Transom Reinforcement

I have a 16 ft Aquaforce, that I recently purchased used, the Hull # indicates it’s a 95. Well I purchased the boat knowing it had some slight cracking where the mercury 90hp two stroke outboard mounts on the interior of the transom next to the lag bolts the longest being on the left side if standing behind the motor and a slight crack on the right. So I purchased a fiberglass kit and filled in the cracks and added 8-9 heavily saturated sheets to these cracks. It cured just find since then I’ve taken the boat out several times running way too fast across rough water, blasting across waves around St. Simons Island Georgia. The fiberglass has since started to peel away from the edges, I thought maybe this was because I did not prep the area first and applied the fiberglass in the 4 – 6 inch crack and put sheets of glass over the black and white splash looking paint. Now I’m thinking the transom needs to be reinforced to help support the motor also when trailing the boat I have to go across quit a bit of train tracks which are really bumpy.

 

As far as the trailering goes, a "transom-saver" bar that braces the OB's lower unit against your trailer's frame, works wonders on this problem.
Your repair probably would have held had the surface been properly prepared. I'd pull it off and start fresh.
The paint must be removed in that area. Then the surface has to be sanded- if there's gelcoat then sand it off, if it's just glass then sand until you are close to the individual fibres of glass (but don't sand through them). Now clean it with a solvent like Interlux Fibreglass Solvent Wash (some just use acetone), that will remove all the wax and grime. Clean it according to the directions, test if water beads, then repeat. When you think you're done, repeat twice more.
Now you can mix up a filler of epoxy and microballoons (or whatever), fill the bigger cracks with it and fillet the joints to a nice round radius. (Definitely use epoxy for everything here, polyester is not nearly as good a choice for repairs like this.) Then you can lay up the repair. Strength comes from being smart in the layup of the cloth, not from using lots of resin- use only enough resin to wet out the cloth without bubbles forming, and stagger the sheets of cloth. Put some at 45 degrees, etc. Let it set for several days, preferably a week, before using it.

 

Thanks do you think a stainless steel plate across the back of the interior transom will help ensure its strength. I was thinking of maybe attaching the motor mounts through it so the weight will be well distributed across the back with two additional lag bolts spaced evenly outside the motor mount.


I attached a simple sketch

 

Using the steel plate will help but listen to what marshmat has to say - he is absolutely bang on. I did the same sort of repair to my transom due to wood rot where the swim grid mounting bolts came through. Chiselled all rot out to bare transom and to good wood on the sides; made a bevelled plywood plug made from 2 3/4" pieces laminated together with West System and high density filler and sealed with epoxy; after wetting out and letting it cure, I rewetted the transom female side and then buttered it with a thick mix of epoxy and filler, placed the plug in and buttered the edges and let cure; used another cover plate of epoxy sealed 3/4" plywood much larger than the inset plug and then poured epoxy behind this plate to gap fill; I used fiberglass tape on the outside edges of the outside plate to lock it to the hull and transom; I then used a good quality 1/16' stainless plate to act as a backer for the through hull bolts to mount the kicker bracket (got rid of the swim grid on that side). A lot of hard, confined space, bent over and cramped up work but it received the "Grandchildren are Safe" Seal of Approval from my Boat Builder Father-in-Law. Overkill, perhaps but I sleep better about it.

 

My own boat has a steel motor mount similar to your idea; I added it a couple of years ago when upgrading to a Johnson J30. The transom on this boat is also braced with a beefy wood-and-glass knee running from the motor mount, down to the keel, and forward to the first bulkhead.
For the metal bar to work, it would also have to have a flange extending lower on the transom, and be bolted there too. An outboard doesn't just create a linear force- it also creates an enormous moment about the mounting bolts, since the prop thrust is so far off the mounting axis. If the mount is reinforced only in the horizontal direction, the mount will twist (viewed from the port side, it will twist clockwise) and fail. It needs to be strong in the vertical direction as well. Mine extends a foot below the bracket hook, and has eight through-bolts and three lags holding it in place.
Keep in mind that metal reinforcements will help to spread the stresses around, but what is actually holding that part of the boat together is the glass laminates and joints. It is here that the most careful attention is warranted.