OK, as I understand it you need to carry as much cargo as possible in a boat
of 1728 cubic inches total volume, can use some reinforcement as long as it
doesn't float and obviously it has to be cheap. Here is another idea.
As others have said a barge shape would be most efficient. It is easy to
make and has a lot of innitial stability. Now for a little math. Fresh
water weights 62.5 pounds per cubic foot so that is the maximum total
weight. You must have some freeboard above the water line so there is a
practical limit somewhat lower than that. Concrete made with portland
cement and sand weighs about .087 pounds per cubic inch. A thickness of
3/8" weights .0326 pounds per square inch.
The shape that will inclose the largest volume with the least amount of
material is a sphere. In this case, half a sphere 14 7/8"" in diameter.with
a surface area of 347.5 sq.in.weighing 11.33 pounds would be the most
efficient but it would not be stable. Next choice is a topless cube 12" on
a side with a surface area of 720 sq. in. and a weight of 23.5 pounds.
Still not very stable though.
An interesting thing about a topless cube though. If you split it in half
and lay the halves side by side you get the same volume without increasing
the surface area. Any other combination of H*W*L with the same volume
increases the surface area. Squash the cube so it is 6" high, 24" long and
12" wide. Now the same cubic foot volume still has a surface area of 720
sq. in. and weights 23.5 pounds but is a lot more stable. In fresh water it
will sink when about 39 pounds of cargo is very evenly loaded. If you are
feeling real lucky you can try for 1/4" thick cement and save about 7 pounds
but it will be very delicate.
To build it, make a block 23 1/4" long by 11 1/4" wide by 5 5/8" high. You
will need some "draft" to be able to remove the block once the cement sets
so taper the sides so the bottom is about 1/4" wider than the top and take
your measurements half way up. Coat it with some heavy grease. Cover it
with hardware cloth or mix some concrete fiber reinforcing in the cement..
Spread on the cement 3/8" thick and work it into the hardware cloth well.
If you are going to try for 1/4" thick, increase the size of the block to 23
1/2" x11 1/2" x 5 3/4" and cut some grooves in the top about 3/8" deep and
1" wide for stiffeners. One down the middle long ways and two across.
When the cement has set to the point that finger pressure will just dent it,
go over it with a smooth, stiff steel trowel or spatula to make the surface
as dense as possible. Cover with a wet towel and let it sit for several
days. Keep the towen wet. Concrete reaches about 75% of its maximum
strength in about 7 days.
--
Glenn Ashmore
I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at:
http://www.rutuonline.com
Shameless Commercial Division:
http://www.spade-anchor-us.com
wrote in message
oups.com...
I am more concerned with the design of the hull. This is a project for
my High School Engineering class, and our teacher wants us to use only
cement to get our craft to float.