cement boat competition question

i know this sounds a little elementary, but in my Engineering systems class we have an assignment to design a boat (or i guess it could be considered a float, because its immobile) out of cement that will hold 2.5 times its weight without sinking. this boat is very small, it has to fit in a box that is one cubic foot. no materials are allowed to be used for the finished boat other than portland cement(mix), and wire mesh. if anyone has any ideas that would be great.

 

Woffy,
Well, a sphere has the best surface/volume ratio, perhaps use a balloon or a deflatable ball for a mold? A round shape should also allow thinner walls.

Yokebutt.

 

well, i guess i could use a spherical shape but the only problem is that i still need an opening to put the weight in the boat/float. how could i create stability so the opening stays out of the water?

 

By making the bottom a little thicker skinned.

Yokebutt

 

No, wait, stupid me, the opening provides the neccessary imbalance!

Yokebutt.

 

but wouldnt it like spin around and take in water?

 

Nope, close your eyes and imagine; with an opening the CG is not in the center of the sphere, but a little bit offset, see what I mean?

 

And of course, with the CG at the lowest point possible, you have a stable equilibrium.

Y.

 

I would go for the shape of a cargo barge. Pratical and proven shapes. Portland Cement barges are also known as Ferrocement barges. Were all the rage in the 50's & 60's. Some are still in use. Make the bottom strongest with wire and cement. Pure Portland Cement powder makes the strongest concrete. You could build a few pratice barges with just wire in the bottom. You may find that wire is not needed. Real concrete boats were built without metal. It was for stress cracks and collision. If it will float /2.5# in the 11" x 11" square hull with sides you pass. right?

 

Maybe I'm missing the problem but everything I thought of seems to work. Say, a poor case of a square box with an open top. Use water as the weight. The pressure will be equal on both sides of the box walls over most of its surface, so no failure there. Only that part of the box sides representing the displaced weight of the concrete needs to be strong enough to resist a collapsing force. Use oil or some other lighter weight fluid in the box and it should get easier.

Now, if they insist on using a chunk of lead as the weight, the problem gets harder and the half sphere looks better.

 

A 3/8" thick bottom 11" X 11" is STRONG, Unless a idiot drops it in. Can you make the 2.5# weight? yes, use a flat plates of steel.

 

You might be interested in this.... a few of my fellow eng students are involved in this project. They build and race a multi-person canoe using concrete.
http://engsoc.queensu.ca/canoe/2005/index-flash.htm
It's a pretty impressive (and strong) boat built using methods similar to what you're looking at.

 

I built a concrete canoe in college. We tried several design mixes. Portaland cement is very heavy! Depending on the strength you need, some of the mixes used hollow glass beads, styrofoam, volcanic rock, etc. for the aggregate. Some of the test cylinders would acually float although they only tested 700 to 1200 psi, they are strong enough for a canoe.

 

yea i know of didfferent aggregates, such as polystyrene and pumice, but all we're allowed to use is portland cement and sand for the aggregate. if that wasn't the case, it'd be alot easier. the only weight reduction i can do is reducing the amount of water i use for the mix, but that decreases the strength

 

oh and marsh mat, about the canoe, that is pretty cool, but my boat will not be going any where; its just to be made for holding weight without sinking.