paper was presented to 1992 Marine Applications of Composite Materials
Conference. I see that all the graphics
and diagrams have fallen out during up-grades of Word versions. They will be found and re-inserted soon.)
FOR RAPID MOLDING OF COMPOSITE MULTIHULLS
'92 CONFERENCE PROCEEDINGS
Kurt Hughes, Kurt Hughes
Composite one off multihulls
have tremendous amounts surface area for a given volume. Consequently considerable time is spent in
molding a composite multi. Builder's
attempts at reducing the molding time are usually spent fairing the work later
on. Any strategies that can reduce
multihull molding time and keep the quality of the work high are desired.
Mold building is a significant
time expense of any composite project.
With a multihull, the problem is particularly acute as it will have
twice the number of molds, and a larger total mold surface area than an
equivalent sized single hulled boat.
Beginning in the Fall of 1988 I
began experiments with possible strategies to mold composite multihulls without
molds or with minimal molds. The
resulting project, a 40' trimaran, was built with two of the following
strategies and will be launched whenever I get time away from the office.
Since that time several other
multihulls have been built using these ideas.
The fastest possible way to
build a one-off multihull would probably be stereolithography. Stereolithography is where X, Y, and Z-axis
lasers, driven directly from the CADD software, beam into a vat of polymer to
form a part. For several reasons
however, this is not yet the answer.
One has to keep looking to find faster ways to rapidly build composite
Building a composite boat is
outside of the nature of materials.
Unlike plywood for example, the amorphous blobs of resin and fabric must
be molded to be useful. One of the
things I do as a designer is look for industrial processes or the results of
industrial processes that already do what I need.
One example is a plate glass
molding table used for molding large powerboats. Large boatyards already use the plate glass molding tables to
create large fair compos site panels.
Both of my experiments were
very successful. The molding times of
the hulls were trivial, less than the time to wax, buff, and pva the mold
itself, not counting a couple of diversions.
The first molding system used
thin tortured plywood to develop the hull shape. Next core was vacuum bagged onto the inside. Finally fabric was bagged both inside and
outside. While not true molding, it did
rapidly define a proper fair surface, and the composites "froze" that
The second system used Cylinder
Molding as a female mold. First, thin
sheets of 3mm plywood had a smooth molding surface put on them. These sheets were then vacuum bagged
together using the Cylinder Molded technology.
The plywood sheets with smoothed surfaces inside were bagged to another
layer, giving more thickness and strength.
The two sides of the mold-to-be were wired together along the keel and
stem with copper wire. The keel where
the two mold sides were sewn together got a cove of structural "bog"
placed in it. Bulkheads were then glued
to the mold on the outside. The result
was a rather traditional 40' female mold in very little time.
The final strategy will use
formica or thick mylar surface spanning former stations on perhaps 16"
centers. The fabric will first be laid
up flat on the formica or mylar, then the whole thing moved onto the forming
stations. The port and starboard halves
will be attached using traditional stitch and glue keels.
I did see multihull hulls and
panels being built in a similar way to this in Britain on my last trip there.
Those hulls were laid up with
no rocker, like a uniform U shaped formica section. To achieve rocker the appropriate bits were cut away from the
forward and aft ends of the keel area.
To force the lot to come together, transverse slits or cuts were made
all along the bottom 15% of so of the hull.
The result was a totally unacceptable level of fairness and repeatedly
chopped full-length fibers. Of course
it is well known that full length properly oriented fibers are the keys to
strength and stiffness.
I see perimeter stiffness as
the key to making this work. The keel
edges must be kept from buckling as they are sewn together.
COMPOUNDED PLYWOOD WITH CORE
The main hull on the project
trimaran was built using this strategy.
It is a system using no mold, unlike the next two, which are rapidly
built molds. Since there was only one
main hull but two amas used in the project, it did make more sense to use this
system for a main hull.
In this process a thin plywood
hull is built using traditional stitch and glue systems. One stitch and glue system is Cylinder
Molding, where two thick nesses of thin plywood are vacuum bagged
together. The resulting panels are full
length and have a transverse curve in them.
The other system that can be used is StressformTM or tortured
plywood. In both systems the identical
port and starboard panels are cut to shape, wired together, spread out and an
epoxy-roving glass and filler keel is poured in. After the keel cures, the spread sheers are brought back in to
final shape with a deck flange. This
will be the final hull shape. Like a
large three-dimensional batten it will be fair and smooth. A composite stem is next glassed in.
The hull panels are built using
traditional CM or Stressform TM techniques. Refer to the Cylinder Mold Construction for
First verify that there are no
holes in the panels. See that the scarphs do not have holes or gaps in
them. Any nail holes or rough scarphs
must be filled with epoxy and bog.
Sand these filled areas smooth.
Any glossy epoxy found on the panel must also be sanded.
Naturally the keel area has
many holes. These are dealt with during
the keel pour.
The keel pour will be done
nearly the same way as a stitch and glue hull using no core. Since these thin ply hulls will usually be
much larger hulls than the ply thickness would suggest, the keel fabric must be
carried farther onto the hull than one might expect. That puts the turn of the bilge out farther. The keel fabric edge transition must also be
more gradual than with a thicker plywood hull.
Both these items are covered in the plans or revision sets. The keel fabric must be placed with peel
ply on top. The peel ply surface and
any bumps must be roughened and knocked back with a small grinder.
The sheer timber will be
installed exactly as on a traditional stitch and glue hull.
The deck flange will be built
and installed exactly as on a traditional stitch and glue hull.
Typically thin plywood hulls
using core will not have stringers.
I am assuming that the core
will be bagged into the inside of the hull.
While core could be bagged to the outside, the fairing job would be much
bigger and since contour core will probably always be used, the scrimm would be
on the wrong side.
Once the thin plywood hull is
compounded to exactly the right shape, the core materials will be cut to shape
and dry fitted.
The core to be used must first
be cut to proper shapes if necessary, all pieces fitted, oriented and each
piece numbered. A dry fit should have very piece to be used in place. A typical core piece will have the number,
orientation and side of the hull noted.
The same steps are done to the
On my project the main hull
plywood was 3 mm Asian luan. The hull
was built using the tortured plywood method.
In fact the folded-up hull was so delicate and fragile that a fan could almost
make it flutter.
I had expected two problems
that did not occur. I worried that the
vacuum bagging might distort the folded up hull. It did seem that the vacuum pressure should press equally on both
faces, but I recall suggestions advising a very robust mold for vacuum
bagging. In fact there was no
distortion at all.
I also was not sure that a
tortured plywood hull this large would have port side exactly match the
starboard one. The largest tortured
plywood hull that I knew about was also 40' long but had very low freeboard,
unlike the 6' wide panel that I was using.
Outside of the standard barely visible recurve in the topsides, there
were no problems controlling the fold up hull, nor in symmetry. The standard requirements of the opposing
panels being exactly the same species and density still apply.
The molding portion of this
hull, building a 3mm thick hull, took about 15 man-hours to do.
On this project I used
1/2" thick contour core balsa, with the Al600 coating. As the core shear of the balsa is between
235 and 360 psi,(1) I worried that the plywood skin would be vulnerable to
rolling shear failure and be the weak link in the layup. The core shear of the plywood was in fact
tested as 713 psi(2) and should not govern.
The fabric used on this project
was Heinsco Rovelock unidirectional E glass.
Each layer was 9-ounce weight and about .009" thick. It is a very high strength glass using
MACM '92 CONFERENCE PROCEEDINGS
polymer fibrils rather than
"slug tracks" or stitching to bind the strands. We were rather beta testing the product and
did have some problems. The polymer
holding the glass together had some sort of memory and was very difficult to squeege
down. Every square foot of glass layup
had perhaps a half dozen little moguls that could not be worked down. I thought the vacuum pressure would help but
I discovered that the fabric arrangement is so dense that air would not
permeate through the fabric, not even under vacuum. I did finally have to use a
dremel tool to take out the
hundreds of memory bubbles in the layup.
The layup was a single layer in
the 0 degree direction and one in +/-45 degree directions. My calculations showed that it was overly
strong(3) in the +/- 45 directions. The
same layup was used inside, vacuum bagged over the core, and outside, without
The two serious concerns that I
did have about this method of non molding composite boats turned out to be
groundless. I consider it to be a very
rapid and successful way to build composite one off multihulls.
THE CYLINDER MOLD FEMALE MOLD
In the next experiment I
attempted to take the tremendous time and fairness advantages that Cylinder
Molding gives and apply them to traditional female molding. The amas of the 40' trimaran were built this
way, and this Spring a 48' foam/glass catamaran on Kauai in Hawaii will be
built this way.
Twenty sheets of 3mm plywood
were used to make the Cylinder molded female mold. These were vacuum bagged into a mold two sheets thick. The first step in this molding process was
to smooth finish the faces of the sheets of plywood which would become the
inside surface of the mold. I flow
coated epoxy and then sanded it smooth, but I'm sure there are faster ways of
doing that job. A plate glass table to
lay the epoxied ply onto might work well, or even some formica might have
worked as an inside surface. This
coating and sanding the flat sheets, plus putting scarphs in the ends of each
sheet took 14 hours. Again, with a
better facing system, that could be a much lower figure. I used standard Cylinder Molding technology
to build the panels. Each full length
panel took three hours to build in all.
A flat surface profile was cut into each panel and these were wired
together along the keel in the regular stitch and glue process. An epoxy "bog" keel was poured
into the wired and spread assembly. The
"bog" keel was hand shaped which meant that it had to be sanded
later. A better choice would have been
to form the keel pour with a large diameter pvc pipe or some other smooth round
pressed into the bogged keel area. That
sanding, which I should have been smart enough to avoid, took two days of
brutal labor to do. The sheer spread
and keel pour took three hours. A
previously built deck flange defined the deck shape of the foldup.
The scarphs joints every eight
feet were filled and sanded if needed.
The female mold shape was kept intact by bogging on external bulkheads
every two feet, in addition to the deck flange. These bulkheads were very rough and took two of us two hours to
Once the mold was waxed, buffed
and pva was sprayed on, I realized that these release agent steps actually took
longer than building the mold itself, ignoring the keel sanding.
This mold stayed joined at the
keel. It could have been cut along the
keel for molding where part of the deck is also molded at the same time. This mold could have been used for a one off
or a small production run.
The hull was laid up with a
single layer of the 9oz Heinsco unidirectional in the +/-45 and the 0 degree
directions in the outer skin. A core of
1/2" balsa was bagged on while the fabric was still wet. I did spread a thin 1/8" layer of epoxy
bog "gel coat" onto the mold before the glass was laid down. That proved to be a serious mistake as the
impermeable fabric trapped air bubbles in the bog. Again, the vacuum would not pull the air out through the
fabric. While the fibers were not
affected by this, the entire bog layer had to be ground off later. With this fabric it would have been better
to have laid the fabric directly on the mold.
After the outer layer and core
cured, the scrimm was removed, and an inner layer of an identical layup was
vacuum bagged on. The nice heavily
rounded sheers that we all like were outside of the ability of this mold. The sheers were laid up separately. These were half length, laid up in a half
round sonotube. These parts had the 0 degree
glass only on them. They were wired to
the hulls and +/-45 glass laid inside partly bonded them to the hulls. Finally a premolded deck was set in
place. The deck, sheers, and hull were
finally and fully joined with a +/-45 layup tying them all together.
These 38' long amas ended up
weighing 312 lb and 318lb each, including connective and rigging
reinforcements, and a solid foam crash bow.
I consider these amas very robust.
CYLINDER MOLDING IN GLASS
The final rapid building
strategy involves Cylinder Molding again, but using no plywood at all for boat
nor mold surface. At the time of this
writing I have not had time to build a hull using this system. When I do, it will again be a female
mold. I will use computer plotted mold
former stations. Each plywood station
will have a slightly different concave curve.
Into the series of curves stations a surface, probably 12mil mylar, will
be placed. Before the mylar is placed
on the mold stations, the fabric will be wetted out and squeeged on a flat
surface. As the profiles of a hull must
mirror each other, the order of
the station formers will be reversed to mold the opposite hand panels.
After these all epoxy/glass
panels are cured, the panels will be trimmed into reflected profiles and wired
together. Again, the sheers will be
spread, a keel poured, and the whole assembly
brought together after it
cures, with a deck flange. Also again,
the core and inner layer of fabric will be vacuum bagged onto the inside of the
hull. After the hull is
"frozen" with the core and inner fabric, the keel wires will be
removed. The glass edges will be ground
back if needed, and the outside of the keel sheathed in a final layer of
I see possible difficulties
with this system if the glass fabric layup is too thin. It would possibly then buckle before
compounding as much as needed to develop a proper hull shape. The best prevention for that problem is some
sort of stiffened perimeter. This
strategy will probably require a couple of hulls to be built using it to
sufficiently understand what is needed.
CONSTRAINTS & DISADVANTAGES
Each of these rapid molding
systems has certain constraints and disadvantges. Also, they all share certain constraints. None of these systems will work with hulls
that need to be fuller than 10/1 on the waterline, or about 6/1 on the
deck. Since few reasonable modern multihull
have hulls fuller than ten to one, that is not a serious problem.
None of these systems will work
easily with complex hulls that need flares or other complications in section.
The first system, with the thin
plywood as part of the hull, probably should have carbon fiber instead of E
glass in the
high load 0 degree direction as
the plywood and carbon have similar elongations to failure. Laminate testing should be done to determine
the best layups to resist multihull loads.
As this system has no real molds, the outside glass layer will need to
be filled and smoothed. Also, this
system only really applies to one off multihulls.
Each of these different rapid
molding systems shows promise in reducing the number of hours spent mold building
for multihulls. The first system needs more study in laminate analysis for the
The other two systems can
probably be improved by input from experienced composite multihull builders.
The last yet untried system
naturally needs some hulls built using it to see if it is as promising as it
appears to be.
(1) Baltek Corp. Reference Book
"Mechanical Properties of Belcobalsa R" P-2
(2) Comtex Development Corp. 2/16/'89 "USCG Certification
(3) Reichard, Ronnal P., International Conference Marine
Applications of Composite Materials, Structural Design of Multihull Sailboats,