Small Catamaran in GRP flat panels

Discussion in 'Fiberglass and Composite Boat Building' started by Jaco, Oct 17, 2021.

  1. Jaco
    Joined: Sep 2021
    Posts: 4
    Likes: 0, Points: 1
    Location: South Africa

    Jaco New Member

    Good day,

    I am interested in building a small catamaran using GRP flat panels for myself. Which design would be best suited for this? Two designs that I like is the Chat 18 (Richard Woods) or the Eco 5.5 (Bernd Kohler).

    Can one just take the plans (for plywood construction), and replace with GRP flat panels? What core density 60kg / 80 kg, core thickness (8mm??) and fibreglass would work? Would the flat panels be much heavier than the plywood?

    I am still in two minds about using marine ply versus GRP flat panels. Although the flat panels adds complexity in the build, I do like the "maintenance free" approach of the GRP (GRP does not rot, as neglected marine ply might) Or is the cost of the added complexity of flat panels not worth it?

    Your time, effort and suggestions will be greatly appreciated,
  2. fallguy
    Joined: Dec 2016
    Posts: 7,796
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    Location: usa

    fallguy Senior Member

    No, plywood core is stiffer than foam core, so the laminate changes. A plywood core is cheaper to laminate because epoxy and glass is less.

    If you build in plywood, as long as all hull penetrations are overbored, filled with pb epoxy and rebore and then sealed; you have little chance of trouble. If the bottom is damaged; it must he dried and repaired.

    The easiest way to build for you is plywood. You will love the easiness of ply.
    bajansailor likes this.
  3. sailhand
    Joined: Jan 2017
    Posts: 142
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    Location: australia

    sailhand Senior Member

    A plywood core is much more expensive to laminate as you should really use epoxy which is three times the price of vinylester and 5 times the price of polyester. The foam will generally require heavier laminates but overall glassing costs are nearly the same due to cheaper resin costs. I would never build anything from ply again. Foam is actually now cheaper in Australia than a good quality marine ply with no voids if you can find it. Depending on the size of the project foam is a lot lighter than ply. If building small, say sub 16 foot, boats in bare ply with short unsupported spans, then ply is the lightest if using stitch and glue and not glassing the entire surfaces.
    The real difference though is in longevity and maintenance and resale value. A foam boat will basically last forever unless you burn it. I have had or been involved with many ply boats and the rot spores spread really quickly. On large vessels this can feel like you are constantly rebuilding your boat just to keep on top of the rot. There are many other factors to consider its really not a simple black and white answer. A nice ply dinghy or kayak kept out of the weather in the rafters of a shed away from termites could last forever, out in the weather with constant heavy use or getting bashed around at a dinghy dock, ply will definitely need more maintenance at greater expense than foam. Foam has almost no waste at all when you build as all the scraps can easily be buttjoined with a polyurethane glue and used up. You only need a stanley knife or a cheap handsaw to cut and shape it and you will have lighter panels compared to ply as your unsupported spans increase in distance. Resale value is where foam really shines compared to ply.
    It amazes me that people think they are saving money building their own designs from ply. A recognized design from a reputable designer will attract much more interest/money at time of sale and there is usually no cost involved in fact its usually a saving in total costs overall. Good designers will save you a heap of money during the build phase and on larger vessels you get engineering you can rely on. They can save you so much weight and structure, expense, build time etc that you actually save heaps going to a designer in both time and money. In saying that I have designed my own boats several times now, however I did this as a hobby which is what a lot of people on this forum do. I would still go to a designer with my own plans unless you have many years experience in boat building and engineering yourself. They can turn your dream into a reality with a lot less heartache and expense in most cases and you still get the design that you dreamt of.
    If your desire is to build and experiment with your own design I would still use foam just for the speed of build if nothing else. No framing, scarfing etc etc just cut your foam to the required shape and glass one side, offer up to the temporary frames etc and glass the other side. When joining panels I use a drywall screw about 2 inches long after placing a polyeurethane expanding glue in the joint. Leave for about two hours and remove screws. Sand or router an external radius on the edges bog the screw holes and glass the entire external surface. Much quicker than my experience with ply which involved frames and stringers planers and jigsaws etc. If you are a super keen woodworker that loves the look of a beautiful timber grain clear finished to a high gloss then go ply and enjoy it.
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