: Hi,
: Just thinking out loud. Why not make the shape of the outriggers like the
: hauls of the Hobbi Catamarans (can't think what it is called,
: assametrical??? SP.) so you don't need any center boards to counter side
: slippage while sailing. It would have to be somewhat v shaped so it would
: dig into the water.

: John

My neighbor has a Hobie cat, but it is put away for the winter, otherwise I'd prevail on him to let me measure the pontoons.

From the few looks I got, the cross section is triangular, with rounded corners. The forms for this can be made from plywood triangles, with the flat side areas covered in 1/4 inch lauan, and the curved corners filled in with a few 1/4 inch strips. The lauan is probably going to be a bit thinner than the nominal 1/4 inch, so your strips will be a tad higher at the junction, but sanding will quickly remove the soft, excess wood.

A few thoughts here:

On the Hobie the pontoons provide the main flotation, and when the pressures of turning, or sailing iwth a wind from the side, causes the boat to lift onto one pontoon, then that pontoon must have the necessary size (and displacement) to support the whole weight of the boat and passengers. When you are using these things as outriggers on a kayak they can be far smaller, as the boat provides the displacement for the paddler, and the outrigger merely adds some stability.

At almost any speed an outrigger shaped like a waterski will give some support. Just ask anyone who has ever slapped the water with the flat side of a paddle -- you'll get the idea. When it is moving, a large flat surface will exert a lot of effort over the patch of water it covers. Sitting still, though, a water ski will give just minimal resistance to tipping. Even so, it will slow down the motion of a tipping boat, allowing the occupant(s) to gain their balance.

Try hanging a 6 to 10 foot long 2x8 as a pontoon and you'll see the effect. With the board laying flat (8 inch side laying on the water) you have it acting like a long kayak paddle blade. It resists movement through the water, keeping the boat from rolling like a barrel. Mounting that same board on edge, with the 2 inch side on the water, you have a leeboard, which drops 4 times more easily (1/4 the surface area in contact with the water) and gives far less resistance to the barrel-like rotation of the kayak. (When sailing, however, side pressures would build on the wider side of a board oriented like this, and it would serve as a leeboard, helping to convert sidewards force from the wind into forward motion of the boat.)

If you go with making a pontoon with a triangular cross section, you can calculate the displacement rather easily. Take the area of the triangle that will be submerged convert this to fractions of a square foot, and multiply that by the length in feet to get cubic feet. Figure each cubic foot at 60 pounds of displacement. Do this operation several times.

for example: I'd start with an assumption that these things would sit 4 inches in the water. (4 inches of draft) that gives me a height on the triangle of 4 inches. If I measure the forms I am using and find that at this point the triangle's base is 3 inches wide, then I get the area by using the formula of 1/2 the height times the base. So, my triangle has an area of 6 square inches. That is 1/24th of a square foot (144 square inches to a square foot). for an 8 foot pontoon I'd have a displacement of 8/24ths of a cubic foot, or 1/3rd of a cubc foot, or 20 pounds. That is not a lot, but it would be about the weight of the pontoon. if the pontoon is 12 inches tall, then it would be 9 inches wide, and the total area of its cross section would be 54 square inches. That is 54/144s, or a bit more than 1/3rd of a square foot, but to make these calculations fast, I'll work with the 1/3rd of a square foot. for a 8 foot length I'd have a displacement of 8/3 cubic feet. That is 2 2/3 cubic feet, or 160 pounds of displacement if the entire pontoon is submerged. Actually it would be a bit more than that, but these are just rough estimates to give you an idea of what size to build them.

to put this another way, you would need to get at least 160 pounds of weight or force on that pontoon to get it under water. Unless that happens the kayak which is attached to it can not turn over, or tilt more than a few degress to the side on which the pontoon is mounted.

The weight of the pontoon and the distance it is mounted from the center of rotation of the kayak provides restrictions that keep the kayak from turning turtle to the side opposite that which the pontoon is mounted on.

For greater stability, it could be advantageous to partially fill a pontoon with water. Assuming the pontoon is mounted on the starboard side of the kayak, adding 20 to 50 pounds of water would make it very difficult to flip the boat onto its port side. At the same time, the would be plenty of reserve displacement to keep the boat from flipping to starboard. Another option would be to place some gear on the struts connecting the pontoon to the kayak. Anything to provide resistance to the rotational motion of the kayak.

hope this helps

PGJ

Y