My boat was damaged over the winter and now has a 6”x6” hole on the rear deck exactly where one would sit. The fiberglass pieces are all still connected but the foam backing is gone. Any advice on how to repair this strong enough to support a 175 pound person?
In my scale of 'THE GOOD, THE BAD & THE UGLY', that definitely rates as UGLY, LOL. However, it CAN be repaired, and if you're merely sailing the boat for recreational purposes, you can also bolster or strengthen the area using flat aluminum straps bolted through the deck. Fix the ugly glass first, then install some additional support IF you need it... using an inspection port for all work. The aluminum straps will resemble battens, and they can be bolted underneath the deck, with stainless screw heads & flat washers all that are visible on deck. Such additional bracing is only a last resort, mind you.
Looking at your photos of that damage, I believe you can simply repair the glass without using any additional support or bracing afterward... it's a fairly large area so it'll cost you a little more in materials, but I've seen & repaired worse. Two options here: cut out the entire area and basically rebuild that section of the deck (more expensive), or fair out all cracks & adjoining areas and start rebuilding the glass that's MIA (less expensive). Install an inspection port that will allow you to access the underside of the deck; after some work on the deck topside, you will sand or grind the underside enough to apply fresh resin & glass where it is needed.
Probably have to do the internal work by hand, but as long as you reinforce the underside of the deck well enough, it won't matter how ugly that repair is... nobody will ever see it. I reckon you'll have to choose a spot and install the inspection port first, as you may need to temporarily brace the underside of the deck before starting any glasswork. Another option is to use thin yet strong line tied to a metal rod or wooden dowel, pushing the rod or dowel through the largest crack and orienting it properly before pulling upward on the line to lift the deck into its proper position. You follow me here?
Sounds confusing, but it's not rocket science... If I were tackling this repair, I'd install the inspection port first, which would allow me to brace the deck from underneath before fairing out all those cracks topside. Then I'd re-glass the topside cracks and surrounding faired-out areas. When you fair out any crack & surrounding glass, go deep enough to reach nice solid glass, no point in trying to re-glass rotten material (what I call trash). Go far enough outward from each crack for fresh resin & glass to form a solid bond. Grind & sand away all the trash before you start rebuilding those damaged areas: the strength of your repair depends upon it.
Once I had rebuilt those damaged deck areas with multiple layers of glass, strong enough to hold the deck in place, I would flip the boat and put her up on sawhorses or some sort of impromptu scaffolding so I could easily & comfortably reach through the inspection port and work on the underside of the deck. As with all glasswork, you want gravity to assist you... do NOT attempt to repair the underside of the deck without flipping the hull, you'll simply waste time & money. You'll want to strengthen the underside of the deck with multiple layers of glass, but first you have to 'rough it up' enough for the resin to form a good bond with the existing glass, aye?
That internal work can be troublesome too, best to cut your cloth or matt to size first and get it near the area to be re-glassed. With multiple cracks such as you have in that deck, I'd probably do the internal work one crack at a time, so to speak. Less messy that way... don't ask me how I know this. You'll want small tubs or resin pots that will fit through the port, perhaps some throwaway brushes with short or cut handles to work with inside the hull. I'm thinking an inspection port in the nearest cockpit wall or bulkhead might best serve your purposes, but I'll have to look at the photos again... this reply is simply to let ya know that the repairs are feasible. A bit of a challenge, but feasible nevertheless, and I know, because I've done this sort of repair before.
Anyway, going back to those aluminum straps or deck braces, you may not need those, depending upon how well your glass repair turns out. I wouldn't worry about 'em at present, just focus upon the glass repair first, and know that it can be done... even with multiple cracks in the deck as seen in your photos. All glass can be rebuilt, and in most cases you can make it as strong or even stronger than what originally existed. In this particular case, grinding or sanding away the trash & fairing out the surrounding areas will be of paramount importance. If you're gonna fix this damage, do it RIGHT, and err a little bit on the side of caution... lay in a little extra glass over the worst areas, underneath the deck and out of sight if you wish, LOL.
Oh, yeah, in case you're wondering, use plastic-wrapped cardboard to back any open cracks or holes as you begin repairs topside... wax paper also works, though you may have trouble removing it once it's time to build in the other direction, if ya catch my drift. Even with the deck braced internally (wooden blocks will serve this temporary purpose, or whatever other objects will fit through the newly-installed inspection port), I reckon you'll have some open cracks once you've ground away the trash and faired out those surrounding areas. Be sure to back those open cracks, you don't want your expensive catalyzed resin simply draining into the hull through those cracks as you lay in glass matt or cloth on deck. Moi, I'd alternate layers of matt & cloth, laying down matt first for all repair work. Choose the right weights of matt & cloth, that's also important.
Don't be fazed by the length of this reply, I'm a long-winded b@stard once I get started, especially after I've had a couple of beers, LOL. But know this: I've done PLENTY of glasswork in my time, and my best friend has done even more, though his specialty was working on surfboards. His family owned & operated the first surf shop in our home town for over three decades, so we're talking about some serious glass repairs over time, AYE? But I was always BETTER at boat repair, which in truth does NOT require as much finesse... board work involves lighter weights of cloth, and a more delicate touch is necessary when it comes to sanding & grinding, otherwise you'll go right through the glassed board into the foam. Again, don't ask me how I know this... LOL.
ALRIGHT, THAT'S ENOUGH FOR NOW, DON'T SWEAT THE REPAIR, I HAVE FAITH IN YA... GET FAR ENOUGH AWAY FROM THOSE CRACKS AND YOU'LL FIND SOLID GLASS UPON WHICH TO REBUILD THE DAMAGED AREAS.
AND DON'T HESITATE TO ASK ANY QUESTIONS, I'D LIKE TO SEE THAT BOAT REPAIRED SO YOU CAN PUT IT TO GOOD USE, AYE???
TIME FOR ANOTHER COLD ONE, I'M ENJOYING THIS COOLER WEATHER WITH POTENTIAL RAIN IN THE FORECAST... CHEERS!!!
Is it too much trouble to post a picture of the entire side of the deck to see where this hole is in relation to the cockpit and how far aft/forward? Interested in the outcome of this big challenging repair. I'll sip on a dram of Talisker if that helps you. Best of luck! Skål!
GRP work takes a long time to do. Lots of gloves, dust mask, work outside if at all possible....fumes are horrid for styrene. Cover a table in polysheet as a work area. You can also use epoxy, be careful as well as it's very toxic to your fertility!
This would be my strategy.
1) Cut the damaged area right back.
2) get a backing sheet (10mm ply I'd look for) to go under the deck and cover in cling film. Screw through deck to secure it all around the hole.
3) Bevel the edges of the hole 25:1 I've heard though I've never gone that far myself
4) Get some of the type of foam that was used and cut accurately to shape and glue in.
5) Lay up fiberglass on the top. I would not bother with gelcoat. Get it smooth and fair (use long sanding board) and then spray paint it.
6) Cut hole for biggest hatch you can in cockpit side. Maybe even two. Access is the thing.
7) Flip boat over on trestles as Cactus says and remove the plywood
8) Glass up the backside.