There's been a lot of discussion about materials and techniques. I suggest
that regulars on this forum will be of limited assistance with these
"engineering" aspects of the problem.

The hull design, on the other hand, is an interesting question.

The boat must float with a heavy load, presumably at rest in a smooth pool,
and have a maximum volume of 1 cubic foot. How will the weight be
measured? I would guess, by adding small blocks or balls of cargo, until a
gunwale goes under, and the craft sinks.

Will the cargo be very dense, such as lead blocks? In that case, you will
want the weight to be concentrated at the bottom center, as ballast. A very
deep hull, (ideally a longish cylindrical shape, with one end closed - as
you add weight, stability increases.) On the other hand, if it's too deep a
hull, it will lack stability when unballasted, and perhaps fail at the
outset.

If the weight to be added is a relatively light (low-density), the hull
shape would be very different, perhaps towering above the gunwales.
Stability will be a limiting factor. Perhaps a catamaran form? What about a
"floating drydock" approach, where the cargo is on a platform at the BOTTOM
of two (sealed?) pontoons - as it is loaded, the platform will submerge, and
cargo will contribute to the buoyancy...

--
Sal's Dad


wrote in message
oups.com...
I have to build a cement boat that will hold as much weight as
possible. It must be made completely out of cement, no styrofoam, wood,
or anything that would provide buoyancy. It can be 1 cubic foot
maximum. How would I go about doing this??

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.

we can use some sort of mesh or something to get the cement to hold a
shape, but it cannot be wood, or something else that would provide
buoyancy that way.

wrote:
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.

I think I would model it after river barges. very flat bottom, straight
sides and rounded up at both ends. Rectangle shape. It is used to haul
tons of coal around here every day. It is also a very simple shape to
produce. Also agree with everyone else that you need a mesh inside to
keep it all together.

There certainly are lots of cement barges plying the costal rivers of China.
Staight sides and flat botton, but probably for ease of construction and
load handling.

Might want to play with the math a little to get a fix on the shape that
encloses the greatest volume with the least surface area. Hmmm. Sounds a lot
like half a sphere. Could cast it inside a basketball...

surfnturf


"No Spam" wrote in message
news:UTeee.7459$Rl6.7151@trndny08...

I think I would model it after river barges. very flat bottom, straight
sides and rounded up at both ends. Rectangle shape. It is used to haul
tons of coal around here every day. It is also a very simple shape to
produce. Also agree with everyone else that you need a mesh inside to
keep it all together.

On Wed, 04 May 2005 05:40:12 -0700, iceman390 wrote:

we can use some sort of mesh or something to get the cement to hold a
shape, but it cannot be wood, or something else that would provide
buoyancy that way.

Use chicken wire, then.

Make the boat a simple sqare box maybe half an inch thick.

You'll have to work out how big each side is based on your cu. ft. limit.

Since it is a school project, I won't say much more than that. Come to
think of it, I'd probably lead you in the wrong direction anyway since
I've never had any training as a marine architect.

Good luck!

--Mac

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.

How is the teacher testing the resulting shape? Is he/she using lead shot,
pea gravel, water. Is the teacher really looking for a boat/barge or the
theoretical practical solution? Lead shot in the bottom of a hemisphere
would be stable enough for a test and give you the least amount of surface
area.
Clark
"surfnturf" wrote in message
news:MIfee.1223692$6l.94600@pd7tw2no...
There certainly are lots of cement barges plying the costal rivers of
China.
Staight sides and flat botton, but probably for ease of construction and
load handling.

Might want to play with the math a little to get a fix on the shape that
encloses the greatest volume with the least surface area. Hmmm. Sounds a
lot
like half a sphere. Could cast it inside a basketball...

surfnturf


"No Spam" wrote in message
news:UTeee.7459$Rl6.7151@trndny08...

I think I would model it after river barges. very flat bottom, straight
sides and rounded up at both ends. Rectangle shape. It is used to haul
tons of coal around here every day. It is also a very simple shape to
produce. Also agree with everyone else that you need a mesh inside to
keep it all together.

wrote:

we can use some sort of mesh or something to get the cement to hold a
shape, but it cannot be wood, or something else that would provide
buoyancy that way.


Dig a hole in the ground, line it with plastic, drive in some pins
to position rebar or position chainlink fencing for reinforcement,
plaster or even spray on concrete. Make it into a basement for a
floating house with several compartments, wait for spring floods to
launch.

Or did you want a speed boat?

Terry K

Terry Spragg ) writes:
wrote:

we can use some sort of mesh or something to get the cement to hold a
shape, but it cannot be wood, or something else that would provide
buoyancy that way.


Dig a hole in the ground, line it with plastic, drive in some pins
to position rebar or position chainlink fencing for reinforcement,
plaster or even spray on concrete. Make it into a basement for a
floating house with several compartments, wait for spring floods to
launch.

It's also been done upside down. Pile up some wet sand and sculpt the boat
you want, cover that with burlap or something (I forget the details,
plaster over smoothly with wet cement mixed with beach sand (mortar),
cover with plastic sheeting (you want to keep the water in while the
cement cures), and when it cures turn turn it over and drag it down to the
water. I read aqbout this being done by someone who could not transport a
boat to a waterside vacation spot. The cement boat was left behind after
the vacation for the use of local residents. Claimed it was a success.
Note that cement has to remain moist to cure. If it dries out before it
cures it will crumble into pieces.

Piling up the sand might be better than digging down in wet locations
where a hole would fill with water. Just a thought.

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