Bouyancy

Jonny Moore has kindly allowed us to reproduce this article:

The idea of adding positive floatation to Casulen II began in 2006 on our trip around the north coast of Scotland. Whether it was the remoteness of this area of coastline I’m not sure but it certainly helped to concentrate the mind on what could go wrong. We had very little experience of sailing on the sea at this point but you didn’t need to be a brain surgeon to work out that if Casulen II was holed she would most likely go to the bottom very fast. Sitting in the cabin, as all Corribee owners will know highlights the fact that these boats are hardly roomy below decks which means that if water started to come in it would not be long before it was full! We were to see only 2 boats between Lochinver and Inverness and for the most part our VHF would not have been of much use as we couldn’t even pick up weather forecasts, probably due to the mast height of Corribee`s and ours wasn’t linked to a GPS at that point.

The weather doesn’t need to be blowing a gale for you to put a hole in your hull, you only need to read through the yachting press to realise the amount of containers, fridges, trees etc. floating around not to mention the odd Basking Shark and all the rocks that litter our shores.

After that trip we discussed this with several people and also trawled the Internet for information. The conclusion was that buoyancy has to be carefully placed so that the boat would float level and the right way up and anyway took up far to much room because of the quantity needed. As I have said we had very little experience to draw on at this point but the nagging feeling that the boat was still better off on the surface even if bow down and upside down remained. At least it’s a bigger object to be seen if it’s still afloat. So in our own simplistic way we started to do the sums. A Corribee has a displacement of 2000lbs laden and seawater weights 64lbs a cubic foot so we needed just over 31 cubic feet of buoyancy. Simple. Well not quite that simple, most woods, with a couple of exceptions that aren’t found in boats, float, so have positive floatation. For every 10lbs of plywood on board it will support 8.1lbs of weight. Even fibreglass has a submerged weight of about 3.5lbs per 10lbs of weight out of water and our Corribee cast iron ballast of 870lbs only weighs 750lbs under water. So ignoring all the wood on board and just treating the weight as ballast or fibreglass we recalculating all this and concluded that if we allowed for 1500lbs we would have a safety margin of 33% and needed 23 cubic feet of flotation. Now that we reckoned we could manage.

20090724_Buoyancy_1_JonnyMoore

When you start looking closely at the inside of a boat there are loads of voids that never get used for anything and the Corribee is no exception. Some are listed below.

  • Under the cockpit
  • Under the rear section of the quarter berths
  • All the voids above the quarter berths
  • Either side of the `Heads` locker

We also put 4” of foam on the bulkhead between the lazerette and the quarter berths as we wouldn’t miss the extra length and store the tender down one and the liferaft and flares down the other anyway. Inside the lazerette we also filled the same bulkhead back to level with the rudder tube and this was all glassed in. Forward 2” of foam was added under all the shelves and new deeper fronts made to cover this. 2” was also added to both ends of the lockers and also on the cabin bulkheads. Up to now we had managed to get just over 20 cubic feet of foam in. We used polystyrene for this. Now there are a lot of arguments for not using this, it is attacked by petrol and also polyester resins and also gives off horrible black smoke if it catches fire. On the plus side it is cheap! I come back to my earlier point, I would rather have the boat on the surface and if more expensive foam were used we would not have done the job.

20090724_Buoyancy_2_JonnyMoore

I don’t know about other Corribees, but Casulen II is a dry boat, she doesn’t leak. Fitting a new front hatch, totally reworking the main hatch so that it has no gaps and slides under a garage, having a tight fitting sprayhood a sealable chainpipe and striping all fittings off the deck and rebedding them, probably helps this. The point being that the foam is not going to fester because it is not going to get wet, if it does we have a far bigger problem so the argument about damp and mould doesn’t exist.

20090724_Buoyancy_3_JonnyMoore

Now for the rest of the floatation. Most of us carry fenders, we carry six, two 200mm diameter and four 150mm diameter, and these are kept in the lazerette and add up to another 2 cubic feet of floatation. Finally we replaced all the bunk cushions with closed cell foam giving a further 6.5 cubic feet. Our total now stands at 28.5 cubic feet and if any of you came on board Casulen you would not realise it was there even if you spent a few days sailing her unless I told you.

I for one feel far happier on board particularly on night passages when you can’t even see what’s in the water. Two other points of reference on this subject are Roger Taylor’s Mingming and Jack Kavanagh`s Pod, which show two other approaches to the same thoughts.

Roger has also written about his thoughts on his site: http://www.thesimplesailor.com

Think about it when you are next crashing through those waves at 6 knots, particularly if it’s at night.

eCorribee writes:

I don’t like foam as it is messy stuff and a fire hazard. 23 cubic feet is equal to approximately 652 litres. A good alternative form of buoyancy for tucking into spaces is old PET soft drink bottles filled with fire retardant expanding foam with their tops screwed on tight and then taped closed. To obtain the buoyancy described, you need to stow 326 two litre bottles in the various voids. For the oddly shaped corners, use the 500 ml small bottles prepared the same way. As you don’t want them bobbing out through a hole in the side if you are holed, it is probably a good idea to wrap them in batches in netting. If you want to see what you can achieve with 15000 bottles, have a look at http://junkraft.blogspot.com/.

Roger Taylor used old shampoo bottles and other plastic containers filled with expanding foam to achieve a similar effect on Ming Ming. He also created a waterproof bulkhead just behind the v-berth and in front of the stern locker and filled the bows and the stern with foam sheets to achieve unsinkability. Full details with photographs can be found in his excellent book Voyages of a Simple Sailor.

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2 Responses to “Bouyancy”

  1. MIrch Says:

    Can I throw a curved ball…. Canoe airbags. You can push them into voids, such s under V berth lockers before blowing them up. Maybe even room to get some bigger ones made for boats? simply deflate when not needed. Simply type ‘canoe airbags’ into ebay and google. lots of different sizes available.

  2. colin powell Says:

    If subjected to contact with water, contrary to popular belief, expanded polystyrene will very slowly absorb water and become incredibly heavy – so should you be tempted to use expanded polystyrene for in-place flotation, I recommend you wrap it in polyethylene and tape it up well.

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