In the process of our ongoing investigation on the effect of time and wind gusts on anchor setting and holding capacity, we performed limited testing at short scope and couldnt help but notice that holding capacity at short scope varied greatly. When recovering the test anchors, some anchors would lift out of the bottom while we were pulling the dinghy over to them, while others required heroic efforts even when the rode was nearly vertical.
Before diving into this report, it is worth reviewing our definition of scope. Although scope can refer to the length of rode paid out, it is usually expressed as a ratio of the length of the rode (from stemhead) to the bottom depth. For example, one hundred feet of scope in ten feet of water is a 10:1 scope .
When sailors talk about scope ratios, however, we are really most concerned about the lead angle, the angle between the rode and the bottom when load is applied. Most small boat perform better when the angle of pull is no greater than 8 degrees-the same as 7:1 scope ratio with a bar tight rode.
During initial setting, when the angle of pull can determine how deep the anchor buries, this angle should be much smaller, closer to zero degrees (see An Inquiry into Anchor Angles, PS February 2017). Generally with small boat anchors, any angle greater than 12 degrees will greatly reduce holding power.
Achieving the ideal angle of pull in normal anchoring conditions depends on the material in the rode as well as the amount of scope. For example, in moderate conditions, an all nylon rode with a 10:1 scope, a nylon-chain-rode with a 7:1 scope, and an all chain rode with a 4:1 scope will all generate about the same angle of pull on the same boat, because the catenary (sag) in the chain reduces the angle of pull. An all-chain rode is the easiest way to get maximum efficiency from our anchors, but there are drawbacks. Chain rode is heavy, hard to retrieve, and adds nothing to the shock absorbing of the catenary when it is sitting on the bottom. In a storm, when the chain can be bar-tight, it absorbs no shock-thus the importance of properly sized snubber (What is the Ideal Snubber Size PS March 2016).
Because many new anchors on the market tout their ability in short scope, we wanted to test some of these claims. Were certain designs truly better suited for short scope? Should cruising sailors start heeding the short-scope examples in their flashy promo videos?
What We Tested
Our testing involved multiple pulls with six different anchors, with the amount of scope ranging from 3:1 to 20:1. All the tests took place in the same general area in two locations, one with fine sand and one with soft mud in the same area.
Building on our prior testing history with these anchors, we tested a 2-pound Mantus dinghy anchor, a 2-pound Lewmar claw, a 2-pound Fortress Guardian, and a 3.5-pound Manson Supreme. We also collected published data on anchors ranging from 45-2,000 pounds, all of which exhibited very similar trends.
Because we wanted to better control the lead angle, we used polyester double braid rode with no chain leader. For more details on the test protocol, see the adjacent article Anchor Scope: How We Tested.
Because the holding capacity of anchors depends on design dimensions, bottom profile, and soil type, we normalized the data as a percentage of the 20:1 scope holding capacity baseline.
We also included data from published literature, including data from yacht anchor manufacturers data based on multi-ton ship anchors.
Interestingly, we found that some design and styles of anchors clearly excel over others at short scope-at least in mud and soft sand.
When compared using this method, Danforth-style anchors have very high holding capacity per weight, new generation scoop anchors (Spade, Manson, Mantus, Rocna) have intermediate holding, and claw (Bruce-type) and plow anchors (Delta, CQR, etc.) somewhat less, although this varies with the specific design and soil type.
However, the holding ability of all the anchors in our test goes down sharply at short scopes that result in lead angles greater than 8 degrees. In other words, the best way to ensure your anchor remains set is to allow plenty of scope, maintain an acute lead angle of less than 8 degrees.
Unsurprisingly, the Danforth style fluke anchor (Guardian), which has a relatively high fluke area (desirable for the soft bottom at our test site), retained most of its holding capacity even at high angles of pull. The 2-pound Guardian remained buried up to the maximum load, and every trial required using the dinghy engine to recover it for the next trial. This agrees with the common difficulty of recovering very deeply set Danforth style anchors. It also makes it clear that you often have to choose between good short-scope performance and easy recovery; you can’t have both.
Scoop anchors provided variable holding, although with the limited sample size and weight differences, we can’t make a fair comparisons between the types. Based on our experiences, we suspect that the larger anchors should yield better performance, but we found it interesting that multi-ton stockless anchors follow the same general trend.
Although claw anchors don’t always set deeply or even dig in completely, they always keep one tine in the bottom, no matter how short the scope. Many cruisers are happy with them so long as they are very conservatively sized. They also work well in hammer lock moorings (in which an a second anchor is deliberately set on very short scope to prevent the boat from sailing at anchor) because they create consistent drag.
We did not test any anchor beyond 3:1 actual scope because holding became very erratic and resetting unpredictable. In firm sand our test anchors might set well at shorter scope, but not reliably. There is also no reason for a cruising sailor to ever anchor with less than 3:1 actual scope at the bottom.
While testing snubbers (Snubber Advisor, PS March 2016 online) we developed a database of wind speed vs. rode tension for our test boat, and with some extrapolation, boats of all sizes. Interestingly, total chain mass doesn’t play a huge role in achieving the ideal lead angle. As long as the chain rode is sized to match the boat (using the anchor manufacturers guidelines), and the grade of chain remains the same, the shape of the catenary curve (sag) at a given wind speed is relatively constant for boats of all sizes.
We looked at grade 43 and grade 30 BBB not because there is a difference in strength, but because the BBB chain would typically be heavier if sized for the same boat. If you have even heavier chain, it will have deeper catenary. According to chain manufacturers we spoke with BBB is becoming harder to find because it is uncommon outside the United States. The same applies to imperial size chain. Although some imperial size gypsies will work with metric chains, if you plan on long-term cruising and expect to replace your chain in a foreign port someday, a metric size gypsy will make sourcing easier.
For this test we assumed the anchored boat would use the correctly matched snubber as specified in our (What is the Ideal Snubber Size? PS March 2016). If you use only a 6-foot snubber, particularly in shallower water, the loads will be much higher and the chain will lift sooner and higher.
While testing in-line tandems (see Tandem Anchoring, PS August 2016) we determined the amount of wind required to lift the first link of the chain off the bottom. While lifting the first link doesn’t mean a well-buried anchor is going to drag-not by a long shot-it is important to the security of in-line tandems, which are more prone to dragging at lower angles of pull (see Doubling Up: Full Size Tandem Anchoring September, PS September 2016).
Lead Angle and Holding
What happens when the rode lifts off the bottom? First, while the chain was lying on the bottom, the boats side-to-side yawing was buffered by the chain dragging on the bottom. Although the chain slides rather easily across sand, in soft mud the chain sinks in and does not move easily. Second, the anchor will begin to feel jerks through the rode more directly. Our work on anchor setting has shown that continuous minor bumps (like waves and minor gusts) prevent soil consolidation and weaken the anchors grip by as much as 30 percent. Keeping your boat lying still at anchor is critical.
Initially, when the first link lifts the lifted chain is still near the bottom and the lead angle is still quite acute. Using G43 chain at 5:1 scope in 7 feet of water, the rode begins to lift at 10-15 knots, but the effective scope (the lead angle) at the bottom remains better than 10:1 (about 6-degree lead angle) until nearly 25 knots. However, if a 60-knot downburst hits, effective scope will drop to 6:1 (about 10 degrees lead angle), anchor holding is cut by about 50 to 75 percent, and dragging is a very real possibility in many seabeds. At shorter scope, the result would be disastrous. If we had used heavier BBB chain, the point at which the chain lifted off the seabed would have been delayed by a few knots, but the result in the 60-knot downburst would be the same.
What about deeper water? In 17 feet of water and 10:1 scope BBB chain doesn’t lift appreciably until over 30 knots and scope remains better than 7:1 (about 8 degrees lead angle) at 60 knots. That 60-knot downburst is still a risk; our anchor is now feeling the jerks very directly and holding is probably reduced by over 65 percent when soil disturbance is included. When we increase the depth to 50 feet, even light chain does a pretty good job of maintaining an acute lead angle, and BBB is scarcely required; better to save weight and carry a longer chain, or switch to fiber after the first 200 feet or so.
Although all of the rode combinations show diminished holding capacity at short scope and strong winds, does this always translate into reduced anchor holding capacity? Some anchors remain tenacious at 7:1 actual scope, but by 5:1 are quite pitiful. These may remain secure in good sand, but they are still only a shadow of their former selves. In soft mud, you will likely find yourself dragging. This sensitivity to the exact scenario (seabed, chain type, anchor type and size, scope, and weather) makes it obvious why a group of sailors can never reach a consensus; a 2-pound anchor that can hold out test boat in moderate weather in fine sand with a chain rode wouldnt even measurably slow its drift over soft mud with a fiber rode.
Anchor holding capacity begins to drop rapidly below 7:1 actual scope (greater than an 8-degree lead angle) at the seabed. Fortunately, a chain rode or even chain leader significantly increases the effective scope by reducing the lead angle at the seabed, particularly in deeper water. However, if the scope is too short and the wind is strong, the anchor will eventually be loaded at higher angles that will reduce its holding.
In our ongoing article on the effect of wave action on anchor holding, we learned that holding capacity also gradually drops when pulsing of the wind and waves can jiggle the anchor at short scope. As a result, when you anchor at short scope in water less than about 20 feet deep, you are performing a delicate balancing act; one moment youre fine but then the gust of wind at the leading edge of a squall hits, the rode pulls tight, and youre off to the races. Add variables such as hobby horsing when the chop builds and soil type, and short scope holding becomes uncertain.
Ease of recovery and short scope holding will always be at odds with each other; you can’t have it both ways. The modern scoop anchors are great for everyday use, but when anchoring for unpredictable weather, other anchors match better with different bottoms. For example, a Danforth-style anchor will hold better in soft mud, but it is poor at resetting, so if the wind veers sharply, youre in trouble.
What is the correct size for an anchor? Is it the one that will hold in good sand at long scope, in fair weather, or the one that holds in soft mud and short scope at 60 knots? Perhaps the oversized anchor we teased our dock mate about, believing the selection was based more on paranoia than engineering sense, is just right for the way he uses it.
What is the best chain? If you have a heavier boat and frequently anchor 15- to 30-foot deep, crowded harbors, good old BBB requires less scope for equivalent rode angle. If you have a lighter boat, anchor in shallow water, don’t worry about crowds and swing room, or anchor in deep water where carrying enough BBB is impractical, higher grade chain is better.
What is certain is that the 35-pound new generation anchor that will hold over 2,000 pounds in good sand at 10:1 scope will only hold a few hundred pounds in soft mud at 5:1 scope in shallow water. This is something to think about before you believe all the hype about short-scope holding ability.
As we pointed out in our previous articles on this topic, the short-scope claims are primarily aimed at daytime powerboaters who are looking to anchor relatively light boats in crowded sandbar gathering spots, or over prized fishing holes. In our view, the cruising sailor should take these claims with a grain of salt, focusing instead on adequate scope a good set that can help you rest easy through the night.