Research, and a bit of humilty: Patching the old thru-hulls

3 Dec

I’ve just done a bit more research on fiberglass repairs and am feeling rather sheepish about my cocky post on grinding the boat’s hull. I’ve since learned that the essential aspect of the 12-1 ratio which we resolutely ignored is less about strength than it is about flex. I had assumed that due to the thickness of my hull we would get plenty of strength even when patching a relatively small area. However it seems there is a possibility of the patch failing precisely because it is stronger (or, more accurately, harder) than the rest of the hull.

The issue here is flex vs. rigidity. The reason fiberglass is an ideal material for boat construction is that it has the ability to flex without sustaining damage. This may be disconcerting when you hear the creaks and pops of a hull flexing in heavy seas but this flex is essential to the integrity of your hull. Thus, when making a repair to the hull it is important that the fibers of your fiberglass patch will bond across a large enough area that they will flex along with the whole hull rather than functioning as a separate piece.The danger in having too small of a bevel in your repair area is that the fiberglass plug which is bonded into the hole will be more rigid than the hull itself and that as the hull flexes under heavy seas it will result in cracking at the hull-patch joint. This, incidentally, is why it would be a terrible idea to try and plug a hole in a fiberglass hull with just thickened epoxy. The epoxy is very strong but it is also very rigid so as the hull flexes it will eventually crack and break loose.

So maybe my cavalier attitude to our hull repair job was a bit unwarranted but it did make sense at the time. The main thing I was struggling with was that doing a repair with the recommended 12-1 bevel would result in grinding out an absurdly large area of the hull just to patch a hole a mere 3/4″ in diameter. What I wish I had read earlier is this article by the folks at West System: Making practical decisions when repairing machined holes in fiberglass boats
It gives a great overview on patching holes in thick fiberglass hulls by plugging the center of the hole and then grinding a bevel in both the outside and inside. This apparently produces a very strong repair and even in a 1/2″ section of hull only requires a bevel of about 5″ diameter.

Unfortunately it’s a little late for all this now as we’ve already patched all the holes using our small, deep bevels. Nevertheless, I think we’ll be fine. I suppose the safest thing to do would be to grind everything out and start again but frankly I don’t care that much about having things perfect. This is a project boat and our goal is to get sailing as soon as possible. If it turns out that our patches start to leak after a few months in the water, we’ll pull the boat and try again but I really think we will be fine. Between my heavily overbuilt boat and the strength and versatility of epoxy resins I suspect there is enough margin for error to keep us afloat. If not we’ll deal with it when problems arise. After all, boats are safe at harbour but meant for the sea, right?

Here is our not-quite-recommended procedure for patching through-hulls:

With all our holes ground out and backed on the inside with a couple layers of thin cloth we started this repair as with any fiberglass work- a quick sand and cleaning with acetone followed by a wet-out with neat epoxy.
This was followed by a small amount of thickened West System 105 epoxy/406 Structural Filler in order to fill any minor gaps between the hull and our patches.

 Again, in retrospect I’m not sure about this step. On the one hand the epoxy/filler mix is more rigid than fiberglass and hence more prone to cracking under flex but on the other hand it seems better to have a little filler in the patch than to have a potential air bubble. West System appears undecided on the subject as well: their Fiberglass Boat Repair manual recommends a layer of epoxy/filler while the article I linked above mentions only neat epoxy and fiberglass. Anyone care to put in their two cents?

With the holes prepped we built up our fiberglass plugs on a board covered in plastic. This was done by wetting out and squeegeeing each piece of fiberglass and then stacking it on the pile. Some of our patches were nine layers deep, even using a very heavy bi-axial weave!

 Once a patch was stacked in one piece we laid this into the prepped holes in the hull. We chose to do this with the largest patch on the outside but again there are differing schools of thought on this.

Another way (detailed in the West System manual) is to lay the largest patch on the inside and the smallest on the outside. This may sound counter-intuitive but the theory is that even if you have to grind a little off the top of the repair you will be grinding down your smallest and least structurally important fiberglass layer(s), leaving your largest layer undisturbed. The reason we chose to lay the small layers on the inside was a fear of creating air bubbles if we were to lay the largest layer across any imperfections in our bevel.

 The final product:

Next time we’ll see how they turned out, and try to fair them smooth with the hull. This will be quite a task I imagine as in places they are a bit uneven!

This article was syndicated from Safe At Harbour But Meant For The Sea: DIY Sailing with Paul Calder


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