Our sailing friend Ian McGehee shares his latest small trimaran project with us here – a Ngalawa style outrigger canoe. Ian has contributed to a lot of great info on this blog over the years, and this time is no different.
Lots of pics to go along with detailed info about how he put this boat together. This appears to be a lightweight, versatile, and stable platform for outdoor fun.
Here’s some info on my windsurfer based ngalawa style amas and adjustable
crossarm system made from thrift store crutches, an aluminum closet pole and
bits and pieces from the hardware store. I’ve been fascinated by the ngalawa
since I first learned of them and this design is sort of a test bed/proof of
concept that allows me to experiment with modernizing their unique architecture
and exploiting the “sprung” nature of the crossarm connections.
The crutches are cheap, very light and strong, and offer lots of adjustment
possibilities along with ease of fitting other standard tube sizes, swapping out
parts, etc. Probably too small to work on boats much over 14 or 15′ but scratch
building a similar design in a larger scale seems very do-able.
(Keep in mind that the crutch idea doesn’t need to use the two piece ngalawa
connection so it might be ideal for kayaks and canoes with fixed amas too. Also
the two piece ngalawa connection with a pivoting fork could work with a simple
tube or wooden crossarm as well.)
Donor sailboard is a 9’6″ epoxy/EPS windsurfer cut lengthwise on the center
stringer, it’s a very thick “floater” style board which is ideal in this
application but other styles of sail/surfboards could work too.
Main hull is a 12′ fiberglass canoe.
Assembly pics show dry fitting, for final assembly I wetted the tape out with
Sika AnchorFix-2 epoxy foundation bolt adhesive for all the layup, and also used
it for securing the screw anchors and in subsequent gluing of aluminum parts in
the ama portion of the crossarms.
Fiberglass tape is FibaTape Extra-Strength 2-3/8 in. drywall tape that has a
multi-directional design that adds a bias ply to the standard mesh pattern.
Besides being strong, cheap and easy to get I chose it because it wets out easily
with thick epoxy and is stiff enough to lay out and handle in all phases of the
build without it becoming a floppy, distorted sticky mess- the self adhesive
quality makes it very nice to dry fit.
Screw anchors are E-Z Ancor nylon drywall anchors rated at 75 lbs each for their
intended use…the loads are different here but they’re also epoxied along their
full length and light enough to use a bunch; there’s 10 on the rear connections
and 15 on the front per ama and between actual use and getting bumped into pretty
hard a couple of times in transit nothing has ever budged.
The four rubber flex receivers that are captured by the aluminum angle forks are
high end fuel hose with a Kevlar braid reinforcement. Wooden blocks and hose
clamps keep the fork ends closed and allow for adjustments and replacement of the
hose portion. Unlike the traditional boats I’ve left that joint unlashed so the
amas can swing freely and there’s only a tether to keep them from folding inward
when launching. As is the friction and weight/pressure on the pinched rubber hose
holds them firmly in place and they act like a car suspension when encountering
wakes or shifting crew weight side to side- since there’s no fixed weight (or
buoy) on the end of a lever like a typical tri there’s far less wave motion
transferred to the main hull and its a very different feeling with far less
rolling and snappy motions.
That said I’ve only had small boat wakes to torture test it all on so far and
realize that friction alone won’t be adequate in a seaway and that the swing arc
would need to be limited more in harsher wave conditions, for sailing, etc. but
all that is as simple as lashing things down. Even lashed hard the ngalawa
connections do flex and that helps soften their motion and prevent stress spikes
that might snap more solid connections, especially with materials this
The inner crossarm receivers spanning the main hull are just aluminum tubes sized
to accept the crutch tubing, epoxied to wooden members that make up the gap
between the tubes of the Y and have outboard holes for lashing them to U bolts
installed in the main hull.
So far the crutches only show a tiny bit of permanent upward deformation, and
only in the front pair where the ama buoyancy is greatest. I intend to keep an
eye on it but if it stays as is I am fine with it and can also adjust the loading
a number of ways- I intentionally did the original setup so that the amas and
connections are sharing more of the total weight of the vessel than they
absolutely need to. I think reinforcing the area where the crutch Y exits the
receiver (where the bend is) could also mitigate any tendency to deform but for
now I want to see where it goes.
My original plan was to use these for a sailboat and I had intended to add an FRP
rod or tube beneath each crutch to make a triangular truss to counteract heeling
forces but still allow some flex, and imagine that the current configuration
without it would fold up quickly under all but minimal heeling or rigging forces,
so this certainly isn’t a universal setup or recommended as shown for boats with
different configurations and purposes.
Water stays could also be used to strengthen them in that regard but perhaps the
easiest method would be to just double up the crutches either vertically or by
arranging them in tandem. Either way for a sail version I would avoid using this
crossarm setup for attaching standing rigging.