The role of height in on required weight to reach neutral buoyancy

[markmud]

Another way to think about this question:

So, an iron boat that weighs 1 ton is floating in a small pool. Mark the waterline on the side of the pool, not the boat. What does the waterline (on the side of the pool) do when the boat is sunk under the water; does it go down or up?

Next, does a one ton block that is placed in the same small pool (without the boat in the pool) create the same waterline as the floating iron-one-ton boat or the sunk boat?

cheers,
m

 

[Ghost95]

Basically what all this discussion comes down to is you can estimate, calculate, and cipher like a double naught spy all day, but your best results will still come from an (and I know you don't want to hear it) in water weight check. Just make sure you'll be neutral at the END of the dive. Experience is the best teacher.

Good Diving!

 

[tursiops]

But I also realize (now) that when that bowl finally gets submerged, and water flows above it, it will sink the same as a 1 ton block of iron. So a tall person who ways the same as a small person will FLOAT better than the smaller person, but once under the surface, only density matters, not body shape.

That's it! Eureka!

 

[Hickdive]

Neutral buoyancy is far more an art than a science.

 

[Neilwood]

Basically what all this discussion comes down to is you can estimate, calculate, and cipher like a double naught spy all day, but your best results will still come from an (and I know you don't want to hear it) in water weight check. Just make sure you'll be neutral at the END of the dive. Experience is the best teacher.

Good Diving!

Agreed - doesn't matter how many spreadsheets, formulas, guesstimates etc you use - the best and only truly accurate way to determine required weight is by a simple weight check.

 

[The Chairman]

What does the waterline (on the side of the pool) do when the boat is sunk

African or European?

 

[84CJ7]

So as I understand it at this point we would need to look up the weight of a given volume of water at the temperature (and salinity if applicable) at the dive site and then measure the weight of the diver with full gear with a tank at 500 psi (or compensate for weight of full tank) and then calculate the water displacement of the diver with all the same gear to determine the difference in weight between the diver and the water they are displacing.
Then we would need to use that difference to determine how much weight would need to be added for the weight of the diver plus gear to exceed the weight of the water he is displacing thus achieving negative buoyancy?
Then we need to check the divers body for proper trim without any equipment so they will have to get into the water naked and lay level to see if they are head or foot heavy and see how much it takes to balance out. After that we calculate the weight and location of all the pieces of equipment on the diver including thermal protection and distribute the added weight for even trim on all three axis.

Sounds simple enough, but I will probably just stick to winging it. Or would that be finning it?

 

[Angelo Farina]

I think maybe I was making a mistake based on the difference between floating and buoyancy.

The point I was trying to make was thus: "Consider a 1-ton block of solid iron. As iron is nearly eight times as dense as water, it displaces only 1/8 ton of water when submerged, which is not enough to keep it afloat. Suppose the same iron block is reshaped into a bowl. It still weighs 1 ton, but when it is put in water, it displaces a greater volume of water than when it was a block." But I also realize (now) that when that bowl finally gets submerged, and water flows above it, it will sink the same as a 1 ton block of iron. So a tall person who ways the same as a small person will FLOAT better than the smaller person, but once under the surface, only density matters, not body shape.

Exactly !!!

 

[Ghost95]

African or European?

African, of course. There's no way a European waterline could keep me buoyant.

 

[Angelo Farina]

Another way to think about this question:

So, an iron boat that weighs 1 ton is floating in a small pool. Mark the waterline on the side of the pool, not the boat. What does the waterline (on the side of the pool) do when the boat is sunk under the water; does it go down or up?

Next, does a one ton block that is placed in the same small pool (without the boat in the pool) create the same waterline as the floating iron-one-ton boat or the sunk boat?

cheers,
m

1) As the specific weight of iron is 7.85, the volume of 1000 kg (1 ton) of iron is 1000/7.85=127.38 liters.
When the boat is floating, the weight of the displaced water must equal the weight of the iron boat, so the boat is displacing 1000 liters. When the boat is sunk, if no air is trapped inside its compartments, it will displace just the volume of iron, so 127.38 liters. Hence when the boat is sunk the waterline will go down, as the total volume of water+boat diminished by 1000-127.38 = 872.62 liters.
2) if the block is of iron, the water level will be the same as the sunk boat. If it is by concrete, or other material having smaller specific weight than iron, of course the level will be higher, but it will reach the level of the floating boat only if the block is made of a material with same specific weight as water (1), or smaller....
If the specific weight is smaller than that of water, despite the volume of the block will be larger than 1000 liters, it will displace always 1000 liters of water, and the excess volume will stay above the water line. Hence the waterline on the border of the pool remains the same as the floating boat when the block is also floating, having a specific weight smaller than water.