Is this a realistic survival technique?

[peterb]

Re: Sums

Well, let's try to put some figures on it. Roughly speaking, a cubic metre of water weighs a tonne (seawater weighs a bit more than fresh, but we're talking rough figures only). YM have recently tested the Malo 41, so let's take it's figures as being reasonable; displacement 11000 kg, ballast 4200 kg. The keel is lead, with a relative density of about 11; when immersed in water it will need 3800 kg of buoyancy to keep it afloat. Presumably we won't want the hull to be awash, we'll need probably half the hull to be above water. I'm not sure of the density of GRP, but if we said an RD of 2 we wouldn't be two far out. Then allowing for the buoyancy of the immersed part to keep the hull (without ballast) afloat we would need about 5100 kg of buoyancy, giving a total buoyancy requirement (including the ballast) of about 9000 kg.

I reckon it's just about possible, but not by inflating a liferaft. A typical liferaft will have a diameter of about 2 m, with a circumference of about 6 m. From memory, looking at liferafts at boat shows, they stand about 2 feet high, so each of the tubes will have a cross-sectional diameter of about 0.3 m. That gives a total buoyancy of less than 1000 kg, as against our requirement of 9000 kg.

But suppose that we supply this buoyancy by an inflated tube running most of the length of the boat. With an LWL of 10.8 m, we would be hard pressed to get in a tube longer than about 9 m, so our tube would have to have a cross-section of about 1 square metre. That means a diameter of just over a metre. But remember, this tube has to be contained in the bottom of the boat, and strapped down sufficiently well to hold the boat up. Even so, it could probably be done.

To some extent I've guessed the figures, but I suspect that they're not that far out. Certainly I'm not out by a factor of 9.

 

[charles_reed]

No.
But probably written to provoke thought and comment.

Or are yachtie journalists as ill-educated and subjective as their national colleagues?

Seriously tho' the point being made was that it was safer to stay in the boat than take to a liferaft. Having spent a short time in one and nearly died of seasickness I would definitely make sure I stepped UP into the damn thing.

 

[oldharry]

Inlcined to agree Charles. One of the sad things about the Fastnet was that although some of the yachts survived, the crews that abandoned them did not.

The advice about 'stepping up' into the raft is sound. As long as it remains afloat the parent boat remains a vastly better survival platform than any life raft. If it can be made to stay afloat when full of water then so much the better, but a sinking boat is not the place to start doing the calculations how to acheive positive buoyancy.

I was taught early on that a person who has come off his boat is in an immediately life threatening situation, regardless.

A thought which I always agreed with on a monday morning at my office desk....

<P ID="edit"><FONT SIZE=-1>Edited by oldharry on 16/09/2002 10:34 (server time).</FONT></P>

 

[AndrewB]

In Summary Then ....

6 people (rtboss1, pugwash, bergman, peterk, spark, andrewb) regard it as an idea at least worth thinking about.

8 people (fatipa, jeremy_w, flyingspud, vthomas, tcm, peterb, charles_reed, oldharry) think it isn't worth considering.

The commonest reason for rejection is that an inflated lifeboat would not support the weight of a yacht (thank you, peterb). However, that may miss the point. With a cabin half-awash in rough seas, it will not take much to capsize and sink it. A liferaft inflated inside will help to hold the yacht upright as it is knocked around, by maintaining air in the upper part of the cabin. Thus the yacht will be less prone to capsize.

I sympathise though with Harry Cudmore (cf jeremy_w). Whatever the balance of statistical odds of survival one way or the other, to discover you just blew your last real hope on a gamble that hasn't paid off, is a situation we all instinctively recoil from.

 

[spark]

Er....

... don't count me as one who thinks inflating the liferaft inside the hull is a good idea. I think it's a daft idea. A liferaft is designed as a liferaft. What I'm interested in is emergency buoyancy that is installed in the boat, preferably sufficient to keep the deck far enough out of the water so that the boat is stable enough to be sailed/towed.

What about:

Cylindrical air bags approx. 2m long x 0.7m diameter (c. 0.77m^3 of bouyancy).
One per bunk, installed deflated under the matress, made fast to strong points on the hull.
Compressed air bottle(s) stowed somewhere low and out of the way.
Auto-inflation valves fitted to the deckhead (so that everyone is very wet before the bags auto inflate).
Manual option on the inflation valves (so that the bags can be filled before everything gets wet).
Inflation valves are regulated so that the air bags inflate slowly (less chance of trapping/damaging the crew).

I am currently building a 30' yacht with a displacement of 3.2 tonnes and 4 berths.

4 bags @ 0.77 = 3.08m^3 of bouyancy
Density of sea water = 1025kg/m^3
Therefore the 4 bags will keep 3.157 tonnes afloat

As the boat is made of wood I reckon when the initial influx of water has been pumped out the cabin sole won't even be awash.

Can anyone see any flaws?

 

[peterk]

Re: In Summary Then ....

hi everybody.
andrew: I once considered it , sure - but decided
firmly against it
for a number of reasons.

mainly: I would not want to have a boat that still floats but is awash
or near awash
in ANY kind of seas
- the force of water in motion is incredible!

- that is why I was so scared when 'Tehani' turned turtle
and stayed that way for a while,
with water coming in -
theoretically she is unsinkable , but the more water she had inside her,
the more sluggishly she moved,
the more likely she was to be
smashed up by the waves.

Apart from built-in flotation
I think I can agree with solidly-built airbags that stay on top of the bunks,
never underneath the coachroof.

...float high, sweet chariot...

...peter, www.juprowa.com/kittel

 

[jimi]

Re: In Summary Then ....

I've actually thoughtof this before .. I've got an 8 man liferaft on the basis of the maximum number of people likely to be on the boat, however I often sail shorthanded where is only two of us. I had wondered whether an 8 man liferaft inflated in the saloon may be a safer option than 2 people trying to get in an 8 man liferaft, particularly in heavy weather.

Jim

 

[HaraldS]

Re: Sums

I agree with this rough calculation and it is along the lines of calculating the size of a system that was called Yacht Saver. A self install kit of several large inflatable bags and a big compressed air bottle. I know of some people who had installed this, but I think the company went out of business.

I too think that if a yacht gets seriously flooded, the chances of holding her afloat with the liferaft in the saloon are rather low. Definitely low enough for me to not want the life raft sink with the boat. If I had several life rafts, well maybe I'd give it a shot.

 

[peterb]

Re: In Summary Then ....

Andrew, I think you also miss a point. If the yacht does capsize, then the "air in the top of the cabin" will now be helping to prevent recovery.