Fiberglass coaming construction

What is outlined below is how I built the fiberglass coaming described in Tom Yosts web pages.

I decided on a fiberglass coaming because I liked the idea of joining the front and rear deck stringers with a somewhat rigid structure. Plus I have quite a lot of fiberglass experience as I built a glass airplane (a Q200) a few years ago. Note that although most of the links to materials below will take you to Aircraft Spruce, Tap Plastics has much of the same stuff. Even West Marine as some of it.

I definitely did not take enough pictures!

I built the mold from a piece of pine cabinet-top stock that I found at Home Depot. I wanted the inside of the coaming "U" to be at least an inch and this stock is 1.125". It's glued up from many small pieces and knot free. I think the piece was $25. It was not quite long enough, but I could cut it in half and glue the halves end-to-end to make a piece that worked.

I left ears on the mold so that I could work vertically. Also I used dowels to position the two halves, then used rubber bands to keep them together. Click on the images to see the full sized picture
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The key to making this layup easy so to lay it up flat with bias tapes, then apply the flat layup to the mold. I used Rutan bidirectional cloth from Aircraft Spruce. This is a cloth that is optimized for hand layups. It's 8.8 oz., just a little heaver that regular boat cloth. It really is easier to work with! http://www.aircraftspruce.com/catalog/cmpages/rutan.php

Here I'm cutting the bias tapes. It's easier to use a rolling cutter then scissors as the tapes have nothing to keep them in shape after cutting. Carefully roll them up trying to keep the width right. I cut them 5" wide which yields about an inch extra on both sides. Don't worry about short pieces, the layup is 7 or 8 layers - just butt the ends together and make sure that splices don't line up one layer to the next.

Here are all the tapes rolled up and ready to use. I marked the length on the end with a sharpie.

Lay down blue tape to outline the built-up layup dimensions. Then tape down a piece of Saran wrap on which you will do the layup. The picture above shows the blue tape, the saran wrap and the first layer of glass ready for epoxy. No pictures of the actual layup - too much epoxy everywhere! Use barrier cream and/or gloves. Barrier cream is great stuff, buy it at an auto parts store. Use a generous amount, maybe a pecan size portion. After the layup is done you can just wash off the epoxy with soap and water. It also keeps the bad stuff from penetrating your skin. Good when wrenching as well.

To minimize air let each layer be pretty wet, then dry it out by letting the excess epoxy soak up into the next layer. Use a squeegee or squeegee roller to bring up the epoxy from the bottom.. Here is a picture of the best part of my layup.

Note that there is very little excess epoxy yet there are no air bubbles that show.

Now the fun part (no pix!). Cut the Saran around the layup, then pick up the whole thing with the saran still stuck to the back. The saran will keep the tape layup with the right aspect ratio until the last minute. Transfer the layup to the mold (you did use lots of mold release, right?). You'll be amazed how easy this is. Remove the saran as you pat down the layup to the contours of the mold working from one end to the other.

Another step with no pictures - I used peel-ply to give the coaming a nice surface that I could easily glue the Velcro to without sanding. I had a roll of bias peel-ply lying around - I have no idea where I got it. Aircraft Spruce seems only to carry the regular stuff. If you don't have bias peel-ply you have to use many small pieces. Still much better than sanding.

Once again no picture - let the epoxy harden until it has the consistency of leather. Not sticky buy still pretty flexible. At this stage use a sharp knife and cut off the excess. This is so much better than sanding! You still have to sand to clean up the edge, but there is really no comparison with cutting and sanding off the hardened excess.

I did the coaming in two layups, overlapping them about 6" at each end. Here I have the previous layup (hardened) put back on the top of the mold while I'm finishing putting the wet layup on the bottom. You'll need to trim each end to the right length while the epoxy is still wet.

I did two coamings, one seven layers and two piece, one 8 layers and one piece. I like the stiffness of the 8-layer one but I wish I had made it two-piece as well. The two-piece packs better.