Anchored Vessels Behavior in Wind

#1

Long, long ago I had a sail boat and from those days I remember now that she would yaw terribly, about 40° while at anchor in a moderate wind.

An anchored boat is an unstable system, mainly because of its topsides that act as a (bad) foresail. Actually, the wind tends to put the boat broadside to the wind, whereas the rode tension tends to bring it back into the wind.

Initially, the boat is stationary, with its rode parallel to the wind, but its bow is headed slightly off the wind (yaw = 1 degree). The windage asymmetry due to this small yaw angle creates a torque that tends to increase this angle, plus a lateral force that pushes the boat abeam. Since the boat is tied to the seabed by the rode, this lateral force results in a near circular motion around the anchor (swing).

At one moment, the various forces balance each other, so the yaw angle rate changes sign. This slows down the swing angle rate, which eventually changes sign in its turn: the boat swings back on the opposite tack.

Now that the pendulum-like motion is initiated, it will last forever!

IMG_4152

For a yacht there is a rather simple cure. Tie a long line to the anchor rode at the bow. Let out the anchor rode until the hitch is 1 - 2 meters underwater. For the final adjustment tighten this stern line, bringing the stern towards the wind until the boat no longer yaws and tie it off.

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Cruise ship Viking Sky in problem
#2

I’ve used this method on sailing vessels up to about 85 feet in length but nothing larger. The mechanics would be more challenging on a ship sized vessel but the effect should generally be similar.

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#3

I think that contrary to the drawing it is preferred to use one of the stern cleats to tighten the line.

I remember that on one occasion the anchor started to drag, got dislodged from the sea floor. This was caused by the strong yawing of the ship, the anchor wriggled loose.

This was also a real danger that could happen with the anchors of the Viking Sky. Luckily that didnot occur and they succeeded in bringing one or more of the engines to life. I donot know whether a tug boat like the Vivax would have been able to control the yawing of such a big ship. In theory a tug could pull the stern to one side so that the wind would come in only on one side of the hull and so stop the yawing.

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#4

I suppose a stern cleat might work depending on the type of vessel and the circumstances but in my experience forward of amidships works best and adjusting the length as prevailing wind and sea conditions change.

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#5

My view here is that you don’t just want to stop the yawing, the goal is to minimize pressure on the anchor. In that case best to keep the ship pointed as straight into the wind as possible as that is going to expose the minimum surface area to the wind.

The one time I had to do this I though that the best place for the tug was made fast aft as the pivot point is going to be near the hawsepipe. But for some reason the tug refused to put a line up aft and stayed forward instead. This arrangement proved to be difficult to control the heading and motion of the ship but we did well enough not to drag, or at least not far. We did not put our beam to the wind which is the high risk situation.

In the case of the Viking Sky it may have been workable in theory but in practice given the wind and seas likely very difficult. I would be very surprised if it had been tried. Maybe a last ditch effort? The wild card would be the estimated time to have an engine on-line.

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#6

Well, for the first time in the 3 or 4 years I’ve been watching this forum, I finally feel as though I can add something. Anchoring is my thing. For 10 or 11 years, I had to anchor my 338’ fish processor in western Alaska in all kinds of weather to get the fish on or off. The additional structure that we had tacked on to the original ship really raised the wind resistance and increased the vessel’s tendency to yaw in the breeze. 60 degrees either side of the wind wasn’t uncommon, and it was sometimes pretty scary to watch the chain fetch up as the ship sailed through the wind and came up hard on the port or starboard extreme. After a few years of watching this, I started doing the following as SOP in a strong breeze: Set one anchor normally with whatever scope is needed, then drop the second anchor underfoot with enough chain to keep it on the bottom. This small amount of resistance was enough to keep the bow from swinging, and it stopped the shock loading of the main anchor. There. I’ve exhausted my expertise.

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#7

Interesting, thx for the pointer Kemal. I thought I read somewhere that while retrieving their anchors the Viking Sky had to cut one chain loose because it was fouled with the bulbous bow? I’m an engineer so I probably don’t have much to contribute to this part, but curious that maybe that’s how it could cross the bulbous bow.

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#8

I can’t imagine a scenario where their chain or anchor would foul on the bulbous bow, especially with all the rocking and rolling they were doing. I can see where they might just release the brake and let the darned thing run free because they need to let it go immediately to take advantage of restored propulsion. Or because the anchor was hung up in the rocks.

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#9

Kemal is right on the money…2nd anchor underfoot works like a charm, used several times in AK on tankers at ballast drafts. Haven’t read about the anchor issues Viking Sky had but I’d imagine it took a bit of time to cut through the chain and not sure I’d do the same, who knows what happens when the chain hits the bulb dropped from above. If it was fouled on the bulb I’d guess they came up on the wrong side of the chain with new found propulsion and weren’t going to slide back and wait for it to come around the bulb after getting closer and closer to the beach!

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#10

4963b

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#11

If it works of course it could be a good tool. But there are drawbacks.

It’s about using friction between anchor tackle and the bottom to dissipate the energy which has been transferred from the wind, a car ship with four of five thousand square meters of sail area, more than most square riggers, is a different animal. Lot of wind energy to dissipate and relativity light anchor tackle.


P&I Loss Prevention Bulletin Preventing an Anchor … - Japan P&I Club

  1. Using the Second Anchor As a Snubber࣭Riding to Two Anchors

To drop the second anchor to act as a check, or snubber, is said to effectively reduce the so-called horsing motion. A suitable scope for the snubber is one and a half times the depth of water. Nevertheless, it is impossible to completely stop the horsing motion. Care should be exercised when deploying a second anchor to avoid entangling the cables and creating a foul hawse, particularly when the vessel is pitching heavily.

When riding to two anchors, there is a possibility that the cables may become entangled resulting in a foul hawse. This method is not recommended for large ships because of the practical difficulties involved in disentangling large and heavy anchor cables and it is likely that outside assistance will be required. Smaller vessels may be able to clear a foul hawse themselves.

The charters recommendation is not to use a second anchor because of the possibility of getting the two entangled. The other issue is if the ship drags the problem of recovering both anchors.

I did actually try a second anchor one time, it did seem to slow the horsing around a bit, not enough that I thought it was worthwhile. The mate and I were on the bow watching and I decided against when the jacking around continued, just happily dragging the second anchor with it, at maybe a slower rate.

The recommendation is to ballast down and/or put the ship down by the head, that dampens but doesn’t stop the horsing, the wind speed it starts is a few knots higher.

Most often the cure is not to anchor, we frequently drift or in high winds stay off-shore and do donuts, the extra fuel burned is understood to be the cost of doing business.

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#12

I suppose that with smaller ships the method with the second anchor underfoot could work quite well, reducing the horsing considerably.

However, cruise vessels have very large windage areas which will result in large side forces and a large yawing moment. The side force and yawing moment are probably too big to handle with this method. Also there is always the danger of fouling or entangling the anchor chains and then what? See here.

The Master, based on his previous experience, had the vessel use both anchors to reduce the swinging speed and angle of yaw of the vessel.

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#13

In them days we didnot sail without a spare anchor on the forecastle deck which was fastened to a ramp. After attaching the chain to this anchor a couple of clamps had to be loosened after which it slided of the ramp and dropped into the water.

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#14

But nowadays there is “safe return to port” so who needs spares?? :roll_eyes:

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#15

I am a frequent user of two anchors and they do sometimes get tangled up. What seems to make for the worst behaviour of all is wind and current opposing. Depending on the vessel, it can do all kinds of odd things that are nothing like your anchorage neighbors either. All I have to do to observe this is look out in the anchorage near where I live and see the unloaded ships swing with the wind while the loaded ships swing with the tide.
EDIT - I am not sure how far you can scale this up, but an old yacht trick is to tie some nylon rode on the end of the chain and tie the nylon to the boat. If you need to cut the anchor loose in a hurry, you let the windlass run free and cut the nylon with a knife in 10 seconds vs. trying to hacksaw the chain or unshackle it under load.

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#16

Using 2 anchors like @Capt_Phoenix’s diagram above is ok as long as wind and current don’t change otherwise the rodes are pretty much guaranteed to get tangled. A line from a point midship to the rode just below the surface per @Dutchie’s diagram is what I used on dive boats in moderate weather to minimize yawing making it easier for divers to climb back aboard.

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#17

Obviously there are drawbacks to using two anchors. Most of the time horsing around the anchorage is expected and lived with, but in 10 years I’ve seen a second anchor used successfully a handful of times in the worst conditions (50+ knots) without issue. It does not stop the movement and it is definitely not used on a day to day basis as there is the threat of fouling, but it is an option to be considered if the circumstances arise.
This is an apples to oranges comparison as everyone’s gear, arrangements, and experiences are vastly different. My experience is with the same 4-5 thousand square meters of sail area, but the gear is heavy and well spaced, monitored continuously, and is retrieved as soon as the situation improves.

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#18

I only used a spread of anchors one time, on the north side of St. Paul Island, when the forecast was for 80 knots. Then, I laid out 8 shots on one side, and something like 4 on the other. Sat there like a rock, and as soon as the weather laid down, I shortened up on both until I was able to retrieve the secondary anchor. No problems that time. My gear was relatively huge, though: original spec was for 7500# anchors, and over time we squeezed 10,000 pounders into the hawsepipes. All on 11 shots of 1 7/8" chain. Made for a good night’s sleep.

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#19

We had more spares such as a cast iron propeller aft. Such a propeller has poor resistance to rapid corrosion and erosion, is less efficient and brittle. On impact with an underwater object it does not bend but breaks. The life of a cast iron propeller must be regarded as potentially very short.

Nevertheless it can serve its purpose because after the replacement of the propeller in dry dock somewhere in the world the ship can sail back to the homeport - safe return to port - while in the mean time a new bronze propeller can be cast. Cost wise not a bad deal. With the much larger ships these days with immense large propellers - Emma Maersk’s propeller has a diameter of 9.1 m and weighs 130 ton - you can forget about having a spare on board!

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#20

I agree, the information I use from the charterer; the wind effect on an anchored ship in ballast for a 3800 TEU container ship and a VLCC, in about 36 kts of wind is equivalent in force as far as risk of dragging as a PCC in 24 kt wind speed.

Typically if the forecast is for more than 35 kts we don’t anchor.

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