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The Rule of 60

As the author discovered, even the most pleasant days at sea can offer trouble if you let your guard down.
On one of those pleasant days whose every attribute can be described as "moderate," we were happily reaching along at six knots under an almost warm North Atlantic sun, making our long, gradual curve down the Great Circle to the Azores. In the fine visibility we noticed a few big ships ahead and astern, slowly working their way along a more northerly track toward Europe. I was at the helm, and as far as I was concerned these vessels were as dangerous as bugs buzzing outside mosquito netting—noticeable but unthreatening.

The closest at the time was a tanker a couple of miles off our starboard quarter, slowly overtaking us, or so it seemed. Without the usual telltale signs of high speed that we see in smaller vessels—the big bow wave, the depression along the topsides, the churned-up wake—the tanker seemed lethargic. With the self-assurance of a veteran of dodge-'em games with slow-moving tugboats on Long Island Sound, I decided that he would cross with plenty of room to spare in about 20 minutes. My attention returned to what appeared at the moment to be the more important business at hand—a philosophical debate about the Creation story in the Old Testament.

I checked my watch: 10 minutes had passed. Time for a progress report. Glancing to starboard, I saw no trace of the tanker. It wasn't astern, either. After some agitation and gymnastics, I eventually found it behind the jib less than a quarter mile away, thundering along with surprising velocity directly into our path. My reaction may not have qualified technically as blind panic. I had enough of my wits about me to shove the helm up hard, but I felt out of control. The shock remained long after we bobbled across the tanker's wake, which was much more agitated than it had looked a few minutes earlier through my far-too-optimistic eyes.

The "should haves" were obvious. In the apparent safety of the open ocean—far from the narrow confines of my home waters—I had violated the basic rule, "Get your eyes out of the bilge." The sailor's worst enemy is neither the weather nor the tides but distraction. Last month in this column I examined a few simple skills for recognizing collision situations using relative bearings to the other boat and the shore. Here, far at sea, I could have used the same principle in a slightly different way. I should have taken compass bearings on the other vessel every couple of minutes. Alternatively, I should have gotten a gauge on the ship over a fixed reference point on my own boat—a lifeline stanchion, for instance, or a winch or a stay. Being careful to stand or sit in a fixed location, I should have checked that gauge frequently. If it or the compass bearing did not change, I would have known that my boat and the ship might soon become entangled in an unfriendly embrace, the mere thought of which still gives me the shivers many years later.

Savvy mariners know that to avoid a fixed object they can simply use the Rule of 60 to calculate their course adjustment using the distance to the object and the distance by which they need to pass it.
If there had been a large object ahead of us—a stopped tanker, say, or an island or a reef—I could have exploited another, slightly more complicated trick called the Rule of 60 in order to calculate approximately how many degrees to alter course to get around it. As we'll see in a moment, the Rule of 60 can also be used to indicate how far to alter course to compensate for drift to one side due to tide or wind.

Mark Smith's drawing above (from The Annapolis Book of Seamanship) lays out the basics of the Rule of 60. Let's imagine that ahead of you in the fog, at a known distance, there sits a stationary obstruction of a known width. You know the compass course to the center of that obstruction. How far should you change course in order to clear the obstruction as you pass it? To use the Rule of 60, you need the distance to the obstruction in miles (A) and the distance-off (B) that you want to pass safely, also in miles. There are two steps: (1) Multiply B by 60, (2) Divide the product by A. The result is the approximate course alteration in degrees.

Two qualifications must be made. First, the result will be accurate to within about two degrees. Second, the Rule of 60 does not work if A and B are the same distance. While it is neither perfect nor all-inclusive, the rule does permit a quick mental calculation to deal with a frequently encountered problem that could be solved otherwise only by spending some time over a chart.

In the example above, the skipper uses the Rule of 60 to determine his altered course (C), so that he can compensate for the set and drift (B) and remain on the rhumbline (A).
The Rule of 60 has another application that is helpful to sailors sailing in tidal currents who lack (or distrust) GPS readouts of COG (course over the ground). The problem is calculating how to compensate for current pushing the boat to one side. Last month I showed a way to do this without making calculations by taking a visual range on objects ahead. Here's how to do it using the Rule of 60:

Let's say that you expect to average six knots over the 12-mile leg to your destination. That gives you two hours of exposure to a current that (according to the tide and current tables) will average two knots. Thus you'll be set four miles to one side. To sail a straight course to the destination, you'll have to compensate by steering four miles to the other, uptide side. The problem is how do you convert four miles into compass degrees.

Here, A is the distance to your destination, and B is the distance that the boat will be set to one side during the passage along A. Go back to the Rule of 60: (1) Multiply B by 60. (2) Divide the product by A. In this example, that's 240 divided by 12, which comes to 20 degrees (once again, that's approximate). That's the course alteration uptide.

If the current is not directly on the beam, you'll have to estimate its side force by looking at current charts or reading the water flow around buoys. Don't forget to factor in any favorable or unfavorable current, which will either shorten or lengthen exposure to side forces.

Now, as neat as it is, the Rule of 60 does not directly solve the fundamental question that I began with: Are you alert enough to know that you have a problem in the first place? Are your eyes out of the bilge?

John Rousmaniere is offline  
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