Surface area of an object effect recovery?

[romad119]

Hello,

I'm going over recovery of submerged objects and have learned about how weight and the volume of water an object displaces effects the amount of lift needed to recovery it.

How does surface area effect the buoyancy requirements? For example, a door being recovered on its side (low amount of surface area upwards) versus it being recovered laying flat (high amount of surface area).

Thanks.

 

[Bubbletrubble]

Regardless of the orientation, the object should displace the same amount of water (all other things being equal), right? Good luck trying to make the door float on its side, though.

Surface area should theoretically have no effect on the buoyant force exerted on an object.

 

[SurDo2]

In a word... No.

 

[solowgregg]

The orientation of the door would not effect anything other than the speed at which the door would rise. Greater surface area equals more resistance, greater density equals more water displacement to raise.

 

[tc]

While the statements above would be correct for a lift in mid-water most of the time thats not where we lift objects from, we lift from the bottom and failing to consider the bottom surface type can be disastrous.

The only way the surface area does not matter is if the object is resting on a rocky bottom or other surface that is so hard that suction has not developed between the object to be lifted and the bottom.

For most bottom surfaces- silt, mud or sand there will be a significant amount of suction developed over time between the object and the bottom. This will of course increase over time as the object sinks further into the bottom.

This suction can require MANY times more lift than is actually needed for the lift itself- resulting in an object rocketing to the surface in an uncontrolled lift if it is not taken into account and dealt with.

You can sometimes rock an object back and forth to reduce the suction, get a pry bar or other object underneath to disturb the suction or break it on one side, excavate under the edge(s) to break it up a bit or even inject air underneath the object to loosen it.

In the door example lifting a door with just the edge embedded it would be relatively easy to rock it back and forth to loosen it unless it was deeply embedded. A door lying flat and sunken in even a little bit would take much more effort and planning to loosen.

Sometimes you can get away with lack of planning for smaller lifts but if you're lifting larger objects loosing control of a lift can easily result in injured divers.

If I recall correctly the Navy had or has a salvage manual that has the formulas for doing the calcs of lifting- if you'll be doing this a lot then this would be a worthwhile investment for you.

 

[Diver0001]

The shape and distribution of mass of the object can also have a big impact on how you lift it.

An automobile, for example, is much heavier on the engine side than the other side and teh centre of gravity can be quite high.

If you tried to lift it straight up in the orientation it would be if standing on land then you would (a) find that very difficult to control and (b) once the object broke the surface the centre of gravity would be too high and it won't behave. IN the case of a car, for example, it would probably flip over upside down once you got it to the surface even *if* you managed to get it to the surface in the orientation you thought you wanted.

Staying with the car example, What TC says is true too. Assuming the car was standing upright on the bottom and it's whole bottom was in contact with the mud then suction becomes a big issue. Think of the bottom of the object as a big suction cup. If you were to try liftin it with one big baloon on each wheel then that would be like trying to pull the suction cup straight off of whatever it was stuck on. But if you tried to lift a car by only one back wheel (assuming you found a point to attach that could take the weight) then it would be like pulling the edge of the suction cup away, which would make the whole object come loose from teh bottom easier. Also in this example, lifting it from one back wheel would leave all of the mass under the balloon so no part of teh car would break the surface and it couldn't move strangely once you got it there. IN other words, the lift would be easier to initiate and more controlled. (incidentally, if you don't want to experiment with a car, you can use an old bicycle and just tie a 20kg weight onto the front forks).

All this is to say that where you attach the balloon(s) has to be consciously and carefully chosen and that poor choices can lead to problems.

So bringing this back to the OP. The first couple of posts are correct that the orientation of an object under water won't affect the total amount of water it displaces but the shape, surface area and mass distribution are critical factors to consider for *how* you approach the lift.

R..

 

[solowgregg]

While the statements above would be correct for a lift in mid-water most of the time thats not where we lift objects from, we lift from the bottom and failing to consider the bottom surface type can be disastrous.

Oh absolutely. The only reason I did not explain that was my feeling that it was not part of his question. And one is taught to "shift" or "rock" loose an object (at least in the S&R class) before lifting if possible. But you are absolutely correct, one must consider suction to not only decide how to lift an object but to stay safe while attempting to do so.

 

[romad119]

Thanks for the information. Seeing other factors is great, I'm the type that always questions things that seem a little too simple, such as the formula being taught, seemed to ignore other things and raised my eyebrow.

 

[romad119]

Here is a link to some Navy Salvage pubs that can be downlaoded that include mention of friction coefficients for various bottom types. Thanks for the tip.

SEA 00C2 Salvage Publications and Technical Documentation