "Prop Gods" they lead me to discover my engine is likely mounted too low. The ACP is about 1/2" under water while running on plane. I was told the ACP should be skimming the surface. The holes on the engine mount appear to be 20mm appart. If I move the engine one hole and the ACP is above the water a 1/4" what would happen? I'd hate to make a change only to find I need to move it back. I'm hoping my estimate is incorrectly low (and I would live in efficient bliss with the change) or the prop would simply draw water up to the plate (and I would live in efficient bliss with the change).

 

[seasick]

You would be better consulting the boat manufacturer gods for their recommendation.A good starting point is having the plate about 1/2 to 3/4 inch above the bottom of the boat. Set the trim to make the plate level with the bottom, ( the transom keel line for a single) and see what the offset is. If the motor is set too high, handling may be seriously affected. The measurement in the water that you mention isn't necessarily a good test since it depends on the trim of the boat at a specific speed and that is affected by fuel load, passenger load, weight distribution, engine/tab trim angles etc.

 

[Curmudgeon]

If I move the engine one hole and the ACP is above the water a 1/4" what would happen?

It could be neigh on to perfect. At anything but extreme trim angles, submerged usually has a slightly negative affect, at or slightly above is usually positive. It's not an exact science, but I'm betting you're in the third hole down, yes? ... :uhm

 

[family affair]

We tested with a light load and half tank of fuel. We trimmed from down to up and the ACP was still about the same depth in the water. I'm wondering if fresh water is contributing to the issue.?
Ya, 3rd hole from top. I can only move one more hole.
On a side note, getting the prop cupped made one he!! of a difference on ride, hole shot, and cruise rpm. I'm hoping the mount height could be icing on the cake!

 

[Curmudgeon]

I kinda figured that's what you'd find. At speed the motor remains basically perpendicular to the water and it's the hull that changes with change in trim. Move it to the last hole and see what happens with the ACV. Do some hard turns trimmed up at speed to see if it blows out ... :wink: