Simple 8 person paddle craft

I need some help designing a vessel that will allow 8 people to sit on and paddle. This will be something like the "Supsquatch"( https://www.c4waterman.com/BoardDetails.aspx?id=66 ), but it will not be used for surfing and it will be EPS foam. The purpose will be to travel ~3 miles in a straight line. The crew will sit down 2x4. The crew will weigh 1200 pounds.

It will be built with the same process and shape as a standard paddle board. I will use 3 sheets of plywood with blocks of EPS foam sandwiched in between. Then glass over it. I am planning it to be 16 feet long, 8 inches thick, and 4 feet wide.

Feel free to address anything I said above or the problems I list below:
1) Will It float the whole crew with the current dimensions?
2) Will the dimensions allow for the crew to paddle easily?
3) Will it be strong enough to withstand calm waters?
4) What kind of hydrodynamics will allow the vessel to move fast in a straight line (vee hull, fins)?

 

It sounds like a race. Also, with a name like that, you might need some grating or a post you can tie people to so you can flog them properly.

If you're "sitting" and paddling on one side, I think you can get the most power with the outside knee down and the inside leg on the foot, so the seat would be something like a narrow bicycle seat or something, something that would be between the legs and not really support so much as give stability.

Water weighs 60# or so per cubic foot, so you can do the math on how much you displace, but a 16x4' paddleboard shape 8" thick should float all you skinny 150# people.

A few strakes or a small keel will help keep you going straight, a rudder would help also.

You don't say if this thing will be used just once or over and over, or what sort of conditions you will encounter such as rocks etc, but if it is a race type thing, light weight is what you want and some of the plywood could be dispensed with.

 

Thanks for putting some thought into it. This will actually not be for racing. It is just to do some island hopping with my friends. I would like to focus paddling to a location in a straight line without getting my friends tired. So the seat idea is good since it lets people paddle with less effort.

This will only be used in the summer. It will be used in water behind barrier islands so the water will be calm. There won't be any hazards such as rocks.

I think strakes will be very useful. Do any of you have opinions on hulls shapes? Vee, flat, etc?

 

flat would be most stable in calm water, rounded front and perhaps a tail coming to a point would have the least drag. Actually a round hull, like a log would have the lest drag, but it will not stay upright (it will roll over). A completely flat bottom hull tends to get knocked around more in rough conditions, that may not be a consideration for you.

Seems like building two hulls and lashing them together in a catamaran configuration would be less costly and less work than building a giant foam and fiberglass plank. two 20'x 3 ft "canoe" hulls, lashed together with several beams, would make a very stable paddling hull. A simple design could be built for about $300 in materials in about 100 hours worth of work. Selected lumberyard materials would work, thought it would not be very light.

There is no strength added by putting a third layer of plywood in the middle. Foam core with plywood top and bottom would be the strongest configuration, less weight too, and give you a durable top and bottom. Of course if you just build a plywood hull with a light lumber frame, it would weigh less and you do not need all the foam either.

Better yet, find a cheap used boat. Or two large canoes, and lash them together. you would spend less money.

 

The length of the boat determines your speed. Two paddlers can keep a 16'er moving at hull speed with 8 aboard. 3 paddlers could keep a 20'er moving at hull speed with 8 aboard. Both assuming an efficient canoe body. Even the 20' will feel crowded with 8 aboard though. What you really want is about a 23' canoe. That gives you a 1,2,2,2,1 seating arrangement and a good turn of speed. I would point out the the passages out to the barrier islands can be quite choppy, and even a big cargo canoe with flotation and part-decks will be only marginally seaworthy on any given day. Something like that pictured below would be way better. You get the benefit of a thousand years of development and a few million builds.

 

If you cover foam with plywood, there isn't any need to fiberglass. If you fiberglass, there is no need for plywood. You need add the weight of the board, so you can estimate 1500 lb . Add food, drinks, etc. and it will conservatively displace 1700 lb. 16'x4'x.75 = 48. The bow and stern will be somewhat finer so the volume may end up at about 36 cubic feet. Subtract maybe 1/3 as reserve flotation to keep it from being a submarine and that gives 24 cubic feet. 24 x 62 (weight of water) = 1488. It will barely float above the water.