Liferaft inflated in a cabin??

[Robert Wilson]

What would be the buoyancy effect on a sinking boat of a liferaft inflated inside a cabin, assuming no puncture(s) from sharp edges, tools, gear etc? Would it possibly prevent a boat from sinking, thereby giving a degree of refuge before final abandonment.

Obviously there would be safety issues for crew e.g. suffocation, access/egress through hatches.

If so, what would be the number of LRs per ten feet of boat-length?

I'm not thinking of trying it, or relying it, but it crossed my mind.

Any thoughts? (preferably not all caustic, drfifting, rude!

 

[35mm]

It would take up all inhabitable space and not add enough buoyancy to keep the boat afloat.... would be my prediction. Far better to inflate the life raft in the sea and jump in it!

 

[Robert Wilson]

It would take up all inhabitable space and not add enough buoyancy to keep the boat afloat.... would be my prediction. Far better to inflate the life raft in the sea and jump in it!

I agree, but I wondered IF it went off accidentally what would be the buoyancy effect. And if the effect was significant then perhaps such a system could be designed.
Just a late night thought as I enjoy my Sunday-evening Rum 'n' Coke.

Sleep well
R

 

[bedouin]

I think it's been discussed on here before.

But to give you some idea - in order to keep the boat afloat if holed you need buoyancy equivalent to the volume of your hull beneath the waterline - that is a lot of air (or equivalently one cubic metre of air per tonne of displacement)

If that is what you are looking for then go for one of the unsinkable designs

 

[theoldsalt]

It depends on the weight of your boat and the volume of the liferaft.
Most boats sink if holed.

The downward force is equal to the weight of the boat less the weight of water displaced. (eg if floating this will be zero). This calculation becomes more complicated if the boat is not completely submerged

The buoyancy of the liferaft is calculated the same way and as it will inevitably displace far more weight of water than it's own weight, the net force is upwards.

For the liferaft to support the boat the net downward force of boat must be the same as the net upward force of the liferaft.


P.S. if the boat is very heavy and the cabin top/deck weak then the net upward force could rip the cabin top/deck off the boat and the hull will sink anyway.

 

[Mistroma]

I don't have liferaft dimensions to hand but made following guesses.

Length per side: 2m
Tube Diameter: 30cm
No. of Tubes: 2

Based on that I'd estimate the volume at around 1,130 litres giving around 1,100 kg buoyancy (in sea water & allowing for weight of the raft). It would be best to inflate upside down in order to trap even more air to give around 2,400 kg additional buoyancy. I didn't detail the calcs. so that anyone checking my rough estimate won't just copy any mistakes.

So you might get something like 3.5 Tonnes of buoyancy and that sounds like quite a lot (if you have a 2.5 Tonne boat).

I think it wouldn't work well because:

1) High risk of getting a puncture on some fitting during inflation
2) Risk of the liferaft floor giving way due to localised forces (you then lose 2.4T of buoyancy)
3) The liferaft would tend to rise to cabin roof and so boat would still be awash if it floated
4) If buoyancy = weight of boat then you'd just be awash. So interior not a very safe place
5) I doubt there would be any room below deck on a 3T yacht with this size of liferaft inside
6) If boat is larger that 3.5T then it would still sink.
7) You've used up you liferaft when very likely to need it

I suspect that you'd be better off keeping the raft on deck until you need to step up into it. Of course you could keep 1-2 spare ones below deck and give it a go with the 3rd in reserve. But who'd keep that number of liferafts on a yacht.

So not a good idea on a small boat and a larger one would sink anyway.

I know that this is a very simplistic view and doesn't allow for other buoyancy (voids, cushions, buoyancy of GRP hull etc.). But even just equating buoyancy with boat weight you get a rough idea.

 

[William_H]

Liferaft inflated in cabin

I would imagine that the life raft would be forced to the top of the cabin so allowing the whole boat to sink except for the cabin sticking out of the water. No good for inhabitation but good for finding the boat. Buoyancy needs to be down low and at the chines to keep boat up and level. olewill

 

[Seajet]

I used to think I'd try inflating the dinghy or liferaft in the cabin to save the boat.

Having dealt with a sunken example of the same class, Not A Chance !

She was only saved by flotation bags in calm inshore conditions, and any attempt inflating a dinghy or raft inside the cabin would just deprive the crew of a potential life saver.

 

[Robert Wilson]

Many thanks folks.
I wasn't seriously thinking of trying it, I was remembering a man who accidentally inflated his L/R in his fishing boat cabin - and was wedged in the forepeak. Luckily someone heard his shouts!
My thoughts were more of "catching the rip-cord" taking a valise out of the cabin (But I KNOW, it should be on-deck in any case!).
I fully accept there would be no crew-space below, sort of thought of crew on deck rather than abandoning.

I certainly can't afford to have three or even four liferafts aboard!
Never considered the deck getting blown off

Very interesting comments and advice. I don't think there is a fortune to be made from that idea!!

R

 

[fisherman]

An old chap, fished with his son, was mending pots in the shed, and found 'a handy pice of cord' under the bench. He mended three or four pots and the cord stuck so he gave it a yank.

"You know that shed was square, well now it's round"

No, you won't accidentally inflate the raft by snagging the ripcord, it's 10 metres long.

 

[Robert Wilson]

An old chap, fished with his son, was mending pots in the shed, and found 'a handy pice of cord' under the bench. He mended three or four pots and the cord stuck so he gave it a yank.

"You know that shed was square, well now it's round"

No, you won't accidentally inflate the raft by snagging the ripcord, it's 10 metres long.

Doh! I should have remembered that.
Go and stand in the corner, boy.

 

[reeac]

I don't have liferaft dimensions to hand but made following guesses.

Length per side: 2m
Tube Diameter: 30cm
No. of Tubes: 2

Based on that I'd estimate the volume at around 1,130 litres giving around 1,100 kg buoyancy (in sea water & allowing for weight of the raft). It would be best to inflate upside down in order to trap even more air to give around 2,400 kg additional buoyancy. I didn't detail the calcs. so that anyone checking my rough estimate won't just copy any mistakes.

So you might get something like 3.5 Tonnes of buoyancy and that sounds like quite a lot (if you have a 2.5 Tonne boat).

I think it wouldn't work well because:

1) High risk of getting a puncture on some fitting during inflation
2) Risk of the liferaft floor giving way due to localised forces (you then lose 2.4T of buoyancy)
3) The liferaft would tend to rise to cabin roof and so boat would still be awash if it floated
4) If buoyancy = weight of boat then you'd just be awash. So interior not a very safe place
5) I doubt there would be any room below deck on a 3T yacht with this size of liferaft inside
6) If boat is larger that 3.5T then it would still sink.
7) You've used up you liferaft when very likely to need it

I suspect that you'd be better off keeping the raft on deck until you need to step up into it. Of course you could keep 1-2 spare ones below deck and give it a go with the 3rd in reserve. But who'd keep that number of liferafts on a yacht.

So not a good idea on a small boat and a larger one would sink anyway.

I know that this is a very simplistic view and doesn't allow for other buoyancy (voids, cushions, buoyancy of GRP hull etc.). But even just equating buoyancy with boat weight you get a rough idea.

That sounds all well and good but nevertheless Sadlers managed to build boats with sufficient built-in buoyancy to make them unsinkable. As the owner of a S29 I'm always mystified by the unobtrusive nature of these buoyancy compartments considering the boat's 3.7 te. displacement - they don't look as if they add up to a liferaft's worth of volume.

 

[Iliade]

Find the weight of the boat in kg then get say 120 empty one litre coke bottles per 100kg.

Stuff these into every orifice you can find as far down in the boat as possible.
If they can be made to stay low, then they will prevent a certain volume of water from entering into the boat in the first place. Just like an Etap only without the ability to slowly absorb water.

Of course you will probably find that there is not enough room, but hey...

I'm sure I read of someone trying the 'dinghy inside' trick and indeed simply popping off the deck. Though that could just be an urban myth?

 

[oldbilbo]

I have 3 helicopter flotation bags squirreled away, looking for an application. maybe I've just found one....

After all, it's how we used to keep wooden dinghies afloat.

 

[vyv_cox]

That sounds all well and good but nevertheless Sadlers managed to build boats with sufficient built-in buoyancy to make them unsinkable. As the owner of a S29 I'm always mystified by the unobtrusive nature of these buoyancy compartments considering the boat's 3.7 te. displacement - they don't look as if they add up to a liferaft's worth of volume.

Exactly so. The amount of foam in a Sadler 34 is remarkably small, the hull has around one inch at the topsides, increasing a little towards the keel but there is none in any of the floor below the sole boards, nor inside the water tank that occupies all of the starboard side seat. The total volume, bearing in mind that it has only around 80 - 90% of the buoyancy of air, must be considerably less than that of a liferaft.

 

[Robert Wilson]

I have 3 helicopter flotation bags squirreled away, looking for an application. maybe I've just found one....

After all, it's how we used to keep wooden dinghies afloat.

Would you deflate and pack them for me? Any idea what size gas cylinder I'd need. Oh, and I don't like the colour, would you have them sprayed dark blue, please?

 

[Searush]

I reckon an inflatable dinghy in the forecabin & one in the aft cabin where headroom is restricted, MIGHT give me flotation, but I would expect a foot or two of water in the main saloon which would mean I would also need a dry suit.

Err plus half an hour of hard work to inflate the two dinghies!

Tristan Jones claimed to have used his dinghy in the forepeak to keep one of his smaller craft afloat & sailing on a voyage. But I don't recall which book it was & there are those who say his creative skills outweighed his memory by a considerable margin.

 

[vyv_cox]

but I would expect a foot or two of water in the main saloon which would mean I would also need a dry suit.

Or perhaps a little more than a foot. Sadler 34.

 

[Searush]

Or perhaps a little more than a foot. Sadler 34.

I doubt that has the bouancy equivalent of 2x inflatable dinks, but I can't be bothered with the calcs for a whimsical concept.

 

[vyv_cox]

I doubt that has the bouancy equivalent of 2x inflatable dinks, but I can't be bothered with the calcs for a whimsical concept.

I might give it a try for the Sadler, as the subject often comes up. A few years ago I did calculate the buoyancy available in a boat naturally, e.g. fenders, half full diesel tank, half full water tank, food containers, etc and found it to be a surprisingly high reserve.