Improvised Boat Project

[KeiJei]

Hi guys!

I'm working on a project, its an improvised boat. It doesnt really need to have all the parts of a real boat, I just need to build something that would carry people and propel on water. More like something that you could use during times of floods.

Anyways, my friend is telling me to use the interior of truck tires, but someone told me it has issues with pressure (water and tire). Will this greatly affect the buoyancy or is this material okay? (Cause I have several unused truck tires here) I know balloons contract when submerged under water...is this the same thing for the interior of tires?

ETA: I have no problem with propulsion.

[hoytedow]

My cousin's house floated in eastern Kentucky during this summer's flood. No propulsion, though. She was in it, so it carried people.

http://www.topix.com/city/raccoon-ky

[J3]

I don't understand the question - the concern is the weight of the truck tire tube when pressurized or what? Surely tubes float... that's where the phrase "tubing" came from before there were brightly colored premade water sports tubes to tow right?

[KeiJei]

^Sorry for the confusion.

I'm a bit worried that: theoretically the tire tubes can carry the load on water (ignoring pressure), but when applied in actuality, it would no longer be able to float and and carry the load (because of these pressure stuff.)

Should I inflate the tire tubes to a certain pressure?

[jehardiman]

The problem is that rubber tubes are elastic where as normal inflatable boats are made of inelastic material.

For the elastic tire tube, the total volume of the tube changes with the minimum differential pressure between the inside and the outside. For the inelastic inflatable the volume does not change until the maximum external pressure exceeds the internal pressure.

So, given that walking pressure is 7 psi, and with a tube and inflatable pressurized to 15 psi, standing on tire tube reduces the volume while standing on the inflatable does not change the volume. Same thing with depth pressure, if you pushed them both down to 33', the tire tube would have 1/2 the volume it has on the surface while the inflatable would still have its full volume.

FWIW, 1 psi is ~ 2 ft water head and most rubber inner tubes cannot support more than a few psi without strange things happening because they were designed to be constrained by the inelastic tire and rim.

[hoytedow]

The inflatable nonstretchable would not retain full volume but would collapse underpressure. surface area of innertube would shrink while surface area of inflatable would remain the same.

[Petros]

The surface area is not realavent, it is the volume of water it displaces. Full inner tubes flaot just fine. Around here on hot days far from the ocean many people just use large truck tires to float down the rivers to cool off. Hence the origin of the terming going "tubing".

A better improvised raft would be to take several truck inner tubes and a large sheet of plywood or OSB, drill holes through the plywood and lash the tires to the bottom. With a deck on it you can carry some supplies with you.

[jehardiman]

Originally Posted by hoytedow

The inflatable nonstretchable would not retain full volume but would collapse underpressure. surface area of innertube would shrink while surface area of inflatable would remain the same.

As Petros suggests, you should review your elementry physics again. As long as the absolute pressure is higher inside the inflatable, it maintains full volume.

[KeiJei]

Originally Posted by jehardiman

The problem is that rubber tubes are elastic where as normal inflatable boats are made of inelastic material.

For the elastic tire tube, the total volume of the tube changes with the minimum differential pressure between the inside and the outside. For the inelastic inflatable the volume does not change until the maximum external pressure exceeds the internal pressure.

So, given that walking pressure is 7 psi, and with a tube and inflatable pressurized to 15 psi, standing on tire tube reduces the volume while standing on the inflatable does not change the volume. Same thing with depth pressure, if you pushed them both down to 33', the tire tube would have 1/2 the volume it has on the surface while the inflatable would still have its full volume.

FWIW, 1 psi is ~ 2 ft water head and most rubber inner tubes cannot support more than a few psi without strange things happening because they were designed to be constrained by the inelastic tire and rim.

But probably at most only half of the tube will be submerged, would the volume still get affected by the pressure even if lets say the pressure inside the tube is higher than what's outside?

Originally Posted by Petros

The surface area is not realavent, it is the volume of water it displaces. Full inner tubes flaot just fine. Around here on hot days far from the ocean many people just use large truck tires to float down the rivers to cool off. Hence the origin of the terming going "tubing".

A better improvised raft would be to take several truck inner tubes and a large sheet of plywood or OSB, drill holes through the plywood and lash the tires to the bottom. With a deck on it you can carry some supplies with you.

Yeah, that's what I was planning to do, use a plywood, but I might add a wooden frame underneath the plywood.

[jehardiman]

Originally Posted by KeiJei

But probably at most only half of the tube will be submerged, would the volume still get affected by the pressure even if lets say the pressure inside the tube is higher than what's outside?

Yes, Boyle's Law applies if the boundary is elastic, the increase in pressure due to submergence is proportional to the decrease in volume. That why it's important to know if the tube is elastic or inelastic. If the tube is inelastic, it will maintain volume until the differential pressure "crushes" it structurally.

[hoytedow]

Originally Posted by jehardiman

As Petros suggests, you should review your elementry physics again. As long as the absolute pressure is higher inside the inflatable, it maintains full volume.

That wasn't in question. Once pressure external is greater than pressure internal the non-stretchable inflatable volume will decrease while maintaining same surface area of containment vessel, whereas the surface area of the stretchable containment vessel will decrease along with volume.

[tinhorn]

Hahahahaha! Asking engineers if inner tubes can float is like tapping on an aquarium.

Yes, they float. I've witnessed it.

[jehardiman]

Originally Posted by hoytedow

That wasn't in question. Once pressure external is greater than pressure internal the non-stretchable inflatable volume will decrease while maintaining same surface area of containment vessel, whereas the surface area of the stretchable containment vessel will decrease along with volume.

True, but as Petros said, irrealavent. Your original comment was that "The inflatable nonstretchable would not retain full volume but would collapse underpressure". This is only true if the external pressure is greater than the internal pressure because the skin exerts no force on the volume of air contained within. For an elastic volume, the skin exerts a force on the air proportional to the stretch. This is additive to the external pressure and the two are always identically equal to the internal pressure. If I increase the external pressure on the in elastic volume, no change in volume to the air occurs until the external pressure excedes the internal, for the elsatic volume, any change in external pressure causes a change in the volume of the air.

[jehardiman]

Originally Posted by tinhorn

Hahahahaha! Asking engineers if inner tubes can float is like tapping on an aquarium.

Yes, they float. I've witnessed it.

And I've seen them sink when mechanical engineers tried to build rafts out of them. YMMV.

As has been said: all horses can run, but a lot of money has been spent trying to learn which horse will run fastest in a given situation.

[KeiJei]

If I'm gonna use it for lets say 3 hours for just a day, will the effect of the pressure issues be a huge factor to consider (coz it might sink right away), or is the effect very minimal that I should go for it since I wont use it regularly anyway?