NathanLee
regular
Reged: 09/06/2008
Posts: 996
Loc: North Sea
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I've started writing up what I think needs doing to Kudu. It's not finished yet (I ran out of lunch break) but I would appreciate your opinions on what's there so far. Essentially, it's everything I wanted to say but I've not polished the writing, as it were.
http://onkudu.wordpress.com/2008/11/06/preparing-a-boat-for-the-atlantic/
Annoyingly, I can't do anything until I have my finances in order, so work on the boat is out of bounds until January. It's frustrating to have all these huge plans but no immediate means to start them. Bloody credit crunch! 
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Around Britain single handed, livaboard, on a 21 footer.
onkudu.com
Around Britain video series
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SAWDOC
regular
Reged: 24/02/2008
Posts: 267
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Hi Nathan,
Good luck with your effort. If your boat displaces 1.5 tons, and you wish to make it unsinkable does that not mean that you will have to fill 1500 litres of hull space with foam? do you have that volume and can you do without it when storing provisions etc? just a thought that struck me when reading your well written aticle......
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Mouton_Noir
regular
Reged: 13/11/2005
Posts: 295
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Hmmm... here's my thoughts...
It's all about density. Imagine a 1.5 tonne boat that was made entirely of wood - everything, including the anchor, chain, and all provisions. How much extra buoyancy would it need? Well, none, because wood isn't very dense; it floats.
Now imagine a concrete boat. Every litre of concrete weighs approx 2.7 kilos. So you'd think we need 2.7 kilos of buoyancy to keep it on the surface... but you don't. While it's in the water, it's displacing 1 litre of water (1 kilo), so only needs an extra 1.7 kilos (1.7 litres of air) to keep it at the surface. To put it another way, 1 kilo of concrete needs 0.63 litres of air/foam to make it float.
So you need to work out the average density of the boat and all its fittings and stores. The metal is very heavy (1 kilo of steel needs about 0.85 litres of air/foam). I'm not sure the specific gravity of GRP - if it's about 2, then 1 kilo would need half a litre of air/foam.
So your 1.5 tonne boat *might* get away with only 1 cubic metre of buoyancy. Which should be achievable.
BUT
If you only provide exactly the right amount of buoyancy, then the boat will be floating with the cabin roof awash. Might save you from drowning, but not from exposure. And the boat ain't sailing anywhere fast. (But might enable you to fix a leak).
So you probably do need more buoyancy to make life livable.
I'm hoping I should manage - Black Sheep is a luxurious 26 feet with some awkward spaces that aren't sufficiently accessible to be good storage. I'm not a fan of foam, so plastic bottles, inflated wine boxes etc are on my list. But to be useful, they should be low down, and fixed firmly to the boat. I'm also rather taken by another suggestion I saw recently, to make all lockers airtight. This gives the added buoyancy of all the airspace around my stores - probably more than 50% - as well as a bulkhead if hull-breach occurs behind that locker.
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Black Sheep. Not black. But sometimes sheepish.
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Nicki_Crutchfield
regular
Reged: 07/03/2005
Posts: 602
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It won't need a litre per gross kilo as things weigh less in sea water. For example 10kg of GRP needs 6.5kg or so of buoyancy to remain afloat. Even the ballast keel will weigh less in sea water than on land. The wood is positively buoyant.
I had not realised that the Corribee is so heavy, my Achilles 24 with a 54% ballast ratio is only 1180kg. As you say, though, it will take a considerable amount of buoyancy, once she is full of stores etc. and space is very limited.
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tdsail
regular
Reged: 11/09/2008
Posts: 46
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Yea I had a similar post...ppl responded with various issues with this...ie the foam absorbs water over time....if some damage/leak occurs its very difficult to inspect/resolve damage...someone has suggested creating 2 or 3 watertight compartments which is an option...
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PacketRat
regular
Reged: 20/05/2007
Posts: 64
Loc: Merseyside
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Couple of thoughts, Nathan, REPF is about 2 1/2 lbs per cubic foot, or 40 Kg foam per tonne of buoyancy, and unencased can be crushed by water pressure at 3 feet down. Then there's stability to consider. Airbags are another option.
If you're after hatch seals, I found a good range and same day despatch at sealsdirect.co.uk.
Robin
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miko
regular
Reged: 09/01/2006
Posts: 40
Loc: S Devon, UK
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Hi Nathan and others,
With reference to adding bouyancy. I was pondering the suitablilty of 'air cushion packaging', think bubble wrap but with fist sized bubbles. It would be easy to stuff and fill voids, and is as light as the plastic film used to make the packaging. Finding a source might be the main issue.
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Mouton_Noir
regular
Reged: 13/11/2005
Posts: 295
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Oooh - that sounds like a very interesting idea!
This guy: http://stores.ebay.co.uk/senditsafely sells them already blown up.
But this product: http://www.airfil.com/pages/airfil_handypak.htm looks useful for increasing buoyancy en route, as stores are depleted.
Definitely worth some thought.
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Black Sheep. Not black. But sometimes sheepish.
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Nicki_Crutchfield
regular
Reged: 07/03/2005
Posts: 602
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Wouldn't they burst under the pressure as the boat settled lower in the water? It would be truly horrible to hit something, float a bit lower for a few minutes, feel smug and then hear an ugly popping sound.
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CPD
regular
Reged: 20/09/2006
Posts: 1870
Loc: Essex
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That would be the Rice Krispies biting the dust.
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Life is just far too short.
I shortened my name from CentaurPipedream so you could spend your time reading something more useful.
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