cavitation plate depth

That's exactly what I've done with two thicknesses of 3/4" grp.Mr E.
The problem is,I'm in between the short shaft and the longshaft height.
I may use your idea with the ally plate and make it higher.
The bolt holes can be used that are already there in the transom.
I used s steel bolts,spring washers and lock nuts,and spaced them far enough apart to miss the engine brackets.
I was considering adding a two inch packing under the brackets,and using the holes in the back of the engine brackets
to bolt the engine on.
Adding two inches would put my cavity plate about 1" below the bottom of the transom/hull.
With the boat being quite light in weight,maybe the extra inch of depth would help in rough water.

 

Even a slab of thick ply would do it, at least 1.5 inches. ( 2 x 3/4 inch glued together with a waterproof wood glue). Easier to work with. You can taper the piece of ply to be wider where it joins the old transom. The height of the actual cav plate relative to the hull, level gives good results if there is no "keel" at the centreline, if there is, go a little lower.

 

Hi Mr E,the boat has no keel,it has a pointed bow that gradually fans out to an almost flat hull at the back.
I have some 3/4" GRP that is fibreglassed on both sides,That's what I used when I originally extended the transom.
Instead of glueing it,I used epoxy resin.