Offshore Log: Essential Forecasting Tools


Our years of sailing in New England and the Caribbean, with their stable, predictable weather patterns, left us ill-prepared to deal with the vagaries of weather in other parts of the world. Faced with the reality of spending weeks at sea at a time, we no longer had the luxury of waiting for suitable weather windows that would encompass an entire passage.

We are not without tools to help us analyze the weather, however. Thanks to weatherfax, you can get satellite photos, surface analysis, and short-range weather prognosis maps via SSB (single-sideband) radio links using inexpensive software and hardware. A big step up the financial ladder, boats equipped with satellite communications can directly access Internet-based weather information 24 hours a day.

When all else fails, of course, you resort to the basics: Look at the sky, tap the barometer, think.

This month, we look at several weather forecasting tools that we have found essential in the last three years and 14,000 miles.

The Aneroid Barometer
A good barometer is more than just the complement to the nice brass clock mounted on a bulkhead. Properly calibrated, it is a valuable weather forecasting tool.

Proper calibration is the key, however. In Tonga, I merely called up the meteorological office at the local airport to get the current sea level barometric pressure, then corrected the ships barometer. The fact that the airport was a few miles away made no difference, as variations in pressure are not that localized in stable weather conditions.

With an accurately calibrated barometer, up-to-date weather facsimile maps, and close attention to true wind direction and velocity, we were able to judge our position relative to a dangerous, developing tropical depression.

Even with the best forecasts from outside, at the end of the day there is no substitute for monitoring the weather exactly where you are. An eye on the sky, a check on the wind direction, and a watch on the barometer will tell you as much about your short-term weather prognosis as the best outside forecast. During our passage to New Zealand we went against the forecast. The local conditions we were sailing in suggested-correctly, thank God-that the depression with our name on it was slowing down, jammed up against a wall of high pressure east of our position.

The fact that we were 600 miles from the nearest harbor at the time made the choice to press on an easy one, of course. Had we been in port, we most certainly would have stayed put.

Aboard Calypso, we have a high-end Chelsea barometer that matches our ships bell clock. However, any barometer is better than no barometer. A $100 barometer that is calibrated properly is more useful than a $400 barometer that has not been adjusted. Our barometer is mounted at the nav station, right next to an old-fashioned, key-wound Maxant recording barograph that we have used for 25 years. A recording barograph is useful for charting changes over time, but you can get the same information by making regular recordings in the logbook of barometric pressure from a conventional barometer.

When we took our clock to a repair specialist in New Zealand for cleaning and lubrication, he suggested bringing the barometer in for the same treatment, as the oil lubricating the barometer escapement dries up, just as in a clock. A barometer with a sticky mechanism can give misleading results.

Dont forget to tap the barometer face lightly before taking the reading. Most barometer escapements are sticky enough that there is a lag between a change in pressure and movement of the needle.

Coretex Weatherfax Software
Sailing from French Polynesia to New Zealand in the early southern spring, it was imperative to have a good idea of the positions and rates of progress of the high and low pressure systems. We tracked these systems using Coretex Weatherfax for Windows 95, a software/hardware combination that allows you to receive weatherfaxes onboard without having a dedicated weatherfax receiver.

Most of the cruising boats we have met in the last two years have been equipped with weatherfax software. Coretex is the overwhelming favorite.

If you have a laptop computer and a SSB (single-sideband) radio, you have all the pieces required to put the Coretex system to work. A single cable containing the demodulator joins the SSB to your computer. Aboard Calypso, this cable is permanently installed in the nav station, but the computer is put away after each use. It takes about two minutes to plug in and boot the computer, turn on and tune the radio, and be ready to receive a fax.

At its simplest, you open the software and hit receive fax when you hear the start tone from the radio. The fax appears on the screen as it is received, so you can toggle between frequencies to pick the best reception at different times of the day.

When you see the end of the fax, hit the stop button, then save the fax just like any other computer file. You can edit and view it at your leisure.

A number of editing tools allow you to re-align pictures and clean them up for easier analysis. You can also select between fine and coarse resolution for clearer reception of satellite photos or more economical use of resources.

Although the software will run on older Intel 486 computers, it works best on computers equipped with Pentium or later processors.

The Coretex software includes a worldwide weatherfax scheduler database, and allows you to program for automatic reception if you wish. This requires leaving both the radio and the computer on-something that energy-conscious cruisers rarely do.

Also included are decoding routines for Morse code and telex. This is a real Windows 95 program, complete with hyperlinks. It is coherently organized and intuitive in use, unlike many pieces of marine software.

On our 3,200-mile passage from the Galapagos to the Marquesas, we received satellite photos twice a day from Honolulu. From French Polynesia on to New Zealand-about 2500 miles-we downloaded four surface prognosis maps daily from New Zealand MetService. These maps were absolutely invaluable, and were a major factor in our decision to sail south at top speed, directly toward New Zealand when we were threatened by a major gale.

A good SSB installation is a must, as is logical programming of that receiver. For example, we programmed the four NZ MetService frequencies sequentially in our Icom M-800s memory. When receiving any fax, we can switch from frequency to frequency without interruption during reception to make sure we are getting the clearest signal.

Online updates to the Coretex software are available via their website at

Although it is a product of New Zealand, Coretex Weatherfax for Windows 95 is available through retail and discount chandleries and marine electronics dealers throughout the US. We recommend a full-service electronics dealer as a source, since they can help sort out any special connection problems or installation issues with either your radio or your computer. Cost is just under $250. (Xaxero Marine Software Engineering Ltd., Riverhead Rd., Riverhead, Auckland, New Zealand).

Mariners Weather Handbook: Indispensable Guide
Steve and Linda Dashew are well-known for their innovative series of big, high-speed Deerfoot and Sundeer cruising boats, and their encyclopedic reference books on cruising. Their Mariners Weather Handbook applies the same thorough approach to understanding marine weather that they have previously used to lead you through the maze of contradictory advice on choosing and equipping a cruising boat. This is an essential book for any cruising sailor who has even the slightest interest in developing a good understanding of weather. At first glance, its bulk seems overwhelming. In reality, however, wide margins and a large typeface have expanded what would have been about a 500-page conventional book into almost 600 pages.

The book is packed with useful illustrations. A large percentage of these illustrations are reproductions of various weather facsimile products. The Dashews do a good job of teaching you how to understand them.

A great deal of emphasis is placed on interpreting upper level charts. These are sophisticated tools which, to put it bluntly, are beyond the useful comprehension of anyone who is not interested in devoting a fair amount of time and mental energy to expanding his or her understanding of weather. For most people, multiple readings will be required before a real comprehension of the relationship between upper level patterns and the surface weather will be possible.

This book is subtitled A Guide to Forecasting and Tactics. The tactical portion of the book concentrates on both utilizing weather features to reduce passage times and tactics for avoiding severe weather. Unfortunately, these sections of the book are geared to those who own large, high-speed boats capable of covering 200 to 300 miles a day under either sail or power. Boats this size can engage weather systems in a way that would be difficult for smaller, slower boats.

A slower boat must generally react to the weather during a long passage, rather than be proactive. The exception might be well-forecast, clearly defined severe weather-a tropical storm, for example-which even a slower boat would try to avoid.

The book is filled with detailed descriptions of various passages made by both the Dashews and others. Thirty pages are devoted to a single passage from Nuku Hiva in the Marquesas, to San Diego, aboard Beowulf, their 78-foot light-displacement ketch. The description includes every weatherfax received and all the e-mail between the Dashews and their weather router, Bob Rice. It is tedious going. As in all of the Dashews books, there is a wealth of information, but in this case, you must have patience to dig it out. This is a book that could benefit greatly from some judicious editing. A big bonus is the inclusion of a complete version of the book on CD. If you would rather sit at the computer than read a book, this may be the approach you take to the Mariners Weather Handbook. In some ways, the CD is better, as the photos on the CD are shown in full color.

Knowledge gained from this book came in handy on the bumpy ride from Tonga to New Zealand. Faced with a rapidly deepening low-pressure system dropping down on us from New Caledonia, our weather forecaster advised us to try to heave to and let the depression pass south of us. This potentially would have left us in the strongest sector of the low, with winds of 50 knots or more.

Instead, with an accurately calibrated barometer, a fresh understanding of the motion of extra-tropical lows, and up-to-date weatherfaxes, we decided to carry as much sail as we could and head due south as quickly as possible. That proved to be the right choice, as the barometer showed the low stalling to our northwest, and we escaped with only a minor beating.

The books $75 price is a lot cheaper than going back to school. (Beowulf Publishing, 12635 Delman Lane, Pineville, NC 28134; 704/544 0919)

A Funky Weather Guide
MetService Yacht Pack is a quirky, 105-page spiral-bound desktop-published guide to weather in the southwest Pacific, particularly New Zealand. The author, professional meteorologist Bob McDavitt, bills himself as New Zealands weather ambassador, and its an apt description.

Through MetService, McDavitt provides, for a fee of about $25 US, five-day forecasts and projected weather maps for cruisers in the southwest Pacific. Countless sailors have relied on his advice to get them across the sometimes-rough waters between Tonga, Fiji, New Zealand, New Caledonia, and Australia.

The Yacht Pack is a brief compilation of McDavitts weather wisdom. It includes a simple tutorial on reading weatherfaxes, descriptions of tropical weather patterns, schedules for voice and facsimile forecasts throughout the region, and straightforward, easy-to-understand introductions to the basics of weather forecasting and analysis.

As a bit of desktop publishing it can be crude, with missing lines of text and difficult-to-follow page organization. At the same time, there is a lot of good information here that is virtually essential for understanding the weather in this part of the world. We bought our copy of Yacht Pack at the Armchair Sailor bookstore in Newport, RI for $35 US; 800/292-4278. (MetService New Zealand Ltd., PO Box 68-429, Newton, Auckland, NZ; 64 9 377 4831, fax 64 9 307 5993; email: [email protected].)

Darrell Nicholson
Practical Sailor has been independently testing and reporting on sailboats and sailing gear for more than 50 years. Supported entirely by subscribers, Practical Sailor accepts no advertising. Its independent tests are carried out by experienced sailors and marine industry professionals dedicated to providing objective evaluation and reporting about boats, gear, and the skills required to cross oceans. Practical Sailor is edited by Darrell Nicholson, a long-time liveaboard sailor and trans-Pacific cruiser who has been director of Belvoir Media Group's marine division since 2005. He holds a U.S. Coast Guard 100-ton Master license, has logged tens of thousands of miles in three oceans, and has skippered everything from pilot boats to day charter cats. His weekly blog Inside Practical Sailor offers an inside look at current research and gear tests at Practical Sailor, while his award-winning column,"Rhumb Lines," tracks boating trends and reflects upon the sailing life. He sails a Sparkman & Stephens-designed Yankee 30 out of St. Petersburg, Florida. You can reach him at