Latest News: Forums Technical Missing buoyancy?

This topic contains 28 replies, has 0 voices, and was last updated by  Algol 9 years, 2 months ago.

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  • #4054

    Algol
    Member

    My W is a mkII, sail no 8242. Should it have foam buoyancy in the ‘boot’?

    If so, what size should it be, where can I get it, and what’s the best way of fitting it?

    #8857

    Bob Harland
    Participant

    There should be some permanent buoyancy in the aft locker.
    Usually a rectangular intrusion – a piece of polystyrene that has been glassed in.
    I am afraid I don’t know what size this should be.
    Perhaps someone with a mark 2 can measure it?

    #8860

    Algol
    Member

    @Bob Harland wrote:

    There should be some permanent buoyancy in the aft locker.
    Usually a rectangular intrusion – a piece of polystyrene that has been glassed in.
    I am afraid I don’t know what size this should be.
    Perhaps someone with a mark 2 can measure it?

    ‘Glassed in’, you say? As in covered in fibreglass. In the front buoyancy compartment it’s obvious there’s foam in there. So, should I see foam in the rear compartment or not?

    #8861

    My previous boat was a mk11 and the polystyrene was held back in place (back against the transom and up against the underside of the aft deck) by a fibreglass retainer. Looking into the bouyancy tank you would have only seen the fibreglass and not the polystyrene, but by putting your hand underneath you could certainly feel it. The polystyrene was actually in quite poor condition so I removed it and replaced it with a bouyancy bag and a number of 5 litre plastic bottles.

    On my current boat (mk1) I removed half of the rear tank bouyancy in order to fit an outboard mounting bracket. Again it was not in the best of condition so I replaced it with closed cell foam. I feel more comfortable with bouyancy bags / closed cell foam / plastic bottles that I can easily give a visual check than polystyrene hidden away behind the fibreglass retainer.

    Jonathan
    W2312
    Shuna

    #8866

    Swiebertje
    Participant

    On a MK2 the foam may be placed under the gunwales to keep the aft locker free for baggage. See class rules:

    34.4 Positive buoyancy units of closed cell plastics foam. Shall be securely fixed within the hull of G.R.P. and composite boats, as follows (lift refers to buoyancy when submerged in fresh water):

    (b) [Mk II (except SD versions)]. One unit providing not less than 40.82 (90 lbs) lift in the forward compartment. Units providing a total lift of not less than 40.82 (90 lbs) under each side deck, aft of the main shrouds.

    (c) Alternative for MK IA and MK II (except SD versions). One unit providing not less than 40.82 (90 lbs) lift in the forward compartment. Units providing a total of not less than 13.6 (30 lbs) beneath each side deck and two units providing a total of not less than 54.42 (120 lbs) lift in the aft compartment. The units shall be positioned according to the official drawing.

    #8867

    Colin Parkstone
    Participant

    First up, yes i am a measurer and should know how to work this out but I just don’t.

    How do you work out how much lift a cubic area of foam has so as to know if it measures.

    Think we could do with this in the rules to help.

    C P 😕

    #8870

    Swiebertje
    Participant

    In sweet water one kg lift equals one liter of displacement. So: the width times the depth times the height of the foam block should be more or equal to 40.82 liters. (W x D x H >= 40.82). If more then one foam block is used, just add up the individual volumes. I never had any problems checking these rules.

    For example: a 15 x 15 cm block of foam that is 1.82 meters long displaces 40.9 liters of water.

    #8871

    Colin Parkstone
    Participant

    Thank You Clever Clogs ! Can I say that to a Dutch Person ? 😉

    Does the Cider Sea count as sweet water ?

    C P 😕

    #8872

    Hi,
    My Mk 2 has a styrofoam block in the rear. It is shaped to fit the aft end of the hull/deck. A tight fit is not desirable as you want any water in there to be able to drain under it to the aft bungs and air circulation can be no bad thing. Mine has a single thin sheet of GRP with a lip on the lower edge holding it in place. The lip is superflous really. This sheet is stuck to the foam with 2 x 2″ vertical strips of glass matting which are also glassed to the underside of the deck. See diag.

    My foam is about 10″ from front face to transom face. You can calculate the right dimension as stated above and maybe check by displacing in a bath or container. Don’t fix it in too thoroughly. You may need to get to bungs, rudder fittings, outboard bracket position etc. The 2 strips fixing mine are only 2″ wide and have just been wetted with resin and applied. One tug would release the whole thing. There are probably easier ways of doing it but that’s what Moores the builder did.Also check if you have bouyancy under the side decking amidships. Mine is foam held in two grp shelves screwed under the deck
    Cheers
    David

    #8873

    BluTak
    Participant

    If you use inflatable buoyancy bags beware that they can and will puncture on fibre/shards of fibreglass. I put a holt bag in my front tank and it was promptly punctured. I subsequently used solid foam. I would use bags on a wooden boat but not on the rough tanks of a fibregass boat unless I was certain there were no sharp bits left by the builders
    Robert

    #8874

    Swiebertje
    Participant

    Maybe empty PET bottles are more durable?

    Or use those PVC sauerkraut kegs canoers use. They can double as dry storage for sleeping bags, cameras and other valuables. But get them from a pet shop, who sell them for pet food storage. They charge less then half from what the average chandler charges. As long as the hatches stay closed the kegs don’t need to be fixed in place IMHO.

    #8877

    Anonymous

    Although I like the thought of using hard plastic buoyancy as in plastic kegs, BDH bottles etc., surely if you fill them with stuff on a trip, they no longer have the same amount of buoyancy as you’ve displaced the air with something else.

    Martyn

    #8879

    Swiebertje
    Participant

    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.

    #8894

    Colin Parkstone
    Participant

    A point on this subject that may help.

    Before the foam block was put in the aft tank it used to be all under the side decks, as in it filled the gap under the sidedeck from aft tank to chainplates.

    It was contained by a GRP cover.

    It was then found to be to much bouyancy at the widest point of the boat when capsized and made the boat float high in the water.

    So the amount was reduced and put in the aft tank,so keeping the same total but distributed about the boat with better result.

    C P

    #8895

    Fantasia
    Member

    @algol wrote:

    My W is a mkII, sail no 8242. Should it have foam buoyancy in the ‘boot’?

    If so, what size should it be, where can I get it, and what’s the best way of fitting it?

    Just in case you did not realise: the ‘boot’ is in fact the rear buoyancy chamber and the additional foam is really only there in case its seals fail and it floods.

    Therefore it is really important that those seals and the catches for the hatches on both the rear and forward buoyancy chambers function correctly. This should be verified with a buoyancy test.

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