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August 23rd

Repairing Serious Keel Damage Part II

Posted by // August 23, 2013 // COMMENT (0 Comments)

Maintenance, ,

Here is the second half of the keel repair Emory and I did on Noah’s Ark. The long gap in between parts I and II is in memory of the two and a half weeks that Emory and I sat twiddling our thumbs waiting for it to stop raining so we could finish the damn thing. Or maybe I got sidetracked… Either way, let’s jump back in.

At the end of Part I we had just re-glued the long crack running up the aft end of the keel, and it looked like this:

The next bit is always the most miserable. In order to repair the keel with new fiberglass laminates we needed to grind down the old glass. I generously offered to go first, leaving Emory to grind the downwind side. The wind was, of course, pure coincidence.

Ideally, you want to grind the old laminate down to an edge with a bevel of 12:1 or larger. This means that if your laminate is 1/8″ thick you will grind 1.5″ in towards solid glass with a nice even bevel. The goal is a sharp feather edge leading gradually to your full 1/8″ thickness.
But Noah’s Ark has a keel which is easily 3/4″ thick! With thick old laminates like this a 12:1 bevel is quite unnecessary and it’s safe to fudge it a fair bit. We did our bevel over an area of about 6″ and called it good:

The holes were drilled to gauge the thickness and condition of the laminate, and for another use I’ll cover momentarily.

Even leaving aside how awful fiberglass dust is, this is awkward, frustrating work. I’ve read about a zillion books and articles on fiberglassing and they all cover this part in a couple sentences, as if the technique is self-evidently easy. Just grind your 12:1 bevel and then move on. If you’re lucky there might be a diagram of a perfect bevel. I’ve never seen instructions on how to actually get that bevel.

This is ridiculous. I’m here to tell you that beveling fiberglass is not so easy! I’m supposed to take a hand tool and evenly grind down from 3/4″ thick down to a knife’s edge over a distance of 5-8″? Just by eye? No way. It’s a real disservice that none of these fiberglass-for-beginners books talk about how to actually do this. Instead, I’ve learned by experience. As usual, what I’ve learned is to stop stressing out about it! You don’t need a perfect bevel for a good, strong repair. I bet even the people writing this stuff don’t get a nice even bevel, and that’s probably why they gloss over how to do it. Me, I just grind away, get it roughly even, and then smooth everything out at the end with filler, if I have to. Seems to work fine. That said, I’ve found a couple tricks that seem worth mentioning.

First off, there’s tools. It’s a glaring omission but almost no one talks about what tools to use when grinding this bevel. The assumption, I believe, is that you will use an angle grinder, but there are so many attachments! I used to use just a hard grinding disc, like you would use for metal:

These are cheap and fast, but a bit tricky to use. It’s easy to cut too deep, and hard to keep the bevel even.

On this repair, I discovered that the job is much easier if you use flexible sanding discs on your grinder. These discs are pretty cheap and you can get them in larger sizes which make for easier beveling. I like to use 6″ sanding discs on my 4-1/2″ angle grinder (with the guard removed). You want a heavy grit, say 40 or 50.

I’ve always been too cheap to go and buy the proper backing pad for these sanding discs. The nice thing about using them without a backing pad is that they’re extremely flexible and will conform to rather large curves, which helps with getting an even bevel. The disadvantage is that they don’t last as long and randomly rip off and go flying. Wear proper eye protection. Next time I think I’ll buy a backing pad and experiment with it to see whether it is flexible enough to be useful. If nothing else, it helps to make a backing pad of sorts from an old cutting wheel which is mostly used up.This spreads the clamping load over a larger area of the sanding disc but it’s small enough that the disc can still flex.

Another trick to beveling is to keep in mind that fiberglass is made up with a series of layers. Picture a piece of plywood. A nice bit of plywood mighty have seven plies. Now if you were to plane a bevel in the end of it, you would see these plies exposed one by one as you cut the plywood down. To keep your bevel even you would just need to make sure that the line marking transition from one ply to the next stays straight across the length of your cut. Fiberglass is similar. It’s a bit harder to see than with plywood but you can usually make out at least some of the different layers. When you’re grinding you can use the transition from one layer to the next to judge how deeply you’ve ground into the layup and if you keep that line straight and parallel to the cut edge of the fiberglass you’ll know that your bevel is even, at least on that axis.

You can see the layers running parallel to the bottom of the keel. Not perfect, but good enough.

Overall, don’t sweat it too much; that bevel doesn’t need to be perfect.
Once we had ours ground in, we faced the next challenge. Unfortunately the reef this boat hit had ripped a couple inches off the trailing edge of the keel, messing up the shape. We needed not just to fix the laminate but to restore the shape of the keel. It had been suggested to me (by experts, actually) that we just use spray foam, but I wanted to use something more solid.

First I tried to just get the shape with a single layer of fiberglass, leaving a hollow that we could fill. This was not successful:

Nobody likes a crooked keel…

Without anything to fill the fiberglass it was hard to get it to conform to the shape I wanted, and it ended up with an odd, crooked hollow tip which I cut off:

Actually, I ended up cutting the whole piece off and starting fresh

For the next attempt I cut another smallish piece of cloth a similar size. When I wet it out I also mixed up a blob of resin with high-density filler. Then I spooned this resin with high-density filler onto the cloth and stuck the cloth up from the bottom. As this started to set I was able to mold it much like clay and managed to get the edge of the keel more-or-less back in shape:

Not perfect, but we were able to work out that unevenness in the trailing edge as we went

With the keel generally keel-shaped again all that was left was to lay up new fiberglass to replace what we had beveled out, and then to fill up the voids. We did the former with two pieces at a time, one vertical piece which was folded over the trailing edge and one horizontal piece folded over the bottom. Where they overlapped (the bottom left, on the photos) we cut one short and left the other long so that this bottom aft section was built up evenly. With each horizontal/vertical pair we alternated the long one so that the bottom aft edge was built up in both directions. Throughout we used bi-axial cloth and vinylester resin.

Mostly built up

Sorry about the lack of photos here, we were sticky and running out of gloves and time!

When we had built everything back up to around two-thirds of the original thickness we let it overnight and then did another round of grinding to even out a few high spots.

This is were we filled the voids in the keel. These voids, which I mentioned in Part I, seemed to be from the original layup. My suspicion was that had they been filled with something denser than air the keel might have suffered less damage in the grounding. Now was our chance to fill them properly.

Adhesion was irrelevant so we used polyester resin for this. We mixed it with a can of filler that Emory had lying around until it was about the consistency of molasses.

Remember, always wear your nicest shoes when doing boatwork: you’ll have incentive not to spill anything on your feet.
We mixed our goop about a quart at a time

Mixing too much resin at once creates dangerous amounts of heat so we filled the void in stages, doing only a quart or so at a time. We poured this filler into the keel through the test holes we had drilled:

All told it took nearly a gallon of resin. Once the keel was full and the resin had set up we finished up with a couple more layers of fiberglass over the top of everything:

At this point I got to work on my own boat and left the rest to Emory. All they needed to do was a little final sanding and fairing. Then the yard slapped some anti-fouling over it, and splashed Noah’s Ark the next day.

Now I know I tend to jabber on about the benefits of heavy old boats, but I think with this sort of repair you can really see why. I’m no expert at this stuff; far from it really. I would not feel comfortable attempting a repair like this on a lightweight fiberglass boat with a thin hull. Fortunately, on a thick old hull like this there’s a lot of wiggle room and I’m confident in our repair. With the added filler I think the hull is at least as strong as before the grounding, and we did this with just a couple days’ labor and about $100 in supplies!

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

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