Hollow fiberglass stringer repair?

Hello from Australia.
I have a Easy Rider 146 (14 foot 6 inch) fishing boat with Force 50HP.
I have to replace the floor in my little boat but I have some questions about the stringers and would like some advice please?
After removing the rotted 18mm plywood floor and the original 3mm fiberglass floor I found the center stringer in need of repair, and a crack in what appears to be a fiberglass stringer, see attached.
I have dug out the rot from the center stinger (3mm thick glass) and plan to fill void with either wood and glass or just glass?
As for the crack, I am no sure, any suggestion please? There is flex around this crack when pressure applied.
I do not know the original construction material used.
After I have repaired the stringers I plan to fit 18mm marine ply as the new floor.

 

That has indicators of trailer damage as the two areas are pretty much directly across the boat.The original construction was likely to have been pretty basic with no exotic materials and you ought to be fine using the same approach.The more or less semi-circular stringer could well be formed over a cardboard tube that has been split,I have been in a few workshops where they save the central tubes from rolls of mat for just this purpose and split them on a bandsaw when a job crops up that needs them.Ideally you would grind a taper from the crack for 3-4 inches either side and winding up with almost nothing at the crack itself.If you go through,there won't be much to support the repair piece and you will have to shape a piece of foam or similar to support the uncured glass.The repair should be as much glass as there was originally and each ply should extend a bit further than the previous piece,with a couple of inches of overlap and the first ply extending a couple of inches either side of the crack.Were it my boat,I'd add a couple of plies that extended out at least a foot either side of the damage to spread the load on future occasions.I'd also take a look at the trailer suspension to see if it has any movement,The type that has rubber strips inside a square tube is notorious for going hard with age and leaving you with a very rigid setup.

The same principle can be used with the outboard stringer but you may need to relieve the underside of the new plywood if the laminate increases in thickness.With a few good evenings work you could be fishing next weekend.

 

That is typical of chop gun construction. The repair can be made with mat and polyester and gring, etc. like wet feet suggests.

 

You’re lucky that the boat didn’t fold up on you!
The loose glass fibers laying around the stringers point to poor chopper gun workmanship on the installation.
I’d drill some exploratory holes in the stringer at various locations to make sure that there is enough solid glass on them to do the work if the wood is gone, then probably grind away the mess and add tabbing anyway.

 

Thank you very much for the feedback.
I did not think of trailer damaged as a possible cause but thinking of when I bought the boat it was sitting on a home made steel pipe trailer that I would not put a canoe on. I did put the boat on a new skid style trailer almost immediately.
The center stringer with the cutouts that are from the rotted spars that used to support the very thin floor and I have dug out the rot. The remaining glass appears to be sound.
The semi circular stringer is on either side of the center stringer and the cracked one is on the starboard side. These stringers are only in the rear half of the boat where the hull is relatively flat.
I will take on your suggestions over the coming weeks and give you an update as it happens.
Thanks again.