Weight of an object in water

[Cablava]

In one of Seahunters threads a topic came up regarding weight,so I thought I would clarify the calculation for you.

The weight of a submerged object is found by subtracting the weight of the volume of water the object displaces from its air weight.

Fresh water weighs 62.366 pounds per cubic ft.
Saltwater weighs 64.043 pounds per cubic foot.
The weight of an object in water then becomes:

Wsub = Wair - Ww

Where
Wsub = submerged weight of object
Wair = weight of object in air
Ww = Weight of water displaced

Since weight can be expressed in terms of density and volume, we substitute the known values of density. For Seawater this becomes

1/
Wsub/sea= Wair-(64.0430)Vobj

Where

Wsub/sea = Submerged weight of object in seawater in pounds
Wair = Weight of object in pounds
Wobj = Volume of object in cubic feet

For fresh water

2/
Wsub/sea=Wair-(62.366)Vobj

Where

Wsub/sea =Submerged weight of object in seawater in pounds
Wair =Weight of object in pounds
Wobj =Volume of object in cubic feet

Comparing Equation 1 and 2 you can see that an object submerged in fresh water will lose less of its air weight to the bouyant force of the water than it would in seawater. The object is therefore slightly heavier in fresh water. For a very dense material such as lead the difference is miniscule, 1 ton of lead will only gain 4 pounds per ton in fresh water. On the other had concrete will gain about 50 pounds per ton under fresh water.


So you want be a salvage man, you should see some of the other boring calculations you need to do, but as Chagy says in the another thread, "if you can’t pick it up add more lift bags", or get a bigger crane.

 

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[chipveres]

Not as accurate but easier to remember: Pilot's rule of 6-7-8. Go juice, either gasoline or jet fuel, weighs 6 pounds per US gallon. Lubricating oil weighs 7 pounds per gallon. Water weighs 8 pounds per gallon.
I learned that from a pilot who quit keeping a logbook after he accumulated 12,000 hours.

Chip V.

 

[Cablava]

Usefull for delicate lifts, but air works real quick if you have enough of it.

 

[SEAHUNTER]

In trying to understand this I assumed some of the following ideas. A gallon is equal to a cubic foot. If a cubic foot of stone weighs one hundred pounds, yet it is displacing a cubic foot of water which weighs sixtyfour pounds, then the stone feels like it weighs thirtysix pounds under water. Is that a reasonable explanation?

Seahunter

 

[ScubaDude]

You are correct.

 

[riobravo]

except for the 1 gallon = 1 cubic ft, that is not correct, the balance of the analogy is supportive

 

[mariner]

Remembering too that the US has a smaller gallon than the rest of the world, so best to use a fundamental equation, like Cablava, though I think it is displaced mass, strictly speaking, rather than weight.

Mariner

 

[Cablava]

An object in water is lighter than in air, the clearest example is go an jump in the water, The human body loses almost 100% of its weight when immersed.

Weight is determined by the density of an object times the volume of that object.

e.g. lead is so dense it is not affected significantly underwater. Aluminium will lose 38% of its weight underwater.


Scott your stones example depend upon what type of stones as some are denser then others. So you need to know the density of the object. Lets look at your cannon for an example. I'll assume its made of cast iron and its weight per cubic foot would then be 450 pounds (wrought iron would be 485 pounds bronze would be 544) lets not get clever and think that it has lost some of its weight over the years, it has but there is no calculation for that.

So we have the density number for the calculation, now we need the volume of the object. so lets say the cannon is 8 feet long and for the sake of this calculation say it was 8 inches diameter uniform along the cannon then you have the bore say 3inches and is 7 ft long. So lets find the volume.



1/ Get the volume of the cannon in this case keep all dimensions in inches

The Volume of the cannon is two calculations, the whole volume minus the bore volume.

Volume = pye*r2*Length 3.142 * (4” * 4”) = 50.272 * 96” = 4826.11cubic inches


Bore Volume 3.142 * (1.5” * 1.5”) = 7.0695 * 84” = 593.84 cubic inches


4826.11 – 593.84 Volume = 4232.27cubic inches

2/Convert to cubic ft and multiply by the density of iron 450

4232.27 / 1728 = 2.79 cubic ft * 450 = 1256.86 lbs


Next we need to get the weight in sea water

The submerged weight = air weight minus the buoyant force.

To get the submerged weight then you need to take the 2.79 cubic ft and make the following calculation using the density of iron 450

450-64.043 = 385.96 385.96*2.79 = 1076.83 lbs



Hence the cannon weighs 180lbs less in sea water.

 

[ScubaDude]

Mike, Thanks for your support and facts for arguments sake. It still boils down to blow up a bag or two, if that doesn't get it add another. Old School salvage.
Regards,
Brad

 

[ivan salis]

this why a sea going ship that is balanced "properly" for saltwater must redo its trim/ ballast as it comes into freshwater -- since it now "weights more" displacement wize -- otherwize its balance would be thrown off possibly causing it to sink.

 

[rgecy]

Cablava,

Great example and explanation!

As for the big stones on the Jupiter site, its pretty cool to be able to lift them or roll them over. Some of them are as big as a car. Just make sure your buddy doesn't drop his end before you do! Those big rocks still hurt pretty good, even under water! Especially when they land on your ring finger knuckle! OUCH!

 

[Jeffro]

Try some half in, half out sometime....

We had fun with this one. Built a coffer dam to help displace some of the weight. Tiny creek, boulder just a little smaller than a small car. Moved it with come-alongs. No air lifting possible here! LOL!

Where there's a will, there's a way.

I like Brad's opinion.... its gonna move regardless of how much it weighs!