Anchoring big ships

[[2574]]

I guess that there are present/ex Merchant/Royal Navy folk hereabouts. I’d be interested to know whether anchoring a big ship is comparable to small boats. Is it 5 x depth and 1/3rd power astern to set the anchor? Are there many different anchor models ? Is catenary important? Must be one hell of a snubber? We see big ships anchored off in 80m of water, presumably therefore they’ve 500m of chain in the locker ready for use?

 

[prv]

One difference is that they'll have an anchor watch on the bridge at all times, so they won't drag into danger with everyone asleep or ashore. I get the impression that a bit of dragging in strong conditions is therefore expected or tolerated more than in yachts.

Pete

 

[Kelpie]

The amount of scope you need can be reduced in deeper water as well. Deep ocean buoys in 3km depths don't use 15km of chain.

 

[dom]

Anchoring ships is a totally different ballgame, basically their anchors are tiny with respect to displacement. As a consequence they always dray and anchoring is really about slowing down that drag - hence the need for a 24/7 anchor watch.

Kukri and others will have all the dets!

 

[Bajansailor]

They definitely do not use power astern to set the anchor - there would be a good chance of something breaking then.
Re scope and catenary, yes, catenary does have an effect with such heavy cables especially in deeper water.
There are many different anchor models - ranging from traditional fisherman to traditional stockless designs like the Hall, to more modern stockless designs like the Pool (high holding power).
You often see S/S Pool type anchors stowed in hawse pipes on larger motor yachts.

Here is a good guide to different anchors for ships -
https://fountom.com/service_warranty/downloads/Fountom_Marine_Anchor.pdf

@Kukri can provide a lot more useful info on this topic.

 

[Hermit]

In the RN, amount of chain cable is calculated using the formula 2x sq root depth. This gives the number of shackles (lengths of chain which are 30 yards/27.5m long) to lay out. An RN vessel will construct a run up plan to that anchorage with marks along the way to measure distance to go aligned with pre-determined engine steps. Most modern ships (with expensive bow domes full of sonar) will approach and drop (or lower if deeper than 27m) in the intended position. They then sternboard away from the drop point laying out the chain in the same manner as a yacht, including snubbing. T23 use a bridle attached to the chain and lowered from the bullring rather then chain hawse to keep the chain away from the bow dome.

A swing circle is then calculated based on the actual position of the anchor at the drop, plus the ship length, plus cable deployed, plus 200m (generally). This is then plotted on the electronic chart and monitored/ship fixed throughout the period at anchor at intervals depending upon the prevailing weather. Engines and steering gear are generally at short notice (shorter notice in worse weather).

 

[crewman]

In the RN, amount of chain cable is calculated using the formula 2x sq root depth. This gives the number of shackles (lengths of chain which are 30 yards/27.5m long) to lay out. An RN vessel will construct a run up plan to that anchorage with marks along the way to measure distance to go aligned with pre-determined engine steps. Most modern ships (with expensive bow domes full of sonar) will approach and drop (or lower if deeper than 27m) in the intended position. They then sternboard away from the drop point laying out the chain in the same manner as a yacht, including snubbing. T23 use a bridle attached to the chain and lowered from the bullring rather then chain hawse to keep the chain away from the bow dome.

A swing circle is then calculated based on the actual position of the anchor at the drop, plus the ship length, plus cable deployed, plus 200m (generally). This is then plotted on the electronic chart and monitored/ship fixed throughout the period at anchor at intervals depending upon the prevailing weather. Engines and steering gear are generally at short notice (shorter notice in worse weather).

Sorry I do not understand the calculation. In 25 m depth . Root 25 is 5 times 2 is 10. So cannot be 10 m chain as that would not reach the bottom. If t is 10 shackles @ 27.5 m scope is 275 m or a scope of 11x depth which seems excessive.

 

[Hermit]

It would be 10 shackles (but most don't carry that much so it would be as much as feasible).

 

[Kukri]

The sizes of anchors and chains and the number of shackles of chain for each anchor are determined by the Equipment Numeral which is calculated by the Classification Society from the drawings and is largely a matter of windage and water effects.

With big ships, say 150,000 tons and larger, it’s important to come to a dead stop before anchoring.

 

[Roberto]

I have a related question: I cannot find it now but I once took a marinetraffic screenshot where, during a violent storm, a lot of ships were at anchor to the east of Cotentin/Cherbourg peninsula: there were probably 20-30 ships and they were all positioned in a very precise way, like a chessboard. It seemed someone gave each of them the coordinates of a spot to anchor: are VTS in charge of that I suppose?
Do they indicate the exact location where to drop anchor, leaving each ship to decide the amount of chain? or they indicate a required final ship position?

 

[Hermit]

With big ships, say 150,000 tons and larger, it’s important to come to a dead stop before anchoring.

Yes! Not many (i.e. none!) RN ships qualify...

 

[Hermit]

I have a related question: I cannot find it now but I once took a marinetraffic screenshot where, during a violent storm, a lot of ships were at anchor to the east of Cotentin/Cherbourg peninsula: there were probably 20-30 ships and they were all positioned in a very precise way, like a chessboard. It seemed someone gave each of them the coordinates of a spot to anchor: are VTS in charge of that I suppose?
Do they indicate the exact location where to drop anchor, leaving each ship to decide the amount of chain? or they indicate a required final ship position?

Not sure of the location you mention but there are often designated anchorage points shown on the charts. In Portsmouth/Plymouth you will see numbered anchorage positions on the chart - there are assigned by QHM. Other places use similar per-determined positions (Suez/Panama waiting zones for example). The points are well enough spaced to provide ample safety for the types of ships that will use them. In terms of cable length - we are only talking relative small distances compared to the area of water so it would appear that they are all very uniform.

 

[STATUE]

The COVID cruise ships in Weymouth bay at first kept floating/cruising around the bay and not anchoring - were they avoiding charges?

They are now anchored, so who gets the money ?

 

[westhinder]

I have a related question: I cannot find it now but I once took a marinetraffic screenshot where, during a violent storm, a lot of ships were at anchor to the east of Cotentin/Cherbourg peninsula: there were probably 20-30 ships and they were all positioned in a very precise way, like a chessboard. It seemed someone gave each of them the coordinates of a spot to anchor: are VTS in charge of that I suppose?
Do they indicate the exact location where to drop anchor, leaving each ship to decide the amount of chain? or they indicate a required final ship position?

At the Westhinder anchorage off the Belgian coast ships are instructed by VTS to pick a spot and check wit VTS before they drop anchor. When they want to get under way again, the same applies.

 

[westhinder]

Ships seem to lose anchors and chains fairly frequently. Every issue of the Belgian NtM has a couple of warnings of anchor and chain lost in position XYZ. The anchorages off our coast are all in open sea though, no shelter behind headlands. I often hear ships opting to drift for a couple of hours rather than drop the anchor.

 

[Hermit]

Ships seem to lose anchors and chains fairly frequently. Every issue of the Belgian NtM has a couple of warnings of anchor and chain lost in position XYZ. The anchorages off our coast are all in open sea though, no shelter behind headlands. I often hear ships opting to drift for a couple of hours rather than drop the anchor.

Indeed. I have also been to muster them after a night of rough weather and only found 1 of the 2...

 

[jdc]

If you rely on the catenary in the chain to keep the cable horizontal at the anchor you have to let out a length, s, of chain such that:

s = sqrt( 2 * lambda * y + y^2) where y is the depth and lambda is the force on the boat from wind etc divided by the weight of the chain per metre.

In reasonably shallow water 2 * lambda is much greater than y so the y^2 term can be ignored, hence the ship rule-of-thumb is 2 shackles x sqrt(depth). [I always thought it was 1.5 shackles, but Hermit is more to be trusted here than my memory].

This implies sqrt(2 * lambda) = 2 shackles, or 55m. Hence lambda = 55^2 / 2 or about 1500.

Weight of chain varies with its size of course, so big ships have heavier chain! But if one assumes 50mm chain then that's about 50kg / metre, so the force could be up to 1500 * 50 = 75 tonnes pull before the chain starts to lift off the horizontal at the anchor. 75 tonnes is the force exerted by 60 knots of wind on a surface around 1000 sq metres so really quite a big ship if viewed bows-on!

 

[newtothis]

If you rely on the catenary in the chain to keep the cable horizontal at the anchor you have to let out a length, s, of chain such that:

s = sqrt( 2 * lambda * y + y^2) where y is the depth and lambda is the force on the boat from wind etc divided by the weight of the chain per metre.

In reasonably shallow water 2 * lambda is much greater than y so the y^2 term can be ignored, hence the ship rule-of-thumb is 2 shackles x sqrt(depth). [I always thought it was 1.5 shackles, but Hermit is more to be trusted here than my memory].

This implies sqrt(2 * lambda) = 2 shackles, or 55m. Hence lambda = 55^2 / 2 or about 1500.

Weight of chain varies with its size of course, so big ships have heavier chain! But if one assumes 50mm chain then that's about 50kg / metre, so the force could be up to 1500 * 50 = 75 tonnes pull before the chain starts to lift off the horizontal at the anchor. 75 tonnes is the force exerted by 60 knots of wind on a surface around 1000 sq metres so really quite a big ship if viewed bows-on!

And that's why the RYA teaches 5 x depth. It saves on having to invest in a scientific calculator to work out scope.

 

[dombuckley]

[I always thought it was 1.5 shackles, but Hermit is more to be trusted here than my memory].

My old admiralty Manual of Seamanship says no of shackles = 1.5x sqrt depth (m).

In my time, there was quite a variety of cable lengths. Type 42s had a single cable of 7 shackles, while Leanders had two cables, each of 10 shackles. Herald class survey ships had two cables of 16 shackles each, while the carriers and LPDs had 22 shackles on each cable, due to the likelihood of anchoring in deep water for extended periods. The only time a destroyer needs to anchor will be in a coastal anchorage , typically about 15-20m, for which 7 shackles is sufficient. On the other hand, during my time in Herald we anchored in 110m for several days, outside a coral reef which we surveyed using the motorboats.

 

[capnsensible]

The sizes of anchors and chains and the number of shackles of chain for each anchor are determined by the Equipment Numeral which is calculated by the Classification Society from the drawings and is largely a matter of windage and water effects.

With big ships, say 150,000 tons and larger, it’s important to come to a dead stop before anchoring.

It used to fascinate me watching vessels coming into Gib for bunkers......mostly in order to keep out of the way! Most seemed to veer a lot of chain after picking up the pilot and heading up the bay. Before reaching the designated spot, a load more would be chucked out very noisily whilst still propelling ahead. Ended up in a kinda handbrake turn. Very impressive.

On the east side of the rock, some vessels would sit for months at anchor in all sorts of weather. Guess that the agents were unable to get cargo? I know some had been arrested for various reasons. A couple of friends of mine used to get very well paid for sitting on them for ages as a kind of human hostage.

I'm sure you know where all the pit stops are but another sight for yachties were the anchorages on banks Mebbe 100 meters deep where your guys would anchor offshore for orders.. Quite something when it's a new experience!