Yawing and Anchor Holding

The more quietly your boat sits, the better your anchor will bite.

9

I can’t leave a boat that’s yawing. I can’t sleep. Each time I’ve had an anchor fail suddenly it was caused by excessive yawing. Once in my foolish youth, I anchored a beach catamaran without a bridle and it sailed off on its own. Through some miracle of beginners luck the anchor caught 100 yards away, I rocketed away from the beach like Mark Spitz, caught up with the boat, and swore and oath to never, ever again allow a boat to yaw at anchor.

Years later on a cruising cat I was preparing to raise anchor, had removed the bridle, and the resulting yawing sailed the anchor out while I was coiling the bridle. Because it was a lightweight aluminum Fortress anchor, there was no chance of resetting. The boat was moving fast enough it simply planed along, 10 feet above the bottom.

And during the testing for this series of articles on yawing the test boat has broken anchors out on numerous occasions; one moment we’re taking data, noting yaw angles and recording rode tension figures that are no more than 10 percent of the maximum holding capacity of the anchor, and the next moment we’re sailing away at 3 knots, trailing an anchor like so much scrap metal, bouncing on the bottom but never catching.

In past testing we learned that yawing just 60 degrees, as measured by helm compass range, can increase rode tension by 50 percent and that yawing 120 degrees will increase it by two to three times (see “Anchor Snubber Shock Load Test,” PS November 2013). The weather sees the boat in profile, greatly increasing wind and wave loads.

When anchored with all chain, boats will often yaw less in lighter winds, but in stronger winds—unless a snubber is used to reduce the loads—they can yaw more violently than a boat on all-nylon rode. Fin keels and lightweight boats yaw more quickly, but the extent of yawing depends primarily on the relationship between the centers of effort above and below the waterline.

However, the higher snatch loads caused by yawing could not alone explain the sudden failures in moderate winds that we have experienced. We’ve tested these anchors in a steady side pull, and they always shifted and held most of the original load. The other common thread was that the failures were never immediate, but took 10-30 minutes in most cases. Apparently some cycling is required.

We puzzled over the mechanism, based on what we have observed during prior anchor testing:

Pivoting Fluke Anchors

If deeply set in a soft bottom, pivoting fluke anchors (Fortress, Danforth, Lewmar LFX) may be many feet underground and drag dozens of feet of chain with it.  Even when yawing through 120 degrees, we wouldn’t expected it to feel the change. In fact, the straight up recovery force can be nearly as great as the setting force.

However, if the anchor is set shallowly, or even partially set, as is often the case in good sand or firm mud, the behavior becomes quite variable. Holding at a 90 degree angle can be reduced as much as 80 percent, beyond which the anchor trips.

Reset is variable; in good sand it may dig in again if the motion is not too sudden, but more often the flukes are clogged with mud and shells, preventing the fluke from pivoting into position. Other times a light weight anchor will just glide through the water like an airplane; the upward angle of the fluke provides lift and the anchor never touches the bottom. In coarse sand a pivoting fluke anchor will often shuffle to face the new force, but it’s not as reliable in this action as other designs.

Scoop and Plow Anchors

There is a continuum of reactions, depending both on the design and the bottom. Anchors with steeper angles (Rocna, Mason Supreme) tend to bury deeply, but may upset instead of realigning if yawed sharply. They have considerable ability to reset if not clogged with mud. Anchors with more shallow angles (Mantus, Spade, Delta) are better able to shuffle around to meet the pull without unseating, but they have lesser holding capacity on a weight basis, either because of the shallow angle (Mantus) or reduced area due to ballast (Spade, Delta). As a rough rule of thumb, deeper set anchors are less likely to be disturbed by direction changes, but may fail more suddenly, and the more shallow anchors will hold less per weight before moving and shuffling (up-sizing is the recommended solution) but may fail more gradually.

Based on years of anchoring and testing, we consider shuffling during active yawing to be a failure.  Yes, an anchor can shift to face a persistent shift, and that is probably a good thing. It will have considerable time to dig in and hold. But during yawing this motion repeats every few minutes, and each shuffle will move the anchor inches or feet downwind. In anchor testing, the most common failure mode is slow dragging followed by sudden failure when the anchor passes through a soft spot, the rode trips on trash, or the anchor clogs with debris. In our experience, any anchor that drags more than a few feet after setting has an unacceptably high risk of hitting something that will result in complete failure.

And so, we believe that just behind poor anchor technique and impossible bottoms, yawing is probably the leading cause of dragging. Gear failure and even poor anchor selection are much farther down the list; we’ve anchored with poor anchors, rope, and rusty chain, but only impossible bottoms and yawing have actually cast us adrift.

Like most anchor testing, the best we can do is document broad trends. Boat design and rode type affect yawing and forces.  Anchor design, setting history, yawing, and bottom type all affect holding capacity. The relationships are complex. You will need to consider how each factor will affect your boat, and which factors you can change in the quest to stay put at night.

Yawing and Anchor Holding
Anchor veer testing took place on Chesapeake Bay, both in soft shallows at dock side, and firm sand from a boat. Loads were applied cyclically at angles between 30 and 60 degrees.

What We Tested

Although we have a lifetime of experience with larger anchors, we began our investigation with 2-pound anchors. We know from numerous past testing programs that even small anchors can serve as reasonable surrogates for their larger brothers, so long as we restrict our investigation to consistent mud and sand bottoms.

They allow for a systematic, laboratory approach, showing trends and generating numbers that can tells us, at least qualitatively, the magnitude of these trends. Hard and weed covered bottoms favor heavier anchors and we did not investigate that. We then tested using 12-pound anchors on our test trimaran; this specific boat is handy because we can rig her to yaw anywhere between 15 to 120 degrees by adjusting bridle and windage factors (see PS November 2013).

Observations

We used 30 degrees as our baseline for measuring degradation, because this is the minimum we observed. Additionally, yawing less than 30 degrees typically has no significant effect on holding capacity. Even a very well mannered boat will yaw 15-25 degrees and the wind will oscillate another 10-20 degrees. These are not strictly additive, since swings and shifts do not always coincide.

Initially, the shank provides as much as 1/3 of the resistance to rotation by acting as a vertical fluke. When first set, the shank is buried in compacted earth. Unfortunately, after just a few yaw cycles, the shank finds itself in tilled earth with no ability to resist rotation. Combined with liquefaction of the soil around and in front of the fluke, the force required to cause rotation and walking drops 30 to 60 percent after 2-3 cycles. Thus, the main advantage of a deeply set anchor is initial stability. However, once shuffling starts the advantage goes away, since the anchor rises to the surface and will never set deeply again.

Setting history matters. If the anchor is deeply set it will feel the yawing less.  A good part of the rode may be underground. The anchor is in firmer substrate; even in loose, non-cohesive sand, the weight of overlying layers increases shear strength. Just as importantly, the shank is now under ground, and if it is broad, that area will directly oppose any force attempting to rotate the anchor. Thus, a deeply set anchor will be largely unaffected by moderate yawing, while an anchor that was lightly set will immediately begin walking about. However, if the deeply set anchor is loosened by a few gusts that coincide with a yaw, it will then walk just as quickly as if it were more lightly set.

Short scope is murder. We ran just a few tests at 5:1 scope with no chain. Because the shank is lifted, soil resistance against the shank is low and the lift-and twist motion turns out to be a great way to dislodge an anchor. Holding capacities became laughably low, as little as 5 percent of full scope straight line capacity. We were able to shuffle anchors with one hand that would take a winch or a team of strong men to pull in a straight line. We did not continue this line of testing, because with chain or reasonable scope this won’t happen. But it is something to remember.

During testing for a future report, we observed that anchors increase holding capacity over time, particularly if provided with resting periods of 20 minutes or more between hard pulls. The obvious a corollary is that an anchor that moves every few minutes is resting in weak, liquefied, churned up soil.

When we moved to on-boat testing we hoped that the 2-pound anchors would hold well enough to generate some data, providing insight into storm behavior. In fact, when allowed to yaw they tripped before we could generate data or get in the water to observe the motion. Even in good sand and 10- to 15-knot winds, when the anchor holding capacity should be 5-10 times greater than rode tension, the 60 degree yaw would pluck the anchor out of the bottom before the pattern was fully established.  We switched to 12-pound Mantus and Northill anchors, and a Guardian G-7. All of them held easily when yawing was limited to 30 degrees. Prior testing and rode tension readings suggest they should have held to over 60 knots. They all withstood 60 degree yawing up to 20 knots in good sand without releasing, although there was some evidence of the Mantus walking and the Northill and Guardian were shifting.  All of them failed in soft mud. We then increased the yaw to 120 degrees. To our surprise, they all failed within 30 minutes even in good sand. Failure sometimes occurred after we had finished our measurements, but before we moved on to the next test.

We then repeated the 12-pound on-boat tests after a hard power set (about 120 pounds in sand, 60 pounds in soft mud for the Mantus and Northill and 120 pounds for Guardian). This is about 10 pounds per pound of anchor, perhaps representative of typical power setting practices using the limited thrust typically available to sailboats. In soft mud power setting was reduced to the threshold of dragging.  In light winds the anchors held at all yawing angles. Diving revealed they were not moving because the chain was restrained by burial in the mud. But when the wind increased to 20 knots, the chain quickly carved a path of disturbed soil down to the anchor. In both sand and soft mud, both Northill and Mantus failed when yawing increased to 120 degrees. In both cases the Guardian stayed put. In fact, it was very difficult to recover.

The Chain Conundrum

As a lifelong multihull sailor I’ve always focused on minimizing weight, and ground tackle is an obvious target. Multihulls don’t yaw much when rigged with a proper bridle and the nylon stretches to absorb wave impact. Simple. We like chain’s chafe and veer resistance, so we use high grades and carrying no more than we need. But as we dug into the effect of yawing on holding capacity, we begin to understand the legendary advantage of heavy chain.

In the opening paragraph I related a number of dragging failures related to yawing. In addition to poor balance, the other factor they shared was rope rode with minimal chain. Chain damps yawing. Sure, once you reach storm levels it will probably lift from the bottom, but until then, even a poorly balanced boat will sit still, or at least the chain near the anchor will.

Chain also reduces the bottom angle. If enough scope is deployed—and this can take a good bit in shallow water and strong winds (See “Short Scope Anchor Test,” PS November 2017)—a few links will probably stay on the bottom. Even as it lifts, it will remain very near, not lifting the shank. While this measurably increases holding capacity in a straight pull, it can double holding capacity when the boat begins to yaw.

A kellet on rope acts a bit like chain, but as soon as it lifts yawing can start and the rode will angle upwards. It’s just not as effective as heavy chain.

So the real advantage of chain, in addition to chafe resistance, is holding the shank steady, even in the face of moderate yawing, and that’s a good use of weight.  But once the chain lifts, much of this advantage is lost.

Yawing and Anchor Holding
PS testers set up 100 feet 5/16 – inch chain at 5:1 scope in a bench test designed to determine the amount of energy the catenary absorbed under load.

Conclusions.

We’re going to avoid anchor-specific conclusions. There are so many variables, the most we can accurately say is that when shuffling is considered, anchor holding with active yawing is extremely unpredictable. What holds one time might pop right out the next, if the anchor walks into a bad spot.

We confirmed the real reason chain matters. It holds the shank still and keeps the shank down in the soil so it can resist anchor rotation. We’d long noticed that a flat shank could serve as a vertical fluke and we finally quantified it. But don’t think of chain as a solution to yawing problems; it’s a band-aid that will fail in strong winds.

If your boat is yawing less than 30 degrees, don’t worry about it. When yawing approaches 60 degrees in a breeze, which seems to be the average of interviewed boat owners in windy conditions, rode tension increased by 30-60 percent, anchor holding has is decreased by 0-55 percent, and your safety factor is reduced by 20-64 percent, depending on bottom, rode, and anchor type. If you can reduce yawing to less than 30 degrees with a riding sail, bridle, or other adjustments, that’s like doubling the size of your anchor.

If the boat is yawing 120 degrees—not that unusual among boats with rope rode—rode tension is increased by 100 to 200 percent, anchor holding capacity is reduced by 20 to 85 percent, and your safety factor is reduced by 72 to 92 percent, which coincides with the failures we have repeatedly observed. It’s impractical to carry an anchor large enough to hold such an unruly boat in all conditions, so you really need to do something about it.  You shouldn’t be sleeping.

Read back through PS articles on yawing reduction. Reduce windage on the bow and add a riding sail or use a second anchor to settle her down before she takes off and hurts somebody. Your fellow cruisers sharing the anchorage, will thank you.

Related posts: Deep Anchors Stay Put in Moderate Yawing

Taking Our Test Anchors Out for a Wiggle-walk

 

Drew Frye, Practical Sailor’s technical editor, has used his background in chemistry and engineering to help guide Practical Sailor toward some of the most important topics covered during the past 10 years. His in-depth reporting on everything from anchors to safety tethers to fuel additives have netted multiple awards from Boating Writers International. With more than three decades of experience as a refinery engineer and a sailor, he has a knack for discovering money-saving “home-brew” products or “hacks” that make boating affordable for almost anyone. He has conducted dozens of tests for Practical Sailor and published over 200 articles on sailing equipment. His rigorous testing has prompted the improvement and introduction of several marine products that might not exist without his input. His book “Rigging Modern Anchors” has won wide praise for introducing the use of modern materials and novel techniques to solve an array of anchoring challenges. 

9 COMMENTS

  1. Drew:

    I enjoyed your article on yawing at anchor. As a long-time owner of a Nonsuch 36 sailboat, yawing at anchor is the story of my nautical life. My sailing experience has taught me that no anchor nor anchoring scheme is wholly immune to dragging or break-out — it’s simply a matter of when, not if, the wind and sea state prove too severe.

    However, I have evolved a set of gear that, so far at least, has mitigated the risk of failure at anchor. I’d be keenly interested in your opinion and thoughts on ways by which I can strengthen my set-up.

    Our primary anchor is a 44# Bruce. From the Bruce’s crown runs 5’ of 5/16” chain to a 9# Rocna Vulcan lead anchor for a tandem anchoring arrangement. From the Bruce connected to an oversized swivel runs 100’ of 5/16” chain spliced to 150’ of rode. That’s it, except we also have a 30# kellet, which is frequently deployed.

    Whatever the anchoring conditions, we always set both anchors with the entire length of chain. Also, we deploy sufficient rode to provide a measure of elasticity and promote the chain remaining on the seabed. The kellet, we’ve found to be helpful but a bit ancillary in holding the chain in position.

    What refinements would you suggest be made to our set-up? All comments and advice are welcome.

    Richard Fried

  2. Richard Freid:

    Tandem Anchors. I invite you to read “Tandem Anchoring” in the July 2016 issue of Practical Sailor. We covered some of this.

    By this time, every anchor manufacturer that I am aware of discourages tandem anchoring and so does Practical Sailor. We tested this many times, on many bottoms, with many anchor types, and the result was always reduced holding capacity, vs. simply using the bower anchor. The reason is that the secondary (in your case the Vulcan) interferes with the ability of the primary (Bruce) to set and reset properly. The pull of the secondary rode is far above the center of effort of the primary’s fluke, and so if the wind direction changes more than 15 degrees, the Bruce will be rolled out and pinned on its back. The back tension from the secondary rode will also prevent the primary (Bruce) from ever resetting once it rolls out. It will just slide along on its back, like a turtle. Additionally, the back tension from the secondary anchor (Vulcan) rode will prevent the primary from setting properly; certain web sites show tandem anchors setting, but if you look closely you will see that in fact, the primary is barely set at all. They also never show what happens when the wind shifts 15 degrees. Finally, the eye on the Bruce is actually a tripping eye, not a tandem eye, and is neither in the proper location nor strong enough for anchoring loads.

    The only time tandem anchors work is on bottoms where the anchor cannot set (rocks, shingle, and hardpan). You will read of a few high latitude sailors, for example, that have found the method useful when anchoring on shale or cobbles. However, it does not help as much as moving to more suitable ground. Additionally, 5-feet is too short for the secondary rode; 20 feet of 1/4-inch grade 43 (weight does not help in the secondary rode) is more appropriate. This allows the anchors to move more separately and helps move the secondary out of the primary’s wake. But again, this is a special situation method and is not recommended for general use.

    A better use for the Vulcan would be as a hammerlock moor. It is the right size and type for this application. It could also be used as a V-tandem, which is described in “Tandem Anchoring.”

    Chain Size. According to ABYC H-40 (Anchoring, Mooring, and Strong Points) 5/16-inch chain is correct for your boat.

    Kellet. I invite you to read “Accessing the Anchor Kellet,” Practical Sailor 2019.

    You really don’t need it and it really does not help much. The simplest way to look at it is that a kellet weighs about the same amount as 15-20 feet of chain, is less effective in most ways, and harder to handle. The only time we recommend kellets is for mostly-rope rodes, where it can help the boat to swing in unison with boats on chain rodes and prevent keel wraps. Even then, we recommend a loop of chain as a kellet, because this can be recovered over the rollers with the rode.

    One final bit of advise is to NEVER deploy only 1-30 feet of rope. You should either anchor on all-chain with a snubber or deploy more than 30 feet of rope. If say, only 10 feet of rope is deployed, that is not enough rope to absorb the snubbing energy, and if it fails (unlike a snubber, which is backed-up by chain) all is lost. Rodes have snapped in this case, when a steep chop came up. If there is more than 30 feet of rope out, there is more length of rope to stretch, and the rope will not be overworked.

    My book, “Rigging Modern Anchors,” is available through Amazon, and covers this and many other anchoring methods is exhaustive detail.

  3. The key to stopping yaw is a riding sail. My boat is a 1998 Hunter 410 monohull which has a stand up head at the bow. The resulting huge freeboard at the bow makes the boat hunt at anchor. Deploying a small, homemade riding sail cuts the yaw by half or more.

    My anchor rode compromise is two boat lengths (80 feet) of chain spliced to a couple hundred feet of three strand nylon. For most anchorages, it is effectively an all chain rode while avoiding the weight penalty of an actual all chain rode.

    https://www.practical-sailor.com/sails-rigging-deckgear/riding-sails-to-tame-those-anchor-dancing-boats

  4. I would rather look for methods to reduce yawing such as anchoring backwards or towing a bucket over the stern. Our boat has a terrible tendency to yaw – friends with the same type of boat more then once had their anchor line wrapped around the keel as a result of yawing. For that reason I only use full chain.

  5. I appreciate your wordy effort to explain “at the end of the day” chain is better. I’m sorry to report, but your heavy use of poor examples of percentages really discredits what you’re trying to say – their margins were so wide, all but useless.
    Experience has taught me just what my RYA captain friends tell me, the anchor is to hold the chain in place, provided the correct scope foe conditions, not the other way.
    Having a fair amount of cruising under my belt and a 100t Master ticket, a proven anchor of the appropriate weight and all chain rode has yet to fail me. I was a Bruce advocate for many years until I picked up a broken chunk of sandstone from the sea floor in a Mistral in St Tropez. Since I’ve not once had a problem with my Manson.

  6. Tony: The data supporting the narrative is in another article in the same issue, “Deep Anchors Stay Put in Moderate Yawing.” This may have caused your confusion. In the print version they were next to each other. Yes, the data is variable, because an anchor’s interaction with the soil varies that much.

    The point of the article is that yawing substantially reduces the holding capacity of the anchor. In lighter weather, chain can damp this out, but controlling yawing to less than 30 degrees is key. I think this is well supported by experience with both yachts and ships.

  7. I am thinking that a wind directional shift will pull the chain and rotate the anchor so that it will need to re-set itself…or not. Is that the concept of a “yawing” motion around a vertical line at the location of the anchor itself? Or is it the rapid or constant “yawing” of the boat around it’s own mast? I want to see it as a “greater that 30 degree wind shift” that will cause the anchor to un-seat and not re-set. An anchor riding sail would probably just keep the boat “into the wind” but not prevent lateral pull on the anchor when there is a significant wind directional change. Clarification on location of the “yaw” please.

  8. I’ve read that a small triangle of sailcloth on the aft stay can prevent the boat from “tacking” back and forth at anchor (that is ‘yaw’, right?) and keep the boat pointed the same direction relative to wind. Does such a sail make a boat point enough off the wind to create excessive additional windage force? Is there a downside to such a sail?
    (My boat is a 23′ swing keel trailer sailer – which begs another question: anchor keel up or down?)

  9. Scott: The article studied the rhythmic back-and-forth motion caused by the imbalance between windage and underwater profile, or short period fluctuations in wind direction, and the reduction in holding capacity that results when an anchor is pulled from side to side. The soil does not get a chance to consolidate around the fluke and the anchor slowly inches forward under less force than if the anchor could rest more quietly. This is different from rotation and resetting resulting from either a persistent shift in wind direction or a tide change.

    Bud: Practical Sailor reviewed riding sails and their relative effectiveness in “Ride Easy with a Riding Sail,” August 2019. Our conclusion was that a simple triangle, such as you describe can work, but that there are also more effective designs.

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