Outboards, how high

[FTW]

Fairly new to boating and have been reading several threads on outboard positioning for best performance. Can someone explain the theory behind positioning. My tinnie seem to be a little low from what I've seen explained here.:-/

[BM]

FTW,

For typical use applications the anti cav plate should be level with the bottom of the hull. On aluminium boats I set the anti cav plate level with the bottom of the keel extrusion, whereas on fibreglass boats simply level with the bottom of the hull (as there is no keel extrusion).

Set the engine too deep and you increase the drag caused by having too much gearcase in the water. Too deep will also lower your full throttle rpm.

Set the engine too high and you may experience cavitation and/or ventilation. Fancy names for prop blowout really. Engine starts to rev higher but boat slows down.

Now thats standard use applications.......

For high speed ski boat applications its not uncommon to have the engine raised 5 inches above where it should be. This application requires special low water pickups on the gearcase (since the gearcase is so high in the water flow) and the use of surface piercing props (props designed to run with one blade effectively out of the water) etc etc

These are your Southern 80 type ski race boats that do ridiculous speeds (100mph) cavitate like sh!t till they get up to a reasonable speed and then go like bullets.

If you are ever discussing props and engine heights make sure it is NOT with a ski race guy as they have usually in my experience completely forgotten conventional prop configurations and typically steer boaters down a very wrong path. Wrong in the sense of general use application.

Cheers

[FTW]

BM,
Will lift the motor an inch or so and give it a try tomorrow. Thanks for the advice.
Kev.

[finga]

It's very hard to say how high as every boat is different.
A bloke here put it fairly easily as
"shove your head over the back and look at the cav. plate. If it's just above the water goodo. If it's below the water raise it and if it's more then a smidge above the water lower it"
Pretty easy.

[Chimo]

BUT when looking to see if the plate is above water you need to be travelling at medium to fast cruise with the the motor trimmed out / up as you would for optimum performance first.

If you check the height and expect to see the plate (s) when you travelling too slow then the motor will more than likely be too high and you will get cavitation / ventilation often and certainly on turns.

Cheers
Chimo