Assuming you have a sterndrive or inboard now.
Attached quick drawing shows what I would consider a minimum. An outboard exerts tremendous forces at the top of the transom; that is no place for 'adjustable' reinforcements or anything else not fixed and permanent. The transom must be rigid and well bonded into the hull
, such that it becomes and integral part of the hull
While iroko is reasonably strong and rot
resistant, it is an oily wood and must be properly prepared if you are going to try and bond it to fiberglass
. That means it must be properly dry, not too smooth, and washed with acetone before you begin.
The wood should be bonded to the fiberglass
skin with resin and some sort of thickening material, I just you glass beads, others will tell you to use milled fibers and/or fumed colloidal silica, but glass beads are quite strong enough if you do the rest of the tabbing and laminating properly...
Any of the structural plastic foams would be a better choice for this application, my current
favorite is Coosaboard. It is about three times the price
of say, 3/4" pressure treated plywood, and doesn't hold screws, but it is lighter and doesn't rot
or absorb water
Isophthalic polyester resin would be my choice for a project
of this size, the real strength comes from the structural members. Any bonding, however, must be done to solid, ground fiberglass substrate.
For proper strength, the transom should extend all the way across the boat, and should be filleted, bonded and tabbed into the sides of the hull. The gussets shown in the drawing should also be bonded in place with whatever you use to bond the transom to the existing skin, filleted and tabbed.
The stringers should be shortened so the new transom goes between the existing skin and the end of the stringer, so that the outboard pushes against the stringer. This joint too should be bonded, filleted and tabbed.
The attached pictures might give you some ideas.
The second and third group of two show a transom reinforcement using plywood where the transom itself was sound but the stringers had become detached. A splash well, battery box
and livewell were incorporated into the reinforcement.
The fourth picture show a failed factory reinforcement.
The fifth shows the beginnings of a more proper reinforcement, to replace that shown in four.
The sixth picture shows the proper amount of grinding to bond a new transom element in place.
The seventh shows two layers of 3/4" pressure treated plywood, bonded in place with new stringer ends also bonded. No filleting, tabbing or inner lamination have yet been done. The gray section in the middle is two layers of 3/4" Coosaboard, installed to isolate the plywood from water
intrusion (this is a sterndrive application). If you use wood in your transom, it is good to do something along this line when you throughbolt the outboard to the transom.