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air pressure and buoyancy?

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air pressure and buoyancy?

Unread postAuthor: lemming124 » Sat Jul 03, 2010 2:08 am

awhile back i saw a very cool homemade kayak/canoe (I don't know the difference) on an image search, however I can no longer find it. I can tell you however, that it was basically to large sections of PVC pipe (probably about 12 inches) joined at the center, with a piece of marine ply and some footholds attached via hose clamps. I really badly wanted to make one, so I set about trying to find some PVC. the only problem is, 12 inch PVC isn't really in the budget of a high school student :( .
enough ranting. My question is, do you think that attaching a schrader to a section of PVC and taking it up to about 80 psi would improve it's buoyancy, to the point where say a six inch, or even smaller piece of PVC would be able to support as much as something twice it's size?
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Unread postAuthor: jackssmirkingrevenge » Sat Jul 03, 2010 2:18 am

more air pressure = more air = more weight = less buoyancy.

To increase buoyancy with a fixed volume you would have to either evacuate the air or will with a less dense gas like helium.

What about lots of soda bottles :)

lemming124 wrote:say a six inch, or even smaller piece of PVC would be able to support as much as something twice it's size?


Worth noting that a 12 inch PVC section might be twice as wide, but it has 4 times the volume of a 6 inch section of the same length
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Unread postAuthor: lemming124 » Sat Jul 03, 2010 2:52 am

i never thought of it that way (logically) :oops:
haha yeh lots of soda bottles, that would look fantastic !
I think I might try the bamboo frame cheap tarp approach! XD
thanks for your reply
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Unread postAuthor: D_Hall » Sat Jul 03, 2010 10:18 am

jackssmirkingrevenge wrote:
lemming124 wrote:say a six inch, or even smaller piece of PVC would be able to support as much as something twice it's size?


Worth noting that a 12 inch PVC section might be twice as wide, but it has 4 times the volume of a 6 inch section of the same length


Depends on your definition of "support". If you're talking about buoyancy, jack is correct. Double the diameter and your volume (and thus, buoyant forces) goes up by a factor of 4.

But if you're talking "support" as in "ability of a beam to support a load," it's much much worse. Double the diameter and your stiffness increases by a factor of *16*.

In the case of a kayak.... Well, you need to take both into account. Obviously you need enough buoyant forces to keep you above water, but if that's all you pay attention to you run the risk of your boat being a bit... floppy. And if the thing is floppy it isn't exactly a good boat.
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Unread postAuthor: Technician1002 » Sat Jul 03, 2010 11:17 am

Remember the volume of the container will displace it's weight in water.

A gallon of water weighs 8.35 lbs. Each gallon of water displaced by the containers is 8.35 lbs of flotation. It is generally good practice to carry extra displacement volume above the waterline so you stay on top instead of simply submarine at neutral buoyancy. :D Plan on enough volume to allow at least 25% of the containers to be above water.
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Unread postAuthor: Ragnarok » Sat Jul 03, 2010 12:18 pm

I might note that pressurizing the system in some cases is beneficial. In the case of plastic bottles, pressurizing them will provide resistance to them being crushed (in turn, keeping their volume up).

However, actually implementing that might prove extra work.

Technician1002 wrote:Remember the volume of the container will displace it's weight in water.

No, it will displace its weight in water.

<pedantry>The possessive pronoun form does not normally have an apostrophe. (An exception being the possessive form when "One" is used as a pronoun.)</pedantry>

But now, having said that, I'm sure people can imagine me goosestepping around alternately shouting things like "Whom can be used to represent the indirect object!" and "Sieg Heil!" :roll:

A gallon of water weighs 8.35 lbs.

Not over here it doesn't. Weighs 10 pounds (at 62 Fahrenheit) - the Imperial system is pretty silly, but at least its fluid ounces actually are an ounce of water.
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Last edited by Ragnarok on Sat Nov 20, 2010 11:25 am, edited 1 time in total.
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Unread postAuthor: SpudBlaster15 » Sat Jul 03, 2010 12:31 pm

Ragnarok wrote:
Technician1002 wrote:Remember the volume of the container will displace it's weight in water.

No, it will displace its weight in water.


It's (Ha!) rather surprising how many people have never heard of a particularly brilliant mind known as Archimedes...

Tech: An object will displace its weight in water, and its volume in air.
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Unread postAuthor: velocity3x » Sat Jul 03, 2010 12:51 pm

http://www.spudfiles.com/forums/pressurized-buoy-t20401.html

Ragnarok
But now, having said that, I'm sure people can imagine me goosestepping around alternatively shouting things like "Whom can be used to represent the indirect object!" and "Sieg Heil!"


You are correct, Sir. That is ONE of the things I am thinking. :lol:
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