Evil, awful fiberglass: Grinding the old thru-hulls

30 Nov

 It may be the single greatest advancement in making sailboats affordable but fiberglass is horrible stuff to work with! Here you see me emerging from the forward anchor locker where I’ve been grinding a 2sq-foot section directly into my own face. Even with a shop vac sucking air out of the compartment it would so rapidly fill with dust (which was also filtering into my foggy goggles via the ventilation holes) that I was doing most of this by feel. No doubt the wost job yet on the boat but we’ll see what’s to come…

This was just a quick side project cleaning up the locker for a little more glass and repainting but I spent that whole day grinding fiberglass. Mainly I was prepping the half dozen or so through-hulls which we are going to be glassing back over:

This stuff, at least, was outside so the nastiness factor was reduced.
Fiberglass is remarkably versatile stuff, so much so that almost any hole or crack can be patched back up so as to be as strong as the original glass. The magic bullet here is epoxy resin. The cheaper polyester resins which most boats are built from have trouble bonding with already laid fiberglass but if the surfaces are properly prepped epoxy creates an incredibly strong bond allowing for repairs which are quick and quite easy. We are using West System epoxy because my father Nigel Calder recommends it (usually a good sign!) and I have found it quite easy to use but any major brand of epoxy should work equally well for these kinds of repairs.
The key thing with an epoxy repair is to be sure the epoxy has a large enough surface area to bond to. This means smooth, beveled edges and no right angles (which concentrate stress on the tiny edge of the angle).The holes we have to patch are all old through-hulls varying from 1/2″ to 3/4″ so the procedure is quite simple. Once again we used West System’s very thorough publication Fiberglass Boat Repair and Maintenance with some modifications for speed and simplicity.

 First, each through-hull was ground down to give a nice beveled edge. West System calls for a 12-1 bevel in order to achieve the same strength as the original fiberglass. We, of course, cheated. My boat was built in a wonderful age when people couldn’t engineer fiberglass down to the last molecule and were forced instead to just pile on some extra to be sure. This resulted in an incredibly strong hull which is solid fiberglass and nearly an inch thick in places! Now imagine following the book to the letter- we would be grinding away the hull in a 24″ circle, just to patch a 1/2″ hole.

Obviously the lesson here is to never follow directions. Instead, we ground everything out to about a seven-inch diameter. We chose this because it’s the natural diameter made by the contour of a well-used 4-1/2″ grinding wheel but if anyone asks I’m going to say it was the result of lengthly and difficult calculations. (Learned-the-hard-way tip: When grinding beveled edges always use an old, worn grinding wheel. When I chucked a new one on the sharp right-angle edge quickly cut jagged grooves into my nearly-smooth bevel)

A fiberglass boat, like the earth’s surface, can be roughly dated by
 its geological layers. Mine appears to be at least Paleolithic.

Once ground out to our exacting standards, each hole needs to be backed so that new glass can be pressed in firmly. To do this West System recommends using a piece of plastic or something similar firmly pressed against the inside of the hull so as t assume the curve of the boat. We, of course, cut some corners. Instead we just went on the inside and quickly applied two layers of fiberglass cloth over the holes, sealing them and providing enough of a backing to allow patching from the outside. (Beveling and patching should always be done from the outside so that water pressure or impact damage will be trying to push the repair in tighter rather than pop it out.)

Once this cloth sets up we’ll be ready to fill the hole from the outside
Thanks to the thickness of my hull this is all relatively easy and forgiving work. It would be a very different story patching a thin-hulled racing boat where much of the strength relies on engineering feats like uni-directional fiberglass! Up next: layering in the patches.

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


  1. Don Bland

    Hi Paul,
    I reckon I own every book your pop has ever written, so best wishes for good on to you ,eh?
    Youre getting off to a great start here!

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