Emergency anchoring while a ship is moving certainly is a risky operation. That is why it was found interesting when I came across Antonio’s quest to study the topic.
May I ask a question about a ship’s anchor gear? If a ship was riding one of her anchors with say full 11 shackles of anchor chains in very rough sea. It was so rough either the anchor will drag or the gear will break. Which case will most likely happened? Drag or break?
My guess is drag. The anchor gear should not break under these conditions. Certainly, the anchor’s brake will not stand a chance to hold the anchor. The stopper should.
I read a while back that the amount of scope put out was a balance between those two things. It’s possible to put out so much scope that the anchor will hold beyond the chain’s breaking strength and so it’s preferable to keep a shorter scope and drag anchor so you don’t lose it completely.
It’s a probability thing. With a load from wind and current alone it most likely the anchor will drag first. Very rough seas, 4-6 meters, would greatly increase the likelihood of parting the chain.
The 1984 Knight’s Modern Seamanship mentions the risk of parting a chain with too much scope out but that’s the only place I recall seeing that.
Going by calculated anchor holding forces and chain breaking force in theory the anchor should always drag first with no sea, but according to reports it’s fairly common that the chain breaks, I’ve seen two ship anchor chains part in my career. I’ve lost count of the number of times I seen dragging
It could be agreed that a ship riding its anchor with full length of cables subjected to a steady force greater than the holding power should drag. A ship’s anchoring gear, and related structural members, should be able to withstand the “normal” stresses from using its own anchor with its full length of cables (If not, why equipped the ship with so many chains as it will be unsafe to use all of them in anchoring). Parting cable in very rough sea, or other situations, should have been caused by sudden tensions/surge that generate “excessive” stress.
Argument: A ship with its anchord should be able to support a holding force equal to the holding power of its anchors and the anchor cables. With the general rule of thumb, a stockless anchor commonly found on merchant ships should provide a holding power equivalent to say three times of its weight. The anchor cable should provide 20 to 30% more holding power in comparison with the weight. Holding force available to the ship can then be estimated. If a situation is not generating stresses outside the limits of the anchoring system, the use of anchors to achieve a goal could be considered “okay”. Sometimes, less anchor cables should be used to account for possible sudden tensions. In turns, the use of shorter anchor cables provide less holding power to achieve the goal. Will the available holding force able to stop the ship? Given sufficient time without breaking, it will?
If a ship is anchored in 80 meters with 11 shots out it is more likely to drag than part the chain. On the other hand if the ship is anchored in 14 meters with 11 shots out that increases the chances of parting (compared to anchored in 80 meters).
Rather than thinking in terms of what “should” happen it is more precise to think in terms of probability.
I can’t find a reference at the moment but I believe a High Holding Power (HHP) anchors such as the AC-14 have a rated holding power of 6-12 times weight.
You might want to have a look at this paper:
P&I Loss Prevention Bulletin
Preventing an Anchor from Dragging
The Japan Ship Owners’ Mutual Protection & Indemnity Association
Loss Prevention and Ship Inspection Department
Off the top of my head the breaking load of the chain is roughly 4 times greater then the calculated holding power of the anchor. However ship’s do in fact often part anchor chains.
“If there is ample sea room, it would be better to reduce the scopes to the amount shown in the table and accept the possibility of dragging anchor rather than risk breaking chain”
In my view, it probably the opposite. Anchor in deeper water could drag easier as the chain in contact with the seabed became less. Thus, there is less holding force provided by chains. Hanging chains in water does produce much resistance.
Even for a very large contapinership, the weight if its anchor could be 10 or 11 metric tonnes (mt) only. The weight of each shackle of chain perhaps 20 tonnes (nobody onboard could tell me this). Therefore, there probably a total weight about 250 mt even with full length in contact with seabed.
A containship of 300m in length could have a displacement of over 100,000mt. At a speed of 2 knots, it has a momentum roughly 100,000 tm/s. The 250mt “anchor” probably has to be dragged a bit to fully stop the ship.
Yes, AC14 could provide holding power something like 6-12 times of its weight. In many cases, vessel fitted with HHP anchors will use HT steel anchor chains. It is lighter with great strength. Howevet, less weight and small size mean less friction, thus less holding power from the chain itself. Merchart ships may use the simple “conventional” one as that weight made very little difference to the deadweight.
It is great to know the breaking strength could be four times the holding power.
For emergency anchoring if only a short stay is run out (which is a common advice) I don’t think the holding power rating matters since we don’t want the anchor to hold and the assumption that the force on the anchor is horizontal is not true.
Ship handling info I’ve seen claim that dredging anchors is an effective means to slow a ship.
Dredging is also used for shiphandling as low speeds, the book I have here mentions that two anchors down on a short stay will hold a ship against a slow or possibly a half bell. That indicates to some degree that amount of force that is developed by dredging anchors
Yes, he did. I made a mistake. I am sorry.
However, is that really acceptable to have ships fitted with 11 or 12 shackles of cables that the crew should not use them all in shallower waters? We were not told about that limit. Should the crew be informed about this limitation if it is really the case.
The strength of the cable as well as the ship’s structure possibly be able to cope with the pulling force generated while at anchor, no matter if it is used in deep or shallow waters. However, what is the limit? We may be able to better estimate the maximum length of cables that can be used in emergency to arrest the ship’s movement without breaking the system or wasting available holding power. A possible educated guess could be made from the likely stresses an anchor system is designed for and subjected to. It is related to the holding power of the anchor system.
It is rather difficult to control a short stay run out of anchor cable in emergency. Free fall object will accelerated at a rate more than 9 meters per second. Through the windlass and the hawsepipe , anchor could still running out a few meters every second. We may need a speed over 8 or 10 knots (~4 to 5 m/s) to catch out the running rate of the cable. We will always saw the cable leading up and down with anything less than that speed. We will able to tell if the anchor cable is riding at short stay or not only after holding the brake. Most likely, we will not able to do anything more if it was not at short stay.
It is also of little help to mariners with the 1.5 depth suggestion. For example, a depth of 30 meters translated into a suggested cable length of 45 meters, two shackles of cable. No more than 10 meters of cable with the anchor could be dragging along the sea bed. With the anchor arrangement of a typical 300m containership, the weight of the anchor together with the cable is no more than 20mt. An anchor will not generate the same amount of holding power when it was dragging. Its holding power is even less than those of a dragging chain. The 20mt mass in motion may provide a drag no more than 25 tonnes force with single anchor, 50 tonnes force with both anchors. It is no match to the momentum (100,000mt at 1m/s) of the ship even at very slow speed (2 knots).
It’s it really acceptable to have a container ship with enough horsepower to go 24 kts if it can’t use full throttle in a confined harbor?
That’s a truly ridiculous question, especially since it was explained earlier.
It’s in seamanship manuals, you should gave been aware of it. People can’t tell you about everything you should know.
In a well protected harbor anchorage you’ll likely not experience the jerking motions necessary to part the chain. Luckily, that’s also where it’s most important that you hold fast. In offshore anchorages where the sea state could shock load the chain you need be prepared to drag on a shorter scope or just get underway.