Rigid Buoyancy

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co256

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I have been looking into the subject of buoyancy and will share my findings with you...

What follows is a comparison of three types of rigid foam, the standard unit for comparison is 1 cubic metre.


2 part Polyurethane liquid weight 35 kg Buoyancy 950 kg Cost £216

Plastazote LD15 weight 15 kg Buoyancy 1000 kg Cost £473

Expanded Polystyrene weight 15 kg Buoyancy 900 kg Cost £91

There was an earlier post on this subject that suggested EPS (Expanded Polystyrene) absorbs water, I have found no evidence to suggest this, in fact quite the reverse. EPS does not absorb water but it will wick moisture to less than 4% of volume, rather like GRP.

It is recommended that buoyancy is retained in watertight compartments, spread evenly around the boat and as low down as possible within the hull canoe body.
 
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EPS absorbs water

If you do a Google search for "eps absorbs water" you'll find a selection of contradictory statments on this subject.
However, I have direct experience of an EPS hot-tub cover which absorbed an incredible amount of water over time via water vapour. When I replaced it, it was so full I could hardly lift it. Even removing it from the moisture source and leaving it to drain for a week made little difference. I'm told this is a common occurrence and the EPS is normally sheathed in polythene to prevent the problem.
Perhaps there are different varieties / grades of EPS and I admit there is a shortage of water vapour on board my boat. However, I don't think it's as cut & dried as you suggest.
 
Does EPS absorb water?

EPS is a closed cell and cannot absorb water. During the process of moulding a block, tiny channels are formed between the EPS particles. If the material is immersed in water these tiny channels can be filled with water. After immersion for more than 360 days, there may be up to 6% water content by volume which has entered the channels.

Even under such an adverse and rare condition of prolonged saturation, EPS suffers little adverse effect. It maintains its shape, size, structure, cohesion and physical appearance. The ability of EPS to resist the adverse effects of moisture is exemplified by its widespread use in floats, marinas and other applications, which involve full or partial submergence in water for prolonged periods of time.
 
"It is recommended that buoyancy is retained in watertight compartments, spread evenly around the boat and as low down as possible within the hull canoe body."

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Good point, but when laying on stores for an extended ocean passage, it is vital to keep the weight of water, tins, tools and heavy spares etc. as low and central as possible. This will help to keep the boat upright, and hopefully avoid the need for buoyancy for survival. For yachts below 25' it would probably be best to reserve the space low down for stores. Scalywag sailed more upright than usual on our transatlantic, but Tesco was then selling curry sauce for 6p a tin, cheap ballast!

I would consider putting a layer of foam glassed in against the hull in the forepart of the boat, as it may give some protection in case of colliding with a semi-submerged object. Each boat will be different, but I would ensure that I could get all my heavy stuff stowed low before considering where to put the foam.

Len
 
I removed my water tank which was fitted in the bow and made the space water tight before having it filled with foam.

I can't recall the foam type but I got the yard to do it as I understood the foam they used (which was the same in the steel raft they use to haul up moorings) expands at such a rate it had the potential to blast my water tight compartment apart if too much was poured in. Having seen the foam demonstrated in a half empty milk carton I realised what they meant.

I understand the 'marine' garde foam is non absorbant but as I haven't yet had the need to test it I can't vouch for it. Have to say I trust the yard implicitly particularly as they have helped me so much on a free of charge basis in order to help get me to the challenges. Am happy to find the type of foam if it helps.
 
There's something instinctively unsettling about heading out to sea in a small sinkable boat.
I opted for the Titanic solution on my 22 footer and fitted two complete bulkheads splitting the boat into three sections. I have only done the one deep sea prep trip which turned out to be a thousand miles of unmitigated glorious weather, so I can't offer any direct experience as to the practicality of the idea in heavy weather. But what I can say is that I had great confidence in my boat and enjoyed the whole experience immensely.
I have never heard of anyone whose bulkheads, trapped air, foam or inflatable bags ever served any purpose. Nevertheless it was transformative in terms of peace of mind, confidence and enjoyment. Thoroughly recommended.
 
Classe Mini Rules - Floatation

Though there are no imposed rules here, it is interesting sometimes to observe others...

Typically, the minis have one volume as low as possible in the bow and two aft, low in the quarters. Having followed various blogs/web sites on boat build/preparation, I have often seen Expanded Polystyrene used.

The Classe Mini is one of the most highly regulated.

J-13-a Floatation

J-13-a-1 Each boat must be constructed with floatation volumes of at least 1200 litres, spread over a minimum of 4 distinct volumes, including the volume inserted in the building of the hull
(exclusive of the deck).

J-13-a-2 Except the hull, these floatation volumes must be located at least at 400 mm below the local gunwale.

J-13-b Stability

J-13-b-1 Besides the above mentioned volumes, boats must be provided with 3 stability volumes :
- Minimum individual volume : 100 litres,
- Minimum global volume : 400 litres (volumes of the hull and deck excluded)

J-13-b-2 These volumes must be located completely above the floatation volumes and can go up to the deck :
- The first one against the bulkhead,
- The two others at the back (one on each side), along the hull planking and at less than 1 metre from the transom.

J-13-c Construction
Floatation and stability volumes must :

J-13-c-1 have an absorption capacity of less than 5%.

J-13-c-2 be laminated or partitioned off to the hull, the deck or the structure so they can resist loads of at least the value of their buoyancy, whatever the heel or the trim of the boat.

J-13-c-3 basic elements of the volumes must answer the following formula (length + width + height) >500 mm.

J-13-d Grandfather clause : articles J-13-a-2 and J-13-b do not apply for boats that can justify a beginning of construction before January, 1st 2010. However, the Classe Mini strongly encourages the concerned competitors and shipyards to make the changes as soon as possible.
 
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