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Old 01-07-2001
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Dobbs Davis is on a distinguished road
A Lesson in Navigational Strategy


Distance races—like the one from Ft. Lauderdale to Nassau—provide a rare opportunity to fine-tune your navigational skills.
Late last year, the race for the Kalik Cup gave participants in the inaugural Crystal Cup at the Atlantis resort in the Bahamas the opportunity to avoid a few more days of cold weather up North and reprise an ocean-racing classic. This 189-mile race from Fort Lauderdale, FL, to Nassau in the Bahamas closely mimicked the old Miami-to-Nassau course used for years in the Southern Ocean Racing Conference (SORC). In those days, dozens of IOR warhorses from the 1970s and '80s competed in the last phase of what was then a three-week series. Because most ocean-racing regattas these days have been cut down to "banging around the cans" over long weekends, this race also presented a significant challenge in navigational strategy across the Gulf Stream to Nassau.

As in any offshore race, on board George Collins' Farr 52 Chessie Racing, we first looked at the weather picture. Weather router Susan Gennett had provided us with maps and a forecast that indicated a large, high-pressure center moving east toward the mid-Atlantic coast of the US. This feature would produce a generous flow of northeasterly breezes across southern Florida, with wind speeds forecast to vary from 13 to 18 knots and directions to range from 010 to 040 degrees true. From the noon starting time until the projected finish the following morning, the weather models suggested that the isobars would push a bit to the south, causing the breeze to shift more to the east while strengthening to the 18 to 23-knot range.

The other important piece of information we received was the size and strength of the Gulf Stream. Jennifer Clark's infrared images suggested the Stream to be about 40 to 50 miles wide in the area east of Ft. Lauderdale, with the maximum current approaching four knots in the western portion of the Stream. This would taper to one knot or less farther east.


After crossing the Gulf Stream, the competitors had to honor Great Isaac Light and Great Stirrup Cay as the two marks of the course.
The shallow Bahama Banks make the course to Nassau a circuitous one for all but the smallest boats. As in the old Miami-to-Nassau SORC races, the Kalik Cup specified competitors to leave Great Isaac Light to starboard, followed by leaving Great Stirrup Cay to starboard as well, and then on to the finish at the harbor entrance in Nassau. The courses and distances for these three legs were as follows: from the start to Great Isaac Light: 58 miles at 090 degrees; Great Isaac to Great Stirrup Cay: 68 miles at 106 degrees; and from Great Stirrup to Nassau: 63 miles at 153 degrees.

With the wind projected to be at 020-030 throughout the afternoon and early evening, we knew we would be in for a long, wet ride hiking out on the rail. Because of the stable weather pattern that was in place, we knew we'd be on the wind or close-reaching on port tack, with the possibility of some spinnaker reaching on the last leg if the wind didn't veer too much. Knowing the wind speed and direction relative to our course helped us anticipate which sails we were likely to use. So we checked that the No. 3 and No. 4 jibs had their battens in, the reefing lines were led in the mainsail, the jib-top reacher was ready, and the reaching spinnaker yarned and packed. We placed blocks on the starboard rail for outboard leads and led short sheets through these blocks in advance, since doing this during the race with the boat rail-down and awash, speeding along at 10 knots, would be a wet and possibly dangerous maneuver. And, although we had countless thousands of ocean racing miles among our crew, a few of us were new to the boat, so we also discussed the mechanics involved in reefing, unreefing, and making headsail changes.


The forecast indicated that the author and his fellow crew would be living on the rail for most of the 189 miles.
Given the weather forecast, we knew we'd be faced with few strategic options other than sailing as fast as possible on the rhumbline for the first two legs of the race. Even though the wind was fresher at 18-25 knots and further east at 035-050 than the forecast, the strength of the Stream allowed us to close-reach with cracked sheets, so we maintained boat speeds between nine and 12 knots. This was particularly true in the beginning portions of the race where the current and breeze were the strongest, and the waves the highest at 10 to 12 feet. In these herky-jerky conditions, the helmsman's course only approximated the route we wanted to follow because he was steering the smoothest path possible through the waves. With help from the mainsail trimmer, this was often the fastest course as well.

Since the course and wind changed little on the first two legs of the race, the real strategic challenge came as we closed on Great Stirrup Cay. Here the heading changed from about 106 to about 153 degrees, and with the wind at a median direction of 50 degrees, that meant a new true wind angle (TWA) of 103 degrees. Many of us on board had enough racing experience to know intuitively when to replace the jib-top reacher with the fractional asymmetrical spinnaker, but because the polar performance curves for these sails had not been well-established for Chessie, we had to reason through our decision as follows:

In most ocean races, there's usually enough sea room to use the try-it-and-see approach, where you set the spinnaker and give up a few degrees of bearing for the added speed. This is especially true if the wind is expected to lift or lighten over the length of the leg—lighter air from the same direction will allow you to sail at a lower TWA than in a stronger breeze. Our navigator Evan Evans reminded us, however, that for about the next 25 miles, the Berry Islands would be under our lee bow, with no more than a few degrees of deviation possible to keep us off the reefs.


The close proximity of a reef meant holding high for a longer period of time on the final leg of the course. 
We knew that in the lighter, 15 to 18-knot breeze we would be faced with having to sail a course several degrees closer to the wind so that the jib-top would be within its range of about 90 degrees TWA. While the wind had been a little more northerly than the initial forecast suggested, it was still expected to veer throughout the pre-dawn hours. Because of this, we reasoned that the best approach would be to sail with the jib-top at a heading farther east of the rhumbline. That way we would be sailing within the range of that sail and positioned to set the spinnaker after an hour or two and sail fast with it for the longest period possible. While it meant sailing a little extra distance (maybe a mile or two), this strategy would also keep us more comfortably away from the reefs bordering the Berry Islands.

Our strategy worked well, since the breeze didn't veer and strengthen quite as much as forecasted, and we were able to carry the fractional asymmetrical spinnaker up until a few miles of the finish, where the breeze veered quickly to 075, forcing us back to the jib top.

While Chessie's PHRF rating of a negative 63 seconds per mile would not win us any prizes, we were pleased to finish only an hour and a half behind Doug Baker's first-to-finish turbo-sled Magnitude. We were also pleased to learn more about the tradeoffs between sails, angles, distance, and boat speed. And for those of us senior enough to remember past SORCs, this was a much faster ride than the old days!

Current Effect on Heading

With the widespread use of GPS, finding our course over ground (COG) was as easy as pushing a button. However, a power failure may force you to consider the following exercise in trigonometry when crossing a significant current flow like the Gulf Stream.

During the Kalik Cup, the heading to Great Isaac was about 090 degrees and the wind was about 040 degrees at 20 knots. It's interesting to observe how the boat's heading changes with varying boat speeds and current strengths:

 

  • One Knot of Current

Boat SpeedHeading
6 knots99.6 degrees
7 knots98.2 degrees
8 knots97.2 degrees
9 knots96.4 degrees
10 knots95.7 degrees

 

  • Two Knots of Current

Boat SpeedHeading
6 knots109.6 degrees
7 knots106.6 degrees
8 knots104.5 degrees
9 knots102.8 degrees
10 knots101.5 degrees

 

  • Three Knots of Current

Boat SpeedHeading
6 knots120.0 degrees
7 knots115.4 degrees
8 knots112.0 degrees
9 knots109.5 degrees
10 knots107.5 degrees

  • Four Knots of Current

Boat SpeedHeading
6 knots131.8 degrees
7 knots124.8 degrees
8 knots120.0 degrees
9 knots116.4 degrees
10 knots113.6 degrees

 




Suggested Reading List

Newport to Bermuda—The Navigator's Race by Bill Biewenga
Thrill and Spills in the Hemingway Cup by Dobbs Davis
Newport to Bermuda Race by Dobbs Davis

SailNet Buying Guide - Traveler Systems

 

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