The heading on your post says "peddle power". Thats a good start for a description of what you are after. You mention 12 LOA feet and 1500 pounds.
First of all, the pontoons that are most often used for the powered type of pontoon boat, are not a good idea for a human powered boat. As a matter of fact they are not a good idea for a power boat either. From a structural point of view, round is a good shape. From a boat point of view, round is a very poor shape. So let us dispense with pontoon designs that are described with words like diameter.
Fifteen hundred pounds is a lot of weight to put on a 12 foot boat. But lets fiddle up the numbers that would yield that capacity. Fresh water weighs 62.4 pounds per cubic foot. 1500/62.4 = 24.03 cubic feet. That is how much water we will have to displace to support 1500 pounds. Assuming that you are thinking of two pontoons then each of them must support half the weight and that means that each will have to displace 12 cubic feet. If we make a box twelve feet long and one foot square, we will have just enough to do the job. With a total load of 750 pounds including its own weight the box would just barely hold the load. Just barely is not good enough so lets make the box 12 inches wide and 24 inches high.and 12 feet long. Now we have some reserve flotation ability. More than that, the top of the box will be 12 inches above the water surface when the box is loaded to 750 pounds. Make two such boxes, join them together with some sort of beams or deck and we have a floating platform that will hold up 1500 pounds and have a fair amount of bouyancy to spare.
Of course we do not want square ended boxes for our boat. We want pointy ended boxes. For pedal power, we will do well to have both ends pointed. But if we pinch the ends we will have lost some of the volume of the original boxes. Suppose we pinch both ends. We will have lost about 20% of our bouyancy but gained both appearance and performance that best befits a boat. Now we have only 80% of the bouyancy so we have to increase the size of the box a little bit. We'll just increase the displacement by 20%. That makes the box section a little wider at 14.4 inches while keeping the 24 inch height as before.
This boat is going to need 12 inches of water to float. If you want to go in shallower water we have to reduce the draft. We do so by making the boxes wider. Just calculate the draft times width (in inches) in such a way that the product of that multiplication is 173 more or less. Thus if we want to draw only 8 inches of water....8" x beam" = 173 sq. in...... and beam is therefore about 22 inches. Thats getting to be pretty wide for a 12 foot pontoon. Your best plan is to make the boat longer or reduce the weight that it must carry.
You'd need a round pontoon of about 21 inch diameter to do what the 12 x 24 pointy ended box could do. The box is the better scheme for this boat. Besides being better it is easier to build. You could use quarter to three eighth inch plywood. Simple, cheap, easy.
03-26-2010, 10:07 PM
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