The PredictWind weather router is the best way to get the most accurate forecast data for your passage. The data available in Map, Graph and Table format is detailed, simply looking at a Wind or Wave Map critical information is missed. Not only should this be your go to tool on passage, after you have established a day to depart using the departure planning, fine tune the departure by using the sail routing.
General Notes on Sail Routing
The weather routing or departure planning will always use the highest resolution available for each model,(1km,8km,15km, 25km,50km) in short the most accurate data.
For a detailed analysis of exactly what we do, please refer to this Help article.
As the weather routing or departure planning is calculated on the PredictWind server, it is not dependent on what GRIB files you have downloaded, making the weather routing file size small, typically 15-25Kb.
This multiple forecast model approach is called 'Ensemble' forecasting, where you can have high confidence if all forecasts are similar. If there are differences in the forecasts and weather routes for each model, it shows the uncertainty in the weather situation.
The GRIB files (Maps) which are still important to download are only used to visualize the routing, giving you a general synoptic weather overview. The big picture.
Tips for interpreting the Sail Routing results
If you are looking at a 'coastal' route, the PWG/PWE forecasts are 1km and will model the geographic/heating effects( sea breeze etc )
For short-term decision-making, you should select the forecast that appears to be doing the 'best job' of matching your onboard observations, barometer, calibrated wind gear and from the wind observations page in PredictWind.
Pro Tip: Mid ocean you should be able to see boats with a PredictWind DataHub providing wind data on the PredictWind observations page, validating the conditions ahead.
In addition, try to understand if there is an apparent reason that the routing is going in a particular direction, i.e. is it due to wind direction or strength? Please watch the animation carefully. The weather routing is not an exact road map to follow, but a general trend, as the timing and placement of the weather pattern is never 100% accurate.
If there is a lot of uncertainty, the routes will be spread out, and it may be best to sail the fastest VMC along the course. But generally, the routes should line up for the first 12-24 hours. You should be updating the weather routing with each new forecast every 12hrs. To check when the forecast updates, please see Update times.
The routing algorithm will find the best route based on your boats performance polar and the forecast. It looks at ALL the possible paths and selects the best route. However, it is only as good as your boats polar and forecast accuracy. The forecast is an average of over 1 hour, but the forecast at the lower resolutions, 8km & 50km, is only output every 3 hours. So, the optimum route does not have the granularity to help with short-term wind shifts and variations in wind speed. So, you must decide how best to use the short-term gains over a 3-hour period. We recommend treating the optimal route as a general guide and fine-tuning it for the exact conditions you experience.
The best route is only as accurate as the forecast and the boat polar you have set. Please ensure you have correctly set up the boat polar and that the boat speeds shown in the Sail Routing Wind Tables seem reasonable to your actual boat speed ( Note : we show SOG so do make some allowance for current and wave action on the boat).
Wind angles we use with sail routing to define upwind/reaching/downwind
% time upwind = True Wind Angle is 55 degrees or less.
% time reaching = True Wind Angle is between 55 and 135 degrees.
% time downwind = True Wind Angle is 135 degrees or more.
Wave Polars - RMS Roll, Vertical Acceleration & Slamming Incidence
Your boats motion in a sea state.
This is a great feature not only for on passage. Looking at this data can help with Pre departure decision making.
RMS Roll, Vertical Acceleration and Slamming Incidence (Graphs and Tables) are only available on the Professional Subscription Upgrade Here. They are crucial safety data sets, and you must review them before setting out. If you're sailing offshore and still need to upgrade to Professional, this is the reason to do it.
To read more about how to interpret these graphs and tables, please look at this article about Wave Polars.