Sunday, January 29, 2017

Pnumatically Stabilized Floatation

Also known as "sea cells". . . .I thought I would put on some notes on their use and operation since we've lived with them for a few months.  PSF floats are basically open ended containers, open side down, trapping air for floatation. 

A PSF flotation cell as opposed to a barrel float.
The Great advantage of the sea cells is the ability to top them off, flotation wise.  Here's the thing.  As water temperatures fall in winter, the air inside pontoons or barrel floats contracts.  In some cases, particularly with flexible barrels, the float kind of collapses.  You can lose upt to 10% of your flotation as the temperature drops.

Open bottomed cells can be easily topped off to maintain the same level of buoyancy, or air can be drawn out to make the cell sit lower in the water.   Here's some of the things we've learned using them, though:

1) There's a temptation to fill them all the way with air, and that's fine, but if you do so during cold water days, when the water warms, the air will bubble out as it expands, then when the water cools again, you'll have less air in them than when you started.  A buffer of water should be left at the bottom to keep the air level stable, even though it will contract with temperature.  This "plug" of water also helps keep the boat stable, as the cell will try to lift all the water with it when pushed upward by a wave.

2)  These things are not hydrodynamic.  Wave action, currents, boat motion, can cause air to be mined out of them, reducing your flotation.  Moving the vessel at any speed is right out.  While topping them off and keeping track of the air level isn't difficult, you do have to keep on top of said. 

3)  All that being said, they do work, and work inexpensively.  There is also a great possible application for collapsible floats or docks as the units could potentially be nested and stacked when not in the water.

Thought you'd like to know.  :)

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  1. Mungo,

    I came up with a similar solution 35 years ago for different problem.
    On large diesel trucks, the fuel pump delivers much more than the engine requires. About 6 out of 7 gallons/litres/firkins of fuel are returned to the fuel tank. The fuel can pick up engine gases bleeding past the injectors, especially if an exhaust brake is used. This can amount to much more than the total volume of the fuel tank per hour, intermittently. So, the tank must vent out to avoid over-pressure, as well as vent in to avoid collapse as fuel is withdrawn. Over-pressure can be especially scary, since the engine can runaway in a scenario such as a long downgrade. To complicate matters, the vent must not allow fuel to escape, even if the truck rolls on its side or upside down.
    So, the airflow is closable by a float. The float must be heavy enough such that a high airflow does not "fly" the float shut. By making the float with an open bottom, it will not float open if the truck is inverted since all plastics are denser than diesel. To assist in the inverted case, there is a ball bearing in the float cage which is normally below the float when upright.
    Most large diesel trucks now use this tank vent. For extra clean operations, the vent can exit into a spittoon.

    1. Clever idea. Hopefully you're getting royalties :)

  2. What is the size of the containers ?????

    1. we have one 60 gal displacement and 4 29 gallon

  3. What ponton-form will you use in the end ????

  4. Still postulating on that. The new version of Floating Empire will actually have a barge-type hull.