Hobie 18F centerboard box repair?

One of the hulls is leaking on my old Hobie 18F. I did not yet find the cause, but probably it will be a crack in the centerboard box (is it the right name even?). Tried to find some material on how to repair it, but found nothing so far. Would be grateful for any tips on how to fix it, and how to figure out the source of the leak in the first place.

 

I think you will need to cut an inspection hole in the deck astern of the case. Ideally you would buy a big 7 inch at least diameter screw top plastic cover, but you may need to do the same thing forward of the case if the damage is there or extensive. Cheap ones need painting and may not work very well, and leak a bit.

If you reinforce the deck internally with extra glass around the opening you make it will help stop the deck collapsing in that area.
You can cut a square hole finish the edges with aluminium angle and a gasket plus hinges and locking device , cam etc, but it is a lot of work and a round hole is better for retaining strength.

Now that you can see the problem you can pour water [dyed if you like]in and watch where it leaks from, fibreglass tape after prepping by sanding 40 or 60 then 80 or 120grit and cleaning,drying may be enough, but a metre of 4 ounce glass cut to small sizes then getting larger to spread the load will work nicely.4 layers finishing with a large one may be enough, or overkill hard to say, it depends how weak it feels and looks.
Your cat is probably polyester resin, use laminating resin, or you could use epoxy.
That's what I would do anyway. test andGood luck

 

Thank you, it helped me starting to think. There is a deck opening for small storage just a bit behind the front aka with screw top plastic cover, and it is already not airtight. I could probably get out the storage box and use that opening to fill the hull with colored water to see where the leak actually is. Maybe I will use some tempera for the coloring.
I don't like the idea of having more of those openings on the deck. What if I just cut the whole deck section above the box (if the leak turns out to be there)? This would give me more place to easily work on the box. When I am ready I could rebuild that deck section after grinding perhaps a 20 cm long taper on the adjacent parts of the decks to have a place to stick the glass. It would need much more material, but perhaps easier geometries to work with. To support the glass while it is curing, I could glue a sheet of plastic under the deck.

Does it look like a sane plan? What could go wrong? (not a rhetorical question, I want to be sure I know what I am doing)

 

If the H18 f is the same as the old H18 all the strength is in the upper flange , so I wouldn't cut through that. The problem with your proposal is finishing the deck if I understand you correctly , it looking good and being strong enough, you may need someone else's advice on that.

 

I'd cut the whole box out of the hull. Cut the bottom out leaving about 3 inches of hull on each side of the slot and a bit more front and back. Mark the hull bottom rectangle. Drill the corners with 3/4 bit, and saw out the hull, not with a jigsaw. It should look like a drawer panel when done.

Do the same on the deck, only bigger. Make the deck cutout 6 inches wider and a foot longer than the bottom.

On the deck, you need to build up a flange inside the hull so the daggerboard housing has something to land on when it goes back in. Turn the hull over, fit a filler block into the deck hole. Make some attempt to bog and fair this surface, cover just the plug with plastic, then fit a solid flange right round that is as thick as the deck - four layers 1208 biax about 5 inches wide feels about right from this side of the computer screen. You have to be able to get the bottom, with its added flange, back in through this hole, with its added flange. If each flange is 2 inches wide, you are good to go.

On the bottom, remove all the core from the outer skin about two inches from the cutout. This core edge wants to be decently neat, square and flat. If the surrounding core is completely wrecked/waterlogged, then replace the rotten areas with medium density foam scrim like Divinycel (and replace the inner veil as needed). The lip area needs to be replaced with a high strength core or solid laminate. Wood, plywood, 300 density fiber reinforced foam, or lots of layers of CSM, but it needs to be build up solid to the old inner surface. The junk that's in there wasn't much to start with, and now its waterlogged. That is likely what caused all this to begin with.

Make a mold of the new inner hull bottom - Mark the inside of the hull 4 inches away from the hole in the bottom. Take a couple commercial carpet squares and trim them to fit those lines and tape them together. Find some dead poly or tired epoxy and mix up about two quarts. Pour this on the carpet and work it in. Place the these in a garbage bag and put in the hull with the carpet side down. Try not to get the goo on the rubber side. Sandbag in place to make a mold. It needs to come out though the top when cured.

Repair the daggerboard box.

Now you can make a bottom flange for the daggerboard box. Cut the carpet squares to fit around the hull bottom of the daggerboard box and secure them so that they extent the inner surface of the hull. Cover the mold in plastic and lay up a flange as you did on top. You're bonding the flange to the box but not to the mold.

Now it's ready to go back together. When you're happy with the fit, apply strips of CSM saturated with epoxy to hull bottom and deck flange and drop the daggerboard box back in. Don't be shy about running 100 little screws into the flanges. You have to fair all that area anyway. Pull the screws, fill with epoxy bog the gaps with epoxy, putty and paint.