Flotation chambers
Below are some thoughts about building flotation chambers with plywood and epoxy construction.
Flotation chambers are important for safety. It is important to make a boat buoyant when flooded, ideally buoyant enough that it can still maneuver in the flooded condition. Unballasted boats offer the opportunity to use flotation chambers to make the boat highly buoyant when flooded. This is an important advantage of unballasted boat types.
It is important to use flotation chambers to allow bailing out of a flooded boat. This means that there is adequate flotation low in the hull to keep vents and other hull openings above the water’s surface. If self-bailing under flooded conditions can be achieved, this is an important benefit to a design.
The interior of flotation chambers should be coated with epoxy and not just painted. They should be treated as if they will someday get wet. Because they are impossible to access after construction, they need to be treated in a way that they will not rot or swell when water gets into the flotation chamber.
Flotation chambers should filled with foam or other material that provides flotation, even if the chamber is holed. There is a weight penalty for using foam or other buoyant material, but it provides an additional level of safety.
Flotation chambers should filled with foam or other material that provides flotation, even if the chamber is holed. There is a weight penalty for using foam or other buoyant material, but it provides an additional level of safety.
Not IF the chamber is holed but when (I have one of the original T-shirts). On my boats I’ve used very large Ziploc bags full on foam peanuts or empty capped 2 liter plastic bottles in heavy duty bags taped shut. Avoid cast in place 2 part foams, longevity can be an issue.
Cheers,
Skip
Don’t fill anything with foam. If a filled chamber is holed, you have a lot of work to scratch the foam out, if you want to repair. Use eg. table tennis balls, plastic bottles which are easy to remove.
Cheers Othmar
Oddly, empty plastic drinks bottles are not much good. They collapse after a while.
Plastic bottles are still possible, if you use the more stiffen PET bottles. Also you can fill thin bottles with cheap insulating foam, and close it afterwards, so that they are watertight, and the foam can not suck water.
If they’re inaccessible how do you dry them out when [ not if ;o) ] they get wet ? Surely it’s better to give them a hatch and fill them with bottles, peanuts, whatever which you can retrieve and refresh while drying the interior. Or am I missing something ?
A hatch is desirable. It can be opened when the boat laid-up, so allow the drying out. Even a good epoxy coat is not 100% effective and some water will be absorbed.
Also gives access should there be any damage to inspect / repair.
And somewhere to hide valuables?
You don’t need to fill a flotation chamber (compartment) with foam or any other material if you have enough compartments. In commercial shipping there are rules for designing for flooded conditions. For example for some classes of vessel it may be assumed that up to 20% of the length of the hull may be holed and that the hole breaches two compartments. You have to design the compartments so that in this condition the boat will not sink and will not list excessively. For personal craft you don’t need to be as stringent (unless you want Lloyds or some other certification), but it is worth considering this type of design strategy for safety.
Mal.
Another consideration that I have heard of, for small boats like ours, is to have your flotation devices (bottles, foam “bricks, whatever) to be inside a net, that is attached at the 4 corners of the top of the “chamber”, underneath the deck.
The idea is that if the hole in the hull is really big, you do not want your bottles or pieces of foam to float away from the boat, through the hole in the hull…
Cheers,
Laurent
Oddly, empty plastic drinks bottles are not much good. They collapse after a while.
The catamaran “Plastiki” used several thousand 2-liter recycled plastic bottles for all flotation (the boat was hull-less; just bottles) To maintain rigidity, they slipped a measured piece of dry ice into each bottle then screwed the cap down tight. As the ice sublimated, the bottles interior pressure went (by design) to 150 PSI. Bottles will remain rigid for a long time when they start at that!
I wonder, how much the high pressure air masses? 150 psi is about 10 bar. Does that mean there is 10 times as many air molecules in the bottle, or is it 2^5th power? 20 liters of air wouldn’t mass much, but 2^5 liters?? Sorry. Senior moment. 😉
Plus, shoot, CO2 (100%) is heavier than air (80% N2 + 20% O2), so add the threes, carry the two… CO2 is a little more than half again as dense as air—so more like the equivalent of 33 liters of air in each 2 liter bottle. ~40 g/bottle; maybe an ounce and a half? I guess we can live with that.
Dave
I wonder, how much the high pressure air masses? 150 psi is about 10 bar. Does that mean there is 10 times as many air molecules in the bottle, or is it 2^5th power? 20 liters of air wouldn’t mass much, but 2^5 liters?? Sorry. Senior moment. 😉
Dave
It’s 10 times the mass. P1*V1/T1 = P2*V2/T2, so there is a linear inverse relationship between pressure and volume. To get ten times the pressure, you need ten times the mass of air in the same volume. So CO2 would normally weigh about 1.9g/l under normal conditions, so it will weigh about 19g/l at 10 bars, which is about 1/50 times the weight of a similar volume of water.
Mal.
It’s 10 times the mass. P1*V1/T1 = P2*V2/T2, so there is a linear inverse relationship between pressure and volume. To get ten times the pressure, you need ten times the mass of air in the same volume. So CO2 would normally weigh about 1.9g/l under normal conditions, so it will weigh about 19g/l at 10 bars, which is about 1/50 times the weight of a similar volume of water.
Thanks, Mal; knew you’d know. 😉 1/50th may seem like a lot; air at ambient pressure is often quoted as 1/800th the mass of water, but in actual units, it’s only what, 18 g/l difference? A typical small boat with crew masses what, 250 kg? That still 600-800 times as massive as a dozen 2 liter bottles, filled with ambient air *or* CO2 at 10 bar, right?
Just did the math, “2 pound” foam masses 32 g/l. Bottles still lighter!
Dave