Buoyancy flotation bags to keep boat from sinking

[scottorious]

I guess some math could give me a close estimate on how much airspace I would need to float my boat if it was totally full of water, does anyone use any inflatable bags to keep their boat from sinking in case of catastrophic Leak. Seems like a better option than depending on a life raft for some scenarios.

[Jdege]

How much does your boat weight?

Seawater weighs roughly 1000 kg/m^3 - 64 lb/ft^3

[scottorious]

9500 pounds with 3700 pounds of lead as ballast.

[denverd0n]

Of course, some of that 9500 lbs. is certain to be wood, which is lighter than water.

[Jdege]

Originally Posted by scottorious

9500 pounds with 3700 pounds of lead as ballast.

So you need 150 cubic feet of volume to float.
150 ft^3 of buoyancy bags would be overkill, though. What's the wetted surface of your hull and its volume?

[scottorious]

Right, I think I'd rather play it safe in this little mind experiment, I'm guessing if it was a good idea I wouldn't need to be asking this question. I just always wonder what could be done to make the boat safer and less sink able instead of trusting a life raft, an epirb and hoping your rations hold you over.

[scottorious]

I actually don't know that, it's a cal 34. Does anyone ever use inflatable bags in this fashion? I just kept getting salvage hits.

[xslim]

There is an Australian company that sells those buoyancy bag, they can be inflated from a scuba tank.

But from what I remember, you need like 6 of them, and they are quite pricy.

[Cheechako]

a cubic foot of air supports 64# IIRC. So a 10000# boat needs about 160 cu ft of air to be neutral. ere is some air in the open space in the fuel and water tanks as well.
I guess one thought might be, is a boat full of water but floating worth saving? Is your money better spent on a life raft?

[Jdege]

Originally Posted by denverd0n

Of course, some of that 9500 lbs. is certain to be wood, which is lighter than water.

The density of the individual materials doesn't matter, only the total weight and the total volume.

[DotDun]

Originally Posted by Cheechako

a cubic foot of air supports 64# IIRC. So a 10000# boat needs about 160 cu ft of air to be neutral. ere is some air in the open space in the fuel and water tanks as well.
I guess one thought might be, is a boat full of water but floating worth saving? Is your money better spent on a life raft?

Since this is a brain exercise.....160 cu ft is (2) Aluminum(80) scuba tanks filled to 3000psi ????

[Cheechako]

Originally Posted by Jdege

The density of the individual materials doesn't matter, only the total weight and the total volume.

Hmmm.... actually it does right? a big piece of timber weighing hundreds of pounds floats in water.... it doesn't need an air bag to hold it up. I suppose it's the difference in density that matters... wood vs water, or lead vs water. A 100# rudder in air doesn't take 100# of effort to lift if it's in water....

[Lepke]

Where will you be? The inflated bags will be at the cabin top.
I've used them in salvage years ago. Usually outside bags with slings to lift and light weight, more flexible bags inside at low pressure, just to expel water. The bags take a lot of abuse from rubbing sharp objects and tend to break up things like cabinets and cabin tops not designed to take the weight of their vessel. Lifting with only inside bags will probably displace bulkheads and could separate most fiberglass 2 piece hulls.
Unless you're near shore, rescue won't include salvaging your boat.

[Jdege]

Originally Posted by Cheechako

Hmmm.... actually it does right? a big piece of timber weighing hundreds of pounds floats in water.... it doesn't need an air bag to hold it up. I suppose it's the difference in density that matters... wood vs water, or lead vs water. A 100# rudder in air doesn't take 100# of effort to lift if it's in water....

Nope.

There's a downward force based on weight and an upward force based on volume. Density can sometimes be a useful shortcut when dealing with homogeneous materials, but when dealing with complex shapes and heterogeneous materials it's just a matter of volume and weight.

[Jdege]

Originally Posted by Lepke

Where will you be? The inflated bags will be at the cabin top.
I've used them in salvage years ago. Usually outside bags with slings to lift and light weight, more flexible bags inside at low pressure, just to expel water. The bags take a lot of abuse from rubbing sharp objects and tend to break up things like cabinets and cabin tops not designed to take the weight of their vessel. Lifting with only inside bags will probably displace bulkheads and could separate most fiberglass 2 piece hulls.
Unless you're near shore, rescue won't include salvaging your boat.

If you want the boat to sail, you'd need to have the bags fill the hull below the waterline. And how would you keep them there? Permanent structures built into the hull and bulkheads? Wouldn't it make more sense to build in waterproof bulkheads and flotation chambers?