Cement Canoe With A Contrarian Approach

Markgor writes: "There is an article in Wired News today about a group of students at the University of Alabama (Huntsville) who entered into the 2001 ASCE/MBT National Concrete Canoe Competition with a canoe that was built to achieve forward propulsion through matching natural resonance."

"Normally, if two objects share an exact natural resonance, the excited vibrations would usually lead the weaker object to fall apart, much like the Tacoma Narrows Bridge did when the equal frequencies of the wind and the structure of the bridge matched. However, since the canoe was designed with a special mix of concrete, it was flexible enough to withstand the vibrations and harnessed it into forward propulsion. They're now talking about its possible use in space, such as interplanetary probes using natural resonance to propel itself."

Guess what - boats are made out of metals too!

Concrete boats aren't news - I held stock in a company specializing in them years ago (did well as I recall.)

Boats can be made out of anything that that doesn't dissolve quickly and has the strength to displace the requisite volume of water. Iron, steel, concrete, waterproofed paper-laminate, whatever - they need not float on their own; it's displacing a volume of water of greater mass then their own mass that is key.

Concrete boats are popular in a number of parts of the world. In Africa they're popular as small calm-water ferries for their low cost, durability, and ease of contruction. Often they're a simple mini-barge with a line crossing the river. To power them one either pulls the line directly or employs a simple mechanism, dragging oneself across the water.

As to the concrete being used in this application - it's made with exotic materials as it has exotic requirements. Light-weight, flexible, etc. aren't usually the priorities for a concrete; durable, high compression strength, low cost usually are. None of this is breakthrough as the materials used in the boat wouldn't likely stand up under a season or two of highway or other civil engineering use.

Rules and Regs

are posted here [masterbuilders.com]. They're both PDF files. The rules are quite complex, explaining the ratios of different types of cement and so forth. Interesting read.


Quidquid latine dictum sit, altum viditur.

In space?

If they aren't expelling mass, they will need to push against something. Perhaps he is suggesting a largish RC circuit that pushes against the earth's magnetic field. This could be interesting, because the power requirements to keep an oscillator going are rather small. But I don't quite understand how resonance would help in that case, since a magnetic field doesn't work the same way as a fluid such as water.

If they can bend concrete...

If they can bend concrete, they may have just saved the national highway system. One of the biggest problems is roads that self-destruct when the soil under them shifts. With enough wet/dry, expand/contract, up/down cycles, our highways are in constant need of repair. If even a small portion of this rework can be avoided with the "flexible concrete", the potential savings are enormous.

Re:Concrete?

Don't confused concrete and cement. Concrete is a simple name for composite construction. A concrete highway is often made up of a composite of steel rebar, cement, gravel, etc.

Cement, on the other hand, is a specific name for a substance, often alumina, silica, lime, iron oxide, and magnesium oxide. Cement is often used in the construction of concrete as part of the composite.

Composite structures != carbon fiber/kevlar/etc exclusively. Composites have been used for hundreds of years to make lightweight, strong things. This is merely the latest example of exactly that.

tacoma narrows

like the Tacoma Narrows Bridge did when the equal frequencies of the wind and the structure of the bridge matched

Just to do a little karma whoring...

Google [google.com] has some nice links [google.com] to video of the Tacoma Narrows bridge moving. This one [airspacemag.com] from the Smithsonian is pretty good.

--

Re:Concrete rowboats

Oddly enough concrete boats [ferroboats.com] are not as rare as they may sound - There are many many large boats made of concrete, and from what I've seen they get real ugly, but last and last. A concrete boat sounds kind of silly at first, but when you consider that a lot of boats are made out of Iron and other heavy metals, its not that strange..

Rubber Roads

..Yeah, and if we could engineer the roads to the right resonant frequency we could save on gas! boing ..boing ..boing .. Probably have to beef up the suspension on the cars that travel them tho' ---this ain't the Dukes of Hazzard..

Concrete?

The concrete is made up of a mix of Portland cement, glass micro-beads (microscopic hollow spheres), latex, acrylic fortifier and water.
Mix these in the right proportions, allow the mixture to dry for 12 hours and presto -- you have concrete so flexible that it will bend and snap right back with nary a crack.

Hmm.. Yeah its got Portland Cement in it, but it sounds like it is primarily rubber and plastic.. Are there no limitation in the rules about the composition of your 'concrete'?

Re:Cavitation + Resonance?

Sure going a few hundred mph is fun for a while. What do you do for kicks after you have a whale embedded in the tip of your vessel?

People Clear on the Concept Unclear on the Concept

From the Purpose, Canoe Design and Past Winners [masterbuilders.com] page at the canoe contest website [masterbuilders.com]

"The trick is to create a concrete mix that is less dense or lighter than water. Regular concrete is 140 pounds per cubic foot, but the students create concrete that is as light as 41 pounds per cubic foot. Water is 62.4 pounds per cubic foot, so the less dense concrete will float. However, students can create heavier canoes that will float, as long as the canoes displace their own weight (and the weight of the paddlers) in water."

The concrete doesn't ever need to be lighter than water. The boat plus passengers and gear needs to be lighter than water. Which means the maximum displacement of the boat (fully loaded volume including outside hull sectioned at a plane level with the waterline) needs to weigh less than the load plus the boat. The concrete itself can be far more dense than water.

Having concrete that is lighter than water means you can make a raft out of concrete. It means your boat won't sink if it floods. It also means your boat has less inertia.

It looks like a minor point, I know, but this is an engineering contest. It's all about minor points and the error bars on them. To put conflicting statements in the brochure is to sandbag the less-experienced contestants.

The real trick is that normal building concrete is much heavier than water (and somewhat absorbent, which reduces its effective displacement in contact with water). But this contest years ago ceased to be about floating a hunk of sidewalk.

--Blair

Like Homer says

In this house we obey the laws of thermodynamics!