# building a floating drydock with 55 gal poly drums

Discussion in 'All Things Boats & Boating' started by corsairf28r, Apr 24, 2015.

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### corsairf28rNew Member

hi thinking of building a floating drydock . I have been planning on using 55 gal poly drums ,pvc pipe and plastic pallets. a few things to think about such as how to push the water out of the drums so that the boat will rise out of the water evenly. should I use a single manifold that feeds each drum or individually plumb to each drum? how much PSI is needed to push the water out.

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### FMSSenior Member

High pressure is not needed but a high volume of air is needed to displace the water in a reasonable amount of time.

Study how the existing floating boat lifts on the market have solved the problem.

Design concerns would be a valve system to guard against leaks and maintain stability in case of individual component failure and/or uneven loading.

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### AndySGraySenior Member

For scuba diving we approximate 30 feet seawater to 1 atmosphere or 15 psi.

So if you had the barrels at 6 foot down, you'd only need 3 psi !

:!:Where this gets dangerous is that the pressure difference amplifies instability QUICKLY:!:

For example
If you had 2 barrels half full at 6 foot down, each is giving about 220 pounds of lift

Now what if one barrel moves up only a foot...

Well, the ambient pressure is now only 2.5 psi...
The air inside will expand proportionately to give a bigger volume and the lift for that barrel will quickly become 256 pounds, this will cause it to rise faster and generate more lift...

Conversely, if the other barrel moves down a foot, ambient pressure is now 3.5 psi - you still have the same amount of air but it now takes up a much lower volume and only gives 188 pounds of lift and would tend to be pushed downwards - all this is happening with the valves all closed and nothing you could do before it resulted in a vicious cycle tipping the boat off the side. Now without the weight of the boat the sunk side will breach the surface in a dangerous manner.

We use this technique to salvage sunken boats and it is often dramatic and sometimes dangerous.

For a small boat, how about a (always afloat) U shaped 'dock' where the boat can be driven in and a sunken cradle raised using winches or hydraulic jacks.

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