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.

Thanks for that encouraging reply - I'm now beginning to hope this really might be possible.



2kg per sq. metre is what I'd like to aim for, so 4 layers of 200 plus Soric would hopefully get close to that and still have enough strength for the job (though the carbon fibre biaxial 300g may be worth a go for the inner layers in places where more strength looks worthwhile). Using kevlar/cf for one of the layers is something I was planning on - I want to err on the side of building the prototype slightly too weak in order to find out where it breaks so that I can learn where I need to add more cloth on the next boat. I don't fancy being in something that breaks up into loose shards, and it's better to have a heavily fractured bit of hull than a gaping hole, so kevlar or something with similar properties is a must - carbon aramid plain weave looks sutiable as I'll have to paint over most of the external surfaces anyway to keep the heat down in strong sunlight, and to make the boat look sufficiently ordinary that it won't attract attention.



I've seen three videos of infusion of hulls (though not CF) and it looks achievable. Two were of similar size to a Mirror and the other was a 43 foot thing being built in India. The way to do it seems to be to have three or more tubes that the epoxy goes in through, starting from one running along the middle, then when it passes the other tubes to the side, epoxy is let in through them as well to make it easier for it to flow the rest of the way from there up to the top edges. Multiple vacuum pumps can be used in combination with a tube right round the rim of the hull and sections of that tube can be clamped off once the epoxy reaches them. It would be expensive to get it wrong though. One particular concern I have is how to deal with strengthened parts where more cloth is used - it could cause the infusion front to be held back there and it might be overtaken to either side and then be encircled, thereby leaving an air pocket.



The way the wood's doubled up where buoyancy tanks meet in the normal Mirror suggests they may have an important structural role in those places, and particularly the ones in line with the mast where high forces are transmitted to the hull, so I'd want to copy that and make them thicker rather than thinner. It's the same at the back where the high forces can be generated by the rudder and the whole stern area needs to be strong enough to take them. It's better to stay close to what's already well tried and tested. The parts of the Mirror that were strengthened in the Mk 2 version of the boat give useful clues as to where the Mk 1 was too weak and overly strong - the storage compartments opened up in the Mk 2 show that the original structure under the mast was stronger than necessary. The Mk 3 also gives some hints as to how to simplify the task of making plastic/CF versions - it eliminates parallel sides and overhangs in the interior so that it can be moulded in a single piece (not including the thwart and centreboard case) which is then glued in. I want to build an interior more like the Mk 2 though as it's much better for providing storage.. (Here are pictures of the three types.)



I can use an old wooden one to make the outer mould from, just so long as it hasn't distorted too much over time. I may rework part of the shape under the front end to improve its aerodynamics because I want to try fitting hydrofoils to it (canting L foils, one to either side of the hull), so the airflow under the hull at a forty five degree angle is important - the boat should have enough sail area to be capable of flying on foils in a strong breeze, and once up on foils it should be able to rip along at a speed approaching that of a non-foiling catamaran. I'm hoping to go on to build a much bigger four-hulled foiler, but I need to learn how to work with carbon fibre first, so it makes sense to start with something relatively inexpensive for a first project. I'll try doing a 12 foot foiling catamaran too after the Mirror.



It isn't the cheapest I've seen, but you generally get what you pay for. What really surprises me about carbon fibre though is how affordable the cloth actually is. Having watched hours of video I can now see that the real expense is in the time you have to put in working with it, so whether you use CF or fibreglass makes a farily small difference to the cost. I looks as if it might be possible to build a much better boat than the best plastic Mirrors for a similar amount of money, and lower cost for subsequent hulls by reusing the moulds.



There are copyright issues that would prevent me from selling them, unless I can get my version recognised as an official Mirror dinghy (Mk 5). That might be possible if the idea of a 3-mode Mirror dinghy appeals enough to the right people: with weights added it could potentially compete as a classic Mirror; without added weights and with the addition of a bowsprit and asymmetric kite (replacing the spinnaker) it could compete as an asymmetric Mirror in handicap fleets (to close the gap on its modern rivals), and with the hydrofoils it could be used for high speed racing. I'm really more interested in using it for travel though, so it doesn't matter if they don't approve of it.