Emergency Buoyancy

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robmurray

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I was reading today about a boat like mine which was lost off Australia after hitting something. It occurred to me - is there not a system whereby you could inflate some kind of emergency air bags to at least keep a yacht afloat? Anyone heard of anythig like this?
 
There was a system back in the 1980's that consisted of shaped bags plus air cylinders etc. Saw it at the boat show. It would only be practical on smaller yachts as there would not be enough volume inside a 40ft boat to support the weight of the keel (without making the bags so large that they virtually fill the boat.)
 
Yes, it is not practical to fit enough buoyancy inside a boat to stop it from sinking if it is holed. The only "unsinkable" boats are those designed with two skins and built in buoyancy through foam between the skins. The larger the boat, the greater the volume required and therefore the less boat volume available for other uses.

FWIIW sinking is very rare, and sudden sinking with no opportunity to take any other action even more rare. Better to take precautions to prevent holing and have well prepared plans for dealing with it. YM ran a series of experiments on the issue a couple of years ago and have published a book and DVD with useful ideas.
 
Are we sinking?!

Not to hijack the thread but this is a topic that''s been on my mind on and off (it usually comes 'on' again when I'm sailing near whales). What measures DOES everyone else plan on taking in the event that they do suffer a hull breach?
 
Pre RCD ,Sweden ,I'm told, had a rule that all boats built in Sweden had to be able to survive a 5 knot collision with a rock. Last year,I tested this(accidentally) with my Rasmus/Nab 35. Took a chunk out of the forefoot and broke an engine mounting,which,unnoticed ,later resulted in a broken prop shaft.
So yes ,there are ways of safeguarding against sinking----buy an old brick wotsit made out of GRP!
 
Tranona said:
Yes, it is not practical to fit enough buoyancy inside a boat to stop it from sinking if it is holed. The only "unsinkable" boats are those designed with two skins and built in buoyancy through foam between the skins. The larger the boat, the greater the volume required and therefore the less boat volume available for other uses.
Click to expand...

Sadlers are a case in point. Apparently the 26 can be sailed when filled with water to sea level. The 29 floats somewhat lower and this photo shows the 34 with a very large hole aft, following collision with a ship.

Floater.jpg


However, what is surprising is just how little of the hull is foam filled. Neither the saloon floor nor the water tank beneath the starboard berth is double skinned. The topsides foam is only 25 mm thick, increasing slightly lower into the hull. There are a couple of thicker areas, around the batteries and the port side furniture, but it really is not a lot. Bearing in mind that polyurethane foam probably only has around 70 - 80% of the buoyancy of air I would have thought that a few relatively small bags would be sufficient to keep fairly large vessels afloat.
 
This was the subject of much discussion and experimentation in YM a few years back. One of their contributors "Aussie" Bray (to distinguish him from the then editor Andrew Bray) actually converted his boat. The conclusion as I recall was that retrofitting was not practical. Two main barriers, the space required to fit bags with sufficient buoyancy and fitting a reliable way of deploying them in an emergency. When you think about it, there is not a lot of free volume in a boat as lots of the interior is filled with furniture that limits the size of bags. The overall volume might be there, but it is broken up into small compartments. Even empty they would encroach on space.
 
vyv_cox said:
Sadlers are a case in point. Apparently the 26 can be sailed when filled with water to sea level. The 29 floats somewhat lower and this photo shows the 34 with a very large hole aft, following collision with a ship.

Floater.jpg


However, what is surprising is just how little of the hull is foam filled. Neither the saloon floor nor the water tank beneath the starboard berth is double skinned. The topsides foam is only 25 mm thick, increasing slightly lower into the hull. There are a couple of thicker areas, around the batteries and the port side furniture, but it really is not a lot. Bearing in mind that polyurethane foam probably only has around 70 - 80% of the buoyancy of air I would have thought that a few relatively small bags would be sufficient to keep fairly large vessels afloat.
Click to expand...

Impressive.
 
I was wondering if you inflated the life raft and dinghy inside the boat as well as putting all your fenders and life jackets in there would a boat with a big hole still sink? How much buoyancy do you need to keep a boat afloat?
 
There is little natual bouyancy in a GRP boat. Wooden furniture is about it. Therefore you would need to support the displacement of the boat with airbags. 1 cubic metre of air per tonne (approx). Considering my boat - 38ft 7.5 ton displacement (not tonnage which is completely different) I reckon that airbags which filled the volume of the forecabin and both aft cabins would do the job. On the other hand a top quality liferaft and a good insurance policy are a more cost effective and practical solution.
 
gavin_lacey said:
There is little natual bouyancy in a GRP boat. Wooden furniture is about it. Therefore you would need to support the displacement of the boat with airbags. 1 cubic metre of air per tonne (approx). Considering my boat - 38ft 7.5 ton displacement (not tonnage which is completely different) I reckon that airbags which filled the volume of the forecabin and both aft cabins would do the job. On the other hand a top quality liferaft and a good insurance policy are a more cost effective and practical solution.
Click to expand...

I was thinking that way too. The volume required to keep my Moody 31 afloat would be about 4.5 cubic metres; obviously more is better! Think of it as 5 builders bags. The volume of aft and fore cabins combined would easily exceed 4.5 cubic metres, and the volume is below the level of the gunwhales, so it would keep the deck on or above the level of the sea.

A further point is that such a system would allow a longer period for find a means of filling the hole. Fothering with a sail would take too long if you knew the boat would sink in a short period, but would be practical if you knew you had bought some time.

The general wisdom is that the safest refuge at sea is the boat itself, and that you should only ever step UP into a liferaft! So, anything that puts off the moment when the liferaft becomes the only option is a good thing.

Of course, you still need a liferaft; as others have said, sinking is a rarity - fuel explosions and fire are probably more of a threat!
 
AntarcticPilot said:
Of course, you still need a liferaft; as others have said, sinking is a rarity - fuel explosions and fire are probably more of a threat!
Click to expand...

Somewhat surprisingly, based on the (limited) data available, fuel explosions and fires on yachts are underway are very, very rare. They usually occur when anchored, moored up or refuelling.
 
I have had an idea, for quite a while now, on how to make a boat unsinkable, but iam not telling you lot

It will also do away with those bloody fenders too

trouble is, it will take alot of investment to get produced



This time next year I could be a millionare.. nay.. billionare :p
 
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