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 topsides.
||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.|
Folded beam: 8'4"
Mast height: 33"
Sail area: 370 sq ft
Weight: 1300 lb
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
he or she builds. (The full kit includes
the fiberglass/epoxy pans for the main hull and
floats, the 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
The floats (amas)
For the floats, the builder starts upside down
with the deck and then 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 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.||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.
|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.|
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 panel.
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 inexpensive.
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.
For more information on both of these projects,
you can contact
www.multimarine.com or 310-821-6762.