By Michael Leneman, Multi Marine
What is the simplest way for a
home builder to build a good, light hull for a catamaran or trimaran? A few
years ago, we set about looking for an inexpensive way to construct a small
trimaran that we had developed as a prototype. The answer we came up with
was unique: to combine a fiberglass molded "pan" with plywood/glass/epoxy
|Figure 1 - A completed L-7 at the dock, Multi Marine’s
new 23’ folding trimaran kit features manufactured hull pans. The
builder attaches plywood topsides to the pans.
It is very time consuming to build and fair the three hulls of a trimaran.
So our concept was to make the exacting, complex, curved, below-waterline
shape of the hull, the pan, in a mold. The simpler, above-waterline shape
can then be formed in easy-to-make plywood/composite panels. This
combination results in lightweight hulls which are easy to build. The
plywood/composite panels bend to a nice, fair curve so that almost no
laborious fairing is required. The pre-molded fiberglass/epoxy pan is, of
course, already fair.
From this concept, Multi Marine's new 23' folding trimaran, the L-7, emerged
(Figure 1). The home builder can purchase pre-manufactured
fiberglass/epoxy-molded pans for the main hull and floats from Multi Marine,
and then join these to plywood/glass/epoxy topsides that he or she builds.
The full kit includes the fiberglass/epoxy pans for the main hull and
floats, glass-pultruded I-beams (tapered), pultruded "C" channel for the
x-arm boxes and daggerboard, mast kit, rigging, sails, plywood, fiberglass,
and foam. West System® epoxy, rollers, squeegees, and other accessories can
be bought from a local dealer or West System, Inc. The full kit costs less
than $19,000, including a mainsail and jib. Any item may be purchased
|Figure 2 - The pan and topsides. The
plywood topsides have been glued to the bulkheads. The next step is to
install the butt blocks, to which the pan will be attached.
The floats (amas)
For the floats, the builder starts upside down with the deck and installs
trapezoid shaped, "picture-frame" bulkheads. The okoume marine plywood
topsides, which have already been fiberglassed on the inside, are then
installed (Figure 2). Next, the butt blocks are put in and the fiberglass
pan is epoxied in place (Figure 3). Once that is done the whole outside of
the hull is fiberglassed. An amateur builder can make this float in less
than 40 hours. We have built 6 floats like this and there is virtually no
fairing... just some microballoon passes where there is a joint.
|Figure 3 - The epoxy/fiberglass composite pan is glued
to the topsides. Fairing is required only along the joint before the
outside of the float is glassed and coated with epoxy.
The main hull
The main hull (if you're building a trimaran) is done a bit differently. The
full bulkheads for the main hull are built first and placed on a strongback.
Then the fiberglass pan is glued to the bulkheads (Figure 4). The butt
blocks are glued to the pan (not the topsides, as in the floats). Lastly,
the topsides are put on.
After everything is set the builder turns the hull right side up, levels the
sheer, and puts on the decks and cabin. The cockpit floor, decks, anchor
locker floor, cabin side seats and lazarette floor are all flat and are
pre-made with foam, glass and plywood. There is basically an entire
mid-height sheer web that runs through the entire main hull.
For the main hull decks, cockpit floor, anchor locker floor and seats, we
use a combination of thin plywood, styrene foam and glass. High-density
styrene foam is not usually used in custom boat construction because
polyester resin eats the foam, and styrene foam is susceptible to pressure
dings. However, West System epoxy resin works great with styrene foam, and
bonding a thin layer of plywood to the top face of the composite panels
eliminates the pressure ding problem. A composite panel made this way
(ply-styrene foam-glass) is very stiff and light. The best part is that the
cost is about 1/3 that of a standard urethane foam-cored, glass composite
The X-arms are one of the coolest parts we came up with. They are fiberglass
pultruded I-beams. The best part is that you can cut the sheer web of the
beams and bend the caps down to make a nice looking, tapered, outside shape
to the beam. The beams look good, they are pre-made, they can't corrode,
they are strong, you can paint them any color you want, and they are
|Figure 4 - The main hull is built by first attaching
bulkheads to a strongback. Here, the pan is glued to the bulkheads
before butt blocks are glued to the pan. The topsides are put on last.
Rigging and sails
To fill out the rest of the basic boat, we extruded our own mast and
designed our own rigging with the added feature of a roller-furling boom.
Full-batten mainsails lend themselves to being furled around the boom,
especially since we use an inexpensive round aluminum tube as our boom. With
a main, jib and reacher, the boat is about the same speed around the race
course as a stock F-31 trimaran with a full inventory of sails.
The Eko-Cat 23
So, now you're sitting around looking at the trimaran floats, and you say to
yourself, "gee, those would make nice catamaran hulls if they just had a
little more freeboard in the transom." Well, this can be easily done since
the topsides of the floats can be changed in a second. It is only the
pan/topside joint that has to remain the same. We have made a few small
power catamarans from old beach cats, so making the power Eko-Cat 23 from
these hulls turned out to be very easy. The mileage is incredible. An 8 hp
motor makes 12 knots and over 20 miles per gallon in flat water. With a 25
hp motor, the boat does 20 knots.
|Figure 5 - A completed L7 trimaran. When folded, the
L-7 is 8’4” wide and easily trailered.
Reprinted with permission from Multi Marine’s from Epoxyworks Magazine Issue
For more information on both of these projects, you can contact Multi Marine
at www.multimarine.com or 310-821-6762.
All contents are copyright (c) 2007 by
Northern Breezes, Inc. All information contained within is deemed reliable
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