Building a hard Bimini, hard Dodger

Why a hard dodger?

I have to ask myself before I do a project on my boat, are you doing this because it's cool or because you need it, and is it really top priority? After much contemplation, I've decided that this project is a go for a number of reasons:

  • The bimini that is on the boat is so old, the canvas will surely blow off in a storm. And it's badly installed, i.e. the supports are right where you climb aboard.
  • I picked up four 120 what solar panels and have to put them somewhere.
  • I need something to hang lines on, mount some instruments and I need to put up the radar pole and mount the GPS antenna (aerial) somewhere. A hard top will solve a lot of this.
  • The current dodger although it can, never get's folded down. Partly because of the effort and partly because I'm scared it'll fall apart if I do.
  • I'm in Mexico and have access to a relatively cheap carpenter who has GRP experience.

Materials for the Bimini

First off weight is a big issue on a catamaran, so this is a paramount consideration in material selection. The solar panels weigh 11kg's each, 24 lb each. Not only is a catamaran's performance very weight sensitive, but putting a lot of weight up high is a bad idea in terms of stability. This is where a number of doubts arise. I'm no engineer, so calculating what strength is needed for the lightest possible materials is, unfortunately, mostly guess work for me. I've looked at a number of materials namely:

  • Wood, mostly to heavy unless I use end grain balsa as a core, and lay it up with fiberglass.
  • Foam's, this is the most likely choice at the moment. There are quite a number of different foams available. PVC foam, Last-A-Foam, Divinycell, Core-Cell etc. I'm still in the process of investigating which is most applicable but it's looking like Last-A-Foam.
  • Honeycomb, this has a number of options but I think, working with this type of material is beyond the skill of me and the carpenter.

It looks like Last-A-Foam is the best candidate at the moment, but selecting the material is only the first step. The foams come in different densities, thicknesses and I'll need to decide what skin (fiberglass) thickness to use. On top of this you have a number of layup methods to choose from. From worst to best, hand layup, vacuum bagging, resin infusion, etc. Without the use of a workshop I'm going to be limited to hand layup and this is a consideration in the configuration of material properties to be selected, in that, a hand layup's bond is reputedly not as good as the more advanced lamination methods, but it'll have to do.

I found some information on what boat builders use for the deck, hull and sides i.e. core thickness, skin thickness, core density and am using that as a rough guide for sizing materials for the supports and panel frame. Fortunately this last-a-foam apparently can be worked much the same as soft wood with standard wood working tools, so that'll make the job a little easier. We'll see.

Hard Top Structure

Besides all the guess work in materials there is some more to be done in designing the supports and frame. Here's my first mockup using a free program Google sketchup.

So far it's just a rough mockup, but using sketchup helps a lot in figuring out where the supports are going to go by drawing in the obstruction like the aft cabin hatch, boom etc. I can also use the angels and measurement help in the building the frame separately to be assembled later. This also helps with the aesthetics of the design.

As for the structure, I've tried to position most of the weight in the center, as you can see with the second sketch the panels are as centered and forward as possible. Important for me is space to get in and out. This seems obvious but the current bimini's supports cross exactly in the wrong spot. The supports will need to handle slamming in all directions and waves dumping on top, so they are going to have to be strong indeed. My thought here is to use oversized foam core 2 inches or more and angle the supports forward and back as well as to the side, to handle side loads although I would imagine these to be less in a catamaran.

Removing Solar Panel Frames

Well, this turns out to be more difficult than I thought. I figured I could save a bit of weight by removing the solar panel frames and placing them within a foam frame. After an unsuccessful first attempt I figured the frames must be glued on. I sent and email to Kyocera and got a quick reply. The frames are in fact glued on as well as screwed together. The glue used is called butyl rubber which to top it off is quite heat resistant, we are talking a number of min at 210 degrees which is bound to damage something before the goo loosens, so that's out. But looks like I can use a solvent like MEK, Acetone, Xylol, Xylene etc. How to get the solvent into that tight fitting frame… not to mention the screws are corroded stuck. Will update the post with the how it goes.

Bimini Cover

For the covering on top and for instrument compartment on the front, I was looking at using very thin plywood, epoxy coated and painted. Although I would like to keep this to a minimum as the more, permanent, solid surface the more windage and with a catamaran, that doesn't have the pronounced keel and lead weight hanging off the bottom it is rather sensitive to windage, which can make docking a challenge, too much windage will also affect her windward performance. For the sides I'm looking into some sort of fabric, with those clear windows sewed in. Probably in a rollup or roll down clip on arrangement.

Well that's all for now, I'll be posting updates as this project progresses.

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.