Monday, October 1, 2007

Pity the Poor Wingbar

Somewhere in Ohio my trailer axle failed and the trailer tipped over on, well, my Moth wingbar. The whole program ground to a halt on the shoulder, and then it started to rain, just as the last light was fading from the sky. It was all pretty catastrophic - one moment you are listening to the radio in air conditioned comfort, with a little John McLaughlin guitar jazz playing, and the next moment all hell has broken loose and you are dragging some amalgam of carbon, wood, rubber and steel down the interstate making all kinds of sparks and ugly noises. I managed to right the trailer and drag it to the nearest off ramp, then rebuild the trailer by replacing the undercarriage with a new one. That got me to California, where my moth was now unsailable. So one of my first projects after setting up the garage shop was to undertake a repair. Just in case anyone else out there ever has to do this, I'll post the steps I used. It goes without saying that you should use full barrier precautions with the epoxy and avoid inhaling too many fumes, and always use a nice particulate respirator with a tight seal when sanding composites. These instructions are aimed at people with some experience building composite widgets, but not much, because that's me! And if you are not of a technical bent, or have no interest in knowing how to accomplish this task, you can stop reading now and go back to YouTube, unless you are at work, in which case you should get up and go for a cup of coffee.

1. Make a new partial circumference carbon sleeve, using the intact bar as a male mold. I used two strips of 5.7oz carbon, wrapping the tube with mylar packing tape as a release agent. There can be no gaps in the tape or you risk it sticking to the bar, and the fewer tape wrinkles the better. Squeegee as much resin out of the carbon as possible before sticking it on the bar, and then wrap the whole mess with peel ply and electrical tape, stretching it a good bit as it goes on to provide uniform tension. If you can find 2' tape great, but it's usually expensive and the skinny stuff works also if you don't mind doing a million wraps. You should overlap about 50%; more is better but anything over 50 takes forever. Make this sleeve twice as long as the area you are repairing.

2. After the epoxy kicks, Take the sleeve off the tube, pull the peel ply off, and admire it for a second. Then cut it in half normal to the long axis. Trim half to fit inside the tube. Sand the inside and outside of the broken tube, and the inside of one of your two sleeves (the other one has a peel ply surface on its outside so it needs no sanding). Now they are ready to bond over the c-shaped broken tube. Lube up the sleeve with thickened epoxy (I used West System High Density Filler, which I think is just milled cotton fibers - a bit heavy but really strong) and jam it inside the broken tube (this will stretch your vocabulary so it's best to not have any kids around while you do it). Take the other sleeve and bond it over the outside of the broken tube. Put some peel ply on or just wrap it with Mylar packing tape, putting LOTS of tension on to compress the outer sleeve against the broken/half tube and against the inner sleeve. If you put too much tension on, you will end up with this section being smaller diameter than the rest of the tube, which is undesirable cosmetically but otherwise of no particular import. As a final step, put the bar into position on the boat, with the ends telescoping over their respective mates. This will ensure that everything is lined up while it goes off. It goes without saying that some tape shoule be applied over the socket in the broken bar to keep goo out, and it doesn't hurt to put a little wax on the crossbar male ends; otherwise the bar may be permanently bonded to the cross bar after this step! Give it a night to kick.

3. When you are done, you should have something that looks like this:

It has a big hole in the end for the front cross piece, but you really should not be able to see that hole, because this part of the bar has a cap to keep water out. So the next steps are to reinforce any partially weakened tubing that was not covered by the sleeves (a good bit in my case) and then make an end cap and bond it on.

4. I had a good bit of bar where two or three of the layers of original carbon had been ground away, but not the entire wall. This needed to be reinforced to get that part of the bar back to its original strength. This is pretty straightforward - just sand the bar a bit, cut some long strips that extend slightly beyond the affected area, put them on and wrap with your tape of choice. It might look something like this while it is curing:

5. Make an end cap. This is sort of a two step process. To brigde the big hole in the end of the bar, you need to put something into it to provide a nice surface for the carbon to lay on, because otherwise it will hang in space and not be the right shape. So the strategy is make a light cap using some sort of foam as a mold, then release that, get the foam out, trim the cap, and then bond it to the bar as a second step with more layers of carbon over the top and tape over those to compress it all. Simple, right? Again, be sure to keep goo out of the socket.

6. After this step, take all the tape off. You will likely have plenty of surface imperfections, bumps etc. unless you have been doing this for awhile. So get out a chunk of 2x4 and some 36 grit sandpaper and make it look right. You have probably used way too much carbon and resin, so some of it needs to come off anyway. Your tramps have to slide over this thing, so there should be no big bumps or ridges on the main part. My corner came out a bit bumpy because I didn't do a careful job with mylar while making the end cap, so it didn't fit quite right. I also used the wide tape on the corner, which was suboptimal because it bunched up inside the corner. So put the mylar on carefully and use narrow electrical tape for best results. I did not grind the bumps off the corner because there are only three layers of carbon there and they are pretty highly loaded. So far it has held up well.

7. After an 30-60 minutes of hard sanding and trimming you should have a functional bar. You will note, in the case of a nice prepreg original product, that the end you just repaired is somewhat heavier than the other end. But don't sweat it - it's pretty much impossible to match prepreg resin ratios with a wet layup, and with some care you should be able to maintain a better balance than this, which isn't really that bad in my view:

That's it. Now rig your tramps and go sailing!

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