Need Advice on engine height

I am currently building an 18 degree full vee 20' x 78" aluminum hull. This boat will get an outboard motor bracket welded to the ouside of the transom. This moves the engine well back from the standard position of being on the transom itself. I have done this many times in the past but I cant recall the optimum height of the transom bracket. I know that moving the engine out and back allows you to raise the engine so that the cavitation plate is a few inches higher than normal(which is level with the bottom of the boat). It's something like for every foot aft you go, you can raise the engine 2" or something like that. Anyone have any input on this? I have a maximum length limit on the package(boat,motor,trailer) so I need to nail this down before I can go very far.

Thanx, Joey

 

This was posted in another thread.http://boatdesign.net/forums/showthread.php?t=1641

 

The only advantage of a transom bracket is better steering.
This arrangement looks cool but is not worth the effort if the boat behaves well without it.

The maximum amount you can raise the engine is governed by the ability of the engine to suck up cooling water. (cooking an engine is definitely not good for your bank balance.) and the onset of prop cavitation. That is assuming a race type supercavitating prop is not being used (cleaver prop)
The outboard on a bracket definitely makes the boat steer better, but adds weight and can put the centre of gravity too far back.
The main reason for raising the engine is because outboard's transom bracket is not the most streamlined object and hanging it in the spray coming off the transom can slow the boat down if it is set at the normal height.
The most the racing guys will lift a motor is with the bottom of the gearbox level with the bottom of the boat (only bottom half of prop working). This may require the use of a remote pitot water cooling pickup.
Be advised that adverse torque effects can degrade the handling of the boat. With two engines and counter rotating props you can cancell some of this out.

 

Thanx Corpus Skipper. Probably will go with that, then fine tune as neccesary.

Joey

 

Hey Corpus Skipper,
Just wanted to let you know I followed your advice and the boat performs perfectly.
Thanks Again, Joey

 

Glad to hear it Joey.