Yawing and Anchor Holding

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.

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.

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