www.iFourWinns.com

The two stowage bins that are incorporated into the lifting engine cover / seats setup on my '04 Funship are made of a very thin layup. The starboard side one as it is has a decent length crack across the deepest part where the base meets the back (front?) wall. I want to repair this and possibly reinforce both bins, as I question whether they could handle anything being tossed in them any heavier than a beach towel.

Any recommendations for fixing this and for reinforcing the bins bottom / rear?

how about posting up a couple of pix, you can work wonders with some thickened West System Epoxy. I fixed a bit of rot in a bulkhead that would have been a major project to remove and replace by digging it all out and then using syringes to inject thickened epoxy a little at at time till the entire void was filled. Now its 100% stronger than plywood and will never rot again. Took me about 4 applications to totally fill the rotted area.

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88 Four Winns 200 Horizon
4.3 OMC Cobra-4bbl
2002 Walker Bay 10/2012 Suzuki 2.5
2008 Walker Bay 8

1998 Jeep Grand Cherokee 4.0/Selectrac
2007 Jeep Grand Cherokee 5.7 Hemi/Quadradrive II

For some reason images I've posted don't show up (may be some issue with the forum server at the moment).
I did post up elsewhere about this, see if this link works:
https://preview.redd.it/cizj0612hd771.jpg?width=1008&format=pjpg&auto=webp&s=7d671cedb48792f69c587f6392e77ebf46fcd0d9

OK what I might do with that, is clean it all up, you might have to remove that old insulation on the side.
Wipe it down with acetone (wear eye protection, gloves) and sand that crack and about 1" around it with 80 grit sand paper, then fill it with some thickened epoxy. I might then go over it with a few layers of fiberglass cloth but just filling it with thickened epoxy may be enough. Is it cracked on the other side as well?

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88 Four Winns 200 Horizon
4.3 OMC Cobra-4bbl
2002 Walker Bay 10/2012 Suzuki 2.5
2008 Walker Bay 8

1998 Jeep Grand Cherokee 4.0/Selectrac
2007 Jeep Grand Cherokee 5.7 Hemi/Quadradrive II

Just cracked on that side, would like to do something to reinforce both sides (at the very least the bottom), as all sides of both tubs are very thin.

If the layup there is thin then you have to use fiberglass cloth (if using epoxy) or mat (if using poly resin) to give it more strength. Its a messy project and hard to make it look good. I can make it strong but making it look good is a challenge. You can use either polyester resin or epoxy. I like epoxy better because most of my repairs have been rot repair and epoxy is more waterproof for protecting wood but for what you are doing poly should work well and that's what the boat was made from in the first place. So for the crack, you can fill after sanding it back with thickened resin and then use resin and cloth or resin and mat if using poly resin to add strength. The restoration forum on iboats.com is very good with advice on these repairs, if you have never done this before I'd try posting up on there.

_________________
88 Four Winns 200 Horizon
4.3 OMC Cobra-4bbl
2002 Walker Bay 10/2012 Suzuki 2.5
2008 Walker Bay 8

1998 Jeep Grand Cherokee 4.0/Selectrac
2007 Jeep Grand Cherokee 5.7 Hemi/Quadradrive II