Boat preparation

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nathanlee

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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! /forums/images/graemlins/tongue.gif
 
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......
 
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.
 
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.
 
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...
 
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
 
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.
 
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.
 
Bonjour

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then hear an ugly popping sound.
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It remembers me a tune called "pop corn" Tu tu tu - tu tu tu tu---tu....


More seriously to double the pressure you must be 10 m under the surface !

It would be already too low, anyway !

Eric /forums/images/graemlins/grin.gif
 
Or perhaps a new method of distress signalling. If you had two sizes of bubble you could use morse code: pop pop pop POP POP POP pop pop pop.....!
 
Darn you, Eric! Now I can't get that tune out of my head!

But thinking about the popping... they would surely pop if you put them in a vacuum, as then the internal pressure would be a lot greater than the external pressure. But uniformly squeezing them in a fluid - I think they shouldn't pop, as the internal pressure would be too low. You would get reduced buoyancy (half, at 10m), but I don't think they should pop.

But there's only one way to find out. I need to find some, and take them for a swim in the river - box them up with a lump of lead and lower them into 6m of water and see what happens.
 
Thanks for all the replies guys. I was checking the thread for a day or two but got no responses, then checked again today and it's full! Some interesting food for thought there.

I think I certainly want foam to some extent. Relying solely on air chambers is asking for Titantic syndrome I think, but, since I have to live on the boat, it would make more sense. The lack of space is very apparent now. With foam, it would be annoying.
That said, the Corribee has plenty of "wasted" space that can't be used even for storage at the moment.. namely under the cockpit floor. That would be ideal for "foaming" although it's in the center of the boat which might not do any favours for stability.

One other thing that has been playing on my mind is buying another boat. I am very reluctant to do so, but again, because I live on her the extra size wouldn't be at all unwelcome.
By the time I've finished making Kudu good, I'll have probably spent about the same as buying something bigger. I was looking at perhaps buying a Contessa 26, or, seemingly similar a Mystere 26.
I'm not sure. I feel somewhat committed to Kudu now.

Thoughts?.. I appreciate them greatly.

Cheers,

Nath
 
Hadn't thought about adding extra floatation to the boat until seeing this post.

Not keen on losing space but was wondering about air bags with gas cylinders along the line of automatic lifejackets so if they get soaked they go off as I guess the only time you need them is if you have hit something / broken a hull fitting etc and the 'oggin' is on the inside. Could secure inside boat one in bow and one under each cockpit seat? Don't know if they exist.

Know the RNLI and salvage boats have some form of rubber air tanks they put in submerged boats and inflate.
 
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