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:
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
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.
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
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.
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.
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.
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
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.
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
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
The figures I'm working off are:
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.