MarineAssist
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Prep work for the new rental dock
Yesterday we were working at Jamestown Marina, installing some large float tanks under the rental office. The marina is getting ready to have some new
docks installed at houseboat rentals, due to the damage sustained in the late January wind storm we had. A company from Georgia has built those docks
and is there on scene working this week and next installing them.
The problem that came up is the fact the new docks sit about 8" higher out of the water then the old docks. The marina asked us to help with a
solution, so we supplied and installed 8 roto molded lift tanks. These are 3'X3'X8' tanks that have holes drilled in the bottom. They are filled with
water to sink them and then they are installed by my divers under the rental office. Once in place they are then filled with air to lift the 30'X30'
floating structure. Each tank lifts ~3800 pounds. We ended up lifting the structure about 6" higer in the water than it was before. By the time the
soda and ice machines are transferred from the old dock to the new one, the height should pretty much match up so no ramp will be needed.
These float tanks work great for things like this. They can also be refilled with water and moved as needed. They can be filled with air to get the
exact lift you need, up to the capacity of the tank. If they lose air over time, a diver and easily top them off again.
It is a solution I thought some of you might think was interesting.
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NautiNuff
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Pretty cool Don.
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E_HILLMAN
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neat, how are the holes plugged?
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MarineAssist
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Gravity! 
Air goes up, water goes down. Air is trapped in the tank and will not come out the bottom unless displaced by water.
Same concept as a Hydro Hoist boat lift.
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E_HILLMAN
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Hmm, that was the part I didn't get. I just wondered if the dock got pushed down if it would cause the air to bubble out any. Wasn't sure about the
dynamics of that. Guess it works the same as a bell? So if it rocked much you might get a blurp?
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^~^~^~^~^~^~^~^~^~^~^~^~~^~~^~~^~~^~~^~
Buy Made in the USA, the job you save just might be your own.
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MarineAssist
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Air compresses as it gets deeper, so bubbling out is a non issue. It expands coming up. All my airbags have relief vents on them to keep them from
tearing from over inflation when they come to the surface from depth. Same reason a diver has to exhale when ascending.
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E_HILLMAN
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^~^~^~^~^~^~^~^~^~^~^~^~~^~~^~~^~~^~~^~
Buy Made in the USA, the job you save just might be your own.
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RELAX
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Archimedes principle, Boyls law and the general gas laws, is how humans deal with the extremes of the marine environment.
The dock might sink some depending on the water temp this winter.
In 1452 Mr. Leonardo Da Vinci, decided to tie a tanned cows udder over the top of his head which was attached to a narrow tube which was kept on the
surface by floating cork tree bark.
In 1797 a German named Klingert is credited with the first salvage dive when he recovered cannons using a metal pot lined with goat skin which was
attached to a compressor which had pistons in it to compress the air.
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Captain Bob
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Thanks for the photos, description and explanation, Don.
Regarding your diving and recovery jobs, I suppose that the airbags, while much more susceptible to rips & tears are just easier to store than the
molded lift tanks?
And, Relax, I like your post as well. I have been reading some very interesting books lately that you might also find interesting. They don't apply
to diving (which you could probably write a book on) but they do deal with the physical sciences. Watch your U2U in next couple of days.
Nobody cares how much you know, until they know how much you care.
-Theodore Roosevelt
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RELAX
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Cool Capt. Bob. thanks!
Looking forward to it.
Now if I just knew how to read and write I would be in great shape! 
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MarineAssist
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Quote: Originally posted by Captain Bob  | Thanks for the photos, description and explanation, Don.
Regarding your diving and recovery jobs, I suppose that the airbags, while much more susceptible to rips & tears are just easier to store than the
molded lift tanks?
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Storage is one concern for sure, but also an airbag will sink when totally deflated. That generally makes things easier on the diver when pulling them
down to a job. Those lift tanks are just a little above neutrally bouyant when full of water. You have to get on top of them to get them under the
surface. The plastic floats it's own weight. They weigh a little over 200 pounds each.
Also, a lift bag typically has a single lift point on the bottom of it, thus keeping the lift above the item you are lifting. That makes it much more
stable. If you tried to lift a houseboat with lift tanks, first you would need to get them under it on the bottom, which would be a task in itself.
Then you would have a heck of a time keeping them under it, once full of air. The air naturally wants to go to the surface, but the weight of the boat
wants to go down. All it takes is a little tilt one way and the tanks would end up on top.
Just ask any of the people that watched me raise that houseboat at Alligator 2 a few years ago. It tilted one way a little bit, and over it went, to
the awe of the crowd! The stern was stuck in the mud originally about 8', so when it started up with full air, there was no stopping it. That little
adventure ruined about $5k worth of my airbags.
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RELAX
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Most people think that the bottom of the Ocean is sand. However it is mostly mud.
I am sure that Don knows it takes between 20-30% of the total lift to first free the wreck from the mud if in fact it is in mud and not hard sand,
which means you have to have 20-30% x-tra bags to do the job which gets costly real quick!
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