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Matters of Weight

These are the carefully considered design elements of an effective anchoring arrangement
Calm conditions allow for a shorter anchor scope.

Calm conditions allow for a shorter anchor scope.

Tether something heavy to your boat and throw it over the side. That is the essence of anchoring. And yet, given the array of anchors that exist, not to mention the breathtaking quantity of research, thousands of years of experience and countless sailors with opinions on the matter, there is obviously more to anchoring than that.

Captain Daniel S Parrott photo

Capt Daniel S. Parrott

An anchor does not hold by weight alone, nor is the function of the rode solely to connect the anchor to the boat. Collectively, the anchor, rode and associated hardware are known as ground tackle. Since we cannot see ground tackle at work on the seafloor, it’s important to visualize what is going on down there, how it all works and why it sometimes doesn’t.

Most anchors have flukes. They provide the broad, flat surface that generates resistance when embedded in the bottom. But if a broad, flat surface were the only requirement, then anchors would look like shovels. They would skip across the bottom, generating no holding power whatsoever. To overcome such futility, anchors have a stock or a tripping palm that points the flukes downward. The flukes themselves are tapered to penetrate the bottom.

The first effect of a strain coming onto the anchor is to straighten out the rode. Next, the anchor drags until the flukes get oriented downward. Further strain (hopefully) causes the flukes to bite into the bottom. Additional strain buries the flukes until the anchor is well set.

The strain that a boat places on an anchor has two components: horizontal (parallel to the seafloor) and vertical (perpendicular to the seafloor). The horizontal component is the good part. That is when embedded flukes provide maximum resistance to the strain coming from your boat. Like nails in a plank, they make it difficult to release the grip by pulling sideways.

Your anchor should be matched to your boat and your cruising grounds.

Your anchor should be matched to your boat and your cruising grounds.

But the rode eventually leads upward to the boat, producing a vertical component to the strain. Like drawing out a nail with a claw hammer, the vertical force will overcome the horizontal force, breaking the flukes out of the bottom—no matter how big the anchor. This is precisely what occurs each time you shorten up the rode to retrieve your anchor.

The obvious purpose of the rode is to connect the anchor to the boat. But the rode, especially chain rode, is also integral to the performance of the anchor itself. Instead of the line from the boat to the anchor being straight, the weight of the rode causes it to sag. This sag has a name: catenary. Catenary acts as a shock absorber between the boat and the anchor. It prevents the full force of the boat from being transmitted to the anchor below, while enhancing the horizontal force nearest to the anchor. Catenary is key in the towing industry for reducing the strain on a towing wire and the points of attachment.

As you can imagine, catenary rises and falls with puffs and lulls. The stronger the wind, the less catenary, and the greater chance that the vertical component of the strain will dislodge the flukes. In a perfect world, the chain nearest to the anchor would lay on the bottom all the time, but that is not always possible in strong weather.

Heavier chain produces more catenary than lighter chain. Vessels equipped only with fiber rode are at a disadvantage in this respect, although the chain leader, intended to prevent chafe on the bottom, does help to a degree. If you are committed to fiber rode for reasons of space and weight, then a longer and/or heavier chain leader likely will enhance the performance of your anchor.

Sometimes, people attach weights called kellets to the rode. Kellets assist in maintaining catenary, as well as the horizontal component of the strain. Some research suggests that kellets are not worth the trouble, but you can picture how the additional weight on the rode will help to keep it down.

On one occasion when a gale was forecasted, I shackled a secondary anchor to the rode and slid it down to the bottom on a separate line, like a tramcar on a wire. Mother Nature blew stink, but we did not drag. I’ll never know what role the secondary anchor played, but it could not have hurt. A further benefit was that this approach sidestepped the possibility of entangling two separate anchor rodes at a location where the tidal current reversed every six hours. In all cases, the best policy is to have a really great anchor, sized right for your boat and the waters you frequent.

Catenary is only part of the story when discussing ground tackle effectiveness. A bigger factor is scope. Scope is the ratio of the length of rode to the depth of the water, plus the distance from the waterline to the chock or the hawsepipe. A ratio of 1:1 scope means the anchor rode is equal to water depth, straight up and down, with the anchor just touching the bottom.

The American Merchant Seaman’s Manual recommends scope of 5:1, with even more in high winds or in a location where sea and swell add to the strain on the anchor. The greater the scope, the more horizontal strain there is on the flukes. With chain rode, greater scope also means more weight contributing to the catenary, obviating the need for kellets.

In general, I find that a 5:1 scope provides peace of mind up to 20 knots or so of wind. The success of that calculation depends on many things: the suitability of the anchor, the bottom type, the size of the chain, the sea state and the amount of windage (surface area) that the vessel presents, along with past experience in any given anchorage. People get away with less scope, and I have too, especially under mild conditions or with crew on deck keeping an eye on things.

It is remarkable to observe the carefully considered design elements that produce an effective anchor. If you are really curious to see how an anchor works, take a small one to the beach, tie a line to it and drag it along until the flukes bury. Better than anything, this will help you visualize what is going on down there when you drop the hook. Turns out, an anchor is much more than something heavy to pitch over the side. n

This article originally appeared in the January 2019 issue.


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