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Author Topic: What stops a ship from pulling an anchor off the sea floor?  (Read 20165 times)

Rob Davidowitz

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Rob Davidowitz  asked the Naked Scientists:
   
Hi Chris,

Love your programmes.  I would really appreciate an answer to this question.

When a super large ship is laying at anchor, what stops the ship from pulling the anchor off the sea floor when it rises with the swell?  I understand that it could be that the length of the chain is more then the depth of the water, which would mean that the chain, from anchor to ship, would be at an angle in the direction of the tide.  But then what stops the very large ship, which is like a gigantic floating cork, from pulling the anchor, which has an extremely small comparative size, along the seabed on a flat sandy seabed with no rocks on which the anchor can hook into.  

Thanks again for your amazing shows.

Regards,

Rob Davidowitz

What do you think?
« Last Edit: 26/03/2011 16:30:03 by _system »


 

Offline Geezer

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What stops a ship from pulling an anchor off the sea floor?
« Reply #1 on: 26/03/2011 17:25:28 »
You're right about the long chain Rob. The chain between the ship and the anchor is made long so that the force of the ship pulling on the anchor tends to drag it along the seabed rather than lift it off the seabed.

The anchor itself is designed so that its flukes dig into the seabed and bury the anchor in the sand and silt. To release the anchor, the ship pulls in the chain until it is almost vertical. When the chain lifts the anchor's shaft so that it is vertical, the flukes also have to rotate and this allows the anchor to release from the seabed.
 

Offline chris

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What stops a ship from pulling an anchor off the sea floor?
« Reply #2 on: 27/03/2011 10:04:41 »
Good answer Geezer.

To add to what's written above, the use of a long anchor chain also means that there's a relatively big strain i.e. the chain stretches a bit, but the increase is distributed over the entire length, so the stretch borne by any individual unit length is low. This is why tugs always tow boats using very long warps.

C
 

Offline lightarrow

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What stops a ship from pulling an anchor off the sea floor?
« Reply #3 on: 27/03/2011 13:07:49 »
Good answer Geezer.

To add to what's written above, the use of a long anchor chain also means that there's a relatively big strain i.e. the chain stretches a bit, but the increase is distributed over the entire length, so the stretch borne by any individual unit length is low. This is why tugs always tow boats using very long warps.

C
Hmmm. this is not very correct. The stretch per unit lenght doesn't matter, the stretch is the same; what counts is the cross section area of the chain: the stretch distributes on it.
A long chain is probably used just to stabilize the pull's variations to avoid chain breakage.
 

Offline Geezer

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What stops a ship from pulling an anchor off the sea floor?
« Reply #4 on: 27/03/2011 20:51:54 »
Although it will stretch slightly, I suspect the weight of the long chain might be more important than its tendency to stretch. Because of its weight, most of the time, the chain won't actually be straight. As a force tends to move the ship further from the anchor, the force has to lift the chain against gravity. As the force diminishes, the weight of the chain will tend to return the ship to its former position.

Of course, if the force is great enough (in a strong wind or fast current) the chain will straighten out. 
 

Offline graham.d

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What stops a ship from pulling an anchor off the sea floor?
« Reply #5 on: 27/03/2011 22:40:55 »
Geezer's right. A long chain is used to ensure that the angle from the ship to anchor to seabed is acute and the force is trying to drag the anchor along the seabed (which it is designed to resist) rather than too much upwards (which it is not). The heavy chain also acts as a shock absorber in that the chain is not normally taut, but curving down to the seabed, then along the seabed to the anchor. Any extra force due to wave action or gusts of wind will tend to try to straighten the chain allowing the chain's weight to act as a gradually increasing restoring force rather than transmitting a sharp pull to the anchor's grip on the seabed. It is usual to use an anchor chain length up to 5 times the depth below the ship.

On small boats other techniques to avoid anchor dragging can be employed such as using a chain close to the achor, then using a nylon rope to give more shock resistance. Personally, I just like using plenty of chain though one has to be aware that the boat then swings more when the tide turns.
 

Offline imatfaal

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What stops a ship from pulling an anchor off the sea floor?
« Reply #6 on: 28/03/2011 13:03:44 »
With respect to the excellent answers above that apply to small inshore vessels and small ocean-goers and to large vessels in good sea conditions - with a very large ship (a 300,000 dwt oil tanker for instance) nothing stops a vessel from dragging its anchor (or losing it).  On Large ships anchors serve as 

Quote
little more than the equivalent of a car handbrake: their purpose is to secure a vessel which has been ‘parked’ under control. Whilst they may be useful in stopping or manoeuvring a ship drifting at very slow speeds of less than 1 knot (0.5 m/sec), they are of no use in an emergency when a ship may be drifting without power at speeds of several knots

From the Donaldson report "Safer Ships, Cleaner Seas"


The following is one of the conclusions of the official enquiry into the Sea Empress Incident by Prof M Disney

Quote
(a)   VLCCs do not have effective anchoring (i.e. braking) systems capable of bringing them under control in coastal waters in an emergency, and scaling laws suggest that they never will. They are simply too massive. If such an emergency anchor is regarded as essential, as it is on all other ships, then severe down-sizing is called for. 

A floating object that, in effect weighs over on third of a million tonnes, and can have max area of each side in the water of around 7200m2 (or almost the same area above the sea catching wind when in ballast) goes where the sea and weather takes it, unless it is under way.   In safe and calm anchorages a standard 20-25 tonne anchor with 14-15 shackles - is sufficient - but as soon as any abnormal braking is required then most anchors will not hold.  Thus anchorage locations are very carefully chosen, are very big, and ships maintain a very close watch.   
 

Offline lightarrow

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What stops a ship from pulling an anchor off the sea floor?
« Reply #7 on: 28/03/2011 16:08:29 »
Although it will stretch slightly, I suspect the weight of the long chain might be more important than its tendency to stretch. Because of its weight, most of the time, the chain won't actually be straight. As a force tends to move the ship further from the anchor, the force has to lift the chain against gravity. As the force diminishes, the weight of the chain will tend to return the ship to its former position.

Of course, if the force is great enough (in a strong wind or fast current) the chain will straighten out. 
I agree on the fact that on a long chain its weight is important, but the chain stretches even for this reason.

Imagine to fix a heavy chain's ends at two trolleys which can move on the same horizontal rail and which are less distant than the chain's lenght: the two trolleys approach each other; this because there is a component of the force also directed along the chain. This means that the chain is stretched. Knowing the angle made by the chain and the orizontal axis at the trolleys and knowing the chain's weight, it's simple to find that force when the trolleys are fixed in position.
 

Offline Geezer

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What stops a ship from pulling an anchor off the sea floor?
« Reply #8 on: 28/03/2011 18:24:16 »
Although it will stretch slightly, I suspect the weight of the long chain might be more important than its tendency to stretch. Because of its weight, most of the time, the chain won't actually be straight. As a force tends to move the ship further from the anchor, the force has to lift the chain against gravity. As the force diminishes, the weight of the chain will tend to return the ship to its former position.

Of course, if the force is great enough (in a strong wind or fast current) the chain will straighten out. 
I agree on the fact that on a long chain its weight is important, but the chain stretches even for this reason.

Imagine to fix a heavy chain's ends at two trolleys which can move on the same horizontal rail and which are less distant than the chain's lenght: the two trolleys approach each other; this because there is a component of the force also directed along the chain. This means that the chain is stretched. Knowing the angle made by the chain and the orizontal axis at the trolleys and knowing the chain's weight, it's simple to find that force when the trolleys are fixed in position.

Yes, I'm sure it will stretch a bit, particularly if it's trying to restrain a 0.3Mt vessel.
 

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What stops a ship from pulling an anchor off the sea floor?
« Reply #8 on: 28/03/2011 18:24:16 »

 

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