If you already know what a Mirror dinghy is, the idea of making one out of carbon fibre might seem rather odd as it would not be race legal, but my plan would be to use it for long voyages. I want to build hatches into it to use the buoyancy tanks for storage, so sticking to the normal design rules is not on the agenda anyway. Mirror hulls normally weigh 45.5kg and are hard for one person to lift out of a canal or river in situations where a trolley can't be used. Ideally I would like to try to get the weight down towards 30kg, if that's possible, but any significant amount of weight saving will pay dividends in situations where the boat has to be lifted or carried. Even if it ends up being the same weight as a normal Mirror though, it will still be much better suited to travel than the standard boat as it will be designed specifically for that purpose, so it will not be a waste of effort or money.

A Mirror hull is normally made out of 3mm thick marine ply [edit - it's actually 5mm for the external panels and 3mm for the internal ones], or GRP/FRP with the modern plastic versions. The wooden hulls are fragile and it's not hard to damage them when climbing on an upside-down hull during a capsize if you don't know where the weak areas are (I've seen a hole in a hull made by a 12-year-old girl in this manner with her knee), but they are more than strong enough if you are careful, so I'm not looking to build a stronger version of the boat. The quality of this wood is lower today than it was in the past, but wooden boats are still being made and are still robust enough for the job. The plastic versions of the Mirror dinghy have to be the same weight as the wooden ones because of the one-design class rules, but they are considerably more robust. My aim would be to try to match the strength of a wooden boat while saving as much weight as possible.

Do you think this sounds like a viable project for a home build using the products available through a company like Easy Composites? It may be that plywood is so strong for its weight that carbon fibre simply can't bring a significant weight saving unless the best manufacturing methods and special epoxies are used. I suspect that large weight savings could be achieved if enough money is thrown at the task. In one real world example, the Flying Phantom catamaran (Pre preg Carbon, Nomex sandwich) weighs 155kg while the ordinary Phantom F18 (Fiberglass / foam sandwich / vinylester) which it evolved from weighs 180kg. That's only a 1/7 weight saving, but the ordinary version of the boat is made using cutting-edge methods too which would also be hard to match without throwing a lot of money at the task. I can't find a good example to allow comparison with a wooden boat to get a real indication of how much weight could be saved. The ancient wooden Shearwater catamaran weighs 120kg while only being two feet shorter than the Phantom, but I suspect it's a lot more fragile.

Anyway, the key questions that I need answers to at this stage are the following:-

(1) If using Easy Composites products, is it possible to make carbon fibre panels with the same general strength as 3mm marine ply while reducing the weight significantly? (CF would doubtless be thinner if the weight is lower, but I would be able to add periodic flanges or strips of extra cloth to thicken the material in places to them to stop them flexing - these flanges/bumps would be hidden inside buoyancy tanks.)

(2) Are the epoxies used in infusion strong enough for this kind of task or are prepregs essential? (Bear in mind that I should be able to cobble together some kind of oven capable of curing the whole outer hull, which should be made in one piece, at 120 degrees to achieve maximum strength.)

(3) Does anyone have a list of commonly-used combinations of different layers of material with various numbers stating their collective mechanical qualities and comparing them to other materials like aluminium, steel and ideally plywood? (For example, I'm thinking of experimenting with a 200 plus 400gsm CF layer on either side of a layer of Soric.)


For those who want a better idea of what the task would involve, this PDF shows how a Mirror is normally put together from plywood sheets. They are stitched together along the edges with copper wire followed by fibreglass tape and epoxy to seal the seams. The structural strength is in the wooden panels themselves - they are not built upon a wooden frame.