A cycle I’ve observed at anchor in strong winds is the ship will drift directly downwind beam-to till the anchor chain comes up bar tight. At that point the ship will pivot into the wind and then the bar-tight chain will pull the ship up wind. At a couple kts, straight into the wind till the strain eases up and the chain drops.
At this point the heading will pay-off till the wind is abeam again and the ship will run downwind beam-to again until all the slack is taken up and the chain comes up bar tight again.
That’s the standard pattern, starts at around 20 or 25 kts of wind. I got relieved by a long time container ship captain one time, no car ship experence, the C/E later told me the captain had anchored and upon observing the horsing around picked up anchor and moved, twice. He
called down to move for a third time but the C/E convinced him it was normal.
The cycle I described is not the standard figure-8 (the GPS track). It was due I think to not only strong winds but in addition a strong cross current.
According to the diagram the extreme jerk does not occur at the positions 3 and 7 where the anchor chain in fact relaxes, but at 4 and 8. At 3 and 7 the velocity is momentarily zero, the sway back point has been reached and the chain relaxes.
Funny that the relief captain obviously never experienced real horsing that is because a container ship also has a very large sail when fully loaded.
Oh, I see, I wouldn’t expect that paper to have the finer details.
As far as container ships, horsing likely, in 20 kts of wind, probably not.
I don’t think it just the sail area, it the ratio of the sail area to the cross-section below the water-line. Also I think house forward makes the car ship directionally unstable at low speeds when pointed up-winds, which is why it’s easier to anchor with the wind on the beam.
On a windy night at an anchorage there will be a bunch of ships anchored and a couple doing donuts. At first light the anchored ships will all be container ships and the ones circling will be car ships.
For computing the so-called critical speed for dragging anchor, for a container ship the wind force is multiplied by thee to get the estimated force on the anchor chain, for a car ship it is multiplied by five.
About the same more or less as the same LOA car ship when in ballast.
This is an interesting report about wind forces on container ships with a lot facts about the yaw. It confirms what you said about the different behaviour of this type of ships.
Wind tunnel tests show that container ships have:
Significantly higher longitudinal force.
Smaller transverse force.
Smaller roll moment.
Smaller yaw moment.
Additionally, a ship with random configuration on the fore deck (forward of the superstructure) and a full container stacking on the aft deck (aft of the superstructure) has a significantly reduced yaw moment for relative wind directions between 0° and 50°. I have
That’s why it helps some to ballast down by the head. But everything has a cost. With a transfer rate of 400 mt/hr it’s going to be a couple hours min to make much difference.
If arrival condition is flat (same draft fore and aft) and the ship is FWE at say 1800 hrs who is going to stay up for two hours pumping ballast? If the pilot is 0500 hrs who is going to get up at 0300 hrs to trim it back? Ship down by the head is difficult to steer/control.
Coastwise the crew is always up against fatigue and work/rest issues.
Single vessels on a long chain always yaw the worst, just like the illustration above shows. Having a fish boat alongside my processor helped to dampen things a bit, but as soon as we got our tramper (250-350’ reefer ship) tied up alongside, we sat there like a rock. No yaw at all. I suspect our aerodynamics changed to that of a basketball when we got the other ship alongside.
One becomes a keen student of anchored ship behavior when faced with the prospect of taking one’s ship alongside another ship anchored in the wind. I only had to do that a handful of times, and nearly all of them worked out well. A thruster would have come in really handy.
After hurricane Charlie I had to beg the USCG to come help us get the anchor up. The anchor, from what I saw on the chain, had buried itself about 8 feet deep in clay and needed the full force of the CG patrol boat to get it loose. I was amazed it didn’t bend. It was also a good thing no one was near us, we had 250 feet of nylon rode out in shallow water and every one of those bands that come across in a hurricane had us going like a bungee cord ride!
Is there such a thing as anchor snubbers on ships or is that just a yachtie thing? We always had 20-30 feet of nylon clipped onto an all chain road with a slack loop of chain to absorb the shock of yawing around. Perhaps that doesn’t scale?
It’s just the limitation of the vessel, I just don’t anchor if the forecast is more than 35 kts. It’s possible to get caught of course but in general it is a situation for example in the trades where there is a steady winds and a squall line passes. Generally if the ship drags it’s only a ship length or two and the squall lines passes.
If the ship does turn 90 to the wind and goes for a run I use the engine, a dead slow bell is enough with the anchor down. Eng dept is told in advance if the situation is iffy.
On the rare occasion where I got stuck in substantiated winds, 50 kts or so I was able to get a tug to dampen the yawing, otherwise the engine and thruster would be used to try to hold position without overheating the thruster or running the engine out of start air, maybe just dredge the anchor slowly ahead.
It’s really no different than any other vessel, if an crowded anchorage is hit by 50 kt winds the vhf comes alive. The dragging ships have to take some action. Same with us except lower wind speed.
@yacht_sailor
A large ship has a remarkably flexible hull compared to a small yacht. At anchor with the main plant shut down, it’s a thing of wonder. You can see the full length fore and aft passageways below the main deck snaking as you would under way but the hollow sound of the steel plates flexing is enough to give me the willies.
The value of the various active forces, yawing and swinging depend mainly on the relative longitudinal positions of 4 geometric points in the ship:
The center of gravity of the boat (G)
The anchor roller (B)
The centroid of aerodynamic forces (A)
The centroid of hydrodynamic forces (H)
In a well balanced ship (underway), G, A and H are grouped close to each other. For an anchored boat (whether it be a ship, a powerboat or a sailboat with its sails down), A usually is forward of G. If this is the case, the aerodynamic force in A generates a torque that tends to swivel the boat off the wind. When the yaw angle increases, however, the aft parts of the topsides, rigging and deck structures become predominant, so A moves aft and finally goes past G, which reverses the motion.
Thus yawing happens when the ship’s centre of windage is forward of the point of lateral resistance in the water. This causes a little yaw to develop whenever there’s slack in the cable and there’s enough wind. The ship then literally sails off sideways until it’s yanked round by the anchor cable at B. Critical wind strengths when yawing starts depend on how big the distance is between centres - anything from 15kts to 25kts can start things going.
A solution to reduce the yawing could be to give the stern of the ship a lot more windage (shift balance). This was also the result of a test in a wind tunnel, as explained earlier, with the model of a container ship.
Additionally, a ship with random configuration on the fore deck (forward of the superstructure) or empty fore deck and a full container stacking on the aft deck (aft of the superstructure) has a significantly reduced yaw moment for relative wind directions between 0° and 50°.
For a yacht it could be helpful, in order to move A aft, to give the stern of the boat a lot more windage by putting up a small mizzen sail aft. Some trawlers even have a small aft mast and boom for that purpose. On a sloop, hoist a small and flat riding sail on the backstay.
If the ship is dragging anchor in the so-called figure 8 pattern, at the end of each “8” the ship will sometimes drag the anchor a meter or two. This can be seen as the ship slowly works it’s way downwind.
That implies that the ship is lifting all it’s anchor chain off the bottom, so why not just run out more chain?
It’s a contest between more wind speed and more chain. The problem is two part, with extra chain, only a portion is laying on the bottom, the rest is being lifted off.
The other part of the problem is a small increase in wind speed negates the extra chain because of the relationship between wind speed and force.
I am not sure that the anchor’s flukes are entirely lifted out of the seabed, perhaps it is more of an enormous yank on the shank that cuts and pulls the anchor through the seabed a couple of meters.
The two ships tied alongside each other can be seen is one new ship with a combined center of gravity and displacement. The combination of these ships will take more force to turn while the aerodynamic force is about the same as for one ship. This is probably the reason that the combination will not yaw easily. Of course there are other factors to be considered like underwater profiles and windage areas on both ships.
