Latest News: Forums Technical Dry test buoyancy tank setup

This topic contains 4 replies, has 4 voices, and was last updated by  Dave Barker 3 years, 6 months ago.

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  • #20851

    suffolktim
    Participant

    Hi guys – I am going through the process of replacing my mk1 hatch seals, but I have decided to make a pressure testing rig, similar to the one detailed in the Wayfarer book, and a few places online.

    I have finished the rig, and all seems to work well. I just need some food colouring dye to colour the water, and it will be as I want it. The only snag I have come up against is a drop of pressure in testing. Its only small, and I have tested all joints…it is definately my stirrup pump, not holding air in the system once I stop pumping. I have also tried my car footpump, and whilst it is better, its still not perfect.

    Could anyone tell me which pump they use in this system, or whether they use a screw down air restrictor in-line to be sure it holds pressure?

    Hope someone can help with this. Cheers!

    Tim

     photo tank tester_zps5qzufis1.jpg

    #20852

    davidhs
    Participant

    Hi Tim

    I did what you did and then found that you don’t need a pump – just blow into the end to get it up to pressure and put your thumb over it for 30 seconds. The pump just complicates things I think (although my tubing may be greater diameter than yours, so easier to do).

    It is also then possible to do the test the other way – suck out some air so higher pressure is outside the tank rather than within it – which will actually be the case in the event of capsize. In my experience the sucking air out test usually yields a better result (which makes sense as the hatches are sucked onto the seals rather than pushed away from them, as would be the case when immersed in water). However, I make sure the worst case test passes.

    The suck-air-out test is obviously no good if you are doing a smoke test to find leaks, and wouldn’t do your lungs much good either!

    I also don’t faff around with food colouring – i’ve never had any trouble seeing the water level.

    Regards

     

    David

     

     

    #20855

    davdor7038
    Participant

    Hi. Tim.

    When I made up my testing rig a while ago, I put a shutoff valve just downstream of where I blow into the pipe to pressurise the system.  IIRC, the test was meant to be held for a minute, which is a long time to keep the pipe in your mouth, especially with the back pressure.

    The shutoff valve is a standard water plumbing shutoff valve, with a quick close 90deg turn lever.   It seals really well against the air pressure.  I used a balloon to over the end of the pipe to check that it wasn’t leaking air from there.

    As it happened, my aft tank was good for nearly 5 minutes, which pleased me no end.

    Regards, davdor.

    #20856

    suffolktim
    Participant

    Hey guys, thank you both for your replies…I have bought a tap, which will do the same thing as your shut off valve davdor. I used 8mm bore pipe, as recommended by several sources when I was researching it, and its biting me a little bit now, as far as getting fittings to fit it. Luckily 8mm is standard in the aquariums world, and I have managed to get the tap below…

     photo tap_zps6wvs8ftr.jpg

    #20869

    Dave Barker
    Keymaster

    I must first emphasise that I have no specialist knowledge of this subject, nor any direct experience. However, I have spoken to some people from one particular sailing club who had a fairly serious incident on a cruise a number of years ago in which several boats were swamped, producing a real-world buoyancy test.

    Their experience was that contrary to what intuition might tell you, the hatch covers tended to be blown off the hatches as opposed to being pressed onto the seals. Presumably this is because the hull, being flexible, was slightly ‘crushed’ by water pressure, thus compressing the air in the buoyancy chambers.

    This doesn’t answer the original question, but perhaps validates the idea of testing by pumping air in rather than by drawing it out. (If possible perhaps it’s best to try both methods?)

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