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

[fsardone]

Chuck, if your numbers are correct, the weight underwater should be 6 kg.

Buoyancy = buoyancy force - weight on dry land - weight of water in flooded cavity
Buoyancy = (20 L x 1 kg/L) - 20 kg - (6 L x 1 kg/L) = 20 - 20 - 6 = - 6 kg.

Weight underwater = - Bouyancy = 6 kg, not 14 kg.

How do you come up with 6 liter cavity?

He actually came up with a 14lt cavity which flooding gives a loss of buoyancy of 14 kg.
Pretty much my conclusion about 10 posts above.
He is saying that the volume of the hardware is 6 lt and therefore the DPV gets 6 kg of positive lift.

Cheers

 

[Steve_C]

Here is one a little closer to you.
Peterborough Lift Lock - Wikipedia

That is cool. They do not even need a motor since dual "boxes"

 

[Storker]

I see now another confusion. The buoyancy of flooded AL80 (- 8.9 kg) is close to the weight of its cavity when it is filled with water (- 11 kg), people just erroneously assume that the weight of the flooded object in the water equals to the weight water in that flooded cavity, when the unflooded object is neutrally buoyant in the water.

Try re-doing the numbers using a 10L 300 bar steel tank

Mass: about 15kg
Internal volume: 10L
Steel volume: a little less than 2L (IIRC the density of steel is some 7.8 kg/L).
Buoyancy when empty: about 3.5 kp negative.
Buoyancy when flooded (or imploded, but if you can do that, you're pretty damned good!): about 13.5 kp negative.

 

[johndiver999]

A bc is filled with air at 10’feet and has a net bouyant force of 20’kg.
The bc, when empty is neutral.
When all the air is vented from the bc, what is the net reduction in buoyant force?
What is the net buoyant force experienced by the bc after venting?
What will be the change if the air is not sImply vented, but instead the bc is filled with 20 kg of water displacing all the air.

 

[Akimbo]

When I was in the Navy they issued the ACR Firefly strobe and military double-ended smoke-bomb flairs to divers (1970s). We used electrical tape to secure them to our knife sheaths. Somehow I "forgot" to turn it back in when I was discharged.

I carried them for decades and to over 200' more than 50 times with no failures. ACR finally stopped making the special batteries for them.

I never checked the depth rating on these old Fireflies but they were very reliable and managed to survive. The housing on the new ones don't look nearly as heavy as the ones I used so I tend to believe it when they say it's not for divers. The form-factor was/is sure ideal.

 

[Dan]

A bc is filled with air at 10’feet and has a net bouyant force of 20’kg.
The bc, when empty is neutral.
When all the air is vented from the bc, what is the net reduction in buoyant force?
What is the net buoyant force experienced by the bc after venting?
What will be the change if the air is not sImply vented, but instead the bc is filled with 20 kg of water displacing all the air.

If the BC is neutral when empty, then the net buoyancy is zero.

You can’t fill empty BC with water in the water. What’s the purpose of this question?

 

[Dan]

I think I see where the confusion is coming from. Assuming the AL80 being neutrally buoyant is wrong. Its buoyancy when empty is 2.1 kp positive. That’s why when its 11L cavity is flooded, the buoyancy would be -8.9 kp (-11 + 2.1).

If a DPV is neutrally buoyant, then its buoyancy would be equaled to the weight of water that would replaced the air in the cavity. Therefore it is good to know the actual volume of its cavity for estimating the buoyancy of flooded DPV.

 

[Storker]

If a DPV is neutrally buoyant, then its buoyancy would be equaled to the weight of water that would replaced the air in the cavity.

No. No. No, no, no. Absolutely no.

Its buoyancy would be equaled to the weight of water that would replaced the air in the cavity, plus the weight of the water displaced by the material.

Dammit, this Archie guy figured it out some 2200 years ago.

 

[fsardone]

No. No. No, no, no. Absolutely no.

Its buoyancy would be equaled to the weight of water that would replaced the air in the cavity, plus the weight of the water displaced by the material.

Dammit, this Archie guy figured it out some 2200 years ago.

^ This!

 

[fsardone]

Problem is that nobody defined buoyancy.
Some discussed of the antiparallel vector.

Here we should be discussing of two forces: hydrostatic force same direction, opposite verse to gravity acceleration and weight parallel and same verse of gravity acceleration.