Scope - PassageMaker

Scope

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In spite of the fact that the location had over 10 miles of fetch, it was our best option, so we anchored and waited. Then, as the hurricane approached, the wind steadily increased, making its way up through the Beaufort Scale and then into the Saffir-Simpson Scale. Although only a Category 2 hurricane, what made it so bad was that it stalled as it approached the coast, allowing it to pound the area—and us—with hurricane-force winds for 24 hours before moving on. Wow, what a ride that was!

Things worked out well for us; our anchors were big enough and their design was well suited to the type of bottom where we anchored. In addition, we made sure that all of the gear, anchors included, had more than enough strength. But, none of these factors would have mattered if we hadn’t also deployed enough scope.

THE NUTS AND BOLTS OF SCOPE

Scope, as defined in Chapman’s Piloting, Seamanship and Small Boat Handling, is “the ratio of the length of line in use to the distance to the bottom of the water as measured from the deck.” The theory behind the use of scope is simple: the longer the scope, the smaller the angle formed by the rode with the bottom—the rode-to-bottom angle—and the better the anchor will hold. Conversely, the shorter the scope, the larger the rode-to-bottom angle and the easier it is for the anchor to get pulled out of the bottom. This is the principle used to weigh anchor and just the opposite of what is wanted if you want to stay anchored.
The easiest, most practical way to get the rode-to-bottom angle smaller is to move the boat farther away from the anchor, and the farther the better. But, how far is far enough? “Far enough” is determined by the rode-to-bottom angle. For the best holding, the rode-to-bottom angle should be no more than 6 degrees, and smaller is better. The bigger this angle becomes, the greater the odds are that the anchor will trip. This point was observed in a British Royal Navy test in which the results revealed that when an anchor’s stock, compared to the bottom, had an angle of 10 degrees, the anchor lost 40 percent of its holding power, and at 15 degrees, the loss was 60 percent (Admiralty Manual of Seamanship, Vol.2).
Fortress Anchors’ literature states it another way: a scope of 10:1 gives an anchor twice the holding power of a scope of 5:1. Alain Hylas, designer of the Spade anchor, says on his website (www.spadeanchorusa.com) that a scope of 4:1 elicits only 55 percent of the anchor’s maximum holding power, while 10:1 elicits 85 percent, but it requires the rode to be laying horizontally on the bottom for an anchor to achieve 100 percent of its holding power. No matter how it is stated, the message should be clear—the more scope, the better the anchor will hold, and vice versa.
In mild weather, it is the catenary that allows the rode-to-bottom angle to remain low. Catenary is the sag in the rode caused by its own weight and because of the low angle it forms with the anchor, it is the basis for those long-used “rules-of-thumb” for scope—5:1 for all chain rodes, 7:1 for half-rope/half-chain rodes, and 10:1 for all rope rodes.
However, as the wind picks up the catenary starts to get pulled out and as the catenary becomes straighter, the rode-to-bottom angle becomes bigger; just the opposite of what the anchor needs to remain set. By the time the wind reaches 40 knots, all catenary has disappeared (sooner for an all-rope rode) and without a catenary, the rode with a scope of 5:1 has around a 12-degree rode-to-bottom angle; at 7:1, around a 9-degree angle; and at 10:1, around a 6-degree angle.
Even the use of a kellet or heavier chain does not stop the catenary from disappearing as the wind increases. Unlike in mild weather, when this extra weight can make a difference, in high winds, the difference in the rode-to-bottom angle that results from using a kellet (unless it is of a weight that would be quite unmanageable) is minimal, if any.
What all this means is that once the wind speed picks up, the rules of thumb become less effective and eventually, when the wind speed exceeds 20 knots, must be discarded. Instead, the focus should be directed toward achieving and keeping a low rode-to-bottom angle. The only way to do this is to let out more scope.
As simple as establishing scope seems to be, it is not as straightforward as it appears. We have seen many boats drag because of too little scope, but one memorable episode stands out in our minds. We were anchored in one of the well-protected coves that dot the Bahamian archipelago, waiting out a soon-to-come tropical depression, when another boat showed up. As we watched this solo young skipper lay out his two anchors, we were suspicious of the amount of rode he put out. Sure enough, when the wind reached about 30 knots, we noticed his boat dragging. He was having considerable difficulty retrieving his gear while simultaneously trying to keep his boat out of the shallows. Our offer to assist was met with an enthusiastic “yes,” and as we helped him recover his ground tackle, our suspicions were confirmed. Anchored in 10 feet of water, with 4 feet of freeboard, and 1.5 foot tides, his primary anchor—though big enough—had only about 50–60 feet of rode, while his second anchor had somewhat less. Since he had no other rope on board that could be used as rode, we combined his two separate rodes into one, and choosing a shallower location, got him re-anchored. This wasn’t ideal, but it was the best that could be done under the circumstances. Fortunately, in this protected cove, it was good enough.

DO THE MATH
To determine scope, the depth of water must be measured at the spot where the anchor is located, and nowhere else. Then, to this depth, add the distance from the water to the bow roller, hawsehole, or chock, plus any additional height that will be contributed by the tide. If conditions will become severe or be of a long duration, also include the depth that the anchor will become buried (a distance that can be considerable when the bottom is soft), the height of any storm surge, and because the boat will have to crest them, the height of the waves. Granted, a few of these factors may be difficult to ascertain accurately or they may seem insignificant, but they are all cumulative, which, if not allowed for, can result in a rode-to-bottom angle that exceeds the anchor’s ability to remain set.
Some literature suggests that any scope beyond 8:1 doesn’t offer much additional advantage when anchoring. While this may be true geometrically, a scope of 8:1 only has a rode-to-bottom angle of approximately 8–9 degrees and as conditions become harsh, this rode-to-bottom angle may prove inadequate. Keeping the rode-to-bottom angle at or below 6 degrees, in other words, 10:1, or even 15:1, pays many dividends such as better holding, having some built-in extra scope in case of miscalculations, and peace-of-mind. These reasons justify building in some tolerance when calculating scope—it is better to err with too much scope, rather than too little.
Knowing how much scope to deploy is only half of the equation; knowing how much rode has been deployed is the other half. To fulfill the second half of this equation, simply place marks of some kind on the rode at pre-determined intervals, say every 25 feet.
During mild weather, when it is reasonable to employ the rules of thumb for scope, it is important not to be fooled into complacency. In spite of how calm it may be, high winds, often in the form of squalls that can develop quickly and show up unannounced, will often blow through an anchorage. To avoid getting caught short anytime the crew may not be able to let out more rode in a timely manner, such as when they are asleep, away from the boat, or otherwise preoccupied, preemptively set scope at a length that would not threaten the anchor’s ability to remain set should a 50-knot squall arrive. To us, this means setting out a minimum of 10:1 scope, even when using all-chain rode.
Contrary to the idea of using long scopes, some anchor manufacturers infer that their anchors are effective on short scopes. While these claims may be true and in certain circumstances short scopes can work, their literature lacks mentioning the important factors that allow the short scope to be effective—information that is critical in evaluating when it may or may not be safe to do so in other circumstances. Without this information, instead of taking a chance on the uncertainty of using a short scope, a safer policy would be to use scopes that keep the rode-to-bottom angle at 6 degrees or less; angles that have withstood the test of time.
One frequently employed method of reducing the length of rode required while still maintaining an effective scope, is using a snubber that is attached to a stem fitting, which is usually located near the waterline. However, before this is attempted, a couple of issues need to be resolved. First, when side-loaded, as can occur when the boat veers, this fitting can break with only half the force that it can carry in straight-line pull, so this fitting must be twice as strong as it would need to be if it was only to carry loads in straight-line pull. Second, if the snubber is dead-ended to this fitting, which is way-down-there-out-of-reach, the process of releasing the snubber, should the rode need to be adjusted or slipped, has now become more time consuming, possibly even dangerous, and this extra time could prove critical.

MAKE THE ROOM
It is a common practice to question the use of scopes of 10:1 (or more) when room to deploy such length is in short supply, but it is Mother Nature that dictates how much scope may be necessary, not some crew-imposed limitation. Any “tricks” to keep the rode-to-bottom angle low have limits. Add to this that everyone has the legal obligation to avoid fouling another boat’s anchorage, as well as to position themselves in such a manner so as not to interfere with another boat’s ability to get under way if the other boat was anchored first. Fortunately, the manner in which situations like these should be dealt with has long been established, both legally and practically, and it is simple: If there isn’t enough room for adequate scope, then the boat should move to a location where there is. Where space is limited, shortening up so others may also be able to anchor might be the considerate thing to do, but this consideration does not extend to the point of putting the boat or the crew in danger. So beware of shortening up too much, as the need for a more lengthy scope could develop and the room to do so may no longer be available.
Even in deep water, where using scopes shorter than normal is a technique that has been proven successful through use, a mariner still has to be careful in unconditionally trusting his boat to this idea. The problems of defining the term “deep” and determining exactly how much “less” scope to use must be considered. Even more important is the question, “At what wind speed does this method lose its viability?” When we discussed this topic with offshore fishermen who routinely anchor in deep waters, they all overwhelmingly supported the use of more scope, rather than less, as the wind picks up. Apparently, this method of employing shorter scopes when in deep water is one that is appropriate for mild conditions and suspect when conditions worsen.
When it is all said and done, anchoring does not have to be an esoteric art. Hard, dirty, and at times, wet work maybe, but simple in its application—a big enough anchor of the right design for the type of bottom, adequate strength in all the components, and sufficient scope, all deployed in plenty of time and chosen to exceed that which Mother Nature dishes out.

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