Traansom Heights Question

[Fastjeff57]

I've always wondered about this and need advise from your experts out there. Three cases of transom height are:

Case One; the cavitation plate is level with the bottom

Case Two; the cavitation plate is an inch above, and

Case Three; the cavitation plate is several inches above the bottom.

In most cases Case One does not cavitate the prop. With Case three (and a proper prop) we know it will cavitate on plane off then catch. But what about Case Two? From what I discovered last week, my usual good grabbing prop went nuts at this height. Is that normal?

Thanks,

Jeff

[sharpeye Mike]

All boats will react differently but that is usually the first thing I do is install a jack plate with some offset depending on the length of the boat that I get, how the boat drives with a stock setup usually will give me an idea of how much offset to add. First thing you will notice when you raise the motor is the top speed will increase. I raised the motor on my 16.5' aluminum fishing boat by about 4.5" and my speed increased by 4 to 5 MPH, cavitation was never a issue on take off but it did cavitate a bit when turning at high speed. The other benefit of raising the motor is you will increase clearance of whatever lurks at the bottom of the river.
You will not notice any difference in case # 2, an inch above is not enough.

[Fastjeff57]

Thanks.

My question revolves around what happened when that the fool thing was only an inch above the bottom--it cavitated like crazy. How bad will it get if I raise it a bunch more, or will the cavitation problem go away?

My theory is that the blade chops down more vertically when the motor is raised WAY up. But when raised just a bit, the blade hit the water more horizontally and cause cavitation problems. At least, that's my theory.

Somebody out there must have gone through this 'raise it a little bit at a time' exercise and knows what happens.

Jeff

[sharpeye Mike]

To be honest with you, I never had that problem, can you give us a little bit of info, boat, motor, prop and the set up you're using.

[Fastjeff57]

Sure. Nine foot shallow Vee with a Mark 78 (mounted on a Mark 55 type tower) running an 18 pitch chopper with lots of cup.

Jeff