How to build a rowing shell using a hot-wire cutter and hand-layup fiberglass
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Step 1: Cut Foam
Foam cutting is an easy way to get a smooth 3D shape. Pink housing insulation foam is also very cheap. If I remember correctly, it took about 10 sheets of 2inx2ftx8ft to build this boat. The process is simple. Cut jig pieces. I used 1/4 in luan plywood. I used a bandsaw for the rough cut, and a large hand sander to get the finish shape. If the shape is accurately drawn, this method can easily get shapes to under 1mm accuracy. The edges of the jigs have to be smooth, or the wire will catch. Make sure you mark the points around each template which you want to have line up when cutting
Building hot wire:
The wire will be best if you can find 1/32" stainless steel cable to cut with. In lew of this, bicycle shift cables are OK, but leave a pattern which needs to be sanded off. Stainless steel wire also works, but it's hard to get it strung on straight. The cable should be mounted to some frame which is very springy, as the cable lengthens a lot when it heats up. The springness lets you set the tension when the wire is cold, and have the same tension when it is hot.
Using hot wire:
Put a voltage over the hot wire cutter which makes it cut smoothly through the foam, leaving little foam fibers behind it. The wire should smoke just slightly once it's cut through the foam. Don't try to force it. It is cutting with heat, so pushing hard will make the wire bend and not speed up the cut. A bent wire means you get the wrong shape. Never hot wire a piece with one person. Get one person on each side of the wire. Mark points on the shape which you want to make sure line up. Number them, so both people are going the same direction around the part. Speed up or slow down to match the other person.
I use spray adhesive to stick the foam to the templates. The propellant in the spray dissolves the foam, so spray lightly from a long distance (1 ft?). Let the propellant evaporate, and then stick the template on.
Step 2: Build a Flat Table
If you don't have a flat table to build on, it is very hard to build the part you want without warping it. If you don't have one, build one.. It saves loads of time.
To make your table flat, build it with many legs. Then use a tight string to define 'flat', and shim the legs until the tabletop matches the string. I neglected to flatten my table, and now my boat has a downward kink in the rear.
Step 3: Mount and Sand
One of the main reasons a rowing shell is so long and skinny is to keep the wave and pressure drag down. This means it needs a very smooth and unimodal surface. Ripples will make it much slower and less efficient.
I made a centerline down my table with a taught string, and mounted all the sections along this. This worked well except that my table curved up slightly at one end, so now my boat has a bit of camber at the tail.
Sanding is equally important. Remember that bit about making a constant curvature hull shape? If you do this, you can make a giant sander to sand with, which will get rid of all of the little ripples and bumps in the hull. I made mine about 4 ft long and just walked up and down the boat with it to shape. This is way easier than sanding and sighting to get the shape.
Step 4: Fiberglass
Once you are happy with the shape of your hull, fiberglass away. I wanted to be able to row in waves, so I used a lot of fiberglass. If you are fancy, putting a few layers of carbon stringer down the bottom can help with stiffness. If you do this, the carbon takes all of the load, so skimp on fiberglass (only enough for a hard shell), and make sure you have enough carbon to handle the loading.
I made my hull with four layers of glass. One (or was it two?) is unidirectional along the hull, two are bidirectional along the hull, and one is bidirectional at an angle. In hindsight, I would have ditched one layer of bidirectional cloth for another uni layer. The cloth weight was roughly 7 oz, 'rutan fabric', which is meant to be good for wet hand layups. This stuff is still my favorite fiberglass due to it's workability. It can be found at Aircraft Spruce.
Learning to fiberglass takes a little practice. It's basically just a process of getting some liquid in a cloth, and getting surface tension to hold it to something. If you do a lot of layers at once, surface tension may not be enough. If you do tight corners, surface tension won't cut it, so avoid both of these things, especially in combination. If you need to do sharp corners, use a touch of spray tack to stick the fiberglass cloth on, and then wet it out carefully. The spray tack holds the cloth in place, even as the epoxy is curing. I would recommend using a peel ply on the top so that the surface may be more easiy painted.
For the bottom of a hull, it should be possible to stick all layers on at once. Get someone to help you with it, as epoxy has limited pot life. Start from one end of your boat, with all of the cloth cut and ready, and work your way to the other end. By the time you are done with the far end, the near end will have set. Eat a lot before starting, as it could be 6 hours of constant work and concentration.
Wear a mask and gloves as it is very easy to become sensitized to epoxy. Many homebuilt aircrafts are never completed because their builders are not careful enough, and end up sensitized halfway though.
Step 5: Sand
If your layup is really good, you will need to do very little sanding. Mine wasn't great, so I put an extra layer of epoxy on top and sanded it smooth. Spray painting your boat so you can see dimples which don't sand and which need filling is a terrible idea. It will fill the room with pink dust.
Oh yes. Sanding fiberglass is really nasty. If you don't wear a mask, you will wheeze for the rest of your life.
Step 6: Get Distracted and Build Another Boat
If your boat is taking too long, it is suggested that you build a second boat in the meantime. Pictured is the spine for a westernized westernized Proa we accidentally built one summer.
Step 7: Build Top
the top of your boat is not particularly important. You can make it look as nice or as shitty as you like. I wanted to be able to take waves, so I built the top very strong for compression. it is made of 2 layers unidirectional fabric and one layer of bidirectional.
The top of the shell is under compressive loads, so it wants to buckle, especially around the cockpit. I added a bunch of layers of cloth to bring the load into the rails on the side of the cockpit. On the rails, I used at least 15 layers of glass to form stiff beams which could bear the compression load. It should be possible to lift the boat from bow and stern and not have it break.
Step 8: Sharpen Your Spoons
I used a sharpened spoon to carve a cockpit in the boat. The fact that it is made of many laters of 2" foam made it very easy to get a flat deck 2" down from the surface.
After cutting out the foam, bind in divinycell. I used a bunch of microbubble to make sure there were no voids. Divinycell is good for this because it can be heated and formed to make the shape.
After cutting and sanding, lay up the cockpit. The cockpit is in compression, so it needs a lot of very straight bits of fiberglass to keep it from buckling in waves. Don't try to do too much at once. Using peel ply and doing two layups in a tight space is usually a better idea than trying to get all of your thickness at once.
Step 9: Paint, Build Riggers, ...
Do everything else I don't have a picture of!
I made riggers out of steel. I made a seat out of fiberglass. Paint everything!
Gel-coat is a pain to use, but makes for a nice finish. I used gel-coat and followed up with spray paint to make it extra-yellow, and so I could see scratches.
Step 10: Row!
time to start building up those muscles. We started with sweep oars, which are a meter longer. This was very tiring. We then cut the sweep oars down and made them into sculling oars. Now it feels fast.
If you happened to build your boat in a room which it cannot exit from, you may be in trouble. Try removing windows.