Anchor Thingy

peteandthira

peteandthira

Well-Known Member
Joined
13 Jan 2005
Messages
786
Location
On boat
Visit site
Dear Anchorers

I have just been handed an enormous shackle, leather covered, with a steel ring welded on at the top, ie away from the open end. It was a present from a liveaboard friend here who uses one to slide a weight down his anchor chain on a separate line in order to reduce swinging at anchor.

Thinking about this, I wonder how effective it is. Surely it will just wander around being dragged across the bottom with a limited reduction in swing.

What about putting a second anchor on this shackle, instead of a weight? One of my spare Fortresses, for example. I am thinking that this will tend to dig in, even if not all of the time, as it drags around and should therefore reduce swinging more efficiently than a weight.

Do any of you regular anchorers use anything like this, with maybe an opinion on my suggestion of sliding a second anchor down it instead?

I apologise for yet another anchoring question to the forum and promise that this is not a troll!

Pops
 
A weight lowered down the chain is called a chum, or angel, and it is intended to keep the chain down, increasing holding power and reducing the swinging circle. You aren't supposed to lower it to the seabed, about halfway down will do. Those that I've met who use the technique say it is effective. Personally I'm of the keep it simple persuasion.
 
The idea behind weighting the anchor chain is not to reduce swing but to reduce snatching when anchored in rough water. The weight is lowered down the chain to ideally halfway. As the boat pulls back on the chain and it tends to a straight line the weight acts to increase the chain tension, so absorbing the snatching effect. That's my understanding anyway. Maybe somebody else can shed more light on your shackle.
 
The effect of a weight on the anchorchain is also that the angle of the chain at the anchor gets smaller, which means a smaller vertical component of the force the chain exerts on the anchor. For this reason the weight should not be half way down the chain, but near the anchor.
 
Oo-err.

I seem to have misunderstood the reasoning behind this weight thing. Keep going chaps, I am still a little puzzled. I haven't tried any of this chum thing before but will give it a go soon!

Pops
 
No, the weight should be near mid-rode, because its working mode is twofold:
(1) keeping as small as possible an angle between seabed and rode increases holding power (static pull case)
(2) damping the vertical movements of the rode when the boat goes pitching (dynamic pull case), and thus reducing the probability to snatch the anchor
 
[ QUOTE ]
No, the weight should be near mid-rode,


[/ QUOTE ]

I am not sure I agree with you. If you have say 5:1 (or maybe 7:1 out if the weather is bad) then your weight is going to spend quite a bit of its time on the seabed in between surges lifting only at times of stress. I am not sure how you would get it halfway along the rode unless you pay the weight out with the anchor chain - which sounds a fairly complex thing to do on a pitching foredeck.

IMHO I would recommend letting the weight out after the anchor is down and then just until it was on the seabed. This should ensure the maximum beneficial damping effects come into play early as soon as a surge starts and in between surges you will tend to lie to the weight.
 
You are probably right. My last post was a theoretical one straight by the book. From a practical point of view, I have never considered using a weight (I agree with Stevie_D analysis), and I had rather carry one more anchor. Back to the matter, the configuration you advise has a strong similarity with the use of tandem anchors , which is a way to get ready for a (+-) comfortable night when things threaten to go awry...
 
Admiralty Seamanship manual Vol 1 states;

"50 lb stone or similar weight is available, it may be secured by a lizard to the cable and allowed to run down the cable to the bottom on the end of a messenger; this will act as a spring on the cable and so reduce the tendency of the boat to snatch".
 
I met Frank Mulville on Iskra in Canouan on his way back to Blighty from Brazil many years ago. He was inordinately proud of two things on his boat, his tecnique of hand-starting his engine which involved a line from the decompressor lever to his belt, and a sudden leap backwards once he got the flywheel spinning. The other was a chum, which consisted of discs of lead on a eye bolt- the weight could be varied by subtracting discs- he said it was the one thing he would never sail without. I didn't have the heart to ask if he had it with him when he lost Girl Stella in the Azores.
 
The weight on the chain as already stated both provides a weight the boat must lift before the chain or rope goes taught but also if positioned fairly well down will help to improve the angle to the bottom of the chain arriving a the anchor.
It is unlikely that the weight unless very large and laying on the bottom (like a mooring weight) will stop the boat swinging around. It may however permit the use of a shorter rode so reducing the swinging or more specifically the radius of swing with tide or wind change.
So even a small weight will help a little in keeping the anchor rode nearer the bottom but one would imagine it should be more like 3/4 of the way down the chain or rope or certainly near the sand. olewill
 
Re: Anchor Thingy - the middle of the \"road\"

Paraphrasing the excellent discussion so far, there are at least two distinct purposes for a chum - reduction in snatching and reduction in the angle of pull of the rode on the shank. Both serve the primary purpose of improving the holding power of the anchor, but there are secondary spin-offs.

Reduction in snatching is achieved by increasing the "catenary". On an all-chain rodes, under light loads the chain hangs more or less vertically from the boat, so a chum won't make any difference to the snatching, but may indeed reduce swing if it's sitting on the bottom. Under moderate loads, whether it is just off or just on the bottom is academic, because if it is working it will be constantly rising and falling. Under heavier loads it will be most effective at reducing snatching in the middle of the rode, provided that it is heavy enough to make a difference to the catenary. But, under heavy loads, one may become more concerned about the angle of pull of the rode on the shank, and for this purpose the chum is most effective fairly close to the anchor.

So my, untested, armless chair, conclusion is that you adjust the position of the chum to the conditions, setting it further down the rode as the wind gets up. Keeping it just off the bottom isn't a bad rule of thumb so long as you remember that to do so will require adjustment as conditions change.

Incidentally, the charming harbourmaster at Port Patrick introduced me to the use of a chum on a mooring warp. Having watched me struggling to adjust warps and springs to hold the boat alongside the harbour wall against a breeze blowing me off, he heaved up a pile of chain bunched together with a bit of polypropylene rope for me to use as a chum on the bow line. Pulled the boat against the harbour wall like magic.

Mark
 
'Strewth.

The extent of the knowledge on here is quite remarkable. It seems that a weight is advantageous wherever it is on the chain.

Does anyone have any idea of what sort of weight is useful, or more to the point, at what sort of weight does it start to become useless? We don't have much room in our anchor well now.

Thanks again for your attention!

Pops
 
Re: Anchor Thingy - the middle of the \"road\"

[ QUOTE ]
Incidentally, the charming harbourmaster at Port Patrick introduced me to the use of a chum on a mooring warp. Having watched me struggling to adjust warps and springs to hold the boat alongside the harbour wall against a breeze blowing me off, he heaved up a pile of chain bunched together with a bit of polypropylene rope for me to use as a chum on the bow line. Pulled the boat against the harbour wall like magic.

[/ QUOTE ]

Good, traditional seamanship stuff - now rarely seen. Tuck away this variant, for when you're weather-bound against a harbour wall, and surge is likely to cause damage. Get a large car tyre for each bow and stern line, and wind the warp spirally around the tyre half a dozen times, secure both ends as normal, and ensure the tyre is hanging in 'free space'.

This tyre will act as a strong 'snubber', and its weight will act as a 'chum' - highly effective. Use two per warp if only small tyres are available......

/forums/images/graemlins/smile.gif
 
Re: Anchor Thingy - the middle of the \"road\"

Useful tip, thanks.

One more for the harbour wall and jetty users. To cope with filthy black vertical "rubbing strips" and wooden piles, for which conventional hanging fenders are less than ideal, carry a stout rough plank to hang over the side (horizontally) between your fenders and the pile.

Mark
 
To make a significant difference, it need to be at least as heavy as half the chain you've got out.

Regarding the use of the chum to avoid snatching; the chain catenary and the chum are not as effective as a nylon rope snubber, and should not be seen as a substitute for the rope. And you need a surprisingly large amount of rope - enough to give you about 1m of spring, that's 10m from attachment to the chain to the bitts, to deal with 1m waves when the rode is stretched with some sort of wind.
 
Yes. I wish I had read Alain Frayssee's excellent analysis before making my earlier remarks - it's too late to qualify them now, but I'll try and make amends by quoting Al.

[ QUOTE ]
An all-textile rode with a kellet [chum] close to the anchor has the same performance as an all-chain rode of same length, with only half the total rode weight. This confirms that concentrating the weight down the rode, if possible, would be much more effective than spreading it along the rode.

Unfortunately, handling capabilities limit the weight of actual kellets around 22 kg (50 lb), which is insufficient in severe wind conditions unless the scope is very large.

In addition, using a kellet does not significantly improve the swinging radius R.

[/ QUOTE ]

Ah well. Nothing ventured, nothing gained (stock phrase to cover the sound of words being eaten). Must get back to designing my perpetual motion machine.

Mark
 
You must log in or register to reply here.

Share this page

Top