Buoyancy flotation bags to keep boat from sinking

[nepal]

The boatbuilders I mentioned are using foam on fixed places. It is more reliable and comfortable. This article explaines a lot.

https://dl.dropboxusercontent.com/u/...%2002-2005.pdf

[OS2Dude]

I would think the low pressure bags inside a compartment to displace water would do the trick if the boat is still afloat. If you're taking on water you just want to keep it out as much as possible. If you can fill the space with air then there won't be that much water to weigh the boat down, would there?

[captlloyd]

You all should watch the video that the moderator posted if you have not seen it before. In a lot of instances it is surprisingly easy to stem a leak enough to save the boat and make more substantial repairs. This is something every offshore should be considering.

[scottorious]

all good responses! Thanks for everything. I do agree it's not the answer the trumps all other answers. A liferaft isnt immune to fire damage either though. Depending on where the raft is stored. Some combination of built in buoyancy, watertight compartments(think James Baldwin) and some flotation bags might keep your boat afloat long enough to make some kind of repair. Cool thoughts though everyone!

[MartinAdelaide]

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....

True but the sea bed is littered with wooden boats, so don't count too much on the buoyancy of wood.

[RPZ]

How about a really BIG bag, a sit harness, and a cylinder or two of helium?

[Dave22q]

One of those great ideas that don't stand up well to careful consideration. I the flotation is not fixed it will rise to the cabin roof but if properly sized your boat will be stable and float. Hopefully you have arranged rescue because your power will be down and your access to any supplies in the cabin will be difficult if not impossible. Your boat may be salvageable but recovering from immersion is not cheap. On the other hand if the flotation is fixed to the hull and you have enough your boat may float but the center of gravity will be a lot higher. The stability created by your heavy keel will be reduced, you will be less resistant to rollover and the boat sinking as you take on water above your flotation, in effect increasing the weight of your boat. It might work but first you have to figure out how to do it. Certainly on my boat there is no room to place 240 CF of foam below the floorboards to support 15,000 lbs of weight.

[jkindredpdx]

I don't think it is a crazy idea. Modern materials are quite puncture resistant... perhaps allow the inflated bags to float to the cabin top. The boat would be swamped, yet stable, and you could survive in the cockpit or on deck while also initiating hull repairs.

I wonder if the deflated bags could be integrated into the headliner like automobile airbags...

[mowerandy]

Originally Posted by Jdege

It's the total volume and the total weight that matter. The average density, if you will, not the densities of the individual parts.

trouble with this statement is that it sounds like you mean the total volume of the vessel, but if the vessel is taking water then that becomes a whole lot less relevant since its volume decreases by a litre for every litre of water she takes on. Volume then becomes the total volume of anything on board that continues to displace water, like airbags, large chunks of Styrofoam, bouyant timbers etc.

So what is on board is most certainly relevant, and potentially very important. Buoyancy calculations based on the vessels intact volume will be completely irrelevant here.

[zeehag]

in 1990 there was a company named yacht saver made flotation bags for inside yachts--as i have heard nothing more from that company i presumed them to have gone out of business never to have been heard from again.........

[Jdege]

Originally Posted by mowerandy

trouble with this statement is that it sounds like you mean the total volume of the vessel, but if the vessel is taking water then that becomes a whole lot less relevant since its volume decreases by a litre for every litre of water she takes on. Volume then becomes the total volume of anything on board that continues to displace water, like airbags, large chunks of Styrofoam, bouyant timbers etc.

So what is on board is most certainly relevant, and potentially very important. Buoyancy calculations based on the vessels intact volume will be completely irrelevant here.

It's the volume that's not filled by water. If the hull is holed, it's the volume of the hull, bulkheads, watertight compartments, foam, flotation bags, whatever.

I thought that was clear.