With thanks to Don and Andrew Landenberger, Don and Andrew McLeod, Ethan Twyman and John Gilmour.
A PDF version of this article is available here: Home Building A Carbon Moth Part 1.
During the late 80’s and early 90’s I was privileged to be witness to the efforts of Andrew and Don Landenberger build a succession of boats starting with It’s Original for the 1986 Nationals and finishing with Thumper Rabbit for the Sunshine worlds in 1995. Their efforts resulted in boats that won the State, National and World Titles. As far as I am aware, Andrew’s is the last non-professional boat builder who has won a world title in a boat that they designed and home built.
Learning from the Landenbergers, I have built three boats myself, using their methods. Returning to the class in 2005 after a 10 year hiatus I was disappointed that the home builder had all but disappeared of the face of the planet. This now appears to be changing, and this article is written with the goal to share the techniques that I used to build my Axeman like boat, Virtual Reality and my latest purpose built foiler Teknologika.
When building a carbon moth you essentially have three options for your mould.
- An open male mould
- A closed male mould
- A female mould
Building the mould
A good, square mould is fundamental to producing a good boat. The mould we require is a male mould of the hull shape. We need to start by making the base that our frames will sit in. The base is essentially made from a “ladder” of 1” x 2” (25mm X 50mm) timber, with a “rung” every two feet that the frame will bolt on to. The line of the frame corresponds to the sheer line of the deck. For my first boat I made the mistake of starting with a single beam in the centre that was a bad, bad idea that was never going to work; but hey, I was only 14 at the time.
The overall length of the frame should be the same as the length of the boat (3355 mm), the width needs to be at least the width of the boat at the chines, but if it is wider, it won’t matter at all.
With the base completed, we now need to move onto the frames. The frames can be constructed from any thick board (around 10mm) that is straight and warp free, such as medium density fibreboard (MDF).
You will need to make a minimum of 4 “standard” frames, at offsets of 615mm, 1325mm, 2035mm, and 2745mm. You can change these offsets if you need to, depending on the shape of your hull. You should add additional frames where required, some moulds have 25-30 frames. In addition you will also need to make a transom and stem “frame”, but each of these are slightly different. The exact number of frames will be dictated by the thickness of your stringers and how much they flex between frames, and how and where the shape changes in your hull.
Care needs to be taken when adding the transom frame in the mould not to produce a boat will be over length. You can do this by moving the rear frame forwards towards the bow the thickness of the MDF as shown in Figure 1, or you can just remember that the boat needs to stop on the inside of the frame. That is the method that I used.
At the stem of the boat, the first frame sits 25-45mm or so from the bow of the boat where a block of Western Red Cedar is placed to allow the bow to be shaped to a fine point, and provide some level of impact protection if you happen to run in to anything. This block is held in to place by two or three screws though the front frame that will need to be removed when we are removing the hull from the mould, see Figure 2.
The exact length of the block needs to allow for both the thickness of the carbon that will cover it, as well as any measurement tolerance that you wish to allow. (I have made all my boats 5mm under length so there is no risk of accidentally being over the legal length. Please take care so you don’t produce a boat that is over the legal length by mistake.
There are two Western Red Cedar stringers that will become the inside of the gunwales of the boat, when these go in the mould you will need to use some L shaped brackets, and screw from the underside of the mould, so they can be easily removed after the foam has been applied.
You could remove these if you liked, however it does provide a good gluing surface for the foam, and it also helps when it comes time to attach the trampolines.
Making the frames
The next step is to make the mould frames. To do this you will need to draw the cross sections of your design, full scale on the MDF then cut them out as accurately as possible. At this point you will also need to decide if you want to recess the stringers in to the frames or make the frames undersize and then screw the stringers on top of the frames. I personally used the latter as you don’t have to go to the effort of recessing all the stringers, and you can use the screws as adjusters to tweak stringer height if required.
To attach the stringers, you will need a good cordless driver drill, and a countersink drill bit. Self-tapping screws are a great way to attach the stringers to the mould as they are quick to install with a good driver drill. You may need some “L” shaped brackets if your MDF is not thick enough to drill straight into.
You will need to pre-drill and countersink all the holes where the screws pass through the mould stringers. So the screw heads sit recessed below the surface of the stringers.
If your design has flares, you will need some extra stringers, right next to each other on both sides of the flare chine, to allow the foam to be stapled down and make a good join.
The exact number of stringers required will vary depending on your design and where you are in the boat. For example, in the middle of the boat you will probably need five stringers across the top of the mould as shown in the diagram. When you get closer to the bow or the pintail stern, you will probably need to reduce the number of stringers to two or three. If you look at the picture below, you can see that there are only 3 stringers running all the way to the bow on the top of the mould.
At this point I recommend that you should use a batten and bend it around the mould, adjusting and faring stringers as required. You should also check that the correct amount of rocker is in the mould.
Making the hull skin
Once you are happy with the shape of the mould, it is time to start laying out the foam core. For Virtual Reality, I used a 4mm foam for the core which seems to provide a good balance between stiffness and weight. The foam is essentially added in three main panels, two rectangles one for each side and two wedges on the bottom of the hull. To determine the exact shape required I recommend that you use cardboard to make templates before cutting the foam to shape.
To cut the foam, the best method is to use a straight edge held in place by a trusty offsider and using a Stanley knife with a sharp blade along the straight edge. With the foam sitting in place, you need to pull it down on to the mould the best way I know to do this is to use poke some wire through the foam and then tie it around the mould from underneath. You will need to press the wire into the foam so it sits below the surface of the foam, then bog and fair it into the foam so it does not affect the surface finish. (The wire will be removed by pulling it out from the inside after the outer skin has been applied.)
The foam will need to be fared and joined, particularly around any chines by gluing on the edges of the foam. For any chine joins, remember to cover the stringers in plastic so the resin joining the panels does not stick to the mould. The opposite is true of the gunwales however where you do want he foam to stick to the stringer. If the foam is applied to the mould with care, it should require a minimum amount of filling, but it is very important that the foam is as accurate and fair as possible. Be particularly careful bending foam around tight curves, and you should establish with some test strips that the foam will bend as required. If it can’t you will need to cut, join and fill thin strips of foam around the curve, or purchase some pre-scored foam that is specifically designed to follow curves. The use of a heat gun can also help bend foam when required. Then fill and fair the joins.
Once the foam is on the mould and faired, we typically applied a very thin coat of resin with a squeegee. This was designed to stop the foam absorbing any excess resin when the cloth is applied. If possible you should do this just before you lay the cloth on the foam to wet it out, and it will help with absorption.
Another thing to consider is foam compression. For areas where the foam will compress, you should replace it with either slab carbon plate or plywood. One example of this would be around the centerboard case, or where the gudgeons or bung will be placed in the transom.
With the foam faired and sealed, you are ready to go with the carbon. For Teknologika each side if the hull skin required about 4sq Meters of 200gsm Twill weave carbon.
Lay your cloth over the hull and cut to shape whilst dry allowing for about an overlap on each layer. This overlap will need to be sanded down later, but unfortunately, there is no easy way to avoid this on a male mould, although peel-ply should help if placed over the join.
If you are using a filled male mould the technique is the same, except that you will use a vacuum bag to suck the layup down onto the mould after laying up the carbon.
Applying the resin
With the cloth in place, it is time to add the resin. Before you start, make up a test pot of resin and let it go hard. This is especially important with new resin that you have not used before. This is for two reasons: the first is to ensure that there has not been a manufacturing fault. I personally have used two batches of epoxy resin that were faulty that never went off. The second is that you want to know how long the pot life of the resin will be, and hence how quickly you will have to work. After the resin has gone off you should have a carbon creation that looks something like Figure 1. The FGI epoxy resin that I use achieves its best strength with a post-cure up to 50 degrees, so in absence of an autoclave I recommend that you place your hull out in the sun for a day or two before removing it from the mould.
Make the cradle
Once the hull has cured, you should cover it in plastic and make the cradle for the boat. Personally I cut a think foam camping mat to shape, and then use CSM over the top. To make a trolley, you will need about 6.5Meters of 50mm x 50mm box alloy. The reason you need to make the cradle now is that you need somewhere to hold the hull whilst you are working on it.
Removing the skin from the mould
Removing the “hull” from the mould is relatively simple, you should just have to climb under the mould and untwist or cut the wires, and remove the screws holding the gunwale and the stem block in place. Once these have all been removed you should be able to lift the hull off the mould. At this point you have to be careful as the hull will be quite floppy as it has no transom and only one skin. It is necessary to tack some timber across the top of the hull nailed temporarily into the inside gunwales. You will need to measure to ensure that the width is the same off the mould as it was on the mould.
The wires that were used to pull the foam down onto the mould can now be removed from the inside, and then the holes in the foam filled with q-cells or an equivalent filler.
Before you remove the hull, you should measure and mark the hull so that you can get it back to the designed width and shape once it has come out of the hull. You should tack some stringers across the gunwhales to keep the boat to shape.
Layup the inside
Once these have been filled, you can lay cloth on the inside of the hull, then cut to shape, and then repeat the layup process on the inner skin.
Adding the transom
Once the hull skin is complete it is time to add the transom. The transom is simply cut from foam with cut outs replaced with plywood or carbon plate for the rudder bolts. The foam should be laid on glass, then carbon wet out on both sides. Once the resin has cured you should cut the edges to shape, and glue the transom into the “hull” using glue powder. Once the transom is in, you should bog a small fillet in the join then carbon over the join inside and out. For my boats I used a then added a triangular mini frame perpendicular to the transom to spread the load of the gudgeons. For modern foiling moths the load on the transom is significantly greater than it used to be, so this area now warrants a lot of reinforcement to avoid a major failure due to foil load.