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Buoyancy

Fascadale

Active member
Joined
15 Jan 2007
Messages
1,372
Location
One end of the A1
Hi All

I have just read a recent PBO which contained an article about the Jester Challenge which mentioned an adaption to a Corribee( Mingming I think) to build in some foam bouyancy thereby making the boat unsinkable.

My Invicta 26 wieghs 4.1 tons. How do I work out the volume of foam required to keep her afloat. Would the same volume of air be as effective ? I had considered filling the lazarette, forepeak, cockpit lockers etc with canoe type air bags and empty plastic bottles all held in place with netting. That all sounds a bit haphazard after what I have now read.

I was also curious that the foam on Mingming was to be enclosed behind watertight bulkheads. Is this to keep the foam in place as well as to stop water getting into the rest of the boat should the watertight compartment be ruptured ?

I ask these questions as I am still thinking about the JC2010 and am considering what changes the boat might require.

Good sailing

Paul
 

kds

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21 Nov 2002
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1,770
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Somerset
"My Invicta 26 wieghs 4.1 tons"
I think you will find that the volume of foam to float 4.1 tons will fill most of your cabin and you will be camping on deck !
Ken
 

JunkMing

New member
Joined
12 Mar 2007
Messages
39
Various marine authorities provide formulae for buoyancy, mainly for passenger-carrying vessels. I don't have one to hand but will be able to post one in a week or so's time. Essentially one cubic foot (whether that be foam or watertight compartment) provides c.63lbs of flotation. However you do not necessarily have to match the full displacement of your hull. Allowances can be made for less dense components, such as timber. Even the weight of fibreglass does not need to be matched fully. Leaving the allowances aside, 4 tons of displacement would require about 150 cubic feet of flotation. This does not necessarily all have to be foam. The creation of watertight lockers for stowage, the use of watertight bulkheads, even, as I use on Mingming, well-lashed watertight containers for food stowage, can all contribute to creating the required volume. I also fill odd awkward and unused space with foam-filled plastic bottles of various shapes and sizes. All this should be as low down as possible, immovable, and distributed as evenly as possible throughout the hull, so that in the case of water ingress the hull floats more or less horizontally.

It is an enterprise well worth pursuing. I personally would not now go to sea in anything but an unsinkable yacht. The difference in the general feeling of well-being is quite remarkable. I have had very bad experiences with liferafts, and hate the damn things. It is a pet gripe of mind that in this era of high-volume light displacement yachts, only two marques, to my knowledge, build in the required flotation as standard. It would be so easy to do in the build stage - but no, the space is required for more berths, more hot and cold running bidets, microwaves, washing machines and all the other essential accoutrements of the modern marina-hugging floating caravan.

Roger
 

2nd_apprentice

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18 Mar 2007
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2,480
Location
Berlin
Roger, if I remember correctly you filled quite a significant space in the v-berth area with flotation foam blocks? Personally I find it very hard to decide since space is also at a premium. I might solve this dilemma by fitting a watertight collision bulkhead filled with foam. So it won't get flooded even in case the hull gets punctured. Unfortunately this will be above the waterline which'll be of limited use but I'd lose to much space otherwise. That's why I thought of adding a few watertight compartments too. The area beneath the cockpit will be one big watertight compartment too.



Here's a very comprehensive article on the topic:

http://atomvoyages.com/projects/UnsinkableBoat.htm

The only thing that worries me, especially since it's a wooden (plywood) boat, is that poorly ventilated areas will be prone to rot. Might call for red lead and/or epoxying everything!
Even though you'll obviously won't have to worry about rot (except with your cockpit superstructure?), what are your experiences concerning mildew etc?

Patric
 

PacketRat

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20 May 2007
Messages
170
Location
Merseyside
Fitting out my Pandora, I decided against foam and bought a liferaft. A ton of foamed buoyancy weighs at least 50 kilos, half fills the boat and makes maintenance impossible. Eventually, inevitable leaks will waterlog the foam, and I'd never feel entirely confident the bow or stern wouldn't end up ten foot under water.

The combination of a liferaft and EPIRB seems a reasonable enough precaution from this armchair. I've heard of accounts from people who have used liferafts - not terribly encouraging - but haven't come across any accounts of swamped, buoyant yachts.

Anyone know of any?

Roger - great stuff - thanks for sharing your experiences.

Robin
 

JunkMing

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12 Mar 2007
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39
Patric - yes, I used some of the forward berth area to create a large watertight area filled with foam. Somewhat like your diagram but extending further aft. This was balanced aft with the lazarette also being foam-filled and sealed. As you say, this is not ideal - it's rather like giving the boat a pair of water wings. It will remain afloat - a rather important improvement - but probably heavily awash. Hence all the additional interior procedures to limit the amount of water inside . The underside of the cockpit is also foam-filled.

I do have ventilation 'problems' with Mingming, as I am fanatical about not having unnecessary openings anywhere. So there has been a lot of mildew here and there. I am thinking of relenting slightly and installing a solar powered ventilator to help ease this. With a ply boat I would definitely epoxy it prior to creating sealed areas.

Roger
 
Joined
20 Jul 2001
Messages
205
Location
Southampton, UK
I've taken several leaves out of Rogers boat ( mainly because he's still with us after surviving some horrific storms - and a major shipwreck - in his long career) and made my Corri as unsinkable as possible. Every spare bit of space that isn't used for storage is designed to trap air. Even if this simply slows down an eventual sinking, every second could be vital. (I carry a small 'helicopter' liferaft.)

I was a bit concerned about ventilation, so I've made my boat very ventable, but equally easy to close up. The anchorlocker (sealable from the outside) can be opened to give a through-draft through the hull, via a watertight hatch.

Also, as with Rogers boat, I'm going to infill part of the cockpit to reduce the swampable area, but with a removeable bridge deck and locker for day cruising. The drains have already been made much larger.

I'll be putting some more progress shots on Steves excellent Corribee website soon, but in the meantime I'm thinking 'trapped air' rather than foam. However, that said, I may still go for some flexible buoyancy bags.

Rogers right - being unsinkable must give you a great feeling of security - once you've become fed up with being scared witless.
 
G

Guest

Guest
Just in case anyone's thinking of using ordinary expanded polystyrene for in-built buoyancy ...

For year's I laboured under the mistaken idea that expanded polystyrene was a water-resistant material, having seen so much of the ugly stuff floating around as flotsam during my diving days.

But eventually someone pointed out to me that it will wick if in contact with water continuously. So I checked some polystyrene plant trays kept on the floor of a wet greenhouse - and sure enough, this stuff was as heavy as sin - and it took many weeks of storage in a hot dry glasshouse to get the water back out again.

So - if you're tempted to use ordinary polystyrene foam, at the very least wrap the blocks up in thick polythene sheeting and tape 'em up well - but even then, still check 'em for water-logging every now and again.

Colin
 
Joined
20 Jul 2001
Messages
205
Location
Southampton, UK
Hi Paul

Just another thought

One system I have seen used a few times is a series of bags triggered by a (large) dive cylinder. These bags are positioned strategically around the inside of the boat, and if you 'discover' one of Napoli's containers, for example, and start to flood, you can quickly inflate the bags. This keeps the boat afloat while you plug the leak - in theory.

In their collapsed form, they hide behind seat backs, or under the bunks, strapped down with the webbing obviously set for the inflated width. You wouldn't even know they were there.

They can, of course, also trash the cabin. (One Swedish blue-water couple had this system fitted, and accidentally set it off a few days out. They had to cross the Atlantic with it inflated, as it was a one-shot system, and they had no liferaft! They're 30ft boat was suddenly very cramped.)

Cheers

Jake
 

CPD

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20 Sep 2006
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Hampshire
Jake, I have heard about the bags also and would be very very interested if anyone knows where they may be available from, or what alternatives could be tried. Doea anyone have any direct experience ?
 
Joined
20 Jul 2001
Messages
205
Location
Southampton, UK
Hi Alan

I've heard you can use a certain type of bag used by divers for lifting heavy stuff off the seabed (although not the open-ended type!). Also, some types of inflatable race markers will also do, along with the larger type buoyancy bags from dinghies. Avon make a large inflatable fender which could be adapted. A lot depends on the valve set up, and how the gas bottles will be connected and triggered. It may be a good subject for a bit more research, so sorry I can't be more helpful at the moment.
 

PacketRat

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20 May 2007
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170
Location
Merseyside
The RNLI have self righting airbags on the A frames of their inshore RIB lifeboats, so the technology is still current.
 

lyina

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8 Jan 2007
Messages
22
I have thought about using inner tubes (available in loads of sizes and cheap ) which could be fixed on bulk-heads or inside cupboards etc, and attached via soft pipework to a cylinder.
 

lyina

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8 Jan 2007
Messages
22
Also inner tubes could be used as emergency/temporary measure to reduce cockpit volume and increase buoyancy.
 

thailand69

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3 Dec 2006
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602
Location
West of Les Écrehous and North of Les Minquiers
Whilst I can understand that rapid auto inflation is important for a lifejacket or Liferaft as you may need it within .05 seconds of pulling the cord..........perhaps you do not need the same facility onboard? given that the odds are that any incidents that result in total flooding in under .05 seconds probably involves something that a few inner tubes won't help much with!

Maybe just having a Dive Tank onboard (dual use?) could be a compromise, albeit requiring someone to hook it up and open the valve...........or even running a compressor off the engine?........or a foot pump!

Although I am not very technical on these sort of things - I would have thought that water displacement would be as important as providing bouyancy and that compartments that could be made watertight as well as any bulkheads would be very useful - also may isolate a hull breach. (mmmmmm, not sure if that last comment is not a bit too "Star Trek" /forums/images/graemlins/shocked.gif /forums/images/graemlins/tongue.gif)
 

2nd_apprentice

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Joined
18 Mar 2007
Messages
2,480
Location
Berlin
Well those "priority 10 force fields" would be most welcome in case of losing the hull integrity but I doubt my Rutland 503 would be up to it /forums/images/graemlins/grin.gif
It might be prudent however if one could quickly resort to a cofferdam as 20th century solution! IIRC one skipper in "Total Loss" had been able to save his boat that way. Griffith mentions them in "Blue Water" (triffic book btw!).

Patric
 

seedog

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18 Jul 2006
Messages
409
Location
Bristol
Totally agree that water tight compartments is the way to initially tackle this. However thinking laterally (literally) what about putting sausage bouyancy bags outside of the toe rail. In the event of a knockdown they will create a faster reaction than any bouyancy held internally and they do not detract from internal usable volume which can also be bouyant albeit as waterproof storage units. I think corned beef cans float, just.
 
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