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        Hard Dodger - Composite Material Selection


        This is the second post on building a hard dodger, go to building a hard dodger for the first post.

        Looking into building a hard dodger for my catamaran, I've entered into the composite material labyrinth. When you deviate from wood, there are quite a daunting number of products and combinations of them to choose from. That said, the advantages of composites are clear, well tested and documented. The primary advantage of composites are weight and maintenance, because the displacement, whether it be a planing, semi-displacement or full displacement hull, is paramount to stability, speed and sea-kindliness. i.e. you overload and you're going to have issues and lighter equals faster.

        Front View of hard dodger 1

        Core Selection

        There are quite a number of cores to choose from starting with wood and going through to foam and honeycomb. For the current project I've been looking into foam because it's easy to shape and light. Yes, I will probably have to make the supports a little thicker but bear in mind that water resistance is many times wind resistance, and a catamaran because of her slender hulls is sensitive to weight. Air density is 1.225 kg/m^3 (0.076 lb/ft^3) whereas sea water is around 1030 kg/m^3 (64.3 lb/ft^3). So to state the obvious water is going to slow you down a hell of a lot quicker than air. But then, as always, it's a trade off.

        With some help from Bruce and friends in Bangkok, I've come down to the following using 6 lb/ft^3 (90 kg/m^3) Divinycell T100. I was looking at Last-A-Foam but read some bad reviews on the aircraft building forums of it turning to powder over time, not sure of the reasons for this but I guess it could be UV and/or vibration. Besides, the price difference isn't all that much. Divinycell is however, cross linked, so needs to be thermo formed if you're looking to bend it to shape or alternatively you can buy scored sheets which have which have various types of cuts into the board to allow it to bend to a curve.

        I would love to be able to sit and calculate the necessary size supports and combinations of materials that'll handle the loads to cut down the weight a bit, but unfortunately the amount of time to do this is just not feasible, so I've simply taken an example of a hard top build for a Prout catamaran provided by Bruce and friends.

        Composite Panel Skin thickness

        For skin, I'm looking into two layers of bi-axial cloth 600 gm/ 20 oz . Here again, besides varying the number of layers, there is a mind boggling amount of possible variations in choices in cloth; thickness, weaves and types.

        Composite Dodger structure Weight

        This is where composites shine brightest. For 3500 lb/in^3 (96 kg/cm^3) of foam we are looking at a weight of around 13lb/5.8kg. Man, don't you wish they would get rid of the imperial system?, problem is living in America all my measuring instruments are now imperial, with numbers like 2'3 6/8ths"+6'1/4" and the like cluttering my head, I thought mathematicians and scientists where about efficiency and results, well there is little doubt which is more efficient system. Ban imperialism!! But I digress. The structure has a

        Composite Dodger Structure Cost Estimate

        I've come up with the first breakdown of cost for the supports based on the surface area of the panels and the area, to work out how much foam, glass and epoxy is needed. This is only for the structure and solar panel supports. There will be additional costs for, material for covering the sides, thin plywood for some covering, paint, primer, solar panels, and mounts/bases for ancillary's I would like mounted e.g. radar mount, hooks for hanging lines, building in wire conduits, instrument and gage mount panel etc. Figured I might as well make use of the additional space.

        I'm looking at around 3500 in^3 of foam and about 6000 in surface area for glass and epoxy. My costs are based on the current price of foam and a write up by west system that lays out the costs for the skin, this plus 20% fat. Odd pieces of Foam can easily be scarfed and jointed together so there should be less wastage than with wood, esp. if you want a nice even grain with wood.

        Costs are estimated in USD to be $970.

        Foam $625, Glass $175 and epoxy $70. Of course we can guess another $100 for the bits and bobs, like mixing and protective stuff for the epoxy. I'm told if you don't wear protective gear with epoxy, your babies will come out with at least three eyes.

        Here are my calculations so far

          qty Length width height Volume in^3 Surface Core Weight Glass & Epoxy Weight Cost Cost Cost
                          Epoxy Glass Core
        width beams 4 53.5 0.5 2 214 539 0.743055508 1.162023217 4.46932 13.4079602 31.65416667
        length beams 6 58 0.5 2 348 876 1.208333256 1.888557214 7.263682 21.79104478 51.475
        Sit lap length 6 51.5 1 0.6 185.4 498 0.643749959 1.073631841 4.129353 12.3880597 27.42375
        Sit lap width 4 56 1 0.6 134.4 360.8 0.466666637 0.777844113 2.991708 8.975124378 19.88
          2 135 1 2 540 814 1.87499988 1.754892206 6.749585 20.24875622 79.875
        Perp 3 82 1 2 492 744 1.708333224 1.6039801 6.169154 18.50746269 72.775
        cross supports on arch 4 57 1 3 684 924 2.374999848 1.992039801 7.661692 22.98507463 101.175
        forward 2 40 2 2 320 328 1.11111104 0.707131012 2.719735 8.15920398 47.33333333
        forward side 2 33 2 2 264 272 0.916666608 0.586401327 2.25539 6.766169154 39.05
        back 2 42 2 1 168 256 0.583333296 0.551907131 2.12272 6.368159204 24.85
        back side 2 40 2 1 160 244 0.55555552 0.526036484 2.023217 6.069651741 23.66666667
        Bases 8 1 2 1 16 40 0.055555552 0.086235489 0.331675 0.995024876 2.366666667
                  3525.8 5895.8 12.24236033 12.71067993 $48.89 $146.66 $521.52
                    weight total 24.95304026 $73.33 $175.99 $625.83
                            20% fat and extra for epoxy

        The figures I'm working off are:

        Density of foam lb/in^3 0.003472
        Epoxy & glass weight per lb/in^2 0.002156
        Core Cost per in^3 0.147917
        Glass cost per in^2 surface 0.024876
        Epoxy cost per in^2 surface 0.008292

        I'll be updating with posts as this project progresses, we'll get to see how the estimates on cost and time match up with the actual.