|A PSF flotation cell as opposed to a barrel float.|
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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