10" f/8 Marine plywood tube revisited.

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The inherent flexibility of my beam and pot design continues to nag. I am tempted to add plywood braces to the primary cell pot support area to spread the load and kill the torque flexure with changing attitude. However, the present, all-aluminium construction would be compromised by the addition of plywood.

Thin marine ply makes an excellent, lightweight, telescope tube if laminated from two [or more] layers of thinner ply. In Denmark marine ply is known as aeroplane plywood and available in thicknesses of 1/2mm upwards. I rolled a 6" tube for my 5" home made achromat 30 years ago from 2 layers of 1mm.[From memory] It was so lightweight I could hardly believe it possible.

I rolled the thin plywood on a series of plywood circles [stops or baffles] using Cascamite waterproof glue for laminating. I remember the difficulty of bringing the long edges perfectly together to close the gap. A perfect [carefully achieved] dry fit changed with the addition of the glue. So there was a slight hump where the edges could not be pressed down quite perfectly. The tube ended up very slightly pear shaped. Spreading the glue over such large area was a nightmare! A 30cm tube would be much, much worse. Epoxy resin might be a much better bet.

The larger radius 30cm diameter tube might better suit 1.5mm laminations but I have no idea as to its flexibility in this thickness. I would need to find a model shop stockists and apply some bending pressure to various samples.

I believe from searching online that this special ply is only available in 1.5m x 1.5m sheets. [Roughly 5'x5'] A standard ply sheet size of 1220 x 2440cm [4'x8'] would be more economical and avoid a seam. I had exactly the same problem with the cardboard tube. Which ended up much heavier after two layers of concrete form tube were glued together to achieve the final length. I shall have to do some calculations to see if the weight and cost of the plywood makes the project completely pointless.

1m^3 of marine ply  = ~700kg  Aluminium has a density of 2700kg/m^3
Or nearly 4x the weight of the densest Birch plywood!

Area of tube material = Circumference x Length.

Volume of materials = Area x finished thickness = [3mm or 0.003m]

30cm tube = 0.03m diameter x Pi [3.142] = 0. 94.26m

x 2m = 1.8852m^2  x 0.003m =0.0056m^3

0.0056m^3 x 700[kg/m^3] = 3.92kg.

So a bare tube, 2m long, made of 2 x 1.5mm layers of laminated plywood weighs about ~4kg or about 9lbs. The weight of the adhesive/resin required shouldn't amount to much.

The bare 30cm tube would probably need some local reinforcing. Cross-grained circumferential strips laminated around both ends of the tube would help. The plywood tube would be no smaller than the present cardboard one but would be about 1/3 the bare weight! It's just a shame the sheets are only 1.5m square. When I really need 2 metres long for minimum wastage and to avoid the extra work required in laminating it up from smaller sheets with close [hopefully invisible] joints.

My attempts to get local builder's merchants to source me some 1.5mm aeroplane/marine plywood has been an exercise in frustration. I am being quoted £100[equivalent] per sheet for delivery alone for just sending it half way across Denmark. I can halve my material costs by buying from Holland. The stuff is made from birch veneers in Finland so it probably passes through Denmark on a lorry to reach Holland. £100 a sheet for delivery does not compute! A plywood scale model of the Concorde would cost more than the original aircraft! The Dutch supplier suggests on their website that they can roll their thinner plywood sheets and pop them into a cardboard box for compactness during delivery. A simple skill obviously not found in the Danish suppliers!

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