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Real-time routing

Jan 1, 2003

One approach to weather routing for voyagers is to get a weather analysis before departure and then work with that for the rest of the voyage. Another way is to do your own analysis and routing en route.

There is an abundance of weather information now available to sailors, with the amount only increasing as communication equipment and systems continue to improve in capability and decrease in cost and size. For example, the introduction this past year of the Inmarsat Mini-M system has opened the door for sending and receiving voice, fax, and data, including Internet access.

Using a variety of methods, from VHF to satellite systems, weather information is available in four formats: 1) charts; 2) satellite images; 3) text; 4) voice.

By using a variety of transmission methods and formats, every mariner is sure of receiving some, if not all, weather products. Charts, text, and voice formats are designed to work together, providing a complete picture of weather activity. A user, though, must determine which format is most easily understood and make it his or her primary source.

Those who learn well by audio methods should rely on voice weather broadcasts, while those who are visually oriented should rely on charts and satellite images. For example, a winter storm moving up the East Coast can be described using text and a surface analysis chart. Both provide details on the developing low and may be used together or separately.

Tools and considerations

Performing underway weather analysis and forecasting requires a good understanding of basic marine meteorology, as well as an understanding of the capabilities of both boat and crew. Weather decisions must be tailored to a crew's desires and abilities and a voyage's objectives. Beating upwind, for example, in gale-force winds for days on endalthough technically feasiblemay not be an appropriate choice for an inexperienced crew on a new boat.

Meaningful weather analysis must be done using accurate and complete information. Without good data only guesses are made as to present and future conditions, so a boat should be equipped with proper weather and communications equipment. Prior to examining equipment options, though, a method of assimilating, processing, and using information is needed.

A proven method used by the U.S. Air Force is known by the acronym ODAA (Observe, Detect, Analyze, Act). Applied to weather, this method requires a crew to: Observe conditions, Detecting changes or unexpected events; Analyze detected features and determine a cause (for every action there is a reaction and for every cause there is an effect); and then Actif action is necessary. ODAA encourages a logical progression of thought and action, which helps greatly in evaluating complex weather conditions.

Weather equipment

Weather is information intensive, and quality information is critical to making intelligent decisions. Therefore, reliable on-board weather systems are important. At a minimum, a boat heading offshore should be equipped with a recording barometer (either electronic or paper), and a single sideband (SSB) receiver.

As important as receiving quality information is knowing how a boat will perform under various conditions. There is little point in deciding to motor into headwinds if a boat's engine is not capable of this feat. Often weather analysis focuses on wind speed alone, forgetting that as seas build to significant height a vessel's progress is slowed and possibly stopped.Daily routineWeather analysis and forecasts are produced four times per day, at 0000Z, 0600Z, 1200Z, and 1800Z (Z being the international designation for Greenwich Mean Time). A few hours after these times, charts and text products are broadcast to mariners. During periods of dynamic and rapidly changing weather, reports are broadcast more frequently.Ideally, on-board weather analysis should be done in conjunction with reception of charts and text reports. Obviously, there are times when conditions are stable and there is little change during a six-hour period, and just as often weather can change dramatically in six hours.

Charts are an ideal format for obtaining and understanding weather features. A good routine is to examine the Surface Analysis, 24-hour Surface Forecast and 48-hour Surface Forecast charts together. This provides a picture of today and an idea of what to expect over the next two days. (Actually, a 48-hour Surface Forecast chart provides information out to 72 hours, as it projects the location of features 24 hours on either side of its 48-hour valid time.)

Upper-level 500-mb and sea-state charts should be reviewed and overlaid on surface charts. Upper-level charts show jet-stream activityin particular, the location of troughs and ridges, which influence development and movement of surface features. Sea-state charts show direction of combined seas, swells, and average wind direction, and are useful tools for avoiding rough conditions.

It is important to accurately plot a vessel's position on each weather chart examined so actual conditions can be compared with those shown on the charts. Comparison of clouds, pressure, wind direction and strength, and seas should be done each time a weather chart is scrutinized.

Projected movement of weather systems, such as lows, highs, and fronts, should be compared with a vessel's intended track. Determine if wind angles and direction will be appropriate for an intended course and destination.

When a daily routine is followed, an overlap of analysis and forecast products occur and a clear picture of weather features develops. Changes in system development become readily apparent.

Up-to-date weatherfax broadcast schedules are available from the National Weather Service's Marine Prediction Center via the World Wide Web at www.nws.noaa.gov. This location also provides additional weather information, including charts for Atlantic, Pacific, and Gulf of Mexico.

Satellite imagery

Real-time visible and infrared satellite imagery is obtained using an on-board receiver and antenna. Imagery shows cloud cover, sea-surface temperatures, and subtle features often not shown on weather charts.

Imagery allows tracking of thunderstorms, gales, storms, and hurricanes, as well as fronts and fog. Zooming, annotation, and color-coding of features assist in determining cloud and sea surface temperatures. Using GPS input, an on-screen presentation of a vessel's position allows precise navigation, and range and bearing capabilitiessimilar to radarallow tracking of specific features. Most satellite systems now work on laptop computers, and are Windows-based using PCMCIA cards and a compact helix antennas.

Routing software

Two routing software programs, Ocean and Kiwi-Tech, use weather data in a format known as gridded binary data (GRIB) and a vessel's performance characteristics (polars) to calculate a least-time route between waypoints.

GRIB data is received on board via a data transfer system such as Inmarsat C or M. Software is run using a standard laptop PC, with a vessel's polars loaded at the time of software purchase. Ocean has been used and proven in several around the world races and circumnavigations, always shaving days off passages.

Shoreside weather and routing assistance is a good resource to have, and though it may not be used regularly can be extremely helpful at times when weather is dynamic and rapidly changing. Shoreside weather companies often have access to more detailed and updated information than is available on board. A listing of weather companies is available on the World Wide Web at: www.nws.noaa.gov/im/more.htm.

Boat speed helps

If there is one tool essential to a safe passage, it is boat speed. Not speed just for the sake of going fast, but speed in the sense of staying with good weather and out-maneuvering bad weather.

Inherently slow boats often cannot maneuver away from impending bad weather, while fast boats usually have many options and can avoid adverse weather. Fast boats are not, as is often thought, inherently uncomfortable or dangerous. A Kanter 53, for example, is capable of speeds between 12 and 15 knots under normal conditions.

Good performance allows a vessel to sail through weather systems, not have weather systems sail through them. On a fast boat it is quite possible, with good planning, to avoid all "bad" weather, keeping up with weather patterns providing desired conditions. For example, let's say we have a high pressure system and two vessels heading east. The high (H) is moving at 10 knots, vessel A at eight knots, and vessel B at six knots. Over a five-day period the high will cover 1,200 miles, and during that time vessel A, moving at 192 miles a day, will stay under its influence. Not until day five will vessel A be at H's outer edge. Vessel B, however, moving at six knots, or 144 miles a day, will be 45 miles outside H's influence on day three. By day five, vessel B is 240 miles behind the high's outer edge.

While vessel A has held onto "good" weather for five full days, vessel B had the high's influence for less than three days and most likely is under the influence of a trough by day four and experiencing a low pressure system by day five. Thus two vessels may follow a similar track but experience quite different passages.

On-board weather analysis and forecasting depend on accurate information, received and recorded frequently and examined by a trained crew. It would be wise to study weather charts and text products before departure and become proficient at interpreting symbols and notations used to describe weather features. Once underway, the key is to keep up to date on the weather situation. Then find a good weather pattern and stay with it!


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