Hi,
I recently decided to install a Mercury Long Shaft 15 HP on my Roberts 25 to replace the inboard. It currently has no outboard bracket. I had a look at the transom on the weekend and I'm wondering if it needs to be reinforced significantly to take the outboard. The fibreglass appears to be quite thin where I was thinking of installing the bracket. I was planning on using a backing plate of 18mm marine ply but now I'm thinking I probably need to do some fibre-glassing etc. Also wondering if anyone has any tips on where to position the bracket.

If anyone is an expert in this area and would do the job (and be paid), please let me know! The boat is in Watsons Bay, Sydney.

With regard to HP. I had a South Coast 25 (same hull, same windage/weight as your Bruce Roberts.
Mine had a 12hp BMW diesel and I found when going north against the East Australia current and a good nor' easterly, I needed ALL of those horses. You are likely to also find that the tides at the Sydney Harbour and Spit bridges will use all of those Merc horses.
You've already said the transom seems thin. At the factory they left engine fitting up to the buyer usually. And so why waste a ton of materials thickening the transon when the owner might fit an inboard?
If the transom seems thin to you then it is too thin to take an outboard bracket without substantial reinforcement. Your idea of 18mm IS NOT overkill in my opinion. Not only marine ply but get some EVERDURE from Whitties and give it a couple of coats.
Because your transom isn't flat but the ply IS, after using the Everdure I'd be bedding the ply onto a thickened bog.
And I also think that the ply should overlap the bracket bolt holes by at least 6 inches or MUCH more if you have the space inside the transom. Because the transom is thin and flexible, but the ply sure isn't, the bedding in bog is essential. And after the holes for the bracket have been drilled thru the ply the holes should then be Everdured also. And the bolts should be sealed thru the transom with Sikaflex.
I think it essential that the ply be bedded non-flexibly. I.E. NOT with Sika.
I always use epoxy resins but I think for your filler, motor body filler would do a good job too.
I also think that the inside face of the transom should be sanded and sealed/prepared to meet the bog with freshly cured resin or Everdure.
FWIW.
Good luck

Why not just go for aluminum checker plate won't rot

The trouble with the cutting board material in large, thick sheets(apart from the cost) is if it decides to warp, which it sometimes likes to do, nothing will stop it, including your transom. I'd also bring the ply reinforcements ,right out to the hull sides, especially if the transom is thin.
if your going for the outboard, I'd suggest you don't muck about with the cheap brackets, go for a Tenob, about $350-400. Mount it as close as possible to centre, while still being able to use the controls (& swivel it, which is handy),