Latest News: Forums Technical Missing buoyancy?

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  • #8896
    Pete Lock
    Member

    My Mk II did not have the buoyancy in the Aft Hatch when I bought it. It has three 25 litre plastic drums stored inside. Which are easy to remove to allow cleaning. The only downside to this is that there is little room for anything else .

    Pete

    #8911
    Anonymous
    Inactive

    @Swiebertje wrote:

    True, but in a 25 liter keg I may have stored up to two kg of stuff (2 liters loss of displacement). This is hardly significant. With two of those barrels I still have enough bouyancy left to meet safety requirements. I can’t imagine storing 25 liters of water (or the equivalent weight) in those 25 liter kegs.

    Some examples: My camera weighs 400 grams. My sleeping bag is about 1,25 kg.

    Ah but the mass of your sleeping bag/camera is not the same as the displacement. In other words, a kg of feathers will not necessarily fill the same space as a 1kg of water. A sleeping bag will inevitably occupy more than 1.25 liters even though it might only way 1.25kg.

    However, I take your point. You will probably have enough buoyancy, but it is worth baring in mind your storage/buoyancy ratios with regards to occupied space and weight.

    Martyn

    #8912
    BluTak
    Participant

    Swiebertje is quite correct. Displacement is related to mass and the volume is irrelevant. If you want to prove it float something in your bath and add first something heavy and small. Then try something the same weight but bulky. You will find it sinks (displaces) the same amount.
    So if you fill your container with a sleeping bag it will make very little difference to the buoyancy.
    If you think about it this is how a foam filled buoyancy compartment works
    Hope this clears up your misconceptions. Robert

    #8913
    krgough
    Participant

    Hmm..

    In the case of our kit inside the barrel, the average density of the barrel has increased. The barrel will float a bit lower in the water (ie. it is now less buoyant)

    In an extreme case – if I partly filled the barrel with lead, it would sink – i.e. negative buoyancy.

    My guess is that this effect is not very significant if you are talking about a sleeping bag, but probably would be if you stuff the barrel with essentials like tins of beer and bottles of whisky.

    #8914
    BluTak
    Participant

    Put simply if the weight inside it in Kg exceeds the empty volume in litres (or thousands of a cubic metre) it will sink! The volume in litres less the weight inside it equals the buoyancy to float your boat.

    #8917
    Swiebertje
    Participant

    Don’t kill the messenger. It was Archimedes (someone from ancient Greece) who said it, not me.
    @Archimedes of Syracuse wrote:

    Any object, wholly or partially immersed in a fluid, is buoyed up by a force equal to the weight of the fluid displaced by the object.

    In the case of sweet water one liter equals one kilo. So, a fully submerged 25 liter barrel has a buoyancy (lift) of 25 kg in sweet water. If it holds a 1 kg object the total buoyancy (lift) is 24 kg. The two barrels I carry have a total volume of 50 liters, well above the 40 liters the rules require. In other words I can store op to 10 kg of goods in them.

    In seawater things get better because sea water is heavier. Have you noticed how your boat floats higher in sea water?

    Why anyone would want to store tins of beer in a waterproof barrel in the aft buoyancy tank is beyond me. Beer is much better stored in the bilge where it stays cool and where you can grab it while sailing and where the weight is in a much better place for boat balance. On my boat booze not allowed, except for medicinal purposes in which case it should be kept within easy reach, because one never knows…..
    Cheers!

    #8918
    krgough
    Participant

    Ah yes 25 yr old single malt “medicine” – I know exactly what you mean 😉

    #8940
    Anonymous
    Inactive

    Purely because I’m enjoying the scientific discussion, not because it is hugely relevant, I’ve been thinking. The principle of flotation is that an object will float IF the weight of the water displaced=weight of object causing the displacement. It does not infer that if you make the object heavier it will displace more water and float. The heavier the object, the more water it has to displace, this being due to the relative densities of water and the object.

    So, if you fill up your storage bottles with water, you are increasing the density of your boat without increasing the amount of water being displaced by a similar amount. Also, vector in the fact that whatever the buoyancy in the tanks of a wayfarer, they need to maintain a density less than that of water to maintain positive buoyancy.

    I think.

    Martyn

    #8941
    Anonymous
    Inactive

    PS Yes of course beer is better held in the bilges. Unless some idiot decides to paint the floorboards black and you can then warm red wine under them in sunnier climes.

    It happened.

    martyn

    #8974
    Algol
    Member

    It turns out the buoyancy is along the gunwhales on my W.

    #8975

    Yup, my MK 2 has that (also see my earlier post). The primary buoyancy is provided by air in the front and rear compartments and the side foam buoyancy. That’s the normal buoyancy total for the boat that keeps it afloat when hull undamaged. The side foam was put in to compensate for the half height forward compartment as compared with full height on a Mk1. The foam fore and aft is what stops it sinking if a compartment is holed. I’ve got both and I wouldn’t be without either! I think they’re built that way. Thank goodness for spellcheck or I’d never spell “bouyancy” correctly.

    #8976
    Swiebertje
    Participant

    The volume of the forward tank is the same on a MK1 and a MK2. A MK2 has the forward bulkhead further aft to compensate the loss of volume. Please note there is a lot of space between the mast and the forward bulkhead on a MK1 and there is none on a MK2.

    The foam, as you noticed, is there to prevent the boat from sinking should the tanks flood. (GRP is heavier then water). Actually the foam in the sides makes the boat harder to recover after a capsize but because the MK2 was designed as a pure cruiser this is a compromise to provide more storage space in the aft tank. Should the tanks be flooded the foam will prevent the boat from sinking but it cannot be bailed anymore because the top of the CB case stays submerged. And that is why safety demands us to check our tanks every year and keep them closed at sea, at all times.

    And thus I avoided the words “buoyancy” and “locker”.

    #8977
    Algol
    Member

    @Swiebertje wrote:

    Should the tanks be flooded the foam will prevent the boat from sinking but it cannot be bailed anymore because the top of the CB case stays submerged.

    Crikey! That’s worth knowing. Thanks

    #9287
    admin
    Member

    Sorry that this thread has gone a bit cold before I realise what has been said….

    The buoyancy foam in my forward tank is stuck to the “roof” of the tank. Now it’s not going to do much good there if the main purpose is to keep the top of the centreboard case above water when swamped, as Swiebertje says, you can’t bail under these circumstances. The buoyancy needs to be low enough down in the boat to keep the top of the centreboard case above the water, I suppose like the SD, but I have read that the SD can be difficult to right after a capsize.

    I wonder if a happy compromise might be to put some foam beneath the floorboards, possibly polyurethane foam shaped into wedges and stuck in (using Sikaflex or a cheaper adhesive) where it could provide some support to the ply (someone has raised this in a thread elsewhere), and enough low-down buoyancy to prevent the hull sinking so low in the water as to prevent bailing, and not so much to impede righting her. So just the right amount.

    If the foam wedges were glassed in using epoxy (=weight, hmmm) they could go a fine job of stiffeneing the hull skin.

    #9289
    Swiebertje
    Participant

    What you are describing in the second section is in effect the SD version.

    The foam is a last resort. It prevents your boat from sinking when the tanks are flooded, that is all it does. Your goal should be never to come in to that situation by preventing your tanks from leaking. When the tanks are in good working order the foam sits around idle and does nothing whatsoever. Once the tanks are flooded it does not matter where the foam is located, you won’t be able to bail your boat, the gunwales will be more or less at sea level and all you can do is wait for the RNLI to save you and your crew.

    Do the annual buoyancy test and forget about the foam (just check its there) it serves no purpose but to save your *** when you failed to properly maintain your tanks.

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