First things first.
:welcome: aboard jayhat. Glad you chose to join us here at iboats...
To answer your question, it you have access to the backside or underside of the hull to mount your trolling motor, a backer plate is a god idea. But lots of people use trolling motor mounting drop in type system like this;
This type trolling motor installation allows you to mount the TM from the top and also gives a little shock absorbing feature as well. However, if you really want to mount the TM solid, the backer plate from underneath will do that for sure. But just understand that unless the TM has some type shock absorbing feature built into it, and some do, every time you hit a stump or something like that, a solid mount will transfer that shock to the hull. And over time that could fracture the fiberglass and make cracks in the hull. JMHO
