Anchors for big ships

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Captn D

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Looking at 3 or 4 ships anchored in Falmouth Bay yesterday in a Force 8-9 and knowing the partisan opinions that apply to yachts' anchors, I wondered whether there was the same variations and opinions regarding those used on proper ships. Most ships anchors, to my untrained eye, appear to be a larger variation of a Danforth used with a lot of heavy chain. I did read somewhere that very large ships (VLCCs etc.) cannot rely on their anchors as a sole means of maintaining position and therefore also use engine power.
 
Captn D said:
Looking at 3 or 4 ships anchored in Falmouth Bay yesterday in a Force 8-9 and knowing the partisan opinions that apply to yachts' anchors, I wondered whether there was the same variations and opinions regarding those used on proper ships. Most ships anchors, to my untrained eye, appear to be a larger variation of a Danforth used with a lot of heavy chain. I did read somewhere that very large ships (VLCCs etc.) cannot rely on their anchors as a sole means of maintaining position and therefore also use engine power.
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This occurred to me reading the ships anchored in St Vaast thread.Modern anchor salesmen seem a bit shy of tackling the shipping Companies?
Surely they cannot possibly be anchored securely unless they have in the very least one Rocna dangling at the bows.:confused:
 
The anchor system in large ships works differently than that used in small vessels. In ships, the weight of the chain anchors the vessel; the anchor keeps the chain from sliding along the bottom.
 
Captn D said:
Looking at 3 or 4 ships anchored in Falmouth Bay yesterday in a Force 8-9 and knowing the partisan opinions that apply to yachts' anchors, I wondered whether there was the same variations and opinions regarding those used on proper ships. Most ships anchors, to my untrained eye, appear to be a larger variation of a Danforth used with a lot of heavy chain. I did read somewhere that very large ships (VLCCs etc.) cannot rely on their anchors as a sole means of maintaining position and therefore also use engine power.
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Most ships have a two stockless anchors as on https://www.google.com/search?q=shi...wCA&ved=0CAkQ_AUoAQ&biw=1280&bih=666#imgdii=_


It is sometimes necessary to deploy both anchors and by letting out additional chain develop a good catenary to do the work. Sometimes the wind and tide will be such that even two anchors struggle and so then plan be comes into action and the engines are started up and sufficient power applied to take weight of the chain. I have seen conditions in the Typhoon Anchorage in Hong Kong where we had the main engine running at about 3/4 full speed at one point to counter the 100MPH+ winds. The Wind blown rain stripped quite a lot of paint of the bridge front! The mate was stationed on the Fo'csle Head to watch the load on the chain .
 
RichardS said:
That's got to be a wind up! :)

If the anchor is stopping the chain dragging along the bottom then it's the anchor which is holding the boat!

Richard
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Perhaps I wasn't clear. In the small boat system the anchor does all the work - the rode doesn't need to be on the ground, as 100% of the "pull" is countered by the anchor digging into the bottom. With a large ship, once the chain is off the bottom, it's no longer anchored - if the anchor itself is dragging along the ground, it isn't expected to dig itself in and provide any "pull". The catenary of the chain anchors and provides shock absorption, using the downward pull (gravity) in the centre of the length of chain to provide the horizontal pull along the chain that holds the ship in place. This of course means there is an equivalent pull on the chain that is lying on the bottom. When you pull a length of chain inline it slides easily as the link in front "greases" the path for the link behind it and so on. So even if the weight of the chain is sufficient to hold the ship still, the lack of friction needs to be accounted for - hence the anchor. It may seem like a subtle distinction, but I hope it explains why you don't see giant Rocnas on the bows of supertankers.
 
> In ships, the weight of the chain anchors the vessel; the anchor keeps the chain from sliding along the bottom.

That's exactly what we did with our yacht, I've mentioned many times it makes sense.
 
I have been thinking the same thing . I overlook St Brides Bay , Pembrokeshire . Three large ships were anchored in the bay on Tuesday night. Couldn't see them Wednesday when we had 80knt winds blowing into the bay but as the storm passed they reappeared from the murk . We sustained all sorts of damage ashore and power loss . It was difficult to stand let alone walk against the wind but they seemed to weather the storm well . I know that ships anchor on weight of chain but exposed to that wind and swell - amazing
 
The same principle works for small craft. If you anchor properly, the weight of the chain holds the boat. In strong winds or tide you may get to the point where the weight comes directly on to the anchor - that's when the dragging starts! Too often one sees boats anchoring with nowhere near enough chain paid out - if they are ahead of you, start getting the fenders ready!
 
This one's good for a laugh, unless you happened to be in charge...


http://www.youtube.com/watch?v=b7pRfix_sNg

I think what happened was the chain didn't run free for some reason with 4 shots on deck and the guys on the brake just kept spinning the wheel thinking to release the brake. It was in fact already released and by the time the chain began to run they'd wound the thing so far they'd disconnected the linkage. result? 102 tons of ground tackle on the bottom and a very pissed off First Lootenant.
 
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I wish these people who think that both boats or ships "anchor on their chains", would throw their anchors away and just put chain out.

If they're correct they will have no problems. If not . . .
 
The anchor is the 'holding point of last resort', and every effort should be made to avoid direct pull on it. That is done by putting plenty of chain out. If you don't do that, one day you'll end up ashore!
 
TimBennet said:
I wish these people who think that both boats or ships "anchor on their chains", would throw their anchors away and just put chain out.

If they're correct they will have no problems. If not . . .
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As I said the anchor and chain work together in the "anchor system" and omitting one part of that would greatly compromise the performance of that system. I also said that small boats' anchors are designed to work alone in that system - that's why it's possible to use an all-rope rode. For large commercial ships, any possible efficiency in cost will be realized to maximize profit; if they could, don't you think they would use rope rather than incurring the added weight (fuel costs) and initial expense of chain? Or perhaps you care to explain why the standard procedure for increasing winds aboard large vessels at anchor, is to put more chain out? That doesn't increase the anchor's weight or holding power, now does it? Or perhaps you care to explain why the RN's formula for scope calculations requires 50% more chain for steel links over the heavier copper alloy chain, irrespective of the anchor?
 
Some factual information

Just before midnight on 30 July 2009, Full City lost its anchor hold and started dragging under the impact of strong south-easterly winds and high waves.
The vessel was now dragging sideways in a north-easterly direction and rolling strongly in the high seas.
Because it was dragging in a north-easterlydirection quite close to the shore, there was little opportunity to regain control once the vessel had started dragging. The vessel ran aground at Såstein after dragging anchor for 35 minutes

The anchor lost both flukes.

The International Association of Classification Societies Ltd. (IACS) is an umbrella organisation for classification societies, of which Class NK is also a member. The IACS issues specific requirements and limitations relating to the design and construction of anchor arrangements.

The following is cited from IACS Req. 2007: Requirements concerning mooring, anchoring and towing:
A1.1.1 The anchoring equipment required herewith is intended for temporary mooring of a vessel within a harbour or sheltered area when the vessel is awaiting berth, tide etc.
A1.1.2 The equipment is therefore not designed to hold a ship off fully exposed coasts in rough weather or to stop a ship which is moving or drifting. In this condition the loads on the anchoring equipment increase to such a degree that its components may be damaged or lost owing to the high energy forces generated, particularly in large ships.
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Source: http://www.aibn.no/Sjofart/Rapporter/2013-08-eng
 
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Cruiser2B said:
PThe catenary of the chain anchors and provides shock absorption, using the downward pull (gravity) in the centre of the length of chain to provide the horizontal pull along the chain that holds the ship in place. This of course means there is an equivalent pull on the chain that is lying on the bottom. When you pull a length of chain inline it slides easily as the link in front "greases" the path for the link behind it and so on. So even if the weight of the chain is sufficient to hold the ship still, the lack of friction needs to be accounted for - hence the anchor.
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Sell this to me a bit more, please. Think of the overall horizontal forces on the chain. There are three, anchor pull, ship pull and chain friction, and they must be in equilibrium so that ship pull = anchor pull + chain friction. What proportion of the ship's pull do you reckon is balanced by the chain friction ("it slides easily as the link in front "greases" the path for the link behind it") and what proportion by the anchor pull.
 
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