Outrigger canoe Ama design considerations

Hi Michel,

You're exactly right, the foam blank is rigid enough to hand layup some fiberglass but would collapse under a vacuum but since my last post I've changed directions on this project. Last Friday I tried paddling a kayak with a single blade outrigger canoe paddle and it wasn't comfortable at all. The seat in a kayak (or in my case surfski) is just too low to comfortably use a canoe paddle. I also wasn't happy with how much I'd have to modify my existing surfski to mount the ama. So I took my very light and shapely ama blank and put it on the shelf and today I started building a blank for "training wheels" for my surfski. I'm about half done with the blank and will post a photo once I've finished it.

I will say that I absolutely love this method of construction. The XPS foam sheets are cheap and very easy to handle, cut and shape. Unlike divinycel and Clark foam, it does not like to be shaped with a sureform, but cuts very easily with a sharp razor knife and 80 grit sandpaper. Gorilla glue works perfectly, sets up in a few hours and creates a very strong bond. I just shave or sand the excess glue once its dried. I haven't tried glassing one of these blanks yet, but I'm using WEST system epoxy which has never failed me in the past.

 

Hello PMU,
I have built a couple of (wind)surfboards back in the mid '80 using styrofoam. We had two methods laminating. First pre coat the blank with epoxy and wait until the resin floats on the blank, then lay the glass on. This is to saturate the foam. Disadvantage is that the glass sticks onto and is difficult to put wrinkles out. The second is method is to lay up a layer of glass and then use lots of resin to have enough resin for foam and glass. Wait 10 minutes and add resin if nessersary. Then lay up the following layers and saturate the resin. Wait a few hours until the resin begins to harden but still sticky, then put on a thick layer of resin as a topcoat to fill the weave of the glass cloth. You can sand this directly.

With the blue styrofoam (1980 around 28 to 32kg/m^3) we used 2 layers of 6oz or 3 layers of 5oz

Have Fun, Michel

 

Thanks Michel. I'm using a very light layup for this build, 6 oz for reinforcements and one layer of 4 oz. I may add another layer of 4 oz if one doesn't feel strong enough.

After I've finished shaping, I plan on smoothing the foam with spackling, then I'll lay out the glass and squeegie / brush the epoxy. I'm using the "slow" hardener with the WEST epoxy so should have plenty of time to work the glass the way I want it.

 

PMU,

Joining the cross arms to the deck of the surf ski can indeed be a challenge.
@DogCavalry made the attempt recently for a pair of "training wheels" on my old surf ski.
The deck's pretty flimsy. He may be able to advise you, I think he made a couple of attempts.

I can tell, it doesn't take much righting-arm or ama buoyancy to make a big difference.
Making them adjustable in height is really useful in the learning phase.

I also wanted to add, waste can be hugely reduced when building from solid foam.
An assortment of 2", 1", 3/4", 1/2" can be used.
A hotwire "table saw" can make tricky diagonal cuts that allow asymmetrical build up.
It leaves very little tooling left to achieve your desired finished shape ready for glassing.
Bullheads can easily be cut in as structurally needed,
although it's pretty hard to cut a perfectly formed blank into pieces,
only to have and glue it all back together.

BB

 

For a 10' long ama, I'd have added a backbone of 1/4" plywood.

I recommend you sand the entire surface with 36 grit floor sanding paper by hand. The shear rating of big box xps is poor and scratching the surface increases the surface area, so 15% more surface is 15% more shear. If the shear is 25 psi, then scratching gets you to almost 30..

Another thing I recommend is precoating the scratched xps with thickened resin. A hotcoat and then immediately go to glass. The harder shell helps with compressibility and makes the part a bit harder to delam from bumping it. The shear and compression ratings for xps are both about the same and both low.

Rolling on thickened resin requires dry laying the glass and knowing exactly where it is going. I did mine in two separate gos. That way gravity was a helper both times.

all the best

 

I agree that you don’t needed a lot of flotation.

FWIW, I made some 4’ ers out of balsa, 6” h 4” w. Easier to shape, glue (tb 3) , and finish (varnish). I started out with an insanely high prismatic, and then cut & sanded them down to taste as observation and experience dictated. Sculpturally streamlined, although one got too small, so I started over. Easy to touch up with varnish, and surprise surprise, didn’t leak. Easy to glue in harder wood where needed Really satisfying.

 

Fallguy beat me to the "abrade the surface" aspect of using xps foam. The only failures I've had with sample pieces were compressive buckling in the fiberglass skins both times in part by stressing the part before the epoxy system cured enough. An issue with solid xps cores which shouldn't apply to your stitch and glue approach is outgassing bubbles. I've not experienced it but know some who have. One of my failed pieces a 64" x 2" x 9" side plank between float and hull was left out in the sun over spring and summer without issue. In retrospect I should have painted it black.

 

Well, I've changed directions three times on this project so far. I started by building an 8 foot ama intending to turn my surfski into an outrigger canoe. Then I actually tried paddling a kayak with a canoe paddle and the ergonomics were all off. So I decided to build a pair of mini amas joined in the middle by a spar of the same construction as the hulls. After reading some of the comments I realized that that wouldn't give me much room to adjust or modify the distance or height of the amas (not to mention the level of complexity this added to the build)... so I decided that I'd keep the two mini amas and connect them to a 1"x3" wood spar (probably the clearest piece of fir I can find at the box store). Here are the amas in their current state. LOA is about 32", beam 6", height 6.75".

Yes, I've learned to score the foam before using Gorilla Glue which helps it adhere. I'm scoring the surface with 80 grit followed by 120 grit and filling the larger gouges with spackling.

 

PMU,

I admire your persistence.

I can tell you, for the next iteration, 20:1 is ideal for length to beam ratio, at the waterline.
Tapering, vertically, can provide disproportionately more buoyancy when loaded, resisting roll.
Conversely, disproportionately less buoyancy when less is needed as load is decreased.
Tapering the stem and stern can compliment these attributes.

Too bad @DogCavalry didn't have anything to add.
He just finished doing what you're doing.
He's pretty busy these days.

 

These are only intended as "training wheels" until I learn to balance the surfski without them. So short and stubby are fine for now. I think my biggest concern right now is that I build it strong enough not to fail and yet be light enough not to become a burden while paddling.

 

Short and stubby times two!
You may have a hard time reaching the speed at which you master the stability issue.
Just trying to help.
Best of luck, post pictures of your results please.

 

I found that the surf ski was so lightly built that I couldn't attach anything directly. Used very large contact surface area, with window and door adhesive that I knew was removable with solvents.

 

My plan right now is to use 1x3 wood blocks with foam pads underneath and straps in contact with the surfski. Then traditional straps to connect the beam to the amas. Do you have any photos of your surfski mod?

 

Be really careful with foaming gorrilla glue if you have asthma (there are warnings on the safety sheet) - even a good mask will not offer a lot of protection- get the biggest baddest mask you can, preferably with outside positive air pressure. Once you are sensitized to GG, it will be almost impossible to work with. It does work well on XPS, and wood to XPS. A good fine spray mist bottle for the water is a must.

Balsa dust can be a problem too.

 

most excellent! looks like an Exocet D2 crossed with a speed needle!

for some reason this made me think of using a windsurfing mast base (not a mechanical one- use a stiff plastic joint) to join Aka to Ama so the Ama could follow wave contours?