Category Archives: Weather

How to get weather information via SailMail

Weather via Sailmail, overview.



You can use the SailMail system itself to receive text or grib weather forecasts at no charge.  The easiest way (free) is to use   For information send an empty email to

Several commercial services format and email grib weather forecasts for a fee.  PredictWind, Squid/Great-Circle, Grib.US, BuoyWeather, Ocens, MaxSea, MovingWeather, Météo-France, and many others are in that business.

You can use SailMail to communicate with a meteorologist or commercial weather service (e.g. Commanders Weather or WeatherGuy), who can advise you on reasonable departure dates and routes, and can send you periodic routing advice during your passage.   We highly recommend this, particularly for new cruisers who have not yet become confident in their ability to interpret weather data.

Finally, there are fax weathermaps and SITOR text weather forecasts that are broadcast by the US Coast Guard and by other HF stations around the world.  Your SCS PTC-II modem can be used in conjunction with your laptop to receive these broadcasts.


Grib files, Saildocs, Weather data

The following link will take you to the Saildocs site.


Saildocs provides custom grib weather-data files per request from data downloaded from NOAA/NCEP and other sources.

Additionally, Saildocs provides document-retrieval for the delivery of text-based Internet documents either on request or by subscription.

Saildocs is supported by the Sailmail Association and can be used by anyone who agrees to the terms and conditions (below).

Maxsea Grib Compatibility

This note is for Maxsea users, regarding grib-display problems with certain versions of Maxsea.

Background: Grib (GRIdded-Binary) files are packed-binary data files which contain weather-forecast data. The grib format is an international standard which is supported by a variety of systems and software. Data in a grib file is stored as a “grid” or array of numbers in a packed-binary format. The grib format allows data to be organized either by increasing latitude with a positive latitude-increment (delta-lat parameter), or by decreasing latitude with a negative delta-lat. NOAA’s GFS model, Saildocs and Raytech use the positive convention; Maxsea and OCENS/GlobalMarineNet use the negative convention. Any grib viewer must support both conventions, and most do.

However there is a problem with certain versions of Maxsea which fail to correctly display positive-convention grib-files for southern latitudes. The data image gets inverted north-to-south (not east-west). The effected releases include Maxsea version-10 PRIOR to 10.3.5 (which was the final Ver-10 release). Version 10.3.5 is OK, and Ver-9 and -11 also appear to be OK. Others are untested, if you have an earlier/later version please check it (see below) and send me a note if you see the problem.

To temporarily accommodate users of Maxsea versions with this problem, Saildocs can also supply negative-convention grib files. In the grib-request message change the delta-lat parameter to a negative number, =
for example:
send grib:30S,40S,168E,180E|-2,2|24,48,72
In this example the “-2” specifies a negative-convention file which will avoid the Maxsea problem. (Complete details on the request-format is available be sending a blank email to:

The simplest way to check whether your Maxsea program has this display problem is to request two small grib-files from Saildocs for the southern hemisphere, one normal file and one negative-convention. Use =
Airmail’s grib-request window and then modify the request message, or simply send the following two lines to:

send grib:30S,40S,168E,180E|2,2|24,48,72
send grib:30S,40S,168E,180E|-2,2|24,48,72

This will request two files, identical except for positive/negative data convention. The grib-viewer must display them identically. When you get these files, open each with Maxsea and check the weather over North Island, New Zealand. If the two files display differently then your Maxsea version is not correctly displaying the positive-convention grib file. A short-term fix is to edit each grib-request before sending to specify a negative latitude increment following the example above. Feel free to contact me with any questions at

The longer-term fix is to update your Maxsea version. Furuno now handles distribution and support for Maxsea. They have acknowledged the problem and have made ver 10.3.5 update discs available without charge for users of prior ver-10 releases which are effected. This update only applies to ver-10 users prior to 10.3.5 and won’t work without a version-10 dongle. Contact Furuno for a copy of the 10.3.5 CD.

Jim Corenman

How to request PredictWind data via email/SailMail

How to download with a Satellite/SSB Connection using an email request

NOTE: This help topic is not relevant for Iridium GO! users.

• Open the PredictWind Offshore App

• Move your start point (green) and your destination point (red) to your desired locations. You can zoom in and out to get a better view of your route. Please ensure your start and destination points are in the water and not on land. Each time you change your route the App automatically selects the relevant high resolution & offshore GRIB files, GMDSS text forecasts & Satellite imagery.

• You can add intermediate waypoints (between start and destination) by clicking on the “+” button next to the start waypoint control – top right of the screen. For the purposes of weather routing, we recommend that you minimise the number of waypoints so the weather routing is free to find the optimum route.

• Click ‘Download’ button from the left menu

• Make your selection of items to download. The total file size to receive via email is shown above the download button.

• Click ‘Next’.

• Choose ‘Email’ tab.

• Make your final selection of items to download. The total file size to receive via email is displayed below the table.

• Click ‘Save Request’ – save the file to your ‘desktop’.

• Open a new email. Send the saved file to by attaching the file to the email. You can leave the subject line blank. Please Note: The email will be sent back to the email address that you send the request from.  You do NOT need to change the email address for your PredictWind login.  

• After a few minutes you will receive multiple emails with GRIB files and PWU files.  Save these files to your ‘desktop’.

• Go back to the PredictWind Offshore App.

• Click File > Open. Please use the ‘ctrl’ button to select multiple GRIB files, and the PWU file.

• The PredictWind Offshore App will then load the files into the app. You can then view the files that you have downloaded by clicking on the different Menus and Views

NOTE: For SSB radio services there are maximum file sizes that these services allow. Please make sure your requests are below these limits.


Pactor I or II:  10 kbytes

Pactor III : 30 kbytes

Pactor IV : 40 kbytes


120 kbytes

Weatherfax Images

Weather charts are created by NOAA and broadcast via radiofax by USCG, and can be received with Airmail software using any of the SCS Pactor modems.

The first thing you need is the broadcast schedule. So go to this web page:  Scroll down and find “Worldwide Marine Radiofacsimile Broadcast Schedules (PDF)” and download a copy. There are five US broadcast stations: Pt. Reyes (near San Francisco), New Oreans, Boston, Alaska and Hawaii, plus a bunch of overseas stations. The US stations are coast guard and quite reliable, the others vary.   The schedule tells you times for each chart, for each station.

Now open Airmail, open the “fax” window (“Get Fax” under Modules menu, or button on toolbar). With radio and modem powered on, select fax mode, select station and frequency. The radio freq should be set (selected freq minus 1.9 khz), and you should hear a characteristic “warbling” fax tone, if the station is transmitting. Try each frequency for the clearest signal. Then wait– Airmail will detect the special tone at the beginning and end of each fax transmission, copes each chart and saves them automatically.

You will note in the schedule that the charts are send in blocks over a couple of hour period, just turn the radio on and open’s airmail’s fax window, and let it run during that period.

Now, all that said, you can get most of the same information from GFS grib data. The zero-hour forecast-time is based on actual conditions, then each forecast is computed from that. The same data is used for grib files, and also to generate the fax charts. The fax charts are reviewed by forecasters, and information added– storm warnings, frontal positions, etc.