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My Rainbow needs repair to stop it leaking through the center case. I was wondering if a boat builder could put in a fibreglass case and fit it extra well to the wooden frame structure. I was thinking of taking to ply off the bottom of the hull to make repairs to the structures in the front bulkhead.
Can anyone offer some ideas here?Attachment 275248Attachment 275249Attachment 275250Attachment 275254Attachment 275251Attachment 275252Attachment 275253
Cases are known to leak and typically are designed to be disassembled, rebedded and refastened. This needs to occur fairly regularly on a actively raced boat, say every few years. On a cruiser, you can stretch this out to every 10 years, depending on use.
Yes, you could make a 'glass case, though it'll be heavier than a wooden one, with few additional advantages. The 'glass case would need to be bedded and mechanically fastened, just like to wooden one, so you'll have similar issues with it leaking in a few years, if actively campaigned. Conversely, you could bond the new 'glass case in place, but now you're married to it and if repairs or damage require you to remove it for some reason, you'll likely have to destroy large portions of the bottom, the case and probably some supporting elements as well.
In the end, it's usually better to have an easily repairable structural element (read disassemblable), then a homogenous composite of differing materials (read can't be taken apart). It's your call.
I'd be inclined to take off 1 side of the bottom ply for ease of access (if you can't get at the case through the deck - which I think is the case on a rainbow?), and then I'd remove the case in 1 piece (if possible - looks possible).
Clean up the case, check its integrity and make sure the leaks are only going to be from the keel to case join. If there's only that to fix, you can reinstall with a judicious amount of bedding compound to seal the case (something like sika 291 leaps to mind), which will be removable later, so you can re-do it in 3-5 years if it needs it. More if you're lucky.
I'd be surprised if the case wasn't made to be taken out in this fashion, from time to time. I guess it's just a matter of whether you go in through the deck, or the hull.
Supplemental note - Plans for the Rainbow are available here, if you want to rebuild the case yourself.
Plans for Sail Boats
Just search for "Rainbow"
I'm not sure of the specifics for this particular boat, but as mentioned, most cases, if properly designed, are intended to be removed, so they can be rebedded from time to time. It's just the nature of the beast - after all, there is a huge hole in the boat, with a big 'ol board applying torque to all it's fastenings.
Most have "logs" along the lower edges of the case, typically running fore and aft down each side. These are lagged, screwed and/or through bolted to the keel, keel batten or possibly just the bottom planks.
The usual course is to remove the log fasteners, then pry off the case (after freeing up any additional support, like furniture bracing, etc.). Clean the faying surfaces, restore fastener holes, which may have rot or have become distorted, then reverse the removal process on a good bed of goo. I prefer polysuphide, but this seems hard to find in your country, so polyurethane would be a good second choice. Choose a less aggressive adhesive version of polyurethane, so it can be removed again, when the times comes.
Lastly, an old school trick to help keep a centerboard case tight against it's perch, atop the keel is a length of cotton string. Yep, cotton kite string works good, amazingly enough. When ready to drop the case back down on the fresh bedding compound, lay the string around the perimeter (in the goo), just inboard of the edge. When you dog down the log fasteners, the string (which is dry) is crushed, along with the bedding compound. If the bedding compound starts to fail and lets some moisture through - the cotton gets wet, swells up and seals things tight again. This simple trick is used on traditional builds, in garboard to keel seams, around through hull penetrations, between skegs and hulls or keels and lots of places, where you need a second line of defense against moisture getting in. The old guys that came up with these tricks, knew their stuff and materials.