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[thataway]

An I/O is not hard mounted to the engine. There is a gimbal bearing. Many I/O have differential joints on the shaft. The mounting bolts will transfer some of the forces from the lower unit to the transom... I have seen owners repair the stingers, and ignore the transom, only to have the transom fail because it was damaged or weak. Although the Arneson surface drives have a more direct attachment to the engine, there is minor force transferred to the transom.

If you want to know there forces and design factors for an outboard: This thread on Boat Design addresses those forces from a engineering/design prospect

Torque or twisting effect of the lower unit are important. Differential thrust on the top or bottom of the mount, both forward and reverse are also important. I recently saw a boat where the splash well was only about 1/4" thick, and there was about 3/16 inch between it and the transom--that splash well fiberglass had been crushed and the upper outboard motor mount bolts were loose.

The thickness of the transom is both a convention and design factor. For example an aluminum or steel transom 3/8 of in inch thick has pretty of strength. The convention for many years has been 1 1/2 inch thick transom for outboards of the size which most of. us use. The most common outboard transom is 2 pieces of 3/4" plywood. A few layers of fiberglass over it is the common production boat. For dinghies 3/4" is often plenty; small boats to 50 hp often only 1". As you get over 100 hp it is often to see over 2". The larger motors are. held on by bolts; the smaller ones with the clamps. I would prefer a plywood transom to a balsa core transom.

The way the force is transferred from the transom to the hull is very important. On occasion there are boats built which seem to ignore this factor. The strength of a transom attachment to the hull involves knees, stringers/grid, or the design of the splash well, as well as the tie in to the sides and bottom of the hull.