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That Damn Tide

When tide becomes a factor, our sport can take on an additional, "interesting" dimension.
A lake sailor with whom I used to sail was fond of boasting that the only currents he knew were the ones in the jam that he spread every morning on his English muffins. I told him that he was missing out on the most interesting challenge of the sea—although I did confess that by "interesting" I also meant "difficult," and even "damn impossible." Nothing makes a day of sailing less easy than a changing tide with its accompanying influence on navigation, sea conditions, and apparent wind.

I was strongly reminded of this rule of thumb while sailing off the south coast of England during most of August. We had 1,000 miles of sailing in the 50-foot Bob Derecktor-built ketch Snow Lion. Most of it was racing: a 110-mile overnight race in the English Channel, the 605-mile Fastnet Race out to Ireland and back, and a total of eight long day races in Cowes Week (that annual gathering of 1,000-plus racing boats), and the extraordinary America’s Cup Jubilee. The last, a floating history of great yachts and great sailors, was the most splendid regatta in history—a vivid reminder of our pastime’s historic roots in technological progress, aesthetic beauty, and sportsmanship.

Throughout that exciting month of racing there was plenty of time and opportunity to study the behavior and consequences of current because there was so much of it. Accurately judging laylines when we tacked and jibed for marks was a constant problem. After a while we became adept at finding ranges (the Brits call them "transits") against which to gauge whether we were holding our own against a side current.

At the America's Cup Jubilee this summer in England, there were plenty of opportunities to experience the tricky tides of this region.
Navigation sometimes seemed the easiest current problem. Take the start of the Fastnet Race. When the 233-boat fleet set out on August 12, the peak of the ebb tide was sweeping us west out of the Solent (the narrow body of water between the Isle of Wight and the mainland) toward our destination, Fastnet Rock, 400 miles away. The fresh southwest breeze was hard on the bow. Banging into it on the 15-mile beat out into the channel between land to port and shoals to starboard, we noticed that the wind speed often changed radically, as did the water’s state. An apparent wind of 25 knots with a three-foot steady chop sometimes quickly turned into 30 knots and four to five feet of broken water. Yes, the wind was puffy off the land, but the main reason for these changes was where we sailed relative to the tidal current. The current and sea tended to be easier in shallow, open water, where the current was relatively slight. Conditions worsened in deeper water and in constricted channels, where the tide ran more swiftly. The worst of it was in the narrows off the Hurst Castle sandspit and the shoals known as the Shingles, where the sea was extremely rough and even breaking.

With a range of between two and four knots, the differences were dramatic. A reliable rule of thumb is that a one-knot current running into the wind doubles the size of waves. Stronger currents have an even more radical effect of making waves higher, steeper, more frequent (with a shorter period), and more unstable, with white water flying everywhere. All this of course affects every aspect of sailing, from sail trim to steering to tactics. Normally the rule of thumb when beating is "sail in smooth water" because, then, a head sea slows a boat. But if a favorable current is running, lumpy seas can be good for you, if a little uncomfortable.

The Solent is an area that sailors claim was made by God for racing, but others say that the Devil also had a hand in its creation.
If the sea state is a ready indicator of current set (direction) and drift (speed), the boat’s instruments provide pretty precise quantitative measures. Comparing the knotmeter and compass heading, on one hand, with the GPS instrument’s Speed and Course Over the Ground (SOG and COG) displays, on the other, you can determine how much current there is and where it’s taking you. The knotmeter and compass show your progress through the moving water without accounting for current; the GPS tells you the effect of the current. The difference in heading and speed are the current's set and drift. A little experimentation will indicate whether taking advantage of a one-knot current boost is worth the rough water that may accompany it. On that windy August Sunday, we intentionally sailed toward rough water because we were racing, even though it slammed us around a lot.

It’s not generally known that current also affects wind speed, sometimes profoundly. A boat sailing closehauled in a favorable current always has a surprising amount of apparent wind. To determine apparent wind speed, add the true wind to the boat’s speed (or, when beating upwind, to about 0.75 the boat’s speed). After the Fastnet start, on the sprint out of the Solent the four-knot favorable tide and our eight-knot boat speed gave us a speed over the ground (SOG) of 12 knots. This transformed the Force 6 (22-27 knot) true wind into apparent wind readings sometimes in the gale range of Force 8 (34-plus knots). Had the current been ahead, our SOG would have been four knots and the apparent wind in the mid-20s. In this blow, some crews severely shortened sail in order to be under better control so they could stay clear of competitors. A few sloops were under jib alone, one boat set her storm trysail in place of her mainsail, and we on Snow Lion eventually doused our main and sailed under headsail and mizzen (the famous combination known as "jib and jigger."

The author's steed, Snow Lion, sports its jib-and-jigger configuration at the start of the 2001 Fastnet Race (lower left). 
The current also affects apparent wind velocity when sailing before the wind. On that point of sail, the apparent wind equals the true wind minus the boat's speed over the ground. For example, while running before a Force 4 (about 15 knots of true wind) into the hard ebb tide on the last leg of the America’s Cup Jubilee Around the Island Race on August 21, the knotmeter showed us making nine knots through the water, but the GPS displayed a discouraging SOG of only five knots. The sum effect was that our apparent wind, which would have been six knots in slack water, was a much harder 10 knots. As a rule, the lighter the true wind, the greater the effect of the current. Sometimes a boat seems to be completely becalmed when running in a fair current and light air.

In the Fastnet, because the Royal Ocean Racing Club started smaller boats early and larger ones later, the crowding was dense as the large fleet squeezed into the narrows off Hurst and the Needles (the chalk towers at the Isle of Wight’s western end). Some people enjoyed this, but most of them were ashore. Yacht racing in England is a spectator sport, and several people who watched from Hurst later told me they had never seen anything like this big fleet pounding its way upwind, bow-to-stern, on the evening of August 12. On the water, the crowding was at times conducive to serious frights. As far as I know there were no collisions, and for that we can thank the fleet’s seamanship skills.

When the wind, sea, and traffic eased, we got our main back up and beat for 30 hours down to Land’s End. After the turn, there followed 15 hours of thrilling reaching toward Ireland until a front came through and left us with some more beating. The conditions were astonishing when we tacked around Fastnet Rock at sunset on the third day, with a rainbow to one side and the lighthouse backlit by the post-frontal clear, orange sky. A fast run got us to the finish at Plymouth 35 hours later, the 24th boat to finish, but with that exciting hard beat in a strong current still fresh in the memory after three and a half days, and with the knowledge that the English Channel and the Solent still had a lot of tidal current left to heave at us.

Defending the Jubilee

I must take issue with my editor and friend Dan Dickison’s concerns about the long-term value of the America’s Cup Jubilee (see Jaded on the Jubilee). Not only was it the most fabulous gathering of sailboats of all kinds in history, but it offered great hope for the pastime’s continuing success. Anyone who has doubts about the value and symbolism of the Jubilee should consider three facts:

First, it was an example of admirable sportsmanship. The Royal Yacht Squadron was sponsoring the nautical extravaganza in honor of the 150th anniversary of its losing the America's Cup.

Second, the Jubilee helped heal old wounds. At the end of the week, the flag officers of the co-sponsoring New York Yacht Club awarded the prize for "The yacht that has brought the greatest benefit to the sport through participation in the America's Cup Jubilee Regatta" to Australia II, the boat that under a cloud of controversy and bitter feelings in 1983 took the Cup away from New York after 132 years. All (or most) is forgiven.

And third, while celebrating the past, the Jubilee celebrated the future. The RYS and NYYC acknowledged the pastime’s continuing vitality when they named as overall regatta winner Bear of Britain, a Farr 52 sailed by a crew of 17-25 year olds and skippered by a former National Optimist Champion. Yachting lives!

Suggested Reading:

Basic Thoughts on Tide by Jim Sexton

Using Tidal Current Tables by Jim Sexton

Understanding Tidal Currents by Jim Sexton


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John Rousmaniere is offline  
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