David Tyler wrote:

This has been done, and offered commercially - at vast expense as far as I remember. Airbags are routinely used by salvage companies for recovering sunken vessels, lifting pipelines etc.

More ideas:

https://www.youtube.com/watch?v=h4JkLUHdlC0

Flotation bags in the rubbing strake — I thought of this too! I'm very amused to see that someone's actually done it, and appears to be selling it. They seem to be focussing on boats that sink unattended. That happened to a relative of mine who fitted a bronze seacock to his steel motor boat. Oh dear.  He set fire to his next boat and burned it to the waterline. Oh dear oh dear.

https://www.glen-l.com/weblettr/webletters-7/wl55-flotation.html

These calculations look familiar. I'll work them through and check them against mine.

David Tyler wrote:

Good sense talked by both Graeme and David. Unsinkability on a ballasted monohull by filling with foam is something that sounds like a good idea in theory, but one that can only be achieved by sacrificing  everything that makes the boat useful - stowage space etc.

Are you imagining that storage volumes will be filled with foam? That's not my intention.  Instead, there'll be a kind of all-around lining.  Yes, volumes will be reduced, but over an area of approximately 30 square meters.

Better to concentrate on eliminating the causes of sinking: through hulls and insecure washboards, mainly.

Why not have it all?  Through-hulls and washboards (and hatches) are all on the list.

By all means put in watertight bulkheads fore and aft, with absolutely bomb-proof hatches through them so that they can still be used for stowage. You might be able to achieve the necessary cubic metre or more of enclosed air that way. Watertight bulkheads would be very much in the Chinese tradition.

On a boat of Tammy Norie's size and shape it's hard to see where I'd put such bulkheads without losing *more* useful space than the lining method.  I'll give this some serious thought though.  It does go against my preservation requirement.

Foam for insulation in a GRP boat is another matter, and much to be desired. For a boat used in the warmer months, closed cell foam of the camping mat kind, 10mm thick, bonded to the inside of the topsides and deck, will be enough. For a boat that is to be lived on in winter, with heat to be generated and kept inside, up to 50mm of foam is good. I did that on Tystie, and was glad of it in the winter.

My plan will result in 30mm in most places.  Can I ask what your experience is with *where* to insulate?  I have heard people say not to insulate below the waterline, but without any real explanation as to why.  I have come up with a few reasons of my own, but I'd be interested to learn from you.

Firstly, many thanks for everyone's comments and feedback!  It is all very helpful.

Graeme Kenyon wrote:

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

Many thanks!  I hope to make more once when I have more time on the water.

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.

The boat does not need gutting to achieve this.  The surfaces I intend to line and the volume I intend to fill are laid out in section 4.2 of the plan, with drawings, measurements, and photographs.  Nothing needs to be disassembled.  This is partly because Tammy Norie is already quite spartan, and has been without any kind of lining since 2015.

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.

Ah, perhaps I need to spell out my overall goals.  This is part of my campaign to sail in the Jester Challenge and cross the Atlantic. I'm planning for the situation where there's no rescue party for many many days, if at all, and where I need to recover the boat as far as possible myself with no rescue party.  This is described (too briefly) in plan.unsinkability.req.haven.

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.

I do not drive! Even if I did, it would be a few minutes compared to many weeks on the ocean.

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.

I thnk this is a very useful part of the discussion. My friend Aaron also asked me about the recovery scenario. I do think this needs elaborating.  In short: she should not be barely floating, or upside-down.  And in any case she should be a better place to be than a liferaft.

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.

My plan is to distribute foam over a great many of the surfaces of the boat, with significant amounts of flotation (possibly foam or airtight containers) in the cockpit coamings, which are approximately 10% of the total planned buoyancy each (take a look at the dimensions table).  In addition, there's a great deal of available volume aft.  Forward is another matter, though.  On Mingming and Mingming II, Roger Taylor did sacrifice forward space for a buoyancy chamber.

... 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.

This is very useful.  A lot of the volume I intend to "fill" is high up in the interior and in the coamings, and nearly all of it is above the waterline.  But I think it would be a good idea to model the distribution better.  I'll think about how to do that.

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.

Isn't this the opposite of keeping the buoyancy high up?  Or am I misunderstanding you?

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.

Well put!  That, and the recovery scenario, are what the test should be for, if I ever get to do it.  I do think it would be fun if it can be arranged.

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.

A quick fix is not what I want. And it's a winter project!

I wonder if it would be possible to dismantle the airbags out of a car from the wreckers yard.

You've reminded me of a proposal someone else made, to inflate air bags using a fire extinguisher.  I found out somewhere that this has been tried, and it's not bad, but you have to have some special arrangement to stop everything freezing.  I should find it and write it up.

Mark, you are right (in theory, in the static situation).

But in practice? When the boat is full of water you have no form stability whatsoever (in contrast to, say, a capsized catamaran) and the situation is likely to be dynamic.

You'ld have to try it in practice but my guess is that a little keel boat, flooded, would be next to useless as a lifeboat unless purposeful, intrusive provisions were made to make it so.

For a little coastal cruiser like Tammie Norrie I don't think it is worth to worry about.

Best, I think -  just make sure, as far as possible, you don't ever get flooded!

Graeme wrote:

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.

This has been done, and offered commercially - at vast expense as far as I remember. Airbags are routinely used by salvage companies for recovering sunken vessels, lifting pipelines etc.

More ideas:

https://www.youtube.com/watch?v=h4JkLUHdlC0

https://www.glen-l.com/weblettr/webletters-7/wl55-flotation.html