Anchor Loads

[boomerangben]

I see in this Month's Any Questions, there is a question about Anchor Loads and why the load in the rode is greater at the anchor than at the bow.

I take the point that the static load of the weight of the chain is pretty much insignificant compared with the peak dynamic loads. But I still cannot work out why the peak dynamic load is greater at the anchor. When the rode snaps tight due motion of the bow, a shock load is transmitted throughout the length of the chain, in other words unless there is a damper in the line, that load will be the same through out the length. So how is the load amplified down the line to end up greater at the anchor?

 

[jimbaerselman]

Where did anyone claim that the load in the rode is greater at the anchor than at the bow? If anything, it's greater at the bow - supporting the vertical element of the chain weight (when the rode's not stretched).

 

[Moonshining]

[ QUOTE ]
Where did anyone claim that the load in the rode is greater at the anchor than at the bow? If anything, it's greater at the bow - supporting the vertical element of the chain weight (when the rode's not stretched).

[/ QUOTE ]

So the pounds in the prow are greater than the load in the rode, since the gain from the chain is greater than the fetch unless stretched?

 

[canalcruiser]

Its definatly greater on the bow as the anchor has firstly the chain lifting off the sea bed the the water to damp it.On the bow if its snubbing the chain will break there if it breaks.

Ive had a link deform by my bow even though i kept adding shock cord.I anchored very badly once and only once!A night not to be forgoten.

 

[boomerangben]

Apparently there was an article on it a couple of months ago in Ym and there is a question about it in this months edition. Like everyone else, I would have said that the static load is greatest at the bow (the catenary and self weight of the rode) and the dynamic weight is greatest at the bow since that is the bit that is accelerating. So why does the expert insist the loads are greatest at the anchor?

 

[rwoofer]

I think that you could argue in a perfect world (straight chain, perfectly stiff) the force should be the same because of Newtons laws - an applied force produces an equal and opposite reaction. I can't help but think in the real world that the chain canternary plus any friction from the chain still on the bottom surface distributes the opposite force so that it is not wholly taken by the anchor.

An analogy would be resistors in parallel on an electrical circuit when the voltage drop is split between the resistors no matter how small (low resistance) one of them may be.

 

[Danny Jo]

[ QUOTE ]
So why does the expert insist the loads are greatest at the anchor?

[/ QUOTE ] I suspect that the expert has based his opinion on the notion that the anchor is firmly embedded (and therefore cannot "give" when exposed to snatches) whereas the bow of the boat is floating (and therefore can dip in response to snatch loads). For reasons given so cogently above, this reasoning is faulty.

 

[silver-fox]

I saw this too and felt it defied the laws of physics!

Basically, unless other forces are at work, if a boat pulls on one end of a chain and the chain doesn't move then that it is because it is resisted by an exactly equal and opposite force. (One of Newtons laws as has already been stated.) That resistance to the pull of the boat can only be provided by the anchor and perhaps some friction between the chain and seabed.

There are other forces at work however and and one of them is gravity which will act on the suspended chain and be resisted primarily by the boat (although if the catenary is pulled clear of the sea bed it will act to a lesser extent on the anchor.

I suspect that the YM journalist got this wrong when he said the greatest forces are exerted at the anchor and that they are in fact exerted at the boat end of the rode.

Hey ho...often wrong but never in doubt!