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 all the replies and links.

Firstly I have to correct something - the "black stuff" is the least expensive CF cloth I've seen (I've only found out today what the "black stuff" is and that it doesn't just apply to all the black cloth sold at EC). It certainly looks like great value - I won't be buying cloth for a while yet, but I'm sure it'll be available again periodically in the future if I decide I need it.

I suggest using MTI Hose as well. It will solve your problem of an uneven flow front leaving dry spots, and considering you don't [seem to] have a lot of experience with infusion, it makes the whole process a hell of a lot more forgiving. It certainly isn't necessary, and it isn't exactly cheap, but it really does lower the risk of something going wrong by quite a bit. Beats stuffing up an infusion of this scale!

Thanks Hanaldo - I hadn't heard of MTI hose before. It looks as if it would simplify the process dramatically and remove a lot of the worry, so it may well be worth the cost. A failed infusion on something this size could be £1000 down the drain, so it has to be done right first go. I've found a thread about MTI hose here http://www.talkcomposites.com/2389/MTI-Hose which expresses some doubts about it, but it occurs to me that so long as there are still some empty areas where the resin has not reached this tube, you can still go on pulling a vacuum on the whole thing after clamping off the resin input to prevent it from forming any resin-rich areas.

Why? there is no air .... it's a vacuum, it will (should) pull the resin into this area.

I was assuming it can't be a perfect vacuum, but maybe it's close enough that the worst that can happen is a few pinholes.

Durepox is available in a few solid colours as well, it will help with preventing scratches etc...

Ah - I see that I should be using it anyway (http://www.resene.co.nz/archspec/products/durepox.htm) as it offers reduced drag through the water. ("Durepox was tested by the Otago Flume Laboratory and results showed that its satin finish gave a 15% less drag coefficient than a high gloss surface on rowing skulls.")

If you intend on painting the surface, scrap the idea of woven fabrics, use UD / Biaxial / Triaxial / Quadraxial fabrics, MUCH stiffer (50-100%) than woven fabrics, given the nature of the curves on the boat, conforming the fabric shouldn't be a problem.

Yes. For the most part, the material only needs to follow shapes that plywood sheets can make, so flexibility of the cloth and its appearance isn't important. UD might be the way to go if I can work out the best way to align the layers and if the cost doesn't go too high (and if infusion is practical with it). I should really spend a few hundred pounds doing experiments (testing panels to destruction) to see what works best and try to judge the best compromise on weight, strength and cost.

Message me if you want about resin and fabric suppliers, your needs are quite specific so I may be able to point you in the right direction...

                                                                          Thanks Chris. It's probably too early in the process to want to take up your time at the moment identifying the perfect materials, but I would like to take you up on that offer when the right time comes. I think I should get some practice in with a wide range of materials and do a bit of testing first, and do a lot more reading, and maybe go on a few courses. Studying a bit of engineering would also be a good plan so that I have a better idea about how to design strength in where it's needed, particularly as I'll need to put extra structure under the mast and across to the sides of the hull if I'm to attach foils at the sides of the boat (this part of the plan may well turn out to be totally impractical without adding a crippling amount of weight, so it may be best reserved for multihull projects - I think I'll drop it altogether for the first prototype).

[Edit: Or maybe not - I've now worked out where added strength to enable foiling is needed and it shouldn't add a massive amount of weight. The vertical sides of the hull need to be extra strong to hold the weight of the crew when there isn't water supporting the boat from below; the underside of the hull needs extra UD fibres running lengthwards to help it stand up to the forces generated by crashing down off a wave, and a beam right through the hull under the mast (side to side) should support the foils adequately such that the hull effectively hangs from it when foiling. I can do tests with cardboard models and weights to check the required relative strengths of different parts of the boat before doing any of it in CF.]

It's going to be a year or two before I'm ready to start building this thing, so at the moment I'm really just planning ahead and looking to see what might be possible. I currently work in programming and intend to get out of it once my current project is complete because being tied to a desk is an extremely unhealthy way to live. There are quite a few things I'd rather be doing, and many of them involve building things where carbon fibre is likely to be the best material to use. I had not imagined until the last couple of weeks that carbon fibre might be a practical material to work with in a small workshop without needing to buy hundreds of thousands of pounds worth of equipment, and this opens up all sorts of new possibilities for me. Another of the things I want to build is an enclosed recumbent cycle with electric assist, so that might be a better thing to work on as a first project as the pieces will be smaller. So many options, so it's hard to choose! But I definitely think this could be my future. What I wanted here, in this thread, was to explore whether my idea of building a weight-saving CF boat in a home workshop is practical or if it's just a pipedream. The answers make it look as if it may be viable, so that tells me it's worth putting in hundreds of hours doing a lot more research and acquiring the necessary knowledge and skills. I can revive this thread if/when I start preparing for the actual build and post pictures to show how it goes. Sadly I need to get my mind back onto my software project in the meantime so that I can cash in on it and raise the funds to start a new life in CF.

Edited 10 Years Ago by David Cooper