Originally Posted by weehaggisuk
I think with the turnbuckles the transom is only strengthened when the boat is stationary
when power is applied they will in effect be loosened off by the power of the motor pushing the top of the transom
I think you'll find the force is rotational, pulling at the top and pushing at the bottom, the prop is at the bottom of a long lever, below the boat. if the prop was at top then yes you would get the effect you describe.
On a more practical note, ive had to beef up the transom of the jeaneau orange plastic tubs, and I can tell you the damage was a tearing at the top of the transom. I used a stainless version of this set up and it worked a treat, keeping the boats seaworthy for many years after the initial tear was seen