Chris has just pointed out to me (very tactfully off-forum) that car airbags won't work because they are perforated and intended to immediately deflate.

I pulled the ripcord on an old well out-of-service liferaft once, just to see what would happen. Something happened all right. The raft came apart at the seams and the perished bottom fell apart. But the infllatable ring popped up in a seriously impressive manner, and stayed up. If that had happened inside a small cabin (with the crew on the side nearest the hatch!) I reckon you would have some instant flotation. I have found with CO2 extinguishers too, the gas seems to last and hold its pressure for many many years.

Perhaps some other inflatable bladder(s) of some kind and a bottle of gas - or even an old liferaft out of its case -  could be stowed up forward as a source of instant emergency buoyancy. Its just a thought.

I loved the youtube clips you have done, especially the mini-junket with Emmelene and Amiina (   https://www.youtube.com/watch?v=MjHW63kcRks    )

Here are some thoughts straight off the top of my (somewhat aged and feeble) head, on your flotation proposal, maybe a bit ignorant but then again, maybe will provoke further discussion:

The idea of gutting the boat, and lining it with foam, surfacing the foam and then re-assembling seems a huge undertaking especially for someone who is not 100% well, for the only immediate benefit that I can see, of improved insulation. As you point out yourself, the best you get from that if you ever sink at sea, will be a nearly submerged boat which will be impossible to pump out and probably not much more visible to a rescue party than you would be just floating in a life jacket. Personally (and I am currently in a similar position, with just a tiny boat to cruise in at present, and nowhere near as fit and agile as I used to be) I would rather spend the time enjoyably sailing, and just try to avoid sinking at sea.

The biggest danger to your life is probably driving your car to the boat and back, perhaps putting your trust in an airbag, which – I’ll come to that later.

You can survive at sea in a partially submerged boat – the amazing story of the Rose Noelle in which 4 men survived 119 days at sea in a capsized trimaran is an example – but the hulk was floating sufficiently high that, even upside-down, it provided a habitable haven in which the men could survive. A monohull awash, barely floating and quite possibly upside down or impossibly cranky would be next to useless I think, and if it did not float upright then you would not be able to cling to it for long.

Unless you are prepared to convert some of your interior accommodation into major buoyancy chambers (such as sealing off the forepeak and afterpeak behind watertight bulkheads) I doubt if your little boat awash would be much use as a life raft.

I have just come back from 8 days coastal cruising in a mini-cruiser (17’) and I have to admit, the thought of sinking at sea never crossed my mind. (I did nearly sink the boat on a rocky beach one night, but had the worst occurred, I would have walked away from that.) This little jaunt gave me an opportunity to think a bit about the best use of a very small interior.

Here are some thoughts. My little boat was originally unsinkable due to the entire volume under the four quarter berths being one large buoyancy chamber. I am not sure how useful this would be to survive a swamping at sea – it would depend on where the boat was holed and which way up it would float. Anyway, that feature had been destroyed by the previous owner who cut hatches in the bunk bottoms with a view to using that volume as storage. I have found (single-handed cruising) that only one bunk is necessary, and that by throwing out the other three squabs I can carry all the food, clothing and equipment I need in plastic containers on top, so the entire volume under the bunks might as well be filled entirely with foam. That space is such an awkward shaped volume as to be of minimum use for storage and once gear ends up on top (which it inevitably does) I am too impatient to be bothered delving under the bunks. I am not going to bother filling the space with foam because I can’t envisage sinking off the coast (except maybe near to the shore) but perhaps I had better “touch wood” while saying that. Anyway, if I were looking for floatation volume, that's the interior volume I would sacrifice first - that gets you to stage 1 (unsinkable) but there is more to consider. My (shamefully vast) experience in capsizing dinghies  tells me that the really useful bouyancy is high up, under the deck, if you want the flooded boat to remain upright and have a chance of providing a survival haven, or bailing out. Its not just a matter of floatation volume, its where its placed that matters.

The area low down under the cockpit in my boat is too valuable a space for water stowage and batteries – heavy things should go down there anyway, not foam. I am afraid I can go a fortnight without a bath while away, so no need for 200 litres of water. 50 litres is plenty for me for a fortnight.

So, my thoughts are, if you don’t want water-tight fore and aft bulkheads, you might consider a horizontal bulkhead at the level of the bunk tops and sacrifice that beneath-bunk stowage space instead. Your proposed floatation test should not be for the purpose of seeing if the boat will float – you can calculate that – but it might be a good idea for the purpose of seeing which way up it floats and how stable it is in that attitude. My guess is that in practice, it still won’t be a practical liferaft.

A quick fix is what you want, so you can spend your time sailing instead of a whole summer working on the interior of your boat. I wonder if it would be possible to dismantle the airbags out of a car from the wreckers yard. I have no idea if this is feasible, but they are very compact and (I hope) very reliable. If you could put a couple of these in the right places (so the boat  would float, and float upright) and devise a way of triggering them, you might have a quick way of retaining your boat interior as it is, and having it pop into liferaft mode in an emergency. Pre-test might be a problem! You would have to rely on faith that they will inflate – just as you do in your car, when you drive to and from your mooring. This might sound a bit crazy, but its an alternative paradigm and maybe it will spark a better suggestion from someone else.