Fiberglass guys. Whats your goto grinder? Mines a Old dewalt 4 1/2" with a clear view 36grit disc.

it died a couple years ago...According to my dad it was from sears.. Looked like an overgrown air impact gun, but was electric with a 2 speed transmission.. you thru that thing in low range with a good 10"36 grit disc and it would eat thru anything in no time. I have searched and searched for another one and have never seen another like it.

Now, the good old harbor freight 4 1/2 angle grinder with 36 grit flap discs.... it just wont die. for smaller stuff and clean up, right angle die grinder with 36 grit roll locks.

For cutting out the old wood in the transom?
use a Multi tool.

same here, i do most of my work with a Fein multi tool. For grinding it depends on what i am working on and what disk i am using. For smaller disks and flap wheels i use a Metabo and for big stuff i have a Makita i really like or an old IR air grinder that works well. The disks themselves? For that again it depends on what i am doing, i like Norton orange fire for the small roloc angle grinder, for bigger stuff its Norton blue or even some big azz floor grinding disks i get from Klingspor (they are super bad azz)

wet marine wood that has not rotted yet is a total pain to deal with . . .