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  1. #1
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    Inside of a fender

    No, not a guitar...

    I am filling 6 fenders with water, so they are as negatively buoyant as possible. I bought really heavy duty ones that have dense plastic, that I reckoned would sink. I bought them un-inflated, but they obviously don't arrive completely flat, because when pressurised with water they still have some air in. You can hear the water sloshing about; quite a bit of air in there. I thought it would be a simple task of pressing the valve when it was positioned uppermost and the pressure in the fender would expel the air.
    Except it doesn't. Water comes out. I have rolled the fender around and cannot get air to come out. If I shake it violently from side to side, an occasional fart appears but it's mostly water. So it looks like the valve has a long tube on the back - though I don't know why it should have.
    When filled with water and in the bath it is a kg or so positively buoyant and I reckon with the last of the air expelled I will reach my goal of negativity.
    Of course I could weigh it down when underwater, but that's not too elegant.
    Anyone seen a fender that's exposing its innards and care to say why no air is coming out? Nothing obvious from googling their design. They are Majoni fenders for what it's worth and have the sort of valve that you use a football spike type inflator on.

    And if anyone is really interested I'll describe why I want some negatively buoyant fenders.....

  2. #2
    Established TDF Member Nickpicks's Avatar
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    Even if it's full of water, most plastic is slightly positively buoyant (hence the amount floating around in the oceans).

    I can't think of any reason to have a "debris tube" type arrangement on a fender, but you could check if there is by sticking an endoscope camera down the inflation tube to see what's going on (how wide is the inflator tube?)

    And go on, I'm curious - why do you want sinking fenders?
    Also, if full of an incompressible liquid like water, they won't have the same shock absorbing qualities as when full of air.
    The major difference between a thing that might go wrong and a thing that cannot possibly go wrong is that when a thing that cannot possibly go wrong goes wrong it usually turns out to be impossible to get at or repair.

  3. #3
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    If you are planning on using it underwater, can't you just cut a small vent hole for air to escape from? Or do you need it sealed, in which case could you cut a vent and then re-seal the hole after?

  4. #4
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    I wouldn't expect a debris tube either! The inflator hole is 2mm across if that, and deep down there's a metal flap that forms the one way valve. The "needle" pushes in and opens the valve and a hole in the side of the needle tip lets air (or in my case water, in). A reasonably tight seal is created by a rubber blob part way up the needle that seals against the outside face of the fender.

    My rib is on a swinging mooring that dries at 0.6m tide height. So pretty well 97% of the time the boat floats. It's sand beneath but with the odd bit of bedrock. Up till now I've used a "hammock" of rubber material - like you get on the floor of a cow shed. It's clipped onto the D rings on the side of the boat so it fits snuggly beneath. When the rib settles onto the sea bed it protects it perfectly. Before setting off, I unclip the clips, the hammock falls to the sea bed and I'm off. When I return, I pick up the mooring buoy, and pull the hammock up via lines attached to the mooring buoy and clip it up. But that's the problem. Even in water the hammock is very very heavy, and lifting it involves displacing an awful lot of water. Very tiring.

    Over winter, I sat down and thought and reckoned that a couple of strips of fenders, running under the boat would work just as well. 8 inch diameter fenders and they would keep the hull well off any rocks. And they are almost indestructible and really cheap for what they are.

    But they have to be negatively buoyant - just a bit, so easily dropped and pulled back up.

    I could weight the loops of fenders with some anchor chain I have, but that introduces nasty hard stuff that is going to damage the hull. TBH I expected the fenders to have a screw in plug like you get on buoys, and I was going to fill them with sand and water.

  5. #5
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    Quote Originally Posted by Spindrift View Post
    If you are planning on using it underwater, can't you just cut a small vent hole for air to escape from? Or do you need it sealed, in which case could you cut a vent and then re-seal the hole after?
    It really needs to be sealed to support the weight of the rib (see my too long answer to Nickpicks.) I suspect that the greasy plastic material that fenders are made of will resist all attempts at re-sealing!!
    Last edited by Tens; 10-05-2021 at 08:10 PM.

  6. #6
    Established TDF Member Nickpicks's Avatar
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    With the "Hammock" could you construct that with a floating buoy at each corner, like a floating dock, which would get left on the mooring so you basically drive the boat into the floating dock which will then settle onto the sea bed under the boat when it dries out.

    That would save having to haul the hammock back to the surface each time you return to the mooring.
    The major difference between a thing that might go wrong and a thing that cannot possibly go wrong is that when a thing that cannot possibly go wrong goes wrong it usually turns out to be impossible to get at or repair.

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    Quote Originally Posted by Nickpicks View Post
    With the "Hammock" could you construct that with a floating buoy at each corner, like a floating dock, which would get left on the mooring so you basically drive the boat into the floating dock which will then settle onto the sea bed under the boat when it dries out.

    That would save having to haul the hammock back to the surface each time you return to the mooring.
    Good thinking Batman. Trouble is, at high tide there's 4m depth of water, so the buoys would need to be on that length of line. When the boat settles onto the sea bed at low tide, chances are it would miss the hammock sideways as there's now 4m of slack in the lines.

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    When I wanted to make a football negative I added salt to the football initially then added the the water

  9. #9
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    Quote Originally Posted by N Bailey View Post
    When I wanted to make a football negative I added salt to the football initially then added the the water
    Think there's too much air in the fender for that to make a substantial difference. I think there's about 0.5kg of positive buoyancy.

  10. #10
    Pedantic Pig Divemouse's Avatar
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    Why do they need to be negatively buoyant to sit neatly under a very floaty thing?
    Definitely don't doubt Dawn - not if you value your life


 
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