Mounting an outboard

[Candyfloss]

The 22ft Pied Piper keelboat has a transom-hung outboard as standard. My boat has a (modified) open stern & came with an 8 horse shortshaft Evenrude mounted on a bracket on the Port quarter which lifted the prop out of the water every time a ripple passed under the boat. I didn't like it, so I ripped it off & am now about to fill the transom back in again to mount the outboard in the center of the boat. I need to figure out how deep to cut the cutout so the outboard rests on the cutout & the prop is the right depth in the water. The plan shows the top of the cutout 500mm above the waterline which seems a bit arbitrary since outboard leg lengths vary greatly.

My question is: Where should the cavitation plate be in relation to the waterline? Just above, just below?

All the websites I can find say half an inch above the bottom of the hull, which is not very helpful, since they are obviously talking about planing hulls, which the Pied Piper is, but not under power.

Any suggestions please.

[wizbang 13]

The anti ventilation plate is supposed to be , theoretically, an inch below.
But, on a performance speed boat, like the former wizbang, it may be an inch or two above.
But, on a sailing boat, one needs to consider the quarter wave , not the wl when sitting still.
AND, you have to consider heeling, even if the ob centered.
A short shaft will not be of much use to you.
I have my sailboat ob mounted on a sled kind of thing, that I can easily and simply adjust it up and down, cuz there is NO perfect one spot.http://farm7.staticflickr.com/6059/6353232327_51a7b32aa5_z_d.jpg The pivot bolt is near my feet, the bracket is the gray thing. Of course this is smaller than your boat, but it works good. I adjust it up n down by jamming a piece of wood under the back.

[John Bell]

The simple answer is you want it deep enough so it doesn't pitch out and not so deep that the power head submerges when the stern pitches the other way.

On my planing dinghy, I keep the cav plate on my 20" shaft motor about 3-4" below the bottom of the transom. I use one of the ubiquitous spring-loaded adjustable motor mounts. I have no idea whether or not this is the "right" way, but it works for me.

[Gib Etheridge]

Regular shaft, top of transom 15" above bottom of hull at transom, long shaft 20" and extra long shaft 25".

[Candyfloss]

Wizzer. A technical question. What is the point of having a cavitation plate if it is 2 inches above the water? Would not the prop be mostly out of the water at that point?

Also, with all due respect, what do you mean by "Will not be of much use to you"? 150 or so of these boats have found quite a lot of use for their outboards, although admittedly, most use them as little as possible, & I understand the setup will not be very efficient at best, but there is literally no alternative, so I am just trying to make the best of what I have.

Check this out; http://youtu.be/DoaSkwxQPgU

[wizbang 13]

Performance speedboats run with the anti ventilation plate above water. Some run with the water intake half out of the water and they block the top half to prevent sucking air. Many performance props DO run partly out of the water. Speed boats, that's why I said "but" , just to point out there is no black n white answer.
By "no use to you" I did not mean an OB , I meant a short shaft. gawd knows I am not Larry Pardey.
The quarter wave changes with speed and heeling, the ob height needs to change with it, that is all I am saying. My deck mounted ob bracket goes up and down with changing conditions.
Bruce