What makes a boat swing at anchor?

[twistedtree]

When anchored, most, if not all boats swing back and forth. Some swing quite a bit. Why? What causes it, and what, if anything, can be done to reduce it?

 

[psneeld]

Like a BAD windvane...wind will eventually load a side and it will sail off until the force on the anchor rode overcomes and now it's weight drags the bow back through the windline and the opposite direction takes over...repeat till a new force overcomes the tendency.

A steadying sail way aft works to a degree, stern anchor adjust deck windage (usually not realistic).....

Lobster boats have riding sails similar to a sail vessel with a mizzen mast that will use them in the same way...

 

[Richard W]

A bad design, or a nature of many non-sailing boats, including mine. It swings in excess of 30 degree either way ... bad for anchor holding power which thends to be yanked from the ground and dragged.

 

[psneeld]

On some boats with widely separated bow cleats...a bridle may reduce the amount of swing...others rig their snubber led aft to a cleat so the boat sits at an angle to the wind to begin with...also useful to point the bow more towards a prevailing wave pattern to lessen roll.

 

[Richard W]

Good points ... need to experiment more.

So far, I have used dinghy as a poor man's storm drogue to reduce swinging with mixed results. Mizzen mast and sail would be an excellent solution ... worked well on a sailboat.

 

[windmill29130]

The bigger and heavier the boat, the less it will move. Buy a bigger boat!

 

[Ski in NC]

My boat swims at anchor too. Annoying, but I got used to it. I have a bottom designed for efficient planing, so there are trade-offs, and swimming is one.

 

[alberto]

Flopper stoppers are useful.
I have installed the Magma ones, and reduces the rolling at anchor by 80%, particularly using 2 sets.
My wife states that is the best "invention" (improvement) I have made to the boat

 

[sunchaser]

The bigger and heavier the boat, the less it will move. Buy a bigger boat!

At first I thought this was a question designed to stimulate the moribund geezers amongst us into action. Now I know, Twisted wants a bigger Nordhavn and needs justification to convince the admiral. Yup, that is the answer, a bigger boat, much bigger.

And Ski, my boat nor occupants don't swim at anchor in the PNW at all, the water is too cold.

 

[Nomad Willy]

I think design elements like a high bow and fwd cabin linked w little windage and keel aft. Not say'in that's the combo but consider a boat w a low bow and a huge cabin right at the stern. Probably make like an arrow and get stable ... especially if the boat is long.

Mostly a matter of where the windage and draft is.

My Willard sails a lot and I haven't rigged a bridal yet. Willy is short so even w her substantial keel along most of her length she sails. With Willy I think it has a lot to do w the bow. Big and tall.

It may have a lot to do w where the attach point is on the bow. The bow eyes on small boats are frequently at or near the WL and they frequently tow well. Same boat from the top of the stem and the're all over the place.

Also I suspect that w a boat that has equal windage and draft along it's length will still sail. The wind seems to have more effect on the bow than the stern. By the time the wind/air gets to the stern it's significantly aligned to the hull.

Not claiming any of this to be true. More like thoughts, hunches and suspicions. But the cure seems to be a bridal. We were at an anchorage sailing back and forth a lot. There was one other boat there at anchor ... w a bridal rigged and he wasn't swing'in AT ALL.

 

[Northern Spy]

A boat yaws at anchor because it is an unstable system trying to find equilibrium. Generally speaking the distributed forces applied by the wind and water change relative to the center of gravity of the boat.

Steadying or riding sails aft or using an anchor bridle to a side cleat forward changes the centroids of these forces and lessens the effect.

 

[Wxx3]

A boat yaws at anchor because it is an unstable system trying to find equilibrium. Generally speaking the distributed forces applied by the wind and water change relative to the center of gravity of the boat.

Steadying or riding sails aft or using an anchor bridle to a side cleat forward changes the centroids of these forces and lessens the effect.

I agree.

I have found little difference using bridle or not in any given circumstance.

For dauntless, it does seem to be the current versus the wind and since the wind is usually less steady in a short period (seconds), the boat swings.

The stronger the current, the less the swing.
Last Summer in Maine, I was in an anchorage, with about 8 other boats: 5 sailboats, 1 Nordhavn and a KK42 and KK48. I noticed that the two Krogens moved pretty much in lockstep, and for more than an hour, the two Krogens were perpendicular to the other 6 boats.

Don't really know if that was a function of current or wind, but clearly we were affected differently, at least for a while. and since, all the boats eventually came back into alignment, I thought it was a current (tidal) issue.

Here is a video I took at the time

Richard

 

[Phil Fill]

A full displacment heavy boat does notr swing as much. I also read that if the house/super was back like the explorer/expidtion the house acts like a vane and does not swing as much. The Eagle house is about the middle of the boat and full displacement so it does not swing to much. As mentioned be for a steading sail bakc by the stern helps reduce the swing.

At our first moorage we where close to the speed marked area so we would be some big waves and rocking, so what I did was hang rocker stoppers. they as plastic cones you can buy West marine for 10 bucks. I had 4 on each side, at the bow and stern, they really helped. They nestled nicely when stored and quick to deploy. I have thought about just hanging them while we are under way with a heavier mush room anchor. I mean under 200 bucks and you have stabilizers and help reduce the roll and swing?

 

[RT Firefly]

Greetings,
P/F. "...hanging them while we are under way...". You CAN'T be serious!

 

[bligh]

Boats that tend to sail around usually have lots of freeboard forward or a large pilothouse/cabin forward. Boats that dont tend to swing are full displacement sailboats or boats with relatively low even freeboard. A good deep keel helps.

Next time you are in an anchorage, observe other boats at anchor. Depending on wind and current conditions, you will notice sailboats tend to swing with the current and power boats tend to swing with the wind. Generally, powerboats swing a lot more than power boats as power boats have more surface area exposed to the winds and sailboats have large deep keels that have mroe exposed to current.

At crowded harbors, you will find the harbormaster keeps all the power boats moorings together and all the sailboat moorings together. He is not segregating the boating community because of the whole power vs sail 'thing'. Harbormasters know that sailboat (in general) swing alike, and powerboats swing alike.

 

[Phil Fill]

Greetings,
P/F. "...hanging them while we are under way...". You CAN'T be serious!

Why not! Hey, once you have a ugly old slow trawler a little more does not hurt. What is the difference does it make if they are plastic or steal. They worked at the dock/anchor so why not when under way.

 

[rwidman]

A boat yaws at anchor because it is an unstable system trying to find equilibrium. Generally speaking the distributed forces applied by the wind and water change relative to the center of gravity of the boat. .....

That's it in a nutshell. What to do about it? Try to anchor out of the current and protected from the wind.

In many places, the current direction and strength will change during the time you are anchored. The same applies to the wind.

My boat doesn't seem to do this much and in any case, I ignore it since there's nothing I can do about it.

 

[twistedtree]

Fortunately the Mrs is totally on-board with the Nordhavn.

People mention greater displacement as being more stable, but what got me wondering about this again is a very experienced Nordhavn delivery/training captain who commented on how much they sail at anchor.

Sure, a shift of wind here or there will cause a boat to move around, but many seem to just constantly oscillate no mater what.

As for sail boats vs power boats, what you say about different windage etc makes sense, yet the worst boat I ever saw sailing at anchor was actually a sail boat. At times I though it was going to hit it's neighboring boats it was so bad. The dock guy said that boat always does it.

 

[psneeld]

Fortunately the Mrs is totally on-board with the Nordhavn.

People mention greater displacement as being more stable, but what got me wondering about this again is a very experienced Nordhavn delivery/training captain who commented on how much they sail at anchor.

Sure, a shift of wind here or there will cause a boat to move around, but many seem to just constantly oscillate no mater what.

As for sail boats vs power boats, what you say about different windage etc makes sense, yet the worst boat I ever saw sailing at anchor was actually a sail boat. At times I though it was going to hit it's neighboring boats it was so bad. The dock guy said that boat always does it.

Like Northern Spy said and I posted in #2...it's about unequal forces and how/why the boat sails back and keeps oscillating....not mater what size or shape boat...obviously the better weather vane or the boat weight that only a hurricane can get to move is going to sail less....and underwater shape with current will also have an effect.

So what works for one boat may not another..like I said about bridles...unless you have a broad bow with wide points of attachment...less help ...no better than just a single snubber through the pulpit.

But also as NS and I posted..if not having much luck and conditions are still reasonable...taking a snubber to a cleat/bridle off the side does work for many boats not matter size shape, etc..etc as now you are preventing the oscillation because you are never allowing wind pressure to the opposite side.

In mild conditions...a good reason for all chain anchor rode as it really limits the swing.

 

[twistedtree]

Yes, I get the unequal forces and all that. Just trying to dig deeper. For example, most boats are symmetrical side to side, or very close to it. So why doesn't it tail straight into the wind and remain stable? And when gusts or whatever yaw it to one side, the counteracting forces should kick in immediately, so why doesn't it damp right out?

I've heard the single line vs bridle argument, and you would think it would make a significant difference, but I've heard lots of people say it makes little to no difference.

Some claim that an attachment point closer to the water line is better than one up on the bow, but again I've heard a number of people say it made no difference when they tried it.

All this suggests to me that we don't REALLY know what the detailed dynamics are. Where are the rocket scientest when you need them......

 

[bshanafelt]

Night and day difference at anchor or mooring with the sail up.


The swing arc is dramatically reduced.


Would be even better if the mast were further aft and the sail larger.

my boat is fairly light at 20k#'s.

The jury is still out on whether the hullform is true displacement or not.(Eric will decide).

 

[Richard W]

All this suggests to me that we don't REALLY know what the detailed dynamics are. Where are the rocket scientest when you need them......

I cannot mathematically express this but I know what makes my boat a bad a$$ boat at anchor. It's not much about center of gravity, or center of buoyancy, as much as about combined center of effort above water and combined center of resistance under water ... very much like with a sailing vessel. More info here: Applied Sailing - Technical Support For Sailors - ASA 106-2: Force diagrams

My bad a$$ boat has center of gravity and center of resistance close to its heavy stern, and center of effort close to its high windage and light bow. This is what makes its bow swinging away from wind at anchor, and when its stern catches with the bow the pendulum starts swinging the other way.

The design is so bad that I cannot swing the bow into the wind stronger than 5 knots while going in reverse at idle RPMs with a single engine. Once I manage to force the bow to cross the wind (higher RPMs ... not good while docking), it falls off the wind to other side at rapid rate and is impossible to control. This makes for a very entertaining docking ... not!

 

[Northern Spy]

the worst boat I ever saw sailing at anchor was actually a sail boat. At times I though it was going to hit it's neighboring boats it was so bad. The dock guy said that boat always does it.

This may be due Bernoulli's principle on the fin keel acting like a sideways underwater wing as pulls it one way, angle of attack changes, then it stalls, the anchor rode pulls the bow around on a different tack, until it stalls....

... or maybe it's just ornery.

 

[twistedtree]

I just found this Rode Dynamic Behavior 3 which help illustrate and explain whats going on. From it, you can see where all our intuitive ideas come into play.

Here's my attempt to summarize:

There are three significant elements at play:

1) The bow attachment point

2) The center of windage when the boat is at a slight yaw

3) The center of gravity of the boat.

The closer the center of windage is to the bow attachment point and further from the center of gravity, the more the boat will sail back and forth. This creates an unstable dynamic.

The further the center of windage is from the bow attachment point and the closer to the center of gravity, the less it will sail.

Sailing happens because, with a forward center of windage, a slight yaw causes the bow to push off of the wind and yaw more and more. As the boat becomes more and more broadside to the wind, more of the boat's side is exposed to the wind and the windage center moves back. Plus, the anchor rode pull is more and more off to the side helping to counteract the yaw. Eventually the process reverses, but just keeps on going.

Intuitive remedies work as follows:

Bridles: These have the effect of moving the bow attachment point further forward, but it's effectiveness depends on how far apart the side attachment points are on the boat. It works particularly well for catamarans, and will be less effective for boats where the two sides of the bridle are close to the center line.

Anchor Sails: These move the windage center further back.

Cool stuff. Now I think I understand.

 

[Richard W]

The further the center of windage is from the bow attachment point and the closer to the center of gravity, the less it will sail.

That is the best take away from this discussion as the cause of swinging.

How to counter it when the center of windage and effort is on the opposite end from center of gravity and resistance ... and a back sail is not an option? Bridles? What else?

 

[N4712]

Nordhavn delivery/training captain w

Is his name John?

 

[Northern Spy]

I just found this Rode Dynamic Behavior 3 which help illustrate and explain whats going on.

It took forever (in computer terms, about 20 seconds) to for me to update and download shockwave (and not install McAfee), to view his graphic, and then it didn't even show the vectors. But yeah, that's pretty much how it works.

 

[twistedtree]

How to counter it when the center of windage and effort is on the opposite end from center of gravity and resistance ... and a back sail is not an option? Bridles? What else?

Perhaps other creative ways to alter the windage profile using canvas, etc. It would be very specific to a particular boat, but a curtain here or there might help.

Also a very long bowsprit/pulpit?

And a bridle with wide attachment points. But it seems there is a tradeoff with that since the side attachment points are typically further aft than the bow roller, so you are improving in one way, but worsening in another. Who knows where it would shake out.

By the way, this suggests that a low attachment point like a tow eye should not be any better than a high attachment point up on the bow. In fact, there is probably an argument that a tow eye would be worse since it is likely further aft than the bow roller......

 

[dimer2]

Stolen from an article showing CE and CLR effects on yawing by Greg Walsh...

Two important design factors are also involved in a typical anchoring situation: Center of Effort and Center of Lateral Resistance.

Center of Effort is the point through which all wind forces tend to act on a boat. It is different for all vessels and it varies with conditions of sail, rigging, deck gear, cargo, and deck structures. It is, in effect, the total average midpoint of everything taken together on the boat above the waterline. An invisible finger pushing on the CE would simply make a boat heel directly over to one side. It is an important factor to consider when a boat is anchored in rising wind conditions.
Center of Lateral Resistance is the geometric center of the bottom configuration. If one were to dive underwater and push on the CLR, the boat would theoretically move directly sideways. CLR can change as a vessel heels, but it generally doesn’t move much.
In most situations the CE and CLR need to be reasonably close for a boat to be balanced and handle properly. Strategy for boat anchoring is also a function of these factors. The CE usually is forward of the CLR, even for an anchored boat. This is because of the traditional shape of a hull without sails - there is more mass up forward. Different styles of boats have more or less mass forward of the beam.
A vessel’s tendency to yaw or swing is enhanced as the CE moves forward of the CLR. The more surface area there is forward of the CLR, or forward of amidships, the more yawing tendency will develop as winds and waves increase.
If the CE can be kept aft - or moved aft - a boat’s tendency to ride properly at anchor will be much improved. That is one of the great advantages of a yawl or ketch rig, or of a vessel equipped with a small riding sail flown from the backstay. These small, flat sails tend to move the CE aft and keep the boat weather-cocked up into the wind. (The sail must be completely flat, however, so as not to drive the vessel ahead.) Considering the awesome power of a gale of wind, any such riding sail or mizzen sail must be tough and very small, perhaps just a few square feet, so as not to be overpowered. Just as often, however, even the smallest riding sail must eventually be doused as the wind reaches the upper ranges of intensity. What may save a boat at that point are efforts the crew has already made to move the CE aft by reducing windage as much as possible forward of amidships (stripping sails, booms, excess rigging and equipment, etc.).
Yaw, of course, is difficult to avoid, since the force of wind and waves almost always strike the boat at some point forward of the CLR The motions of yaw and swing seem almost inconsequential when anchored in light conditions. As wind increases, however, the tendency to yaw and swing can combine with catastrophic results. As soon as a boat’s bow blows off the wind (the boat turning on its vertical axis) it will begin to swing off to one side because of the force of wind on its side. As more of the boat’s side is exposed the tendency both to yaw and swing will increase. Before long, in the worst situations, a boat may be swinging back and forth like a pendulum at the end of its anchor rode, each time yawing so as to present a different side to the wind. The acceleration of swing in each direction can dramatically increase the forces involved, especially at the apex of each swing. All the work of stopping the swing, reversing the bow, and holding up to total wind force falls directly on the anchor and its gear. Total loading on the anchor gear consists of pure wind force per square foot, plus additional forces for acceleration or momentum of swing, surge, current, and wave action.

 

[Richard W]

Two important design factors are also involved in a typical anchoring situation: Center of Effort and Center of Lateral Resistance.

Right, this is what I said #22 ...