Hi,

I have an outboard motor bracket that I want to mount on the stern of my fibreglass boat for a 3hp auxiliary outboard (18kg).
My question:

1. Should I use an underlay on the outside of the boat between the fibreglass and the bracket? I have seen other boats show little cracks after years around where the bracket was mounted. Should I use plastic or wooden materials for the underlay?
2. Should I use a back panel on the inside of the boat to spread the load (weight) of the bracket? Should I use a plastic or wooden panel for the inside?
3. Should I use an adhesive or silicon around the screws and the brackets to avoid water collecting in the spaces around the bracket and the screws?

Thank you for any tips and assistance :)

cheers,
Yamaha24ft

Depends on the layup of your transom - panel thickness and stiffeners etc.
But agree good idea to use both.
Firstly would radius the 4 corners of the two unequal angle sections which bolt to the transom. Say 10mm radius but not too large that would encroach on the present hole to edge distance. And add another 2 bolts - one on each unequal angle contact face midway between the 2 there already. 316 stainless bolts.
For the inside back panel would use a marine ply piece say 50mm larger on width and height than the shape of the contact face of the bracket. Say 18mm ply thickness. 20mm radiused corners. Epoxy glued to the transom inside face after suitable sanding and acetone cleaning.
For the outside underlay using marine ply would become a maintenance issue. A piece of t=12mm white acetal plastic could be useful - 25mm larger on width and height than the contact face. 10mm radiused corners. Adhere with Selleys 655FC or similar.
On line checks indicate your bracket is all stainless - if so would just use the 655FC to seal all bolt holes. If not (and aluminium frame with ss fasteners) use Tefgel to isolate the ss fasteners from the aluminium, and 655FC to seal them. Use bolts not screws.

@r13 awesome :) thank you for the detailed responds. That all makes perfect sense. Good to know that I was also on the right track :)