Liferaft on a small boat?

My dad comes up with that argument. It's flawed in so many ways.

Making a 52,000 ton ship unsinkable is slightly more of a complicated problem than a 20' 9" dinghy.

Also, saying that something was tried almost 100 years ago and failed so we shouldn't try again is... a bit defeatist don't you think /forums/images/graemlins/smile.gif

If you calculate the right volume of foam, the only way you're gong to get the Corribee to actually sink is by breaking it in half. Whatever you hit to manage that has probably just killed you anyway, so problem solved.
 
But, by putting the foam lower down in the lockers that you wouldn't otherwise use, you're preventing them from filling with water to start with - so the additional weight just isn't there...
 
This is fine on a dinghy with a double floor and transom draining.

But if the foam is in the lockers at the botton of a displacement hull then any water which enters the craft will be higher up.
Iain
Thus reducing the stability.
 
But this just means the light floaty stuff is at the bottom and the heavy watery stuff is on top which will make it unstable and invert.

I'm inclined to agree with the foam under the decks idea from a stability point of view.
 
I think you guys are missing the point a little.

If the foam is lower down, then it will be displacing enough water to keep the hull reasonable high up, out of the water, thus there will be no weight above the foam at the top.
 
And, the water level is not going to be above the sea level, so the boat is not really going to be top heavy, plus is still has the keel(s)... unless that's the reason for your wet feet, but then you can kiss goodbye to being upright flooded or not.

I think. /forums/images/graemlins/tongue.gif
 
If you're going down the foam route: make sure that wherever the foam is situated, whatever structure is above it can support the full weight (albeit distributed) of the boat - in the case of the Corribee, something over a ton if you include the outboard, personnel and stores. If you locate the foam in just one or two places under the deck, it might rip those sections of the deck clean away, unless they have been previously reinforced. (designers tend not to focus on upward forces applied to decks)

Personally, I'd be inclined to place foam as low down in the hull as possible, in order to try and keep the decks above the water - that way you stand some chance of bailing-out if and when you manage to plug the hole(s) and if the sea-state permits.

Once your decks are awash, you're in big trouble if far offshore: the boat might be 'stable' (in the sense of being upright), but you'll be low-down and completely dead in the water and unable to steer the boat and thus at the mercy of every wave from then on. Also - if the hull is completely immersed, the chance are that your food and water supply will be ruined (and inaccessable anyway), and your electronics will probably be non-functional.
Unless you had a survival suit on at the time of the impact, you'll also be at risk from impending hypothermia.

On balance, I think foam (or similar forms of flotation) as low down as possible in the hull is the far better option, as I believe this will optimise your chances of survival.
 
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Personally, I'd be inclined to place foam as low down in the hull as possible, in order to try and keep the decks above the water - ....On balance, I think foam (or similar forms of flotation) as low down as possible in the hull is the far better option, as I believe this will optimise your chances of survival.

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You seem to be suggesting that putting the foam down low is not likely to capsize - or at the least - put the boat on its side.
POSSIBLY.. if there is not much foam buoyancy you could get away with this - but you would still compromise stability, even if the boat floated upright.
Suggested reading: "Centre of buoyancy" and "Free surface effect"
 
French regs say that anything over 16ft 6in has to have a liferaft if it goes 6 miles from shelter. (If it is shorter it is not allowed more than 5 miles out)

Which makes it illegal to sail a French Wayfarer accross the Channel - and can you imagine a Liferaft on a Drascombe Lugger?

Commonsense has to come into this somewhere. There is a size of boat below which a liferaft will interfere with the use and working of the boat and unless you are expecting danger (e.g. by sailing out of reach of shelter in the event of a forecast storm) I reckon a Corribee is below that size.
 
Dear All,

Thanks for taking the time to respond to my question. I am not really surprised by the diversity of views on the subject - it just illustrates what a dilemma it is. I am not contenplating a major re-engineering of the boat to make it unsinkable but I shall continue to ponder the liferaft issue with a few glasses of malt over the winter!

Best Regards.
 
Foam down low scenario:
If the inside of the boat is awash with water the LIGHTEST part will be nearer the bottom (keel notwithstanding).
So:
Flat calm, boat full of water, centre of gravity is higher than centre of buoyancy because of or contributed to by the foam - but balancing there.
A slight wave sends the free water inside the boat to one side - the centre of gravity is now no longer sitting balanced over the Centre of buoyancy so it falls over - sending the rest of the water to the side and making the effect worse.

Scenario with the foam high up:
Inside the hull the centre of buoyancy is also high, hopefully higher than the centre of gravity so that an upset is resisted by the COG trying to return to its lowest position - under the COB.
In addition high up reserve buoyancy fills space that water would have sloshed around in and thereby limits free surface effect.

Phew!
 
I see - so if the foam means you avoid water inside then it works, but if it doesn't it is a bit like loading a car on your coachroof with its handbrake off.
 
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You seem to be suggesting that putting the foam down low is not likely to capsize - or at the least - put the boat on its side.
POSSIBLY.. if there is not much foam buoyancy you could get away with this - but you would still compromise stability, even if the boat floated upright.
Suggested reading: "Centre of buoyancy" and "Free surface effect"

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Hold on. The buoyancy of a boat is low down as it is. You can't get lower than the hull (the keel doesn't float).
 
What a fantastic thread! ASK ANYONE who has been saved because they had a life raft what they think - then think about buying a can of builders foam!

ASK my mate Bob why he was in the water 1 mile off shore coastal cruising on a calm hot sunny day - he wished he had a life raft!

He could have sat in relative comfort and watched his 16ft boat burn.

make the boat unsinkable, what ever next!

/forums/images/graemlins/laugh.gif

FIND SOMEWHERE TO PUT THE LIFERAFT
 
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Foam down low scenario:
If the inside of the boat is awash with water the LIGHTEST part will be nearer the bottom (keel notwithstanding).

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Not necessarily - the cabin sole area will be holding at minimum a quarter of a ton of seawater which will be offsetting the buoyancy. Admittedly this will have a tendency to slosh about, which certainly isn't good.

Another approach (which is something I'm currently pondering), is the practicality of fitting a matrix of interlaced (think 'noughts and crosses') bulkheads, to a height of (say) 12" above the LWL. There would then be a trade-off between the compartmentalisation thus created, and the practicality of life below deck with such structures in place. I think the bottom-line is that - other than sailing a steel hull - there probably isn't a perfect solution, otherwise folks would surely have adopted it ages ago.
 
The buoyancy (centre of buoyancy) of the boat is at the centroid of the immersed section of the hull. (roughly)
The centre of gravity is at the balance point of the COG of the hull and COG of the keel.
for the boat to be stable in most conditions, the COB has to be above the COG.
Take a boat and say, half fill it with water:
The centre of buoyancy is now fairly high and the centre of gravity VERY low. So as long as the COB stays high the boat will float upright. Once the water starts to slosh inside the COG will move about, up and to the sides, and you may get a capsize condition.

Now, if you fill the bottom of the boat with foam then half fill whats left with water, the centre of gravity will be MUCH higher, greatly increasing the chance of a capsize.

While the boat is still afloat the centre of buoyancy will be not be affected other than by the change in waterline altering where the centroid is. But once the boat is awash the centroid (centre of bouyancy) becomes governed by the centroid of the FOAM and the centre of gravity by the nett weight of boat and keel - question - which would be higher?

If the COG is lower than COB with the boat awash then it will wallow generally upright, conversely if the COG is higher than COB when awash then it will lie down or capsize.

My guess is that sufficient foam to keep the boat afloat (ie displaces the dry weight of the boat - less the boats own displacement while awash and immersed), installed low down would make it unstable for the reason above.
And even if it didn't capsize its still bad engineering.
 
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Now, if you fill the bottom of the boat with foam then half fill whats left with water, the centre of gravity will be MUCH higher, greatly increasing the chance of a capsize.

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But it doesn't work like that. The Corribee hull 'as is', is full of air - with no complaints about Centre of Buoyancy or anything like that.
Foam (and NOT builder's foam !) is nothing more than trapped air. So you fit (in the case of the Corribee) something aound 35 cu.ft. of this trapped air 'low down'. It doesn't matter whether this 'trapped air' is in the form of foam, air held behind a bulkhead or within a locker.

Seawater enters. OK - that's bad news.

The seawater will rise to somewhere near the top of this foam or other forms of trapped air, and that's as high as it will go. If the seawater ever rises above it, then the boat will sink - period - demonstrating that there wasn't enough buoyancy fitted. You will NEVER get to the point where the hull becomes top-heavy and unstable - it will have sunk long before then.
 
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