I can't see how a part of the hull isn't structural? It's things like the table inside that aren't structural - pretty much all of the hull will be supporting the superstructure, stopping water getting in, taking the forces from the keel and rigging, etc. rather like a monocoque construction on a car. The fact that the whole thing doesn't fail when you remove a bit doesn't mean that it's not structural - just that you're inside the safety margins (or at least the breaking point). The hull will flex when the boat is underway in a heavy sea, during gusts and when you go alongside. If you have an impact or maybe take the ground heavily then the plug could be forced out, especially as a large amount of resin holding the plug will be resisting shear forces, which resin isn't good at unless reinforced with fibres.
As said above, definitely chamfer the edges of the hole and lay mat across it using epoxy resin. For a 3" hole in a hull thickness of say 3/4" I'd chamfer it back so that the chamfered area on the outside was 6"- 9" and not much less on the inside (I'm assuming the hull is solid GRP). If the hole is largish then zinc mesh or chicken can be used under the first layer to provide a backing (at least it can on cars - I can't see a problem with corrosion on a boat as it will be completely sealed under resin). Halcyon's photo and post shows how to do it perfectly. I'd be interested to hear what Chrissie or another experienced surveyor would say though. I'd regard their opinion as definitive.
Last edited by grumpy_o_g; 19-04-12 at 02:00.
Hi Vic and All.
The plug repair has 2 possible failure modes. One is to be pushed from the inside out wards. If you have a chamfer on the inside of the hull then the new glass and resin will form a step so that as long as the step of f/g remains strong the plug can not come outwards.
In the same way the plug could be pushed from the outside into the hull. This is a more likely scenario from water pressure or sitting on a rock. It is the f/g that is outside the plug that extends out over the original hull that stops it being pushed in.
Now a 12 to 1 chamfer is the sort of thing that as said is required to retain the structural integrity and flexing characteristics of the original hull. (before the hole was made) Because the hull never showed any problems having the hole then it is unlikely that you need to recover the original strength. However it will always be weaker or more liable to flex than the original solid hull.
So if you don''t need the original strength (before hole was cut) then it is just the size and strength of this step that you are concerned with to stop the plug being pushed in.
Vic I would certainly advocate some chamfering of the outside of the hole. The more you take out especially in depth the more new f/g you can put in while keeping the outside of the hull fair. The thicker the new f/g the stronger the step.
I would not agree that a plug be used from another cut out unless this is drastically cut down in thickness.
In both epoxy and polyester a best attachment is made to resin which is either still wet or not fully cured. Even after a day or 2 a good chemical bond can be made with new resin on day old stuff. But as soon as you try to bond new resin onto old f/g you are relying on mechanical bond.
So if you try to bond an old plug into the hole you might lay fibreglass cloth over the outside say 1cm larger than the hole. So the cloth when hard can not be pushed into the hole but if it is thin it will not be stiff enough to take the pressure. But the plug is only stuck to this thin fibreglass as well so could be pushed in. If the old plug is cut down in thickness it will act as a mold supporting many layers of f/g cloth and resin which has as strength of its own.
Now all this is theoretical. A plug with a bit of sikaflex or just a bit of filler and a patch or ply backing on the inside, would probably last a lifetime but I would rather do it with the best strength easily achieved.
I did one years back where the log transducer was. I am not sure i could even find it now certainly it has become just part of the hull. I did chamfer both sides. good luck olewill
This was done on ours (GRP) by the previous owner. He left the old skin fitting and stopcock in place, closed the stopcock, removed the handle and fitted a blank in the open end. I check it when I check the others and it's perfectly fine still. When we're out of the water I check that the closed ball is still there too
In my case there was no fitting and in the OP's case I imagine he needs to remove the fitting to make way for the new holding tank.
It strikes me that a bronze plug with the same size external flange as the original fitting and long enough to just get the nut on inside would solve the problems in most cases.
All you need is a bit of bronze bar and a friendly machine shop or a small lathe and you could soon have one.
July 27, 2016