Making Your Own Chop Strand Mat

I needed to make some chop strand mat to help bond in the frame supports for the manual bilge pump. I dug deep into my storage lockers to find my fiberglass materials and was shocked that I had run out of chop strand mat and all I had was a very large piece of woven cloth.

Part of being a cruiser is being creative. You need creativity because you don't always have access to all the tools and materials that you need. This means that you need to get creative and figure out ways to make the tools and materials that you need out of what you have on hand.

Chop strand mat is simply a mat made out of small strands of fiberglass. Woven cloth is simply made out of long strands of fiberglass woven together. I didn't have chop strand mat, but I did have fiberglass cloth, sharp scissors, and plenty of time!

4x4 inch sections of cloth were cut out of the fabric and I proceeded to unravel the cloth. Fiberglass cloth is very easy to pull apart, you just need plenty of time to do this. Start at a corner and pull a single fiber at a time. Continue to pull the strands out of the cloth until you have pulled the whole 4x4 section apart. If you lay all the strands in the same orientation, the next step is pretty easy and straight forward.

I simply pick up the bundle of strands and begin cutting them into 1 to 1.5 inch sections. It is best to cut slowly that way stray fibers don't go flying through the air and into your cushions!

Cutting the fibers over a bowl will catch the small strands as they fall off the clump in your hand. With all the fibers collected, you can mix them up to randomize their orientations to each other. 

Now you have a bowl full of chop strand mat, ready to soak up the resin like a sponge to help bond the wood frames together.

Beveling the Rabbet

The rabbet needs to change bevels as it progresses along the keel from stem to stern. The easiest method to get the bevel right on a small build such as this is to eye ball it!

The rough cut should be made about 1/2 an inch proud of the actual rabbet line. At this point, a batten can be placed on the frames to see what angle the planks will reach the rabbet. With this angle in mind, adjust the plane to this similar angle and give it a few strokes, then test it again.

If you can see light under any part of the batten, you need to do some more trimming. Light between the frame and the batten means that the rabbet is proud and needs to be trimmed. Light under the rabbet means you trimmed too far and need to trim the frame a bit to compensate (this should not happen though as it means you trimmed too far).

Once there is no light under the frame and the rabbet, you are done! Now you move to the next station and adjust the bevel. Once all the stations are done, you will need to blend the spans between stations. Resting the batten on the chine log works well for these parts as it serves as a point of reference.

Once the bevels are all established, the rabbet can be cut into the keel by resting the rabbet plane on the bevel and sliding it along the keel, cutting in a nice and clean notch that flows with the bottom of the hull.

Carl's Ride

Carl is a wooden cat on a swing and he is suspended from the salon hatch where he swings about as we rock around with the waves.

When it is calm, Carl stays pretty still. When the wind pipes up, Carl really starts swinging!

Chop Strand Mat

Chop Strand Mat (CSM) is pretty much "Fiberglass Sponge". It offers little strength to the final product, but it does a great job of holding the resin in place. 

When you are bonding a core material to the hull, you need plenty of resin in place to bond to the coring material. If you do not have enough resin present, the core material will not properly bond to the hull and delamination will present itself later on in the life of the hull.

Yes, chop strand mat is weak, but its objective in bonding coring is to hold the resin like a sponge so that the core material can be properly bonded and the coring will supply the needed strength to the hull. 

If you are bonding two pieces of wood together, chop strand mat can be placed between the two pieces of wood to help hold more resin between them. This will keep the resin in place as it cures the two pieces of wood together. If you use only resin with no chop strand mat, the resin can run and drip out of the void between the wood and a weak bond will develop. 

When bonding materials together, chop strand mat works very well to hold the resin in place as it cures and produces a stronger bond, holding the materials together more predictably.

Fine Tuning the Port Rabbet

The port rabbet line was cut and shaped using hand planes and a rabbet plane. Once the bevels were set to match the frames, it was time to refine the rabbet. 

The rabbet plane was set on the bevel that lies with the frames and floors and allowed to cut into the keel. This produces a clean, crisp notch where the planks will lay. By laying the rabbet plane on the bevel with the frames, the notch will change direction with the hull. If the rabbet were cut in a stagnant angle, the frames would not lie flush up against the notch. By allowing the rabbet plane to flow with the contours of the hull, the rabbet will also flow to match. When the planking is put on, the planks will also follow this same curvature and lay flush into the rabbet of the keel.

The forefoot was a bit trickier to cut. The rabbet plane can not cut up to the edge of the stem, even with the blade moved forward to act as a bullnose plane. The very edge needed to be cut out with a very sharp hand chisel. Just like the plane, the hand chisel takes off a tiny scraping of wood with each passage. Over time, the forefoot will take shape and the rabbet line will come to life. 

It is important to ensure that the depth of the rabbet is uniform and deep enough. If the rabbet is wavy, so will the planks. If the rabbet is shallow, the joint will not be as strong and the hull will be more prone to leaking.

Using a piece of wood as a template will make it easy to make the depth of cut uniform. The wood template can be slid around the rabbet to ensure appropriate depth. Any shallow spots are easy to identify and then plane down a bit further.

Once everything is cut to size, the whole area can be cleaned up with sand paper, but special care must be taken to avoid rounding off the sharp corners of the rabbet. If these corners were to become rounded, the strength of the rabbet joint is reduced.