Dear team, I have watched your fantastic video on making a new car bonnet in carbon fibre and this has given me the idea that I could do something similar with my very heavy centre boards on my Hobie 18 catamaran. The centre boards are about 1.2m in length and 35cm wide with a large handle and small flange at the top. The foil section is symmetrical (like an airfoil basically). The existing foils are very heavy and cumbersome to use. I am thinking that 2 moulds could be made that would basically create the new foil in 2 halves (length ways). Making the moulds and the 2 halves would be the straight forward part, it's the joining to create something stiff, strong and water tight is more tricky to visualise. Alternatively I could be barking up the wrong tree. I have attached a rough sketch of the foil and your advice would be welcome. Mike Cook

You can make the 2 halves and bond them together, perhaps with a foam core filler to stiffen it a bit more.

Alternatively you can hand lay up around a sculpted foam core and accept a bit of sanding and smoothing necessary to get a decent smooth finish.  You could then paint the final finished fin or lacquer it if you want to show off the carbon.

If you have problems with stiffness, you can add UD carbon to the layup or premade carbon tube/rod down its length in the core to stiffen it up.  You could even bond carbon strip on the inside of the moulded fin before bonding together as that would also add UD carbon down its length.

Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant

Hi Warren, thanks for your reply, I like your ideas for adding stiffness. I am trying to avoid having to scuplt a core if possible. When thinking about bonding the 2 halves, how would you build in a flange on which to apply the adhesive or would you build up some thickness around the edges and apply the adhesive there. Also could you describe how you would build up a mould please. Many thanks. Mike

What is UD Carbon? 

UD Carbon is unidirectional carbon. Its a fabric that has its carbon fibers oriented in only one direction. It uses tracers (I think that is what they are called) to hold the fabric together.

Its great for adding strength to an area that is stressed along the axis of the fibers's orientation. Imagine trying to bend a long tube or pipe, where the fibers are running down the length of the tube. When you try to bend the pipe, you are trying to stretch and elongate the carbon fibers along the one side of the pipe. Because carbon fiber has a high tensile strength, it resists the stretching, thus making the part stiff and more resistant to bending or deflecting.

This is why you often see UD carbon fiber being used on bike frames, tubing, or in this case, the foils that Cookie is talking about. Its a really good fabric to become familiar with using.

Sorry for the long winded answer.

What he says lol.


If you are designing the fin from scratch then you could design a bonding flange into your mould.  Using a core makes things a bit simpler in that sense as you are bonding both to the carbon and the core.

Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant