If you want a serious answer - you would be struggling to get enough buoyancy in that way to stop the boat from sinking - still less floating in a useful way (e.g. with enough freeboard)
For sailing boats only
Explosive keel bolts, shed all that dead weight instantly, resulting in the yacht turning turtle and placing the hole above the waterline. Immediate fix.
Save trees, eat beaver.
There was another system of internal bags in the 1970-80's called 'Unsink'.
I wouldn't bother...
last season a chum's Anderson 22 settled on a badly laid mooring sinker ( half tide mooring ! ) with the concrete & big steel ring etc a good foot proud of the seabed.
I had a call from a chum at the club who could see her sinking, but by the time the owner & i got to her all we could was remove the outboard engine and watch her fill on the flood tide.
We couldn't get at the large hole to fit a fothering mat.
At the next low water we attached 4 large bags around the hull, each bag roughly 3' long x 3' dia.
Even with these she only just floated off the sinker, it was way too dodgy for anyone to enter the cabin from the rescue boats, the stern was a few inches out of the water and the bows a few inches under.
We towed her backwards with a large traditional diesel launch, using a bridle tow to help steer her.
The boat kept 'taking over' the tow, going sideways despite a smaller rescue boat streaming from her bows as a drogue.
It was a VERY hairy tow ashore through the moorings; in the open sea she wouldn't even have made a usable raft.
I'd stick to 'unsinkable' foam buoyancy in a double hull, or a liferaft.
Last edited by Seajet; 07-05-12 at 13:09.
Is that based on any particular calculation, or just gut feel?
Originally Posted by bedouin
The book I mentioned includes calculations both for the authors' wooden boat and (if I remember rightly) for a "standard" GRP boat. The volumes of airbag required didn't seem implausible.
Consider the amount of foam between the hull and liner in, say, a Sadler 26. Imagine you went in with a crowbar and scrapers and hacked it all out - could you stack what you extracted on top of the five and a half berths? (which is where airbags are generally put, or rather under the cushions). Intuitively I think you could. Plus of course for a given buoyancy, pure gas takes up less space than plastic foam.
Airbags didn't succeed for various reasons, including cost, inconvenience of having them in the boat in normal use, the emotional appeal of a whole spare boat in the liferaft cannister, and no doubt others. But I can't see anything to
suggest that they didn't, in the narrow sense, actually work.
A few years ago I calculated how much buoyancy was present in the average boat, provided by the things that most will have. There are the obvious items like fenders (only four in Tom Cunliffe's case, something like 12 or 14 in mine) but half full fuel and water tanks, holding tank, a wide variety of containers and suchlike add up to a surprising amount. I suspect that not all that much inflatable buoyancy would be needed.
Originally Posted by bedouin
having seen the little episode I describe above, - photo added now - I am absolutely convinced air bags are impractical and insufficiently effective.
The boat was awash, even staying on deck would be untenable and absolutely no hope of self recovering to a vaguely steerable / sailable / habitable condition.
I had always wondered about inflating a liferaft in the cabin rather than lose the boat - airbags being I think impractical and vulnerable in normal sailing.
Now I would be reserving a seat in the liferaft pronto !
This photo shows a Sadler 34 that was hit by a ship, removing a sizeable piece of the port aft topsides. Although far from being sailable, the boat is definitely floating, was recovered and repaired. The volume of foam in a 34 is surprisingly small. The hull beneath the sole boards is single skin, as is the water tank beneath the starboard settee. In most of the topsides it is only up to 25 mm thick, apart from a couple of areas where it is more. The areas between internal mouldings and hull contain foam but again the volume is not huge.
that looks very similar to the state of the Anderson in my post & pic above, when 'afloat'.
Unless that happened in a remarkably calm sea and / or close to help, I can't see it being viable, nb the waves will be washing over + in & out of the boat.
I do fancy the idea of Sadler style 'unsinkability' though, they don't suffer too badly from lack of stowage - other build issues with double skins & foam though, whereas the chances of internal airbags which have been chafed sat on & impossible to test seems a long shot after my experience.
So a Sadler ( or maybe Etap ) with a liferaft too as Plan C seems a good bet !
Edit; thought I'd add, the Anderson in the pic had no equipment at all on board, no engine, sails, fenders & lines, anything...
Last edited by Seajet; 07-05-12 at 16:17.
A bit of both - my 32' boat displaces between 5 and 6 tonnes - so to keep that afloat you would need about 6 cubic metres of buoyancy below the level at which you want the boat to float. As it happens my boat also has 6 berths - each of which has a surface area of about 1 sq metre so in order for the boat to float I would need an airbag 1m tall covering each of the berths - there simply isn't that much room - only 1 of the 6 berths actually has 1m of headroom above the entire area.
Originally Posted by prv
Another way of looking at it is that you need to replace the entire volume of air in your boat this is below the waterline.
Maybe for an AWB - but I don't have a holding tank, my water tank is flexible (so no air) and my fuel tank holds 30L It would be very generous to think that all sealed spaces add up to as much as 0.5 tonne - maybe add a bit more for the fabric of the boat itself.
Originally Posted by vyv_cox
In the case of the Sadler - 25mm may not be much but it adds up if distributed across most of the hull and topsides.