A Physics Brain Teaser

[Man Overboard]

Walrus
I thought of a million things that could go wrong, so decided the safest thing to do is stay clear of the rig. You are correct, ascend to quickly, and you will suffer the bends, and could die. I was most afraid of the door coming loose from the chain, or the drum rupturing, and then helicoptering down on top of us. we swam a fair distance laterally to be safe.

[Man Overboard]

Hansp77
Did you pull it out on shore and realise it wasn't a door?

Ha! I guess it wouldn't have been that big of a deal, as we were just seeing if we could do it. A group of divers a year or so earlier went to much trouble and expense to lift an 8 foot high anchor off the bottom of one ship that had sunk. It took them about a week, and some expense. They got it to shore and on to their trailer when the owner of the salvage rights showed up with the local authorities. If you find gold, don't tell anyone.

[Raggi_Thor]

Tom, you are keeping half of Europe awake!
The next 10 minutes I'll be in my car, when I come home I hope you have another hint :-)

[Man Overboard]

Ragnar,
Now I can't just give away the answer. Read post # 30 again especially quote 2.
Just as with a boat you can't design for just static states of equilibrium, I too should have thought more carefully and considered dynamic forces.

[Raggi_Thor]

Now at home with wine and cheese :-)
...
I think the speed was so high the the door jumped out of the water, up in the air, and made some sort of problems.

[Raggi_Thor]

Hmm, the drum is very light compared to the door...

/RT

[Man Overboard]

Now at home with wine and cheese :-)
...
I think the speed was so high the the door jumped out of the water, up in the air, and made some sort of problems.

Ragnar, you better cut back on that wine a bit.

[DanishBagger]

Damn, I couldn't get back until now.

I'm glad I was close :-D

I can just imagine you trying to move all that mass while wearing flippers.

[Poida]

I am beginning to think that this is not a case of what did I do wrong with the imformation that I have given you, or with the applied priciples, rather than what did I do wrong that I haven't told you about.

This could include not filling up your own cylinder with air before descending or forgetting to put on your boyancy jacket.

For example, nobody that I know that does this sort of activity would drop drums down anywhere near the object you want to surface and then drag them across the ocean floor. They attach a line to the object up to the boat so the drums sink down to where the object is.

I know you left the bungs out of your boat so half way up the door met your boat on the way down.

[Man Overboard]

Danish Bagger
I can just imagine you trying to move all that mass while wearing flippers.

Danish has it correct. Even though the drum had only about 10 lbs of negative buoyancy, there is still 249 lbs (113 kilograms) of mass due to the water inside the drum. A diver is for the most part neutrally buoyant; so it is not very easy to stand on the lake bottom, as you tend to float around. If you grab the drum, and place your feet on the lake bottom you can lift the drum, but the motion do to lifting creates inertia. Once you get the drum moving in a direction, it just keeps right on going. My dive partner tried swimming up and grabbed it like superman, trying to swim as hard as he could to direct it to where we wanted it to go, but that wasn’t very successful. Eventually we decided rolling it would be best due to the fact that as soon as it floated up at all it was completely uncontrollable.

Poida
For example, nobody that I know that does this sort of activity would drop drums down anywhere near the object you want to surface and then drag them across the ocean floor. They attach a line to the object up to the boat so the drums sink down to where the object is.

Now this would have been smart, had I known moving it was going to be so difficult. I have since learned that this type of work is done with heavy bags, kind of the shape of a hot air balloon. The bags can be moved in place at will, and then opened up and filled with air after they are in place.

[rayk]

Man Overboard,
I am thinking you would have had to move a couple of hundred pounds of water 10' across the bottom, while you have almost zero bouyancy...
Like a butterfly pushing a brick...
While salvaging a 30' boat in 30' of water, my friends and I came across the same problem. We used a 20lb anchor on the drum(44gal) and pumped in a little air to get ours off the bottom and swam them around.

[Man Overboard]

Rayk
We used a 20lb anchor on the drum(44gal) and pumped in a little air to get ours off the bottom and swam them around.

Hey, now that is good idea. I remember trying to add a little air to do the same, but since there was no significant weight to the drum, the drum would just want to float up, you then had the problem of the little amount of air in the drum expanding, giving the drum more positive buoyancy. Ryak, we didn't have a sharp guy like you around, so we had to learn by trial and err.

[rayk]

We had to figure it out after sinking the first ten drums in a big heap.....not too close to the boat, dont want to ding the wreck up....

The rest slid down a rope tied to the rails on the other side of the wreck.

It seemed such a simple plan....