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Thread: Chum

  1. #31
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    Default Re: Chum

    Quote Originally Posted by AngusMcDoon View Post
    A kilo of lead has a density of 11.3 kg/l, so a volume of 0.088 l. That volume displaces 0.088 kg of water, so the effective mass of the lead in water is 0.912 kg. For aluminium with density 2.7 kg/l the effective mass of a kg in water is 0.63 kg.
    Fail.
    You are confusing mass with weight.

    Both matter in this instance, the weight is what initially deflects the chain, the mass provides inertia.
    but a physically large, low density weight would have more damping, as it needs to move more water out of the way to straighten the chain.
    When a boat starts yawing at anchor, the damping effect of moving the rode through the water becomes important.
    But I don't see the situation as exactly amenable to accurrate mathematical analysis.

    I wonder if a weight plus somesort of 'flopper stopper' would be useful in some conditions?
    I just went for lead, because I had some and it's easy to cast into a block that stores nicely in the bilge, doesn't rust etc.

  2. #32
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    Default Re: Chum

    Use of the wrong word doesn't invalidate the argument that it's the weight that does the work in increasing the force needed to straighten out the rode. You might be able to measure the difference in drag between equal masses of ally and lead in a lab, but wouldn't notice it in use. I doubt very much a bucket would make a significant difference.
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  3. #33
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    Default Re: Chum

    Quote Originally Posted by lw395 View Post
    Fail.
    You are confusing mass with weight.

    Both matter in this instance, the weight is what initially deflects the chain, the mass provides inertia.
    but a physically large, low density weight would have more damping, as it needs to move more water out of the way to straighten the chain.
    When a boat starts yawing at anchor, the damping effect of moving the rode through the water becomes important.
    But I don't see the situation as exactly amenable to accurrate mathematical analysis.

    I wonder if a weight plus somesort of 'flopper stopper' would be useful in some conditions?
    I just went for lead, because I had some and it's easy to cast into a block that stores nicely in the bilge, doesn't rust etc.
    Not confusing mass & weight at all, but thanks for your concern. I could have said weight, but then I'd have had to use units of Newtons, & poor old Andrew would have got even more confused, so that's why I said effective mass. I very much doubt water resistance of a lump of metal really has much effect. I would expect the water resistance of the 30m of rode as it swung about would be greater, but still small.

    The weight of a same sized mass of different density under water is less, & the force required to lift it is therefore also less. If it didn't matter then the cheapest 10kg chum would be a bag full of 10 litres of water, & that would only be useful in Andy's weird world.

    I didn't fail mechanics or fluid dynamics as an undergrad, by the way. Thermodynamics was my bugbear.
    Last edited by AngusMcDoon; 06-06-19 at 11:54.

  4. #34
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    Default Re: Chum

    Quote Originally Posted by Seajet View Post
    I'm not an anchor expert

    Which clearly shows Angus, now let us men get on with proper advice
    Have you boffed up on Archimedes' principle yet, or are you still claiming that there's no difference on the density of the material a chum is made of? The principle is taught to 12 year olds in school so you must have forgotten it. I'm choosing to identify as a hermaphrodite today, so I should be allowed into your manly discussion.
    Last edited by AngusMcDoon; 06-06-19 at 11:44.

  5. #35
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    Default Re: Chum

    How about 10Kg of wood?
    Last edited by SteveGorst; 06-06-19 at 11:39.

  6. #36
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    Default Re: Chum

    Quote Originally Posted by SteveGorst View Post
    How about 10Kg of wood?
    Beat you too that with my 10kg bag of water idea. Or what about using my inflatable dinghy? That's got a mass of 14 kg. Think of the water resistance that would have swinging about down there.
    Last edited by AngusMcDoon; 06-06-19 at 12:02.
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  7. #37
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    Default Re: Chum

    Quote Originally Posted by AngusMcDoon View Post
    Not confusing mass & weight at all, but thanks for your concern. I could have said weight, but then I'd have had to use units of Newtons, & poor old Andrew would have got even more confused, so that's why I said effective mass.

    The weight of a same sized mass of different density under water is less, & the force required to lift it is therefore also less. If it didn't matter then the cheapest 10kg chum would be a bag full of 10 litres of water, & that would only be useful in Andy's weird world.
    Not really true, you are missing the ppoint of inertia.
    A scuba diver is weightless underwater, a force of a few ounces will make him rise or sink.
    But he still has a mass of say 80kg, and when you shove him, you have to shove aside a mass of water for him to move.
    The water he is displacing has mass as well as viscosity.
    So if your weightless scuba diver was holding onto your chain, the force required to yaw the boat around, oscillating the scuba diver through the water, would still be related to his mass. And the mass of the water which has to move around him.

    There is a bit more to kellets than a simple static weight in the middle of the line.

    My lead brick moving through the water damps the oscillation because it takes energy to move a body through water at speed. Viscous drag.

    In a snatching scanario, where every little wave is jerking your rode by lifting the bow, the weight clearly helps by deflecting the rode, meaning the bow can rise before the rode becomes bar taut, but the damping of the weight and rode moving through the water as this happens is probably significant too.

  8. #38
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    Default Re: Chum

    Quote Originally Posted by AngusMcDoon View Post
    Not confusing mass & weight at all, but thanks for your concern. I could have said weight, but then I'd have had to use units of Newtons, & poor old Andrew would have got even more confused, so that's why I said effective mass. ....
    Sometimes 'redneck units' like pounds have their uses....
    Is effective mass supposed to be the equivalent gravitational or inertial mass?

  9. #39
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    Default Re: Chum

    Quote Originally Posted by lw395 View Post
    Not really true, you are missing the ppoint of inertia.
    Explain to me this difference in inertia of 10 kg of aluminium & 10 kg of lead...

  10. #40
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    Default Re: Chum

    Quote Originally Posted by lw395 View Post
    Sometimes 'redneck units' like pounds have their uses....
    Is effective mass supposed to be the equivalent gravitational or inertial mass?
    Ok, I should have used units of Newtons. Mea culpa.

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