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.