|Other than a solid log, the foam and
fiberglass ama is in my opinion the easiest to build. I'm using the
method employed to build tens of thousands of surfboards since the
1960's. The ama design used here as an example would suit a shunting
or tacking canoe of 16'-18' (4.8-5.5M) It can be connected to the
cross beams by either a direct (Hawaiian) connection or an indirect (almost
everyone else) method using struts.
You may wonder why a lightweight construction method is used to build an ama that is partially used for ballast. My philosophy is that if you build it light you can always add weight in the form of water bottles or lead dive weights. If you build it heavy, you have no options. In very light winds or when paddling, you don't want to be moving any unnecessary weight.
Begin by cutting out a 4mm (3/16") plywood shear web drawn to the side view profile of your ama design. As the ama will most likely be longer than the plywood sheet, butt the panels together and laminate a patch of fiberglass cloth over the joint on each side.
There are two main types of polystyrene foam available. The most common type seen is used in packing materials and cheap ice chests. When crushed, you will see individual beads. You don't want to use this type as it is open celled and water can migrate around and through it. The other type is closed cell and will not absorb water. The one I'm experienced with is the light blue colored Styrofoam made by Dow. It comes in several thicknesses and you may have to glue layers together to reach the requisite thickness. I've glued it with either polyurethane glue or epoxy resin mixed with microspheres.
Another option is the more expensive polyurethane foam. The advantage of using polyurethane is that it can be glassed with the less expensive polyester resin. Clark foam is the white polyurethane used in surfboards and is exquisite to shape.
Use the plywood shear web as a pattern to draw shapes on the foam. In this example we need 75mm(3") of foam glued to both sides of the web. Stack the layers together with wet glue and weight them down until the glue sets.
Make a cardboard pattern of the top view shape that you want and draw around it onto the foam with a marking pen. In this case I only needed a pattern for the first 600mm (2') as much of the shape is a constant section.
Roughly shape the blank with a handsaw being careful to cut well outside the line. A Japanese pull saw is particularly good at this (and everything else).
The remainder of the shaping can be done with several different tools. The pros use an electric hand planer and this works very well if you have one. You can use a vibrator sander with 36-40 grit paper and keep it moving. Draw long straight or curved lines or chines with the marking pen for guidance. A long sanding board like the ones used in automotive shops will also do the job. The whole job can be done with the hand sanding board and will ensure professional results. Shape everything in a multichine shape first and round over the edges last. This helps to insure a fair shape as you can sight down the chines.
Finish sanding with 80 grit paper.wrapped around a block or piece of foam. You will find it difficult to avoid gouging the soft foam when shaping. Don't worry about it as microspheres were invented for this problem.
When you're satisfied with the shape, lay the ama on its side in foam cradles and cut the 330 gram (10oz) fiberglass cloth to glass one side at a time. Cut the fabric to overlap the ply web by at least 12mm (1/2"). Don't try to get it to form over the ends as the fabric is too stiff for that. Stop 12mm short of the tip. Apply masking tape a little beyond the edge of the fabric to prevent resin from running down over the foam.
If you have gouges to fill, fold or roll up the fabric, mix a stiff batch of resin and microspheres and fill with a putty knife. Don't overfill as we're not waiting for it to harden. Spread the fabric back onto the foam and begin wetting it with the resin. Use either a brush or a foam roller followed by a plastic spreader or squeegee to remove all excess resin.
Allow the first side to cure, roll it over in the cradles and carefully block sand the edge of the cured fiberglass to allow the next piece to overlap without air bubbles. If you have some really ugly spots, you can fill with the stiff microsphere mixture before you glass the second side. Repeat the steps from the first side, again using the masking tape to stop the runs.
After the second side has cured, apply additional coats of resin with a brush. Thin coats are best as the resin runs easily.
To avoid the difficulty of wrapping glass around the sharp ends, cut about 12mm(1/2") of the ama off with a hacksaw. Dig out some of the foam and mix a batch of stiff epoxy filler. Use the harder microfiber additives to make a strong mixture. Fill the end and build it out to the original length. Shape down fair with the ama when it hardens.
A direct connection between the ama and cross beam ('iako) usually requires a curved beam to get the ama low enough. The attachment shown below is used on the Ulua design. Begin with a 40mm thick piece of wood for the center of the raised pylon. Epoxy glue it to the top of the ama and fit foam fairings in front and behind the center wooden piece. An easy way to get the bottom of the foam fairings to fit perfectly is to tape some coarse sandpaper to the top of the ama and move the foam back and forth over it until it matches the surface of the ama. Glue the fairings in place and after curing, shape them with coarse sandpaper wrapped around a dowel or tube. Cover the entire pylon with two layers of fiberglass cloth large enough to spread out at least 50mm (2") onto the ama. The use of many small round patches of fiberglass cloth will greatly simplify getting around the angular shape. After curing fill and fair with epoxy fairing compound.
The dowel glued through the beam protrudes about 12mm (1/2") below and fits into a hole drilled in the top of the pylon. The whole thing is tied together with strips cut from a tire innertube. It's quick and easy to rig and allows the very important small degree of flexibility that prevents stress concentrations and structural failure.
Indirect connections are most common throughout the Pacific region. They have the advantages of keeping the ends of the cross beams high and out of the waves and they don't have to be built curved. They range from the incredibly complex to the simple forked stick.
Attaching struts to the ama is as simple as cutting a loose fitting hole, pour in some microsphere mixture and insert the strut. After curing, form a small epoxy fillet around the strut where it passes through the fiberglass skin. Have your crossbeams attached to the main hull and lashed to the struts before bonding them in place to insure that everything will fit.
In spite of what you see in the last photo, it is recommended that you paint an epoxy/foam ama with light colored paint, as excessive heating in the sun can soften the epoxy enough to cause small blisters that have delaminated from the foam. If you do get a blister, drill two small holes in it and inject epoxy resin with a plastic disposable hypodermic syringe and weight it down flat until it cures.
Direct questions to Gary Dierking