On The Hook: Anchor Math and Management Part 1 - PassageMaker

On The Hook: Anchor Math and Management Part 1

Believe it or not, there is much more to simply tossing your anchor overboard. Check in with cruising contributors, Rudy and Jill Sechez and discover part 1 of the anchor math equation.
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Anchoring sounds easy: Stop forward motion, lower the anchor, back off, set the anchor, let out enough scope, then belay the rode and maybe attach a snubber, too. But, if this is all there is to anchoring, why do so many crews have trouble setting their anchors and why do so many boats drag or break free? 

Before anchoring, even before buying ground tackle, you should determine what the load will be on the ground tackle. This data establishes what size and strength the various bits and pieces are that make up the ground tackle—the big pieces, as well as the smaller pieces. If the ground tackle is already onboard, this same data will give you a pretty darn good idea about your system’s breaking point, and whether you should switch to bigger gear. 

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The data is based on boat size, wind speed and the exposure that the boat will have to seas and waves. Although the data for calculating these loads is beyond the scope of this article, you can obtain the numbers through the American Boat and Yacht Council (ABYC), boat designers and naval architects, and books such as Earl Hinz’s The Complete Book of Anchoring and Mooring.

Whatever the source, this information should be kept aboard for references.

Sizing gear based only on boat size, anticipated wind speed and the exposure to seas and waves may still result in ground tackle that is inadequate. Other factors also need consideration—wind gusts, duration, current and the boat’s ability to freely oscillate.

Wind gusts: When calculating the load on ground tackle, it must be established whether it is sustained wind speed that is being used or maximum wind speed, the difference being wind gusts. The importance of wind gusts when sizing ground tackle lies in the fact that the gusts add significantly to the load. Wind force does not increase in linear fashion as the wind speed rises; it goes up exponentially. As the speed of the wind doubles, its force quadruples and so does the load on ground tackle. 

A wind gust can typically exceed the sustained wind speed by 30 percent, but these gusts pale, by comparison, to the less frequent, but stronger gusts, gusts that can exceed the sustained wind speed by 70 percent. Even worse, under the right circumstances, some gusts will even exceed 70 percent. What all this means is simply that ground tackle needs to be sized for the loads that the wind gusts produce, not the lighter loads produced by the sustained wind speed.

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As an example, for a 35-foot boat in 40-knot sustained winds, the load on its ground tackle will be around 900 lb. Gear sized for this wind speed may not hold in gusts that reach 50 knots, producing a load of approximately 1,200 lb. on the ground tackle. And of course it will be even less likely to hold in 70-knot gusts, which can put over 2,000 lb. of load on the ground tackle. And these figures do not even include surge loads!

Surge loads: The loads imposed on the ground tackle by seas and waves will increase the strain the ground tackle beyond that imposed by wind alone. When surge loads are involved, a boat anchored where there is moderate protection from seas can have twice the load on its ground tackle than from wind alone.

Granted, for those scientists among you, these estimates may seem imprecise, but when it comes to sizing ground tackle, they are plenty close enough and easy to use. This is a rule of thumb that we use for surge loads: In moderate protection from seas, double the wind load. In little or no protection from seas, double the wind load, then add 50 percent more.

Let’s expand on the example of a 35-foot boat in 40-knot winds that’s anchored in moderate protection from seas. The load on the ground tackle, when figuring in wind gusts and surge loads, would probably reach 4,000 lb. or more. No wonder some boats drag!

Surge loads should almost always be part of the calculation. Even in generally protected anchorages, clocking conditions can mean periods of exposure to waves that will make you wish you had upsized your anchor.

Duration, or the length of time that any set of conditions remain in effect, is also an important consideration in anchoring. Gear good enough for a 20-minute squall may be woefully inadequate for a storm of any duration, even though in both events the wind speeds are the same. 

There is no magic line delineating where the boundary is between short and long duration, except what common sense tells us. Since a long duration requires heftier gear, our rule of thumb in such cases is to upsize every component in the ground tackle at least one size from that which may be adequate for the same conditions for a shorter duration.

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Having the ability to pitch, roll, yaw, heave, sway and surge, as a boat will do when anchored to a single rode, allows it to present the least amount of resistance to wind and seas. With the boat anchored fore and aft, where it has lost much of this freedom, in particular the freedom to yaw into the wind, the load on the ground tackle can be as much as 50 percent greater when the wind comes on the boat’s beam.

So, the rule of thumb that we follow for sizing gear for a boat that will be anchored fore and aft, or tethered in any other manner as to reduce its ability to freely oscillate (this pertains to boats tied up in a slip, too), is to add 50 percent more to the calculations for the load on the ground tackle. 

Currents are often thought to contribute so little to the load on ground tackle that they can be ignored. An example that is presented in Earl Hinz’s book is that a 5-knot current acting on a 40-foot boat produces approximately 300 lb. of load on the ground tackle. Think about it. A 300-lb. load on the ground tackle of a 40-foot boat is equivalent to a 15-knot wind. 

This doesn’t sound like much, but should the ground tackle be marginal to begin with, an additional load like this could result in the boat dragging, tripping its anchor, or deforming or breaking gear. So, the rule of thumb that we follow, when considering currents is to upsize to the next level any anchor that is at the upper range of its holding power or any gear that is at its upper range of its Working Load Limit (WLL).

THE BIG FOUR

Most literature on anchoring focuses on the proper technique for setting an anchor, creating the illusion that this is all that there is to it. However, anchor problems usually stem from shortfalls of another kind. I’m referring to the “big four” of anchoring—size, shape, scope and strength. This installment of the anchoring series will deal only with scope. I’m saving the other three for the next issue. 

This is our basic rule for scope: 10:1 for all rope rodes, 7:1 for 1/2-rope and 1/2-chain rodes and 5:1 for all-chain rodes. These rules work because the rode near the anchor is lying on the bottom due to its catenary, creating, what we will call, a rode-to-bottom angle that is zero. But, again, these rules apply only in mild conditions.

Click Here to Watch The Fortress Anchor Tests in the Chesapeake

As the wind picks up, the rode’s catenary starts to get pulled out and as the catenary is pulled out, the rode-to-bottom angle increases. When the catenary is gone—something that usually occurs before the wind hits 40 knots or sooner for an all rope rode—the rode-to-bottom angle is six degrees with a scope of 10:1, nine degrees at 7:1 and 12 degrees at 5:1. An anchor with a rode-to-bottom angle of 12 degrees has approximately 50 percent less holding power than it does with a rode-to-bottom angle of 6 degrees.

As catenary is pulled out, less and less rode lies on the bottom, which means that the rode also has less and less friction with the ground. These two factors, a bigger rode-to-bottom angle and the rode having less friction with the bottom, create a double-whammy to undermine holding power.

When it comes to scope, our rule of thumb is that as the wind speed increases, the focus on these 10:1, 7:1 and 5:1 ratios need to be dispensed with. Instead, the focus needs to shift to keeping the rode-to-bottom angle small—6 degrees or less, the equivalent of 10:1. Since it is impossible in the real world of anchoring to know exactly when this transition should occur, it is best to err on the safe side and achieve a low rode-to-bottom angle before the wind really kicks up.

Anchor Vitals: Learn The Parts and Functions of your Anchor

Later, the crew may be unavailable to lengthen scope in a timely manner. Later, the crew may be off the boat, asleep or otherwise preoccupied and not aware of the weather. Think about what would happen if one of those quick-appearing, unexpected squalls or evening thunder(less) storms blow through the anchorage.

Short scopes may be okay in crowded anchorages—a common courtesy—but if the plan is to use a short scope, the boat must still be anchored in such a manner that will allow for more scope to be veered and a competent crew should remain onboard, alert for changing weather. Courtesy does not trump your responsibility for the safety of your vessel or crew.

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