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.
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Ian writes:
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
lightweight.
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.
Hello Ian.
Nice to see you experimenting with this stuff. I like to do this myself. I did not see a sail on it. I assume this will come later. I would recomment a large old camber surfboard sail. This is cheap has a a great default curve in the sail. And flipping the cambers over is no problem whatsoever.
Hope to see more testing video’s…
Hi Dutchy,
I do intend to experiment with a sailing version of this idea eventually but for a number of reasons this particular boat probably won’t be it in any serious way; the 12′ canoe has very limited freeboard and is an older boat that is pretty heavy on its own so with all the extra gear needed it would be borderline unsafe with much heeling.
Without the amas it wasn’t much fun at all with two people because of the constant need to maintain balance..all canoes are like that to some degree but this one is narrow and pretty round bottomed and had no margin for error. Even with the extra drag of the amas it’s faster paddling now because before you couldn’t lean into it and really pull without risking a capsize.
The other thing is that the area I intend to use it most is on the bayou in the picture where overhanging trees, very shallow depths with lots of things to snag on, lots of very low bridges and a fair amount of powerboat traffic (including commercial barges) make sailing impractical. Eventually I’d like to add a small electric trolling motor to this one and also have some ideas for a pedal operated sculling oar system that this would be a good candidate for, that also wont require registering it like adding a sail or motor would.
Really cool concept Ian. I would be very interested is seeing your pedal system as it develops.
Decades ago Eric J. Manners (GB) has developed and extensively tested a 20ft trimaran with hydrodynamic well working wings and an extended cockpit to prevent capcising when these
floats submarined. His creation seems to be the perfect modern ngalawa version.
Hello Manfred,
Thank you so much for this comment; I’m always interested in seeing other examples of this form being adapted in the modern age and had heard of Erick Manners from reading AYRS publications from the early days of modern trimaran development, but had never run across this particular boat which I think is the “Flying Wing” (?)
I was able to locate a 1961 article in the AYRS archives that describes that boat and his “Trifoil” ama design, which is remarkably similar to a halved surfboard as far as relative dimensions go and oriented almost exactly like the traditional ngalawa concept.
That article also has an interesting commentary about foil assisted trimarans and diverging opinions at the time about whether a low aspect water ski -like foil or a high aspect blade -like foil was the best option… (pp 88-91)
https://www.ayrs.org/repository/AYRS074.all_A5.pdf
The ngalawa style outriggers and exploration of low aspect ratio planing amas in general was mostly abandoned as tris became more mainstream but it’s fascinating to see how many people were aware of and working with this style of ama back then, with some very positive results.
This comment from Manners in particular was very satisfying to see in light of what my little experiment has shown so far, it’s virtually identical to what I posted-
“The hydrofoil arrangement gives the FLYING WING an exceptionally smooth ride with no “wave shock” from the floats. Nor is there a quick motion when a float meets a wave which can be annoying in a trimaran”. Indeed, Erick has actually made trimarans with spring loading in the floats to try to overcome this.”
It’s also interesting to note that in that era the low aspect ratio water ski/surfboard -style outriggers were treated as just another type of hydrofoil, which is probably why I had never seen that particular boat before when searching for examples of modernized ngalawas, but not hydrofoils as we think of them today.
In that publication is another article I’d seen before describing a “Melagasy outrigger configuration” using plywood surfboards (“JEHU” p 40) that reports very positive performance…and in 1955 had a configuration very much like l’Hydroptere, especially when they added tilting leeboards and realized that they acted like foils when canted in their stowed position-
“When fitting the lee boards to Jehu, the only logical way of having them retractable was to hinge them and retract them inwards. Hinged struts held them in position, either up or down. It was, of course, quickly seen that if they were angled in at the Baker angle of 40°,
they would function as hydrofoil stabilisers. We soon tried them as such and, to our delight, we found that the surf boards could be completely lifted off the water by the angled-in lee board. This occurred at a relatively low speed; about 4 knots. At speeds greater than 5 knots no sitting out was necessary and the craft sailed close hauled very nearly with the crew placed just as they wished and the surf board raised off the water.”
it’s also worth noting that in an earlier AYRS publication it is established that Victor Thetchet’s pioneering designs used an adaptation of the low aspect ratio planing form idea, but performance suffered in large part due to the horizontal orientation of the ski-like portion of his amas-
“In Madagascar, the float is given the shape of a water ski but is thick enough to act as a float, too. As shown in Fig. 2, it is angled outwards so that, as it planes along, it gives a certain amount of lateral resistance which is useful because neither centre nor Iee-board is used. This angle of dihedral also give a certain amount of lift on the lee side (quite distinct from the dynamic lift of the planing under-surface. It also reduces the wetted surface at very low speeds when the float can be trimmed to touch the surface by only a very small amount of the lowermost corner so that it does not drag a lot of water behind it .
Victor Tchetchet of Long Island uses a modification of the Madagascar float (Fig. .3). Because there is a centreboard, the float is placed on the water with no dihedral angle and the float under surface is horizontal…. This system has the fault that it drags water behind it when it is being pressed down by a weight of wind, close hauled.
When running free, the float will plane well due to its speed, though the entrance could well be given a V to lessen its resistance.”
https://www.ayrs.org/repository/AYRS007.pdf