Weight of wood used for Mirrors

Those numbers on the kind of wood supplied in Mirror kits is exactly what I needed - it turns out that it is extremely light stuff. I was wondering if it might be best to use carbon fibre for the whole hull, but it looks as it would be hard to save a lot of weight there due to the weight of resin that would be taken up by the core (which I suspect will be needed between the CF layers to provide sufficient stiffness). It may be just about possible to get it down to 2kg/m² without it being too weak, but that would be using a startlingly small amount of carbon fibre cloth. A bigger gain could be made with the transom as it should be much easier to make a thiner and stronger piece to replace that (with structure used only where it's actually needed), and that's why I'd like to know what it normally weighs. I think it might just be possible to get the hull weight down to 35kg - that would make a substantial difference to the ease of lifting the hull, but it might turn out to be too weak. (Remember that A. J. MacKinnon had to lift his Mirror out of a canal without help from anyone, so any weight saving here is a huge gain if you end up in such an extreme situation.) A more radical option though would be to make a two-piece hull that can be taken apart - that would likely lead to a heavier boat rather than a lighter one, but with two halves that could perhaps be only 25kg each and dead easy to lift and carry. Now, that would be a fun build!

I was thinking about splitting it lengthways rather than sideways, but it's now obvious that doing it sideways would enable it to be lighter and would avoid disrupting the structure under the mast (and also where the rudder attaches) so it's a much better idea. Good to see that it's been done before with at least one boat. The best plan would be to have the two parts slot together vertically in some way such that they don't need to be bolted together below the waterline or inside tanks. It should be possible to have them held together by shape with just a couple of bolts at the top to prevent vertical movement in the slots. Probably best to use metal for the slots, but the distance of travel in the slots needn't be more than an inch, so the metal parts needn't add a lot of weight. Even so, it could easily end up with two halves each weighing 30kg. I'd obviously prefer to have the whole boat weighing 30kg though if it can be done, so I want to keep exploring the carbon fibre route first.

I've been looking at different core materials. Nomex honeycomb is awkward to work with and hard to stick on properly, so delamination could strike at any time. Closed-cell foam cores are good if they're thick, but for a lightweight Mirror they'd need to be thin. They use the same amount of epoxy to glue them in regardless of how thick they are, so the gains simply disappear if they get too thin. M200 uses less epoxy than M80 because the holes in the surface are smaller, but it weighs so much more itself that it again cancels out the gains. With 2mm Soric (which makes construction easiest) you already have 1kg/m² once the epoxy's filled all the voids within it before you've even added any layers of carbon fibre cloth, so it doesn't leave much scope for getting a low weight structure. Without any core you get something so thin that it suffers from a serious lack of stiffness, so a core is essential. Stick a sane amount of CF cloth round Soric and the weight instantly goes up to the same as plywood, though likely with greater strength - I'm thinking of 300gsm of biaxial cloth on one side of Soric, 200gsm plain weave on the other, then 200gsm of kevlar/CF (which is vital to avoid dangerous shards being generated in a smash). Once the epoxy's infused through it all, that would get to a weight of 2.3kg/m². It may be a fair bit stronger than 5mm marine ply though, so perhaps only strips of Soric could be used instead of a continuous core, relying on the greater stiffness of thin materials on a small scale to hold their shape between the reinforced sections. By doing this it may be possible to get the skin of the hull down to 1.6kg/m², making the idea of a 30kg Mirror hull look just about achievable if the strength of a hull built like that turns out to be sufficient. By this method, something more like a frame would be built by the periodic inclusion of Soric strips while the areas in between them would act like canvas stretched between the reinforced areas. The strips would run primarily vertically on the sides of the hull and from side to side across the floor, while support lengthways is often already there due to the thicker gunwales and effective flanges where tanks join and where the surface bends round corners. I'll need to do experiments to see if this construction method can create viable structures.

That would involve carrying extra heavy equipment which I'd rather avoid, but I'm also thinking about the need to carry the boat past rapids over rocky ground, so there really is no substitute for getting the carry weight down. (I do also harbour a few crazy ideas though about making a foiling Mirror with canting T foils attached to the sides and two long rudders with winglets at the back, but that's another business altogether.)

I've seen people lifting currachs with considerable ease (on TV) and carrying them over their heads. The bulk and shape is not as awkward as a Mirror, but weight is clearly the key factor in enabling this. A 30kg Mirror dinghy (which isn't necessarily possible to build) should be 1.5 times easier to pick up and carry than a 45kg Mirror regardless of any complication caused by the centre of mass being some way to the side of you. That distance certainly adds to the problem, but it adds it in direct proportion to the weight.

I've thought of another way to increase the stiffness of a light hull thanks to another thread here, and that's to pump air into the tanks to perhaps 1.05 times atmospheric pressure. You would only need to do that if going out in rough conditions. Even if the hull is of fully normal strength, this would be a good way of making it more robust in a heavy sea, so it would be worth designing that in anyway. (You can see the effect if you get a plastic bottle and try squeezing it first with the lid off, then with the lid on - in the latter case, you feel the resistance go up dramatically and soon reach the point where you can't compress it any further, even though the pressure inside has not gone up massively.)

I'm not worried about having to remove the mast as that takes little time. The important thing is to make possible some journeys that would otherwise be impossible due to the need to carry the boat over rocky ground (I'm thinking boulders here) to get past a rapid.

Your point (Gernot) about damage to the hull in shallow water is a good one - carbon fibre is the worst material to use for a hull in that respect. Any river journey would need to be planned carefully in advance with a lot of checking on Google Earth when the river's low (I don't know if it's possible to see different versions taken at different times though - in most cases they probably only shoot it once) so that all hazards can be noted on maps. I don't like the idea of scraping the hull on anything anyway, no matter what it's made of - it's something you just don't do with a sailing dinghy. In difficult waters it would be worth attaching a frame to the bow to feel for rocks and stop the boat before you run onto them - not much use at high speed, but it would be good for seeking out temporary parking places in the shallows.

I'm not planning to build anything of this kind any time soon, but I do want to give it a go within the next few years. I might make a wooden Mirror first and use it to make a mould for the carbon fibre hull. I have a lot of learning to do first about engineering and building with composites, so it'll likely take a couple of years of planning before I start building anything beyond test pieces.

No connection with me, but you should really be thinking the other way round and NOT let them in if you find that it's an existing member wanting to join again under a commercial identity (unless they explain their reasons clearly and have a very good excuse for doing so). What may have happened though is that someone working in that industry has read this thread after happening upon it via a Google search and wants to make some helpful suggestions here, or it could be a spammer using an automated system to hunt out possible targets to try to sell to. The only way to find out is give them a go, keep a close eye on events here, and then block them and delete things after the event if necessary.