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At first, the idea behind OhMyWind stemmed from a frustration that?s quite common among weather-savvy boaters: a vast amount of data exists?sometimes freely accessible?but bringing it all together to answer a practical question like ?When should I set sail, and how long will it take? ? quickly becomes a real headache. OhMyWind was born out of this problem. Since then, the project has taken the form of an open-source sailing trip planner that combines weather, sea conditions, polar charts, and currents to help prepare for a crossing.
A project centered on weather data accessible to boaters
OhMyWind is first and foremost an independent software project. Its approach is based on utilizing existing data rather than generating its own weather forecasts.
Wind data is sourced from Open-Meteo. Depending on the region and the forecast period, several models are included in the data chain: Arome from Météo-France, Icon-EU from the DWD, ECMWF IFS, and GFS from NOAA.
This architecture addresses a reality of marine weather forecasting. No model offers all three of these features: a very fine grid, global coverage, and a long forecast lead time.
Arome, with a reported resolution of 1.3 km over its coverage area, is of particular interest to coastal navigators for short-term forecasting. Icon-EU extends the analysis up to five days. ECMWF IFS and GFS become more important as the forecast horizon extends further into the future.
However, the project goes beyond simply displaying a wind arrow on a map. Its goal is to transform this data into information related to a sailboat?s actual movement.
That's where things get complicated. Two boats leaving the same port under the same wind conditions do not take the same amount of time to reach their destination.
Turning a Weather Forecast into a Sailing Plan
To switch from the weather to navigation, OhMyWind must provide the boat's speed.
The project uses seven categories of sailboats: 20-, 25-, 30-, 40-, and 50-foot cruisers, a racer-cruiser, and a 40-foot catamaran. Each archetype corresponds to a speed profile.
This polar chart shows the boat's theoretical speed as a function of the actual wind speed and wind angle. The software can thus distinguish between a close-hauled, beam reach, and downwind course.
But a polar chart is still just a theoretical representation. A cruiser loaded up for a three-week vacation?with its dinghy, full tanks, and a hull that needs a good scrub?doesn?t sail the same way as the boat used as a reference.
OhMyWind therefore introduces a performance coefficient that can be adjusted between 50 and 100 percent, with 75 percent as the default value. This allows the boater to gradually adjust the model to match the behavior observed on their own boat.
The project also calls for the use of the engine. The user can set the sailing speed at which to start the engine, as well as the engine's cruising speed. The route estimate then factors in this switch when the wind becomes too light.
The idea is not to replicate every maneuver performed on board. In particular, there are no fuel consumption calculations. Rather, the model aims to prevent a summer lull from artificially turning a few hours of sailing into a seemingly endless crossing.
OhMyWind doesn't want to decide the route for the sailor
The project's positioning warrants clarification. Despite its travel time calculations, OhMyWind does not present itself as a routing tool designed to automatically find the optimal route.
It doesn't generate a multitude of virtual routes to select the one that would theoretically get you there the fastest.
The boater plans their route. OhMyWind then analyzes that route.
The route is divided into sub-segments, typically about 10 miles long. The system retrieves the expected conditions for these segments, determines the wind angle relative to the course, and queries the boat's polar curve.
Tacking is also taken into account. When the destination is too close to the wind?s path to be reached directly, the program calculates an efficient angle of approach based on the VMG, then estimates the actual progress made in the desired direction.
And this distinction has an important implication: a route proposed by a boater may still be a poor choice. A numerical calculation does not validate either its safety or its nautical suitability.
French political movements give the project a more local focus
OhMyWind operates globally, but part of its development efforts are focused on the French coasts of the English Channel and the Atlantic Ocean.
The reason can be summed up in one word: electricity.
A global oceanographic model may be sufficient for describing large-scale offshore structures. It becomes much less suitable, however, when preparing to navigate the Raz de Sein, the Fromveur, or a strait where the current converges over a short distance.
The project therefore built a cascade of three springs.
The first level contains the SHOM?s C2D tidal current atlases. Nine atlases cover approximately 13,000 points, located primarily in the English Channel, Brittany, and along the Atlantic coast.
When this data does not cover the location in question, OhMyWind uses the MARC PREVIMER harmonic atlases. Their grid resolution goes down to 250 m in certain areas of Finistère and southern Brittany.
Finally, the SMOC product from Copernicus Marine Service, distributed by Open-Meteo, provides global coverage with a resolution of approximately 8 km.
Above all, this hierarchy makes it possible to address a very practical problem. In certain stretches of the river, the current is not merely a minor correction applied to the ETA; it determines the entire navigation process.
OhMyWind therefore projects the current along the boat's direction of travel. The speed through the water (STW) then becomes the speed over ground (SOG).
For a recreational sailor preparing for a coastal crossing, this part of the plan is undoubtedly more critical than knowing an ETA down to the minute. Choosing the wrong tide phase can cost much more time than the few tenths of a knot gained through better sail trim.
An open-source project that also shows its calculations
Another unique feature of OhMyWind is that its code is released under the AGPL-3.0-or-later license.
This decision involves more than just free access to the software. The polar plots, calculation methods, and harmonic predictor used for MARC can be viewed in the project repository.
For navigation software, this transparency makes it possible, in particular, to understand what factors determine a crossing time or a difficulty rating.
The methodology ( https://ohmywind.fr/methodologie ) explains, for example, how the sea reduces a sailboat's theoretical speed. The calculation takes into account both the significant wave height and the direction of the waves. Head seas hinder progress more than following seas.
The project also assigns a complexity rating to each passage on a scale of 1 to 5. The maximum wind speed and significant wave height form the basis for the classification.
A situation where the wind is blowing against the current can raise this level, as can short, choppy waves with a steep crest. Here?s a concept that will resonate with those familiar with areas with strong tides: 20 knots of wind and a 2-knot current don?t tell the same story when they work together versus when they work against each other.
But this note is not a safety bulletin. It takes into account neither the crew's experience, nor their fatigue, nor visibility, nor traffic, nor the boat's actual condition.
A marine laboratory as well as a design firm
OhMyWind therefore remains a work-in-progress whose significance goes beyond the simple question of whether the estimated time of arrival will be accurate to within ten minutes.
It combines into a single chain elements that sailors typically manage separately: weather forecasts, waves, sailboat performance, tacking, tidal currents, and ground speed.
The choice of open source adds another dimension. The assumptions are not merely used; they can be examined, discussed, and modified. Contributions focus in particular on polar coordinates and the expansion of available data.
For the recreational boater, the result primarily offers a way to plan and compare a voyage. Then it?s a matter of doing what sailors have been doing since long before the advent of algorithms: comparing the plan with the official weather forecast, nautical charts, the boat itself, and what?s actually happening out on the water.
Because OhMyWind can calculate the course you give it. It still doesn't know if the crew will want to tack for eight hours.
