Pop bottle style pontoon

I've seen those, the plastic seems much softer....?

 

Drink Bottles are made of Polyethylene terephthalate
http://en.wikipedia.org/wiki/Polyethylene_terephthalate

Roto-molded boats are made of Polyethylene as well, but there are many types, all dependant on their uses.
http://en.wikipedia.org/wiki/Polyethylene

The drink bottles are created with a hard surface to make them food grade, to stop them holding bacteria.

The different boat manufacturers choose the type of Poly compound that is the best performer for the price, and there is no need for a hard food grade surface on boats.

 

The standard 2 liter / 3 liter bottles display very interesting properties.

Durability and pressure retaining capability is better than you would think.

Quite a while back, I built some compressed air / water "toy rockets". Look them up they are cool. However, I am an engineer that some times gives into pushing things harder than most would. We made some that would fire of around 100' up and then come crashing down pretty hard. Many of the landings were onto an asphalt road and the external surfaces took a real beating. Although I had proof tested the bottles to the max my compressor output, 125 psig, I kept waiting for a failure due to the damage. Failure never happened.

These bottles are discarded and left exposed to the sun. Most old discarded plastics crumble after a decade or so. Not this type of plastic. I do not have any real test data, but I will wager that the percent reduction in tensile strength for this material due to UV is pretty minimal.

The manufacturing method may not really work out. However, it they could make something like an 10' bottle the same diameter as a 3 liter bottle, I assure you they would be very useful. Stack one on top of another inside of an external sleeve and put glass on foam ends on them and and you would have a decent small cat hull. Attach your cross beams at where the ends slip over the "bottles".

 

Lets see one.
Enough talk.
Have you ever tried to cut one with a knife? Easy easy.

Ever seen a sharp rock or glass in the water? There goes your boat.

 

You mean on the sides? An aluminum can is weak on the sides too!
Try cutting the top or bottom where it's a little thicker!
Interestingly enough, You can quickly rip an aluminum can apart if you "krinkle" it a bit and twist it back and forth a few times.

The plastic bottles take a lot more beating until they rupture.
I bet the force to puncture an aluminum can is less, and I bet it takes less force to continue to cut through the aluminum.


I've dropped aluminum cans on the ground and had them rupture and spray cola everywhere, I've never seen a plastic bottle rupture.

 

We make aircraft of aluminum. We don't make them with soda bottles.
There are a lot more issues than you are willing to talk about.

 

Fair enough argument, but I am willing to talk about any issues, thats the entire reason I brought this up .

To be more specific, MOST airplanes are built from aluminum, not all.

 

Many planes also used to use canvas as the skin, but they had strong frames to support it. It was possible for a pilot to put their foot through the skin of a wing, if they weren't careful, but the plane would fly ok with canvas wings.

The big problem with boats using canvas, is that there are lots more stones around water than up in the air.

Likewise, thin plastic might be great for planes as a skin, but the forces on a boat hull are far more severe, more often.

The thin PET bottle material would certainly hold water out if it had a strong frame, but it would be punctured easily.

You could use roto-moulded techniques to create a much stronger skin (same sort of plastic), like those commercial dinghies and kayaks, but then the expense of the material, and more importantly, of production goes up. Building and using a roto-mould is very expensive (over $50,000 to build a small mould) , so you have to build a lot of hulls to make it worthwhile. Once the moulds get bigger than ~18ft, then the production techniques get a lot more complex as well.

If aluminium, fibreglass and wood weren't cheaper to build from (both in cost of labour, material) , then almost every boat in the world would be roto-moulded. In fact, only the smaller, mass produced craft are built using plastic., because it just isnt cheaper once you get over a certain size.

Its not enough to propose that a material is cheap - you have to also factor in the design, building and usage as well. For a test example, next time you are in a camping shop, sit on top of a roto-moulded kayak, and watch it sag. Note also how heavy it is.

Now, if you can locate someone with a plywood kayak ( with or without fiberglass on top ) you will notice that it sags a bit less, but more importantly, is a lot lighter.

This is the best demonstration of how Polyethylene does not scale up well in boat building.

 

You don't make aircraft with aluminum soda cans.

 

Exactly.

Normally aircraft are made of Bud cans.

Soda cans are for land transport.

 

Exactly right.

 

The Roto-moulded kayaks use a different plastic though (NOT PET), and also use a different structure than a pontoon.

When PET is used to make pop bottles, it gains strength from being stretched to fill the mould, as it is blown. The bottom is melted on to the blown section.


For making a pontoon, A 24" x 30' rotating drum could be used, and a thin heated PET sheet maybe 5" wide could be applied to the spinning drum, back and forth, at the proper temperature that it is being streched as it is being applied.
Once the tube part is finished, the ends could be melted on.

PET costs about 1000-1500$ per ton depending on quality and quantity.

 

Parkland,

Have you found the tensile strength of PET(12,000#/sq in.)?
When you do lets compare to aircraft grade alumnimun (70,000#/sq in.) and graphite composite.
Note that PET is 1/2 the density of aluminum, so you need 2 square inches to get 24K#, but only 1 square inch of Aluminum to get 70K#. So for equal weight aluminum is 2.9 times as strong.
The Tensile stiffness of PET is 423K#/sq. inch. Aluminum is 10M#/sq in. That is 20 times as stiff, and plastics loose stiffness rapidly with higher temperatures which aluminum doesn't. Have you ever picked up an aluminum mast on a hot texas day? It gets hot enough to almost burn your hands. The PET will be a limp noodle. At room temperature enough PET to be as stiff as an aluminum 1 sq inch bar will be 10 times as heavy.

This is a bad material to consider.

Note that the two jokers above actually started out with a good point. Nice looking vehicles guys - are the cans full?

Aluminum cans are pure aluminum - very soft and weak. Aircraft or even boat masts are made of alloyed aluminum which has mechanical and heat treatments applied to get much higher strength.
Most of the aluminum in Home depot etc, is 6061T6 which is the basic boat building aluminum - good corrosion resistance (1/2 the strength of aircraft stuff). Compare that to any PET you can get.

Please check out the numbers your self: http://www.americanplasticsupply.com/images/spec-page1.jpg

How does PET do in the presence of common chemicals? Too bad if normal cleaning solvents cause it to fall apart (I don't actually know but it is a serious concern)

 

I don't know any scientific or engineering data to back anything up.
Again I'm not trying to say I'm building one, but I think it's an interesting discussion at least.

PET in the presence of chemicals? Again not 100% sure what the answer is, but remember aluminum reacts with things too, so the answer might be give and take.

I understand what you mean about the PET has less tensile strength, but it also might have properties that make it more durable in certain situations.
Go throw a full can of pop against concrete, and throw a full PET bottle.

I still think PET could make wonderful pressurized pontoons.

 

Parkland,

There is nothing left to talk about.
Can you cut a pet bottle with a knife - you never answered that one and it doesn't take Engineering BS to answer.
There is nothing in a saltwater enviornment to really attack aluminum, including cleaning solvents. If you are serious about this you need to find out.

I just told you that beer cans are not useful material for boats, take it or leave it.
I have never found the need to throw a boat hull against concrete, but any material will be destroyed grinding on concrete. A pet bottle will last a lot less time than normal marine grade aluminum, or fiberglass. A rubber hot water bag for heating up sore muscles will not bust thrown up against concrete, but you would not make a boat of it. You keep bringing up useless comparisons.

You need to research some engineering facts is you want a discussion. With someone else, I should not have wasted my time trying to help.