Anchor Line: Six Degrees of Separation (Web Extra)

Author:
Publish date:
Dangling

Seamanship contributors Rudy and Jill Sechez have some strong opinions about anchoring. Here are some additional arguments for their more-is-better approach.

SCOPE

Even though some authors suggest that any scope in excess of 10:1 is unnecessary because the change in the rode-to-bottom angle is so insignificant, we disagree. While this may be true when only the geometry of the rode is considered, there are other aspects to this issue that need to be considered.

If the height of the tide, the height of the boat’s free-board or the depth of the water was misjudged, or during harsh conditions, in which the height of the storm surge, the height of the waves and seas and the depth that the anchor may have buried had not been factored into the equation, the scope, having been miscalculated, may not be at 10:1 and, in fact, may be significantly less.

In conditions such as this, where these heights may be misjudged or unaccounted for and, regardless of where the rode or snubber may be attached to the boat, a scope of 15:1 or more may prove to be the scope that is needed to keep the rode-to-bottom angle at or below that critical six-degree point. Veering out a little extra scope is not only an acceptable move, it is a wise move. As with the hard components in the ground tackle, scope, too, should have a “safety factor”.

KELLETS

Although a kellet is a good mechanism for reducing the boat’s swing room when the conditions are mild, as the wind picks up, even with a kellet installed, the catenary will still disappear. Granted, the catenary will disappear at a slightly higher wind speed, say 43 knots instead of 40, but disappear it will, leaving the anchor dependent on that all critical rode-to-bottom angle.

So, as the wind becomes strong enough to start raising the rode off the bottom, a kellet, unless it is so large that it is not likely to even be carried on the boat, is not a substitute for enough scope. The only way to keep the rode-to-bottom angle to 6 degrees or less in severe conditions; with or without a kellet, is with a scope of 10:1 or more.

MULTIPLE ANCHORS

There are some anchors or circumstances that require a short scope to enable the anchor to set, or re-set should it trip. Ironically, this short scope can also cause the anchor to trip. When this is the case, it is kind of a catch 22, because if the boat was laying to a scope that is long enough to maximize the anchor’s holding power, should the anchor trip, it probably won’t then re-set on its own, due to the length of scope deployed.

This dilemma can be resolved, and setting out multiple anchors is often the only way to resolve it. Each, once set, with adequate scope to maximize the anchor’s holding power (which is a scope of 10:1, giving a 6 degree or less rode-to-bottom angle). This amount of scope, plus using a pattern that eliminates any side loading on any one anchor, eliminates or at least minimizes the likelihood of any one anchor tripping.

This article is a Web Extra for the July/August Seamanship article "Anchor Math and Management: Part 1."

Related