One fluke or two?

Question: I have two questions about anchoring. My husband and I just returned from a 12-day bareboat charter of a Beneteau 351 in the British Virgin Islands, where it is possible, due to the clarity of the water, to observe the position of an anchor set while snorkeling.

Let me first describe how we set an anchor. For example, in 18 feet of water we would come up into the wind, drift to a stop, then lower the 35-pound CQR anchor until it reached the bottom (typically of sand) and appeared to dig in. We would then play out the chain rode in 10-foot increments, pausing to assure that the anchor remained buried, until we had reached a 4:1 ratio of scope.

Then we would back down upon the anchor at 1,800 rpm for 30 seconds to test the strength of the anchor set. Finally, my husband would go over the side and snorkel down the anchor to check its set. Often, he discovered that only one of the two anchor flukes was buried. Despite several attempts at redeploying the anchor, we were rarely able to set the anchor with both flukes buried. Nevertheless, the anchor sets typically held throughout the night in the 20- to 25-knot trade winds. The fact that the anchor did not have both flukes buried, however, led to considerable anxiety, which we assuaged by checking the boat’s position every two hours throughout the night.

Our first question is whether both flukes need to be buried for an anchor set to be considered reliable. A related question is how to check the anchor set reliably when the water is too murky to see the anchor.

Our second question regards the appropriate anchor type for use on the following bottoms: a) sandy bottoms covered with grass, b) sandy bottoms covered with small boulders or dead coral heads, and c) hard shelf bottoms with a light covering of sand. Mardah WeinfieldHighland Park, IllinoisAnswer: It is good to see someone with a healthy curiosity about anchoring, and you raise some interesting points. I have had similar experiences. Diving on my anchor after anchoring two weeks in 20- to 25-knot trades I found the CQR still on its side.

Given your boat description, I suspect that setting the anchor at 1,800 rpm would load the anchor less than 400 pounds. This load is consistent with the anchoring results you have experienced with your 35-pound CQR.

Both flukes do not need to be buried to produce a reliable set, as long as the rode leads cleanly away from the anchor, the pull on the anchor remains horizontal, and the strength of the bottom is sufficient.

The anchor will continue to dig in until it is completely buried. The deeper it digs, the stronger it gets. This means that, if the load increases beyond the setting load, it is possible for the anchor to increase its holding power. A key point here is that the pull on the anchor must be horizontal, which depends on the size of the rode, the scope, and the pull. This is why, when wind velocity increases, adding scope boosts the holding power of the anchor.

It is possible that during the setting you describe in your letter you were actually pulling upward on the anchor because of the short scope, which would impede digging. I suggest you change your setting procedure to use as much scope as you can and slowly increase the engine power in steps to maximum rpm in reverse. The anchor doesn’t know, or care, whether it is wind or engine that is loading the rode.

This setting procedure not only sets the anchor, but actually tests the strength of the bottom. Which answers your second question by providing a good way to check the set of the anchor in any conditions. If the anchor holds under these new setting conditions, making reasonable assumptions for prop size and engine power, I would expect the anchor to hold the boat you described in winds in excess of 40 knots with some caveats on dynamic conditions. Therefore, you can sleep soundly until the wind gets above 40 knots.

By definition, an appropriate anchor type for the various bottoms you list is in reality any anchor that holds during a proper set. Anchoring can be a process of trial and error during setting. Maybe the question is really, “Which anchor should I try first to reduce the number of trials?” For sandy bottoms covered with grass, I think the best bet might be a CQR because of its weight. In sandy bottoms covered with small boulders, I might try a Bruce first. Hard shelf bottoms with a light covering of sand are not the best conditions for any of the typical lightweight digging anchors. I suspect a fisherman-type anchor with its high penetration angle and small flukes might provide the best chance of holding under these conditions.

By Ocean Navigator