|From Ocean Navigator #87 |
Although access to space-age weather data has by no means eliminated all romance and adventure from offshore sailing, it certainly alters the character of the exercise. Given enough advance warning, storm avoidance and weather-based route optimization become viable techniques even at modest four- to seven-knot voyaging speeds. And in an era when the global climate appears to be growing more volatile, most sailors are grateful for all the help they can get.
In contemporary offshore racing, the science of wind/weather strategy now goes far beyond simply sniffing out wind with the aid of weatherfax charts and satellite images. Performance data for the boatboth real-time and computer projectedis routinely used in conjunction with weather data and forecasts to facilitate sail selection, sail trim, and point of sail (wind angle) in keeping with an overall game plan. And because the sailing speeds of many contemporary ocean racing boats, monohull and multihull alike, are two or three times greater that those of most voyagers, the range of strategic options expands proportionally. For example, powered-up racing boats with skilled crews can often take advantage of weak, erratic zephyrs to dance around high pressure systems that would trap less fortunate cruisers for days. Likewise, it’s not uncommon for a racer to “hitch up” to a fast-moving depression and ride a single intense system for a 1,000 miles or more, much as a surfer catches a long ride from a good wave. Nevertheless, many of the wind/weather strategies perfected by racers can work for voyagers too, albeit with less dramatic results and perhaps a somewhat lower success ratio.
Knowledge is power
Using accurate positional information (from GPS) in conjunction with weather charts, satellite images, or wind field plots that are overlaid with a lat/long grid, it is easy to determine exactly where a vessel lies with respect to existing weather systems. By also establishing the present wind conditions and extrapolating for boat and weather system movements, an experienced navigator can usually judge whether the wind is most likely to shift left or right, and build or fade during the next few hours.
Predicting the tracks of weather systems and, particularly, the passage of fronts is the main uncertainty when it comes to anticipating open-ocean weather developments over a longer time scale. But even by focusing on a time frame that extends only six to 12 hours into the future, it is usually possible to accelerate passages by deviating toward areas of increasing wind strength (known as “pressure” in racing lingo) and/or by positioning the boat to take better advantage of changes in wind direction.Because wind strength and direction are almost never constant for any appreciable time period, it’s no great surprise that soldiering resolutely along a great circle route will rarely turn out to be the optimal course for either an ocean racer or a passage-maker. Nevertheless, the most surprising lesson to emerge in the electronic sailing era is the sheer magnitude and frequency of heading alterations that are usually required to sail an optimal course. And, as a general rule, the faster the boat, the greater the course deviations that will potentially pay off. This principle is being hammered home time and again as the 1997-’98 Whitbread round-the-world race unfolds.
Boat performance and wind measurement
Many blue-water voyagers as well as racers are equipped with the means to receive and display weather charts and satellite images, making it possible for both groups of sailors to use weather-based routing strategies. However, a well-prepared offshore racing crew will probably also have access to computer-generated boat-speed predictions (often presented as polar plots) for various true wind angles and velocities. Furthermore, the raceboat will be equipped with a carefully calibrated knotmeter and accurate wind speed/direction instrumentation.
Using this equipment and polar plots, the racing crew can evaluate whether the yacht is being sailed efficiently (i.e., achieving its “target speed” for a given wind velocity and point of sail). If not, the crew will fiddle around to improve sail trim, wave steering technique, etc., until they zero in on the target number. In highly integrated systems, the current target speed may be displayed right beside the actual speeda great way to keep the night watch on their toes.
And the value of these tools extends further. By knowing the targets for slightly stronger and lighter winds, it becomes feasible to improve sailing efficiency through gusts and lulls. For example, let’s say the close-hauled target in 10 knots true wind is 6.2 knots boat speed with the apparent wind at 37°. If a 14-knot gust rolls through, the temporary new target might be 6.6 knots at 35° apparent. An experienced, seat-of-the-pants helmsman will reflexively luff up a little in the gust, thus preventing the sails from stalling as the higher true wind velocity shifts the apparent wind angle aft. On the other hand, a crew that understands targets will respond by easing down the main traveler and perhaps easing out a couple inches of jib sheet, thus allowing the boat to accelerate to the new, higher target speed as quickly as possible before heading up and simultaneously sheeting back in to achieve the new apparent wind angle target.
But “sailing the numbers” will only pay off if the computer-generated targets accurately reflect the actual performance envelope of the yacht, and if the on-board instrumentation accurately measures boat speed and wind speed/direction. Knotmeter accuracy is often degraded by off-center installation or misalignment, minor impeller fouling, or by disturbed flow beneath the hull. Digital displays make it easy to hint at two-decimal accuracy, but many cannot be trusted to even one-tenth of a knot.
Measuring apparent wind direction and speed is even more problematic. The preferred sensor location at the masthead minimizes gradient effects caused by air flow retardation down close to the waves, but guarantees that the sensors will whip around wildly as the boat plunges in a seaway. Sophisticated electronic damping is required to stabilize the readings without destroying accuracy. Upwash from the sails also affects the readings, especially at higher heel angles, so it’s common practice to mount the wind sensors well forward of the masthead. In addition, heel sensors and accelerometers are sometimes used to upgrade the accuracy of the masthead data.
Correlating boat speed with apparent wind speed and direction will enable a microprocessor to resolve the vectors for true wind direction (relative to boat heading), true wind speed, and velocity made good (VMG, rate of progress through the water in the direct upwind or downwind direction). True wind speed is useful in deciding when to change headsails or reef, and true wind direction is a help in assessing tacking or gybing angles. However, VMG, although intriguing, is rarely more than a rough-and-ready indicator of upwind/downwind efficiency. The delayed response of the VMG meter makes it worse than useless as a helming aid; and the cumulative errors imparted by leeway, wind measurement inaccuracies, and knotmeter bias usually result in VMG readings that are suspect for navigational purposes as well. For estimated time to a waypoint, it’s generally better to use the speed made good (SMG) function on the GPS (which computes speed over ground on course).
For voyagers, the most valuable integrated wind function is undoubtedly magnetic wind directionobtained by interfacing wind and boat speed instrumentation with an electronic compass. Magnetic wind is derived from true wind angle and boat heading plus a pre-programmed allowance for leeway (which ideally should vary with boat speed and apparent wind angle). For instrumentation skeptics, a magnetic wind direction reading can be readily verified by simply luffing head-to-wind to check whether the computed value coincides with the boat’s main compass.
Continuous information on wind direction enables a crew to track wind shifts in a straightforward, intuitive way regardless of alternations in sail trim and heading. And recording the actual wind direction and speed as a series of notations on the weatherfaxes is a good way to cultivate weather savvy.
On the other hand, if you don’t own top-quality, integrated wind instrumentation, don’t despair. The average voyager who isn’t inclined to trim sails and alter course with every gust, lull, or minor wind shift can get along fine without this fancy equipment yet can still benefit considerably from some of strategies used by racing sailors. For night sailing, even a modest wind speed/direction package is useful in deciding when to reduce sail and in helping avert accidental gybes and broaches when fatigued. On the other hand, a basic knotmeter and lighted Windex at the masthead will also get the job done. If the budget is tight, good navigation and communications equipment should ordinarily be the higher priorities.
Some useful strategic concepts
In racing, the term strategy normally applies to the use of wind, current, and geographic considerations to sail a course as quickly as possible. The term tactics, although sometimes used interchangeably with strategy, is better reserved for ploys aimed at achieving gains relative to other competitors or minimizing the risk of losses. Tactics are obviously of little concern to the average voyager, but strategy should be.
The development of accurate wind instrumentation in the mid-’80s opened the door to a new level of racing efficiency in shifting winds. A technique that has come to be known as “Wallying” played a key role in Dennis Conner’s 1987 America’s Cup win aboard the 12-meter Stars and Stripes. Sailing upwind in a shifting wind, the best average VMG can be obtained by sailing targets that are significantly lower and faster than would be optimal in a steady wind (provided, of course, that you manage to tack in phase with the shifts). It works because a yacht that foots out below a competitor sailing the same tack will gain separation or leverage, even if her VMG is slightly lower, as long as steady wind conditions prevail. But then, when the wind swings toward her side of the course, the separation is translated into an instant lead. Exactly the same principles apply when gybing downwind on a series of reaches.
A willingness to sail “off course” in exchange for extra speed is not something that comes easily to most sailors. But in the early portions of a long passage, if you can sail 20° off course and 30% faster by bearing away to a close reach (or heading up from a near run), it’s probably worth doing as long as it doesn’t send you into danger or steer you wrong with respect to the upcoming weather. With good weather information, it’s usually not especially difficult to avoid the really big mistakes, but if you have absolutely no idea what the wind will likely do next, consult the pilot chart and consider simply playing the odds. If nothing else, it’s more interesting than simply slavishly following the rhumb line, and it is very gratifying when it pays off.
The seventh running of the Whitbread round-the-world race has brought major changes, including a single class of boats (Whitbread 60s) and a new scoring system based on points rather than accumulated sailing time. Although the longer legs in the nine-segment race are weighted a little more heavily than the shorter ones, it’s tactical sailing and finishing orders that will determine the winners this time around.
In the 1993-1994 Whitbread, Chris Dickson’s Tokio had built up a seemingly insurmountable time lead on the first four (of six) legs and was poised to win easily until she dismasted during Leg 5. The new scoring system is keeping the overall race very much closer, so despite breakaway performances by a few boats in each of the first two legs, it’s still anyone’s game. With four legs completed, only 78 points separate the first six yachts in a fleet of nine. Considering that the difference between first place and sixth in the upcoming southern ocean leg from Auckland, New Zealand, to SÃ£o SabastiÃ£o, Brazil, will be 70 points; it’s clear that this is still a wide-open race.
Whitbread 60s are powerful, water-ballasted boats with huge rigs and highly refined sail inventories. In light and moderate air, these boats can match wind speed almost knot for knot, which means that a crew who finds just a touch more pressure can easily gain 50 miles or more in a day. Leaving Cape Town after the start of Leg 2, Swedish Match broke with the fleet and headed south, angling away from Australia. The move paid off enormously when she tied into a strong system and bolted for Perth, building up a lead of about 900 miles.
The subsequent two legsPerth to Sydney and Sydney to Aucklandhave been closely fought, tactical contests from start to finish. Shorter course distances probably had something to do with this, but it also appears that crews are adjusting to the new character of the Whitbread, and now cover one another fiercely to minimize opportunities for another breakaway. Thanks to satellite tracking and frequent on-board reports, the racers are fully aware of each others’ positions and speeds. All have access to the same detailed weather information, including sophisticated “wind field” maps.
It’s interesting that the most successful Whitbread teams so far have been stacked with inshore racing talent, while other crews, some with considerably more Whitbread experience, have so far come up short. Thanks to the information revolution, a global race can now be sailed using much the same strategies that bring success in round-the-buoys events. For voyaging sailors, it’s food for thought.