Why Boats Yaw At Anchor

by Arlyn Stewart
  

Two years ago my boat suffered its closest encounter with disaster in many years of cruising.  We were approaching our planned evening anchorage at the end of a long sailing day and had to hurry to beat an approaching thunder storm.  The primary anchor was set and only moments after backing it down, heavy rain and high winds drove us into the cabin.  I was glad that I'd taken care of top side chores prior to reaching the anchorage.  The sail cover was well secured, the halyards belayed with the jib halyard wrapped around the roller furled headsail and binoculars, camera, and other cockpit items had been stowed.

It wasn't long until the wind shrieked through the rigging.  Perched near the companionway and the GPS / sounder combo,  depth was noted at 12-13 feet and remaining steady.   After watching for a few minutes and confident that we were holding well,  I relaxed, taking a seat on the port settee and glanced out the windows to shore where lights from the shore side cottages were observed which instantly stirred awareness that we had dragged, the cottage lights were too far away. 

I lurched to the cockpit and assessed that we had drug two thirds the way across the bay.  It was imperitive that the secondary anchor was deployed quickly.  Fortunately it was stowed in proper fashion for immediate use and was handed over the cockpit coaming and released.  Waiting for the proper scope to pay out offered my first opportunity to take full stock.  The wind was fierce and rain pelted and stung my face causing thought that it was hail but with no ice under my feet, it was obviously rain.  The boat was abeam to and straining at the primary anchor rode and heeling 15-20 degrees.  The wind rather than off the bow, was off the forward starboard quarter.  It seemed from the strain on the anchor rode that while it was dragging, it hadn't broke out.  Enough daylight remained to see the rocky shore to leeward, I feared we would be shortly upon it unless successful efforts were made and quick.  

Not more than thirty feet of rode payed out for the secondary anchor when the wind abated and the dominant part of the micro burst winds moved east.  The primary anchor now held and the secondary anchor never came into play having not reached the scope allowed it.

This incident motivated me to understand anchor yaw and provide solutions.

Rode Stretch

Some feel that the cause of anchor yaw and sailing on the rode is rode stretch.  That as the drag of the boat against an increasing wind stretches the rode,  when the wind slackens or when the momentum of the aft boat movement reaches the limits of the rode stretchability, that the energy stored in the rode then collapses and pulls the boat back to windward where that momentum will leave the rode now loose enough to have the bow blown off on a tack and that the subtleties of wind direction influence which tack it is blown. 

Boat Balance

Others argue that its about boat balance and that the keel acts as a pivot point and that the rigging serve as sails.  That because the mast and furled headsail are ahead of the keel, that they present a force that blows the bow around the keel.  There are several problems with that theory with the greatest being that many power boats are as notorious about yawing on their rode as sailboats and they have no deep keel or rigging structure.

Lifting Foil 

My belief is that the culprit force is the lifting foil of the hull sides.  That when the wind blows past the hull sides, the hull is lifted in the same way an airplane wing lifts.  That if the foil is given an attitude of attack, lift will be generated that will cause the boat to be pulled sideways.  Because of the restriction of the rode, the movement is an arc on the rode.  At the end of the arc, lift combined with momentum swings the boat through a tack.  Now somewhat broadside to the seaway, she may experience momentary rolling at the end of the tack before being pushed off the wind to resume the hunt to the other extreme.  I don't think that either rode stretch or boat balance account for the energy necessary to bring the boat well into the wind. 

Gaining Perspective

The story above with the very high winds may yield some clues and answers, In that instance, the boat hung on a tack instead of swinging and lay there straining with her rode abeam and the wind on her bow quarter.  In lesser winds, I'd always known her to turn at the end of the arc to sail to the limits of the other end to then repeat over and over again.  Why then did she stay in high winds but turn in lesser winds?

Looking at the sketch to the right, it is easy to see why the boat will turn at the end of a hunt and sail the other way.  With the angle of attack at position A, it will lift to B where the side lift movement becomes arrested by the rode as it can climb no higher into the wind and lifting force and momentum tacks the boat through the eye of the wind to  C.  There, it experiences a momentary broadside to swell and rolls badly likely throwing occupants of the V berth against the hull sides.  (from personal experience)  Hull lift has now changed to the starboard side. At D, the lift force takes over again as the hull straightens somewhat on the wind and the roll problem abates as the boat sails its arc toward E and eventually to a furtherest position to starboard and another tack change and cycle.

The down pointing arrow from the hull depicts the lift caused  by wind over the hull sides.   The up arrows against the centerboard and rudder depict the lateral resistance of the water as the boat is being blown leeward. ______________________________________________________ Estimated elapsed time of drag was 4-5 minutes Distance of drag was 1950 feet Drag rate was between 6.5 - 8.1 ft per sec Drag speed was between 4.5 and 6.6 mph

What is harder to explain is the yaw that occurred in the very high winds experienced in the story above where the yaw was arrested and found balance instead of swinging and tacking.  I'm certain of the following observations.  The wind was on the starboard bow quarter and the rode led off abeam.  The only place those two observations fit each other is if the boat is located where depicted in the drawing to left.  For the boat to go to that position and stay, it seemed that two forces had to be at play.  The hull lift force would certainly answer the posture to port of the anchor... but why did it not tack as normally happens in lesser winds?   What other force was involved?  After seeing no other wind forces,  water forces were examined and it soon occurred to me that as the boat was dragging anchor, there might be hydro forces generated.   How fast and in what direction?  Would they be enough to have effect?  From the chart below, we drug just under two thousand feet  in something less than five minutes, the numbers to the left were crunched. They fit. We were sailing backwards  pretty fast.  It began to make sense, the opposing force was lateral resistance of the rudder and centerboard opposing the hull lifting forces and restrained the boat from swinging and tacking as it would if the anchor were holding.   This effect should not be confused with the boat balance theory above.  In this instance, the boat was making leeway and causing  lateral resistance forces to come into play.

Other observations are that the wind was very high, estimated in excess of 50 mph and probably exceeding 75 mph.   Wind blew so strong that there was no distinguishable perception that the boat was sailing backwards.  Evident now is why the boat heeled so far over with no sails aloft,  it was tripping over the keel and rudder.

Why then did she stay in high winds but turn in lesser winds?  The question was flawed,   She would likely tack in high winds just as she does in moderate winds if she were not dragging her anchor.  I wished that I could remember what followed when the winds abated and she came solid on her anchor.  Had she then started to tack?  I don't remember and probably didn't observe.  What is remembered well when realizing that the wind had abated was gazing at the rocks to leeward and getting hard on myself for my indiscretions which had put us in harms way.  For not realizing that the bay was a constant depth and not having paid the price of getting drenched to set the second anchor.   In the end, I got both drenched and scared. 

A host of things come to my mind now like a Monday morning quarterback, the most significant that it would have been much better to hit the MOB button and watch for a distance rather than a depth change. If the gps had been left on the main screen instead of switched to the sounder screen to monitor for weeds when setting the anchor... I may have noticed the boat speed reading.   If a riding sail had been deployed, if we'd drug at all it would have been in line with the wind and out the bay to safe open waters. 

Stopping the Hunt

Two traditional methods have been employed to limit or stop hunting.  The first is to set two anchors.  Spaced with about a forty five degree angle the boat is held in check from sailing too far on a tack thus limiting the momentum to swing its beam to the oncoming sea and suffer roll.  This method is quite effective with the drawbacks that a wind shift will somewhat void the effort.   Micro burst in my opinion are the greatest danger and while most will hit from the southwest and can be predicted, thats not always true.  A large tall thunder head can produce down drafts that turn into straight line winds that will travel in all directions from the cloud, totally unpredictably.  The second is to use a riding sail which, has the effect of vanning the boat into the wind and limiting the yaw and hunting.  Riding sail designs are interesting and varied.

A third method is to anchor by the stern.  While some argue that doing so exchanges the rigging in front of the CLR for a position aft of it, I believe the real reason that this eliminates hunting is that it takes away the airfoil provided by the hull.  A wide flat blunt surface to the wind doesn't provide a proper leading edge to the foil which,  destroys laminar flow over the hull surfaces.   I've personally never tried this but will.  I wonder if the oncoming swell might lap the stern and be noisy.  One of my primary reasons for enjoying anchoring is the ability to regulate ventilation at night but I see no reason why anchoring by the stern wouldn't just reverse the air flow if the hatch boards are left out... which we normally do.

Since the above event, I've employed a fourth method in combination with a riding sail and experienced good results.  Forming the anchor rode into a bridle cleated on both forward cleats has a dampening effect to the boat setting up an angle of attack and gaining hull lift.  The bridle serves to balance the load and shift the load on the opposite side to the sail tack thus helping to leverage the boat back toward center on the rode.  It does this while momentum is yet minimum and is quite effective.  I use a modified bowline in a bight to form the bridle.  The bridle is especially effective compared to a single chock on one side of the bow.  A setup with an anchor roller may benefit less.

Observations

While I had backed the anchor down well and it held well, high winds exerted far greater forces that exceeded the anchors abilities.   I don't believe the anchor ever broke out.  If it had, I think the bow would have blown off wind or we would have felt the boat lurch.  Instead, I think it plowed a furrow across the bay straining at the rode the whole way.  The incident gave a new perspective about some of the dangers of lying at anchor.  Changed is my primary anchor to the heavier and better holding Horizon Claw with the danforth now the secondary.  Two anchors are always set now when a storm threatens.  A  riding sail was constructed and used in combination with an anchor bridle.  The comfort found by combining a riding sail and anchor bridle was discovered last summer during a thirty six hour blow as the difference between sleeping and not,

While most sailors likely understand that sails are lifting foils, many may fail to recognize all the various foils at work with the hull and its appendages.  A boat like the Catalina 250 that carries a wide beam and lot of freeboard is subject to great forces during a micro burst.  The 3:1 length to width ratio with high freeboard provides a thick foil with extra length, both of which contribute to higher lifts than a narrow beam with minimum freeboard. That lift can exert far more load on the ground tackle than simple hull and rigging drag resistance alone. 

To illustrate this, examine the track of the drag as shown on the chart.  With winds from the WSW, instead of dragging toward open water out the bay, the path of the drag was toward the shoals to the ESE.  Why did the path of the drag skew from the wind direction?  Again, look at the second diagram above and see the pull on the anchor for the answer. Clicking on chart will enlarge

It is very strange how the senses work in such circumstances.  Its like little snap shots are taken of various things that don't seem ordinary, these to stand for years as memories of the event.   One of those is the deployment of the secondary anchor.  I remember the effort to hand the anchor over the starboard coaming not wanting to bang up the hull with the boat heeling well to port, of wanting to insure that the anchor vaned into the flow as I felt only one chance for this to work would be offered.  Next, I recall watching the scope pay out and that it went off the beam which seemed odd and caused me to look forward and observe that the primary rode was straining beam to the boat.  This serves as another testament that my observations were correct and that the drawing represents accurately how it was.

It seems important to point out that dragging might carry the boat anywhere within a ninety degree arc off the wind.  

We drug within 630 feet of the rock infested shoreline.  The rate of drag would have put us there in another 78 - 97 seconds had the micro burst endured.

Conclusion

The experience related above provides an interesting model.  Hull foil lift that yaws the boat at anchor carries a potential for both discomfort and danger.  The experience betrays a hull lift force that will vary with wind strength and during a micro burst challenge ground tackle.  Perhaps most interesting from this whole experience is the application to sailing a close hauled course to weather.  Examine the position of the boat again carefully.  It lies on a course typical to that of a close hauled beat to weather.  It can now be better appreciated why this design and other mono hulls like her have such a difficult time with leeway when weathering a course in high wind conditions and experience tack angles of 130 degrees or more.  In addition to all the other leeway forces that have been well understood, that inflicted by the hulls leeward lift needs added.

Copyright 2004   Arlyn Stewart