Chum

[noelex]

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.

+1

The desirable effect of the chum is mostly a function of the weight. In a vacuum all materials of equal mass will have the same weight, but as we have boats not spaceships we need to consider the in water weight. The weight or the effective downward force of the chum will depend on the density as well as the mass of the material. More dense materials will produce more downwards force than a chum of identical mass made from a less dense material, as Angus has correctly illustrated above.

So a chum made out of lead will produce almost 50% more of the desirable downward force than a chum of identical mass made from aluminium.

There are some other properties of a chum. The drag or friction can be an important benefit but this is not as simple to analyse as it is dependent on many other properties of the chum such as the shape. Inertia is one property that is related to mass not weight, but adding inertia is a very minor benefit of deploying a chum.

A simple way to look at the issue is to consider a chum made from a buoyant material such as wood. It could be the same weight as lead chum but would have opposite to the desired effect, although surprisingly there is some rare situations where a buoyant “chum” has some application.

 

[lw395]

.....although surprisingly there is some rare situations where a buoyant “chum” has some application.

Sometimes a mooring buoy alters the dynamics of a boat on the mooring, a bit like that?

 

[AngusMcDoon]

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.

If you consider the inertia & water resistance as part of a chum's effectiveness, fine. All I'm saying is that downward force is a major part of how a chum works, & density of the metal affects that force, contrary to Andrew's 'advice' that it doesn't.

 

[lw395]

If you consider the inertia & water resistance as part of a chum's effectiveness, fine.

Yes.

All I'm saying is that downward force is a major part of how a chum works, & density of the metal affects that force, contrary to Andrew's 'advice' that it doesn't.

Density of the metal will clearly affect the force due to (weight minus buoyancy)
It may affect the F=MA and force due to viscous resistance in the opposite direction....

 

[GHA]

If you consider the inertia & water resistance as part of a chum's effectiveness, fine. All I'm saying is that downward force is a major part of how a chum works, & density of the metal affects that force, contrary to Andrew's 'advice' that it doesn't.

If the wind is well up then quite likely that the weight will be floating about hanging off a fairly straight chain just when you want the benefit of a bit more damping so good chance water resistance and inertia will do next to nothing.

Probably.

Get a good snubber instead

 

[AngusMcDoon]

If the wind is well up then quite likely that the weight will be floating about hanging off a fairly straight chain just when you want the benefit of a bit more damping so good chance water resistance and inertia will do next to nothing.

Probably.

Get a good snubber instead

Or just use string like I do, & leave all this hefty metal stuff for the lead encumbered boats to lug around

 

[noelex]

This is a photo of a typical “chum” underwater in light wind.

An additional line is often incorporated to position the weight in the desired location and to help with retreval, but this was not used in this example.

Judging by the corroded, thinned chain attaching the chum this device was frequently used by this skipper:

 

[Neeves]

I would hate to be thought of as being controversial and I am honestly fascinated.

What is the science behind a kilo of lead being better than a kilo of say,. cast iron or even a kilo of aluminium.?

Jonathan

I was actually looking for the simple answer, not the arguments - I had to work out the simple stuff and discard the arguments.

I find the idea that corrosion might influence decisions on use of steel odd as galvanising seems to work quite well and I believe is quite accepted for chain and anchors. Similarly anodising is also well accepted. Abrading lead on the seabed might be frowned upon in some quarters.

Having posted I did wonder of the value of designing a chum/angel (whatever) that reduced the propensity of a yacht to veer and (hobby) horse. Something with a bit of surface area - maybe a bit like a radar reflector. Lead would not really be very appropriate (being soft) but thick plates of aluminium or steel (suitable coated for corrosion) might make an interesting concept.

With so many experts contributing the idea might make an interesting topic and further the value of the chum/angel, as I think was suggeted by Seajet (though I continue to favour simply deploying more chain, as Vyv suggested, or use of a decent snubber).

Jonathan

 

[AngusMcDoon]

I was actually looking for the simple answer.

The simple answer is that a given mass of different materials weigh less in water as the density of the material decreases. As the effectiveness of a chum is at least partially due to its weight in water this leads to lower density materials being less effective that higher density ones.

For low density material anchors, which means aluminium alloy, it's a double whammy. For a given size (by which I mean length/width) it has a lower mass, & that mass has a lower proportional weight in water.

 

[Neeves]

For low density material anchors, which means aluminium alloy, it's a double whammy. For a given size (by which I mean length/width) it has a lower mass, & that mass has a lower proportional weight in water.

And now it can truly become an anchor thread.

Jonathan

 

[lw395]

Explain to me this difference in inertia of 10 kg of aluminium & 10 kg of lead...

The aluminium needs to move 3x the volume of water when it moves. That water has inertia. Try moving a nice light oar through the water, you will feel drag and also inertia.

 

[lw395]

And now it can truly become an anchor thread.

Jonathan

Yup.
Because no two anchoring scenarios are exactly the same two days running, it's not an exact science.
I made a lead weight, because I had some lead and a good weight of lead fits nicely under tha cabin sole.
When the motion of a boat at anchor is not what I like, I experiment.
I've found a lump of lead some way down the rope rode, off the seabed to be a useful tool.
I've pondered about why that seems to help.

If you're happy at anchor, fine, I can't help fix a non-problem.
If not, I'm trying to share my experience of what I've found to work and some understanding of what people observe.
And challenge the simplistic drivel people recite of course.

 

[thinwater]

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.

Show us the maths. Unless your boat is really galloping, the drag of a brick moving through the water at few knots is a rounding error. But show the maths so that we can better understand. Numbers.

 

[Neeves]

Show us the maths. Unless your boat is really galloping, the drag of a brick moving through the water at few knots is a rounding error. But show the maths so that we can better understand. Numbers.

But if the brick were replaced by a 3 dimensional cruciform plate (suspended from the bow loosely attached to the chain and freely supended), still with mass but now also surface area might the maths look different?

It could look like, actually be, a Northhill or a Bullwagga. and maybe made from aluminium.

It might not bet be suspended from the bow, maybe the stern (so no longer attached to the chain)

I don't know - I'm simply wondering.

Jonathan

 

[noelex]

The aluminium needs to move 3x the volume of water when it moves. That water has inertia. Try moving a nice light oar through the water, you will feel drag and also inertia.

The effect you are describing would be more correctly termed drag rather than inertia. Inertia is related just to mass. For example the inertia of an oar is the same in air as it is in water (because the mass does not change). The oar is more difficult to move in water because the drag is higher.

It would be nice to see some numbers but like Thinwater I suspect the drag in water when suspended above the seabed of most Chums is not a significant factor in their performance. However the drag (or friction) of the chum on the seabed can be a useful feature. You can see in photo on post #47 how the chain is relatively free to roll around this seabed, whereas the lead chum has sunk into the substrate and developed some grip, resisting the sideways movement. The difference with and without the chum will be even more pronounced with rope rode. However, the friction on seabed will be lost in moderately strong wind as the rode and chum is lifted.

This extra drag on the seabed can be useful property of chum. One application is to slow the boats response to a new wind direction so that for example a boat with predominantly rope rode responds in similar way to neighbouring boats using all chain rode, potentially preventing conflict in a crowded anchorage. Thankfully, few anchorages are crowded enough to require this level of detailed tuning, but tricks such as these can occasionally be useful.

 

[Neeves]

It is common in these thread to guess, as Thinwater said

'Let's see the numbers'

then we will not need to guess.

We have had far too many guesses - that are proven wrong (and dangerous)

Sadly guesses become gospel through constant repetition - surely we can get above that?

Let's see the numbers

Jonathan

And letting a chum drag on the seabed - it was an established practice in RN to lower a second anchor to the seabed, sufficient to add a bit of drag.

 

[lw395]

Show us the maths. Unless your boat is really galloping, the drag of a brick moving through the water at few knots is a rounding error. But show the maths so that we can better understand. Numbers.

Can you show me the maths of my boat yawing at anchor in Whitsand bay?
I don't have the maths of the problem, let alone the solution.

I do have a lead brick that helps.

 

[AngusMcDoon]

The aluminium needs to move 3x the volume of water when it moves. That water has inertia. Try moving a nice light oar through the water, you will feel drag and also inertia.

That's called hydrodynamic drag, not inertia. There's an equation for it, called the drag equation. It's hard to write in this box, so here it is...

https://en.m.wikipedia.org/wiki/Drag_equation

Let's do some numbers on a 10 kg cube shape steel chum. Volume of steel is 1.25 l. Area of each face is 0.012 square metres. Coefficient of drag is 1 for a cube. Density of water about 1000. Let's assume a velocity of 1 m/s. Plugging the numbers in gives a force of 12 Newtons. A kg weighs about 10 N in air where I live, so a 12 N force of a 5000 kg boat is, in my opinion, trivial.

Happy to be proved wrong by a better analysis. I know a more complex chum shape may give more drag, but I don't think it will be great, & I can't do the sums anyway.

 

[lw395]

That's called hydrodynamic drag, not inertia. .....

There is drag and there is inertia.

It takes more force to start moving a paddle through the water than to keep it moving at a steady speed.
That extra force is due to the inertia of the water.

 

[lw395]

....
Let's do some numbers on a 10 kg cube shape steel chum. Volume of steel is 1.25 l. Area of each face is 0.012 square metres. Coefficient of drag is 1 for a cube. Density of water about 1000. Let's assume a velocity of 1 m/s. Plugging the numbers in gives a force of 12 Newtons. A kg weighs about 10 N in air where I live, so a 12 N force of a 5000 kg boat is, in my opinion, trivial.

Happy to be proved wrong by a better analysis. I know a more complex chum shape may give more drag, but I don't think it will be great, & I can't do the sums anyway.

But that 12N at right angles to the rode, which it is deflecting through a small angle creates a catenary effect. It may easily be modulating the tension in the rode by a factor of 10 more?
And if it's bent the rode you also have to allow for the drag on the rode itself as it straightens.
What is the mean tension in the rode?
Looks like it could easily have effects which are significant on the force actually applied to the bow of the boat by the string?
Which I know, because I have observed it to have an effect.

Of course mooring with all chain is similar, except you need to add a long snubber to gain the elasticity of a rope rode.
And it was easy to find some old lead in my Dad's shed. I didn't find 50m of high grade chain in there.