How do you optimise your sailing route based on weather forecasts?

• Satellite comms onboard
• GRIB files
• Weather routing software
• Polars
• Limitations

Satellite communications onboard

Over the years, communication systems onboard yachts have become better and better, allowing more data to be downloaded and at faster speeds. Only a handful of years ago it would take for ever to download even the smallest of files, but nowadays, using systems such as the IridiumGo, you can get access to relatively large files, giving a much better overview of the weather situation around you and for a longer period of time, making sailing safer and faster.

The IridiumGo is a stand-alone system, but it can be installed with an external aerial and hardwired into the boats 12v system or it can run from it’s own internal battery. For an around the world trip it would certainly be prudent to have the external aerial (it allows for a more stable connection) and also to wire it in to the boats power supply as it draws relatively little current.

The unit is accessed via mobile phone and an App which you can use to send and receive texts and emails, download weather files (either directly in the PredictWind app or via email) and make and receive phone calls. The IridiumGo effectively acts as a Wi-Fi hub for your mobile.

Although data transfer speeds have improved somewhat, they are still slow compared to typical landline connections with a file of 100kb typically taking around 15-20 minutes to download. For this reason we usually wouldn’t download grib files larger than this.

The IridiumGo is a great easy to use piece of communication equipment, low power and reliable. Making a phone call on your mobile, from the Southern Ocean, is a surreal experience.

GRIB files

GRIB stands for General Regularly distributed Information in Binary form. It is a data format used in meteorology to store weather data. It was first defined in 1985 by the World Meteorological Organization and is the format used by all modern weather routing programs. It compresses the weather data into a binary format which saves space and allows it to be transferred easily.

There are many weather models which produce weather files, some are more accurate than others in various situations. Some work better offshore and others model the weather better in certain situations. It is the experience of the weather router that comes to the fore in which weather model to use. Some of the more popular weather models in use are described here:

GFS: Stands for Global Forecast System from NCEP. This is used by most free weather websites/apps. Higher end grib files now use the GFS-FV3 model. It’s the first significant upgrade to GFS in about 40 years. Unlike the previous GFS model, GFS-FV3 is able to simulate vertical movements such as updrafts, a key component of severe weather, at very high resolution. So far, tests suggest that the FV3 model has more accurate five-day forecasts, as well as better predictions of hurricane tracks and intensification. Although the new FV3 core has shown improvements over GFS it remains ranked 3rd for accuracy behind ECMWF(1st) and UKMO(2nd).

ECMWF: Stands for European Center for Medium-Range Weather Forecasts and is highly regarded by Meteorologists and top Navigators around the world. The ECMWF High-RES model consistently rates as the top global weather model from a national weather service with the highest rating scores. In March 2016 ECMWF increased the resolution of their model to a record-breaking 9 km resolution, which is currently the highest resolution global model available. ECMWF data has a very high acquisition cost, and this is why the data is not widely used by many weather websites and has been traditionally used only by top yacht racing teams and meteorologists.

SPIRE: Is a truly innovative company with the largest nanosatellite network in space. Spire uses a unique technique of measuring the earth’s atmosphere with 3x more radio occultation data than any other commercial entity. This gives an advantage in forecast accuracy for remote locations. The Spire model is #1 for wind speed and direction accuracy using data from offshore weather buoys. It is #2 behind the ECMWF for land-based weather stations.

UKMO: Otherwise known as the “Unified Model” by the UK Meteorological Office has a long reputation as a market leader in forecast modelling. UKMO has very similar accuracy to the ECMWF model offshore and is slightly behind the ECMWF & Spire models for the land-based weather stations.

HRRR: Stands for High-Resolution Rapid Refresh and is a NOAA real-time 3-km resolution, hourly updated, cloud-resolving, convection-allowing atmospheric model, initialized by 3 km grids with 3 km radar assimilation. Radar data is assimilated in the HRRR every 15 min over a 1-h period adding further detail to that provided by the hourly data assimilation from the 13 km radar-enhanced Rapid Refresh.

NAM: Stands for North American Mesoscale Forecast System and is one of NOAA’s major weather models, which in this case covers most of North America. NAM is a mesoscale model, which means that the numerical analysis is able to model land, and other features, at a higher resolution than in a global model, leading to improved forecast accuracy.

AROME: Is a small-scale numerical prediction model, operational at Meteo-France since December 2008. It was designed to improve short-range forecasts of severe events such as intense Mediterranean precipitations (Cévenole events), severe storms, fog, urban heat during heat waves. This model is highly regarded by top racing navigators and beats the ECMWF forecast.

So there are lots of models to choose from, the skill is choosing the right one.

Weather routing software

Once you have the GRIB file, you then need some way to work out the optimum route to your destination. This involves a lot of number crunching and there are many computer programs to help with this. We use a range of software depending on the situation. The French ‘Adrena’ program is a very popular choice but comes at a high cost. This is used by a lot of professional teams, such a Volvo and Vendee competitors. PredictWind have a range of options, a free version and a paid one, the paid one having more features. PredictWind also have their own ultra-high resolution 1km grib files and their software is pre-installed on the IridiumGo so you can download weather files and import them directly into the routing program, rather than downloading the grib and then separately importing it. (PredictWind was developed for the America’s Cup in New Zealand). qtVlm is a powerful free open-source routing tool available online which is becoming popular and has a lot of options, you can also link it to your onboard instruments via NMEA and use it to build your own Polars.


There are now many different routing programs available to the offshore sailor – the most important thing is that whichever you choose, you need to be able to use it. It is no use having the best, most complicated program if you can’t use and understand it. You also need to remember that sailing solo you will be affected by fatigue, so if you are doing your own routing onboard, you need to fully understand the routing program. Learning this will be time well spent.

However, any routing software is useless without having accurate Polars…


Every boat and sailor combination is different. Has different appetite for risk and deals with fatigue differently. Polars are a numerical (and graphical) representation of a boats performance in a given wind strength and wind angle. Many boats have a set of base polars which come from VPP software from the designers, however you need to be cautious using these for actual routing as these are theoretical polars and don’t take into account fatigue or damage to the boat or sails (some of the higher end routing software can take this into account though, allowing you to specify which sails you have up and a % of their original performance to allow for sail degradation as well as stacking of ballast and trim settings).

If people don’t want to build polars from scratch, we normally advise teams to start with a base set of polars for their boat, or one very similar. Then go out sailing and see how they compare to the base set, modifying them as they go. Obviously, the more time can be spent on this, then the more accurate representation of their actual performance it will be, so the routing itself will be more accurate.


With any weather forecast it is very important to remember that it is just that – a forecast. There is no guarantee that the weather will behave exactly as it says. Weather systems can move faster or slower and change direction. The skill is in spotting these trends and modifying your routing accordingly. No one model is perfect, but some work better than others in certain situations. For example, in the Southern Ocean, the models generally under estimate the wind strength and you can often add 10% or more to the forecast wind speed.


Weather routing yourself around the planet is very rewarding, but there are also compelling arguments to use an external router. And expert, sat at home with a high-speed internet connection can download multiple high resolution GRIB files and run them through different routing programs. They also have the added benefit of not sitting on a cold wet boat pitching around at sea and they usually aren’t suffering fatigue. They email routing suggestions, usually daily, and hear feedback on the local weather from the sailor, allowing them to make adjustments to their routing to be more accurate. This takes the pressure off the sailor so they can concentrate on getting the most out of themselves and their boat and not worry about which way to go and what is coming up ahead of them.

About the author

Alex Alley is a multiple world record sailor who has raced around the world with a crew and in 2019 set off to break a solo non-stop around the world record in his crowd funded Class 40 ‘Pixel Flyer’. Unfortunately, disaster struck 1000 miles south of Australia in the Southern Ocean and he was forced to abandon the record attempt and make his way to Adelaide under jury rig.

He has raced Open 650’s in France and out in the Atlantic as well as many other classes both inshore and offshore.

With over 150,000 miles of offshore racing, much of it single handed, he now speaks at conferences and events about his experiences. He has written 2 books on teamwork and leadership, and helps teams in various disciplines with logistics and weather routing for their challenges.