I've just had the transom on my boat reinforced with a stainless plate across the top 2 mounting bolts for the outboard across the inside of the hull.
The repairers mentioned there was some "rot" around where the bolts were and advised this was only a "patch" and should this be an issue in future, the best repair method would be to cut the transom out and put a new one in.
Are there any tests that can be carried out on fibreglass that can detect rot or weakness? We do Ultrasonic testing on steel at work and was wondering if there is any procedure for testing fibreglass in a similar fashion?
It would be good to know the extent of the cancer, if any before I was to go down that path.
Cheers for any advice.
Kearnsy
Usually a timber plate reinforced with glass I'd say it'd be in the timber, post a few pics
Its not the fiberglass that rots in your transom its the ply in between which rots.
Easy to tell if its gone soft, when you tighten up your engine mounting bolts they just sink/squash into the transom.
My boat is in the same condition not sure I want to spend the quoted $4000 approx for a new transom on an old boat. 
Thanks for the replies.
@T11 there is no ply on the transom. This is what it looked like before the repair.
I picked it up today and it now has a piece of 80X6 stainless steel flat bar running across the inside across the bolts as a brace. This seems to have stiffened it up a lot now , however would like to know the extent of the rot , if any at all.
I first noticed this when I was washing down after a trip to Rotto. There was no definite incident that would have contributed to it that I know of. I have though pushed the boat rather hard in the past into swell and chop where the whole boat has "bounced" and shuddered into oncoming waves.
As you can see, I was lucky not to lose the 130hp off the back 
I can't imagine the fiberglass would actually "rot".
I think it's a term loosely used to refer to an area around bolt holes and mountings which have been flogged out due to continuous point overload, such as might be applied when hammering over chop.
The fibers break up and the resin is hammered to powder.
The damaged area then fills with salt or dirt or whatever and gives the appearance of it rotting away.
The stainless steel plate acts to distribute the load over a wider area and provided that it is properly secured to the remaining structure so that it doesn't just transfer the overload to another equally small overloaded point, it should fix the problem.
I'm assuming the flogged out section was dug out and properly glassed up again, rather than just bogged over.
If the damaged sections were not properly repaired then the damage will just be repeated at the mounting points for the plate.
If you use a g-clamp in the area suspected you can hear the fibreglass crackling as you tighten it. On other areas it won't compress. As T11 said, it's not the fibreglass, but what's been used to re-enforce it that's rotting. If the area is cut down using a grinder, new fibreglass will bond easily to the repair.
Mm tap the hull with the back of a screwdriver or use a engineers plastic hammer, if it sounds hollow it means the fiberglass is delaminating of the core. Could have a couple of reasons, Fiberglas not properly bonded to core, water in core, if wooden core means rot. Even waterlogged fiberglass will rot. It is Called osmosis. Only options are get the grinder out and prepare to be itchy :-)
