A Physics Brain Teaser

[Man Overboard]

The following is a true story, I laugh at it now. An intuitive designer should be able to catch my mistake.

I started diving with my father when I was about 7; some 40 years ago, so by the time I started wreck diving (lake Superior) in college I had a fair amount of experience behind me. On one particular wreck, I came across a medium size door that I wanted to salvage. My plan was to lift the door to the surface with a container filled with air and tow it to shore. I calculated that the door weighed between 150 and 180 lbs. Realizing that I would have to displace that much weight to lift it to the surface, I looked around for a suitable container. I decide on a 30 gallon drum that weighed about 10 lbs. I attached chains to it, another 3 lbs. I took the drum to the dive site, and filled it with water and let it sink close to where the door was. I donned my dive gear, grabbed an extra air tank and descended to the site. But, alas, I encountered a problem. Who can tell me what I overlooked?

[Trevlyns]

Duh!
How will you displace the water in the 30 gallon drum?!!

[Trevlyns]

Duh!
How will you remove the water in the 30 gallon drum - now firmly submerged in water?!!!

[Raggi_Thor]

I think you where able to fill the drum with air, but that the preassure became a problem when you approached the surface? How deep where you?

[Poida]

Firstly you wouls have to pump the air in and have a valve and a pressure relief valve at the bottom.

As you pump the air in, you have to release the water, or force the water out at the bottom. No good doing it at the top or you will simply vent the air. As the air (incidently I work in the metric system and I can't be bothered converting to work out working out bouyancy) displaces the water and bouyancy occurs the vessels will rise lifting the object. The pressure relief valves will insure that the inside pressure does not exceed the outside pressure to an extent that the vessels burst.

Don't anchor your boat above the rising gear as it can surface at quite a rate of knots and damage your boat. And, as you are lifting it doesn't always lift vertically and still damage your boat.

[safewalrus]

Thus resulting in the boat passing the door on the way down! Makes for an interesting way to go home!

[Man Overboard]

Ragnar
I think you where able to fill the drum with air, but that the preassure became a problem when you approached the surface? How deep where you?

Poida
Don't anchor your boat above the rising gear as it can surface at quite a rate of knots and damage your boat. And, as you are lifting it doesn't always lift vertically and still damage your boat.

These are good points that I actually had thought of. After dropping the tank, I moved the boat some distance away.

Trevlyns
How will you remove the water in the 30 gallon drum - now firmly submerged in water?!!!

Look at the diagram and consider if you think this would be a problem. The hole in the bottom of the tank was about 3 inch / 7.6 centimeter in diameter.

[Raggi_Thor]

I am getting curious :-)
No problem with the air in and water out.
But did it accelerate too fast?
Did you get a jet effect when the hole thing started to lift after 90 liter of air and the remaining 23 liter of water was pushed out as the water pressure dropped?
Or did the door start to rotate, like a helicopter?
Or did you reach the surface and then the drum started to take in water and sunk again?

Can you give us a hint?

[Man Overboard]

Ragnar,
I see that you are a Navel Architect, which makes me feel a little better; at age 19 I didn't consider all of the physics involved in my plan. The oversight isn’t readily apparent, but it is an issue that has implications for some of today’s boat designs. I will give a hint after the post has been up for 24 hours.

By the way, all of the incidences that you mentioned where a concern, but as it turned out, lifting the door went quite smoothly.

[Raggi_Thor]

I'll have to sleep on it (if you can say that) :-)

[longliner45]

did you tie a rope to the drum?

[SamSam]

It all rose so fast the door got the bends and was no longer usefull? Sam

[DanishBagger]

You forgot to calculate the weight of the scuba tank?

That when filled with water, the drum would be rather hard to keep "floating" with the hole underneath, before it filled up?

The lift would have to overcome the suction, so the attachment point was ripped off of the door (it being rather heavy and all)?


I can't wait for the answer - I really like this sort of thing, but at the same time I really hate it.

Promise you will back with the answer!

How many litres are there in a scuba tank, btw? Never dived, but, no it couldn't be that simple, could it?

[DanishBagger]

Come to think of it, after rereading your post, the most likely in my opinion, is that you couldn't manhandle the waterfilled drum, while at the same use the scuba tank.

[Poida]

I thought from your description that you was hosing the scuba tank directly into the drum, hence the reason I put in the pressure relief valve so the drum would not burst from the pressure of the scuba tank and also the extra pressure from surfacing would not burst the drum.

From your sketc I don't think it would work because where is the pressure coning from to displace the water? The air in the drum would reach 1.8 bar and then not be able to push anymore water out.

Also if you want to hose the scuba tank up to the drum you would come out of the gauges outlet to give you full pressure and not through either the first or second stage regs.

Anyway whats the prize for getting this right?

Poida