Displacement of Scooters at Depth - Spun off from the A&I Discussion about Nothernone

[Dominik_E]

Under water the same as if completely flooded. We sorted this out already. Something that is already completely flooded can not implode at depths reachable to a human.

 

[boulderjohn]

How much would it weigh if it imploded?

I have never seen an imploded scuba tank. I have never seen an imploded scooter. I don't know what it looks like when that happens.

I assume it would be deformed to some degree. If so, it all comes down to Archimedes' Principle. Because the volume of a flooded scuba tank or scooter does not change, the difference in buoyancy would be 100% due to the difference in weight between the water that entered that volume and the air it displaced. If the physical body is distorted, then the volume would change, and both would have to be considered. As anyone who has taken AOW should know, if you take an empty but capped water bottle to depth, it will implode, and its total volume will be far less than on the surface.

Let's say that instead of imploding, a cylinder has exploded. In that case, it would be wide open. The volume of the cylinder will have changed dramatically. In terms of volume, there is no longer an internal volume to consider, and for buoyancy, the only thing that would matter is the weight of the metal itself and the volume formed by the volume of the metal--the same as a lead weight.

 

[boulderjohn]

If a completely flooded scooter somehow managed to implode actually not.

How does an already flooded scooter implode?

 

[michael-fisch]

My Gavin 33Ah from 20 years ago was impossible to lift with my DiveRite Classic Wing and Drysuit fully inflated. I had to disassemble it underwater, and take the Tail, Nosecone and Tube out on 2 trips. A friend was able to salvage the batterys with trim weights about 2 weeks later, they had to weigh around 32 kilos. Respect Michael Keimes!

Michael

 

[boulderjohn]

I think the negative buoyancy of imploded scooter and fully flooded scooter would be the same because the difference in volume are occupied by the same density of water, plane physics.

As I wrote earlier, I have never seen an imploded scooter. It's a pretty rare event. If an imploded scooter and a flooded scooter have the same volume, then, yes, their buoyancy will be the same.

 

[Dominik_E]

boulderjohn, as I have written twice, something completely flooded can not implode, as it is for practical purposes incompressible. No disagreement there. The thing is that you arrive at the right conclusion (implosion leading to massive loss of buoyancy) via a misunderstanding. In terms of buoyancy loss, there is absolutely not a difference between complete flooding, implosion or explosion (for the case of explosion count all pieces). Any volume generates positive buoyancy only if filled with something lighter than the displaced medium.

The second part of this, beginning with "If the physical body is distorted":

" Because the volume of a flooded scuba tank or scooter does not change, the difference in buoyancy would be 100% due to the difference in weight between the water that entered that volume and the air it displaced. If the physical body is distorted, then the volume would change, and both would have to be considered."

is wrong, sorry...

Cheers,
Dominik

 

[runsongas]

Batteries in good scooters are traditionally floodproof since a 45V 50Amp battery pack that shorts out over 0.1 seconds will give off approx 81 Megawatts. Got no idea how much energy 4oz of TNT can develop, but it can't be much less.

Michael

Just under 2kg (1.936kg) of TNT equivalent for 8.1 megawatt seconds.

 

[Storker]

I assume it would be deformed to some degree. If so, it all comes down to Archimedes' Principle. Because the volume of a flooded scuba tank or scooter does not change, the difference in buoyancy would be 100% due to the difference in weight between the water that entered that volume and the air it displaced. If the physical body is distorted, then the volume would change, and both would have to be considered. As anyone who has taken AOW should know, if you take an empty but capped water bottle to depth, it will implode, and its total volume will be far less than on the surface.

Let's say that instead of imploding, a cylinder has exploded. In that case, it would be wide open. The volume of the cylinder will have changed dramatically. In terms of volume, there is no longer an internal volume to consider, and for buoyancy, the only thing that would matter is the weight of the metal itself and the volume formed by the volume of the metal--the same as a lead weight.

I'm interpreting this as a statement that an imploded tank has a different buoyancy than a flooded tank. If that's correct, you're wrong. Sorry.

Let's run the numbers and look at an Al80.

As best as I can find out, the mass of an Al80 is 31.3 lbs, or 14.2 kg. Let's disregard the mass of the valve, because it doesn't matter for the net result here. The internal volume of an Al80 is about 11L, and the total volume is internal volume plus the volume of the aluminium. Now, Al has a density of 2.7 kg/L, so the volume of the material is 5.26L. Total volume of an intact Al80 is thus 11+5.26=16.26L, and the net buoyancy is 16.26-14.2=2.06kp. It's some 2kp or some 4lbf positive, minus the weight of the tank valve. And of course neglecting the mass of the air, which is only 0.0135kg, so it's quite ok to neglect that.

First, I'll flood the tank. Let's disregard that the density of sea water is slightly higher than that of freshwater, because once again it doesn't matter for the calculations. Now its total mass is 14.2+11=25.2kg, and its net buoyancy is 16.26-25.2=-8.94kp.

Next, I'll thoroughly implode the tank. I'm so good at that that what I have now is a solid Al ingot with a mass of 14.2kg. The volume of that ingot is 5.26L. And its net buoyancy is 5.26-14.2=-8.94kp.

So no matter if I flood the tank - or scooter - or if I implode it completely, its (negative) buoyancy is exactly the same. As not only I said, physics.

With the obvious caveat that I might have misunderstood you.

 

[Charles2]

As anyone who has taken AOW should know, if you take an empty but capped water bottle to depth, it will implode, and its total volume will be far less than on the surface.

And if you remove the cap from the bottle and fill the remaining, although deformed, space with water, then replace the cap, it will weigh the same as it would if you had just flooded it.

Let's say that instead of imploding, a cylinder has exploded. In that case, it would be wide open. The volume of the cylinder will have changed dramatically. In terms of volume, there is no longer an internal volume to consider, and for buoyancy, the only thing that would matter is the weight of the metal itself and the volume formed by the volume of the metal--the same as a lead weight.

​

Flooded, exploded, or fully imploded, the answer in your earlier example is still 24 pounds.....unless, you somehow change the volume of the aluminum.

 

[BRT]

I don't know the air volume of the scooter he used, but I can make a comparison to a scuba tank, which has an internal volume that is 100% air. I am looking at a chart as I write and rounding everything off for ease of understanding.

The scuba tank weighs roughly 35 pounds on shore when empty.
When empty, it is neutrally buoyant, meaning it has no apparent weight in water.
It's physical capacity is 11 liters.
A liter weighs about the same as a kilogram of water.
11 kilograms of water = 24 pounds.

If filled with water, this scuba tank would weigh about 24 pounds in water as opposed to 35 pounds on land.

This is not correct. You would have to know the volume of the cylinder metal and offset that with the weight of that volume of water. The inside volume filled with water is a nothing burger, not positive and not negative. The negative buoyancy is the difference between the weight of the cylinder and the water it displaces. When you flood the cylinder it displaces less water.

Edit: I see what you did there. If empty it was neutral, then flooded it would be negative by the weight of the water. For that case you are correct and if you know the volume and the negative or positive buoyancy empty you could easily calculate what a flooded cylinder would weigh under water. Sorry.