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

[Dan]

Dan,
are you purposely being dense?
If the tank is neutrally buoyant and you then add 10 Kilos of water to the inside of the tank, you have not caused any change in its displacement but it now weighs 10 Kilos more.

Michael

No, are you?

It weighs 10 kg more in dry land, not in the water, due to buoyancy.

 

[michael-fisch]

Technically, the displacement change IS exactly why it gets 10 kg heavier.

The initial mass or weight or external volume of the tank is irrelevant to the change in buoyant force.

Ships sink because they displace too little water when they fill up with water. Filling of the tank reduces the total displacement.

You have got it wrong too.
A neutrally buoyant tank with an internal volume of 10L and a weight of 15 Kilos displaces 15 Liters of water.
If you add 10L of water to the inside of the tank, it will still only displace the 15L that it displaced back when it was neutrally buoyant but it now weighs 10 Kilos more.
If you don't believe me, try it out in a hydro shop, the full tank will not displace a cc more that it did back when it was neutrally buoyant.

Michael

 

[Dan]

Technically, the displacement change IS exactly why it gets 10 kg heavier.

The initial mass or weight or external volume of the tank is irrelevant to the change in buoyant force.

Ships sink because they displace too little water when they fill up with water. Filling of the tank reduces the total displacement.

It should be less than 10 kg heavier in the water due to buoyancy.

Buoyancy force doesn’t change. It equals to the external volume X density of water.

What changes is the bulk density of the tank that contains water that is used to be air in it.

 

[michael-fisch]

No, are you?

It weighs 10 kg more in dry land, not in the water, due to buoyancy.

As a neutrally buoyant tank it has no buoyancy in any medium that it is neutrally buoyant in. O will always equal O.
A boat floating on the surface of the water will displace only it's own weight in water, if you add a few tons of weight it now displaces more water. A closed box or tank weather filled with Aluminum or Gold will still have the same displacement if it is completely underwater.

 

[BRT]

If the tank empty weight is 35 lbs and filled up with 24 lbs (11 liters) of seawater, then the tank would weigh 11 (35 -24) lbs underwater.

So if it were a much smaller, thicker walled tank, made from the same amount of aluminum, that would only hold 1 lb of water it would weigh 34 lb under water?

 

[Dan]

You have got it wrong too.
A neutrally buoyant tank with an internal volume of 10L and a weight of 15 Kilos displaces 15 Liters of water.
If you add 10L of water to the inside of the tank, it will still only displace the 15L that it displaced back when it was neutrally buoyant but it now weighs 10 Kilos more.
If you don't believe me, try it out in a hydro shop, the full tank will not displace a cc more that it did back when it was neutrally buoyant.

Michael

Don’t forget the weight of the tank. If the tank weighs say 14 kg, then you floods it with 10 kg of water, the total weight of the tank is now 24 kg. The buoyancy force is as you said, 15 kg. Therefore, the net negative buoyancy would be 24 - 15 = 9 kg.

 

[Dan]

As a neutrally buoyant tank it has no buoyancy in any medium that it is neutrally buoyant in. O will always equal O.
A boat floating on the surface of the water will displace only it's own weight in water, if you add a few tons of weight it now displaces more water. A closed box or tank weather filled with Aluminum or Gold will still have the same displacement if it is completely underwater.

Correct. And your question?

Buoyancy of a loaded boat equals to the weight water volume that being displaced. Heavier boat would be floating lower than lighter boat as the added weight increased the buoyancy of the boat. If you add a ton of weight to the boat, the boat would displace a ton of water & float at lower position, closer to the sea surface.

 

[Steve_C]

There is a marvelous boat elevator in northern germany I saw once. there is a sharp drop between two plains. To connect canals they build this boat elevator. Lower canal ends in a box that can be closed at both end. Then the box and water in it is counter weighted so that aside from friction zero work is done in lifting the water. Neat part is that when a boat enters the box it displaces water equal to its weight. So that no matter the size of the boat or its contents the box and its contents is always the same. Still operating close to 200 years later. Not sure if this is the one I saw but gives the idea
Niederfinow Boat Lift - Wikipedia

 

[Pedro Burrito]

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A ScubaBoard Staff Message...

Thank you for your continued interest in this physics discussion. This has been moved from: Accident & Incident Discussion - Northernone - aka Cameron Donaldson​

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[grf88]

There is a marvelous boat elevator in northern germany I saw once. there is a sharp drop between two plains. To connect canals they build this boat elevator. Lower canal ends in a box that can be closed at both end. Then the box and water in it is counter weighted so that aside from friction zero work is done in lifting the water. Neat part is that when a boat enters the box it displaces water equal to its weight. So that no matter the size of the boat or its contents the box and its contents is always the same. Still operating close to 200 years later. Not sure if this is the one I saw but gives the idea
Niederfinow Boat Lift - Wikipedia

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