Fiberglass and Epoxy- Repairing an Outboard Motor Mount

11 Jun

Although I’ve been stuck here in Marathon Florida longer than I would like, we’ve been busy as bees during our layover. I’m here with one of my crew from the New Orleans – Florida Keys leg of our trip and before we set off on the next leg, up the East coast to Maine, we’re doing repairs on the boat which they are caretaking. The first big job was the transom/outboard well, which had been underbuilt and was sorely in need of repair. After a couple good yanks on the outboard the mounting face ripped right out!

That big empty space in the center of the photo? That would be where the outboard is supposed to mount…

Time to break out the fiberglass. And I thought I was done with this sort of nasty work… Still, as much as I haven’t missed the mind-altering fumes and constant itch, I have been enjoying myself. There is much to be said for versatility of fiberglass, the ability to play chemist, shaping solid structures out of liquid and cloth. It can be rather fun, once you’re done with the grinding, and it feels good to be working with my hands again.

The first step, as always, is the worst. You need freshly ground fiberglass all around your repair area. This is best done with an angle grinder equipped with a 4-1/2″ grinding wheel for tight spots and a 6″ sanding disk for flat areas. We ground out a long bevel in all directions that we planned to lay fiberglass cloth.

Then we poked around for a bit of wood to build up the structure of our repair. A good piece of hardwood is as strong as quite a few layers of fiberglass and when I’m doing structural repair I use wood wherever possible to keep the materials cost down. Properly encapsulated in fiberglass it ought to last the life of the boat.

This bit of mahogany happened to be a perfect fit so all I had to do was give it a rough sanding to give the fiberglass something to key to.

Our next move was to lay out supplies. I’ve learned the hard way that when doing glasswork there is nothing more important than proper preparation, including having every tool you expect to need on hand.

For this repair we went with fiberglass mat and cloth, as it was what we had on hand. We used West System epoxy and fillers.

I’ve also learned to always lay out a good-sized board for cutting cloth and another for wetting it out, even if I don’t think I’ll need them for a particular repair. This is the stuff that’s easy to forget until you’re having to scramble for it.

With everything together we went and cut out all of our cloth beforehand. Tending towards working on the fly this is another lesson I’ve learned the hard way. Glasswork go so much smoother and with fewer interruptions if you cut and dry fit every snippet of cloth and mat before you begin.

This cloth is quite old and pretty grotty-looking, but it worked ok

Finally, time to get started.

One of a few reasons why I tend to use West System epoxies is their measuring system. For about $15 you can buy a set of pumps which are calibrated so that one pump of hardener will catalyze one pump of resin. This makes it easy to pump out only what you need, saving waste and making it easier not to rush. For most repairs I will keep the same brush and mixing container until they get too tacky to use but mix only enough epoxy to wet out one or two pieces of cloth a time. This way if I run into any snafus I’m not rushing to sort them out and put on the next layer before my epoxy starts to kick.

The first step with any repair it to coat the entire repair area with neat resin (called wetting out). I took care to saturate the wood as much as possible. The unthickened epoxy penetrates into the grain of the wood giving it protection from moisture and a stronger bond with the fiberglass which we were about to apply.

After wetting everything out we thickened a couple squirts of epoxy with West System’s structural filler, a product which gives the epoxy greater strength and density. You can use all kinds of stuff to thicken epoxy, ranging from sawdust and talcum powder to all kinds of highly specific commercial fillers. The main thing is to use a high-density filler when you want strength and a low-density filler when you want easy sanding. We used our thickened epoxy to stick our board in place, holding it in place with a stick jammed against it on the inside of our repair area. Fiberglass really doesn’t like to conform to sharp angles so we globbed on extra at the corners to fill in space and round out the angles.

With our board in place we started applying fiberglass. Fiberglass mat is made of randomly chopped strands of fiberglass and has good bonding properties, sticking to wood or old fiberglass with more tenacity than cloth so it is best to start repairs with a layer of mat. Following the mat is a layer of fiberglass cloth which is woven from long strands and provides much more strength than mat. We laid up two layers of each, starting with mat on the bottom for a good bond and ending with cloth for a smoother finish. It is also possible to buy variations of fiberglass, known as bi-axial cloth, which have layers of mat and cloth stitched together and are a bit easier to use but we used what we had around.

We cut our pieces so that each would overlap and only have to fold over one axis, making them much easier to lay flat than if we had used single large pieces
It’s not very pretty, but it is quite strong! The stains on our fiberglass spread when we added the epoxy but didn’t seem to interfere with our bond

With the outside built up we waited until the epoxy had set enough to let us remove our clamping stick and then started on the inside. Again, our first step was to soften all corners and fill potential voids with thickened epoxy.

Then we laid up our mat and cloth, a couple pieces at a time. There is almost always a point in my fiberglass work where things start to gel up and little voids or mishaps need attending to and often the glove supply is running low and for some reason or another when I go back to review what I did I realize that I just stopped taking photos. This is just before that point:

From here I don’t even have a shot of the finished repair! It seems we were too caught up in the next task, which was far more serious. So for now I’ll just say that everything went well and the new motor mount seems far stronger than the old. Almost as quickly as we finished it we had forgotten it, because there was this to contend with…

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


  1. Jared.Wilburton

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