Routing and safety precautions

With the advent of powerful weather-routing applications on our tablets and smartphones, as Weather4D and SailGrib WR, at really affordable prices, Routing democratizes for boating. The quality of these applications already seduced the amateur sailors, and even some high-seas professional riders, but also a growing number of leisure boaters. Riders use the routing in the essential interests of performance, achieve the fastest route from one point to another, so leisure boaters are more focused on getting safer and more comfortable routes. For them, the concept of speed is used primarily to avoid periods of bad weather, and the optimized route to escape from areas where sea and wind conditions can be dangerous. The routing is also used in this : even in powerboating, you may try to avoid too much wind speeds or wave heights to ensure ypur crew comfortable and serene trip. But what credit grant routing ?

Can we believe routing ?

I note, to browse a number of boating forums, both in France and overseas,  Many practitioners to throw themselves on this tool in a little smug way, and without actually taking the measure of its limits. It seems to me important to remember that the routing is but a tool to help in the boater decision, all the components is based on hypothetical predictions : weather, currents, Polar speed, What makes it perfectly random, therefore requires an absolute caution.

• Weather and currents predictions :

Enormous progress has been made for decades to predict the weather on the planet, on land and sea, the main purpose of preventing natural disasters, but also to meet the requirements of air and maritime traffic, rail and road, Agriculture and how many other uses. Boaters now have more and more reliable weather forecasts, for areas more and more reduced. Some forecast models can provide data with 1 km resolution, one hour steps. at range of 24 to 36 hours, all the 4 km range up to 48 hours, 16 km range up to 96 hours. This is what is called the Fine grid models (¹). The «global» models that can cover the world offer forecasts for shipping with large grids 30 to 120 Nautical miles range 8 up to 16 days. This is the case of the NOAA GFS model(¹) distributed for free worldwide. These long-term models are for large areas a reliability that can be approximately call 90% to 3 days, 50% to 5 days, and trend beyond. They allow to see the distribution and the evolution of the phenomena, across the North Atlantic in example.

Forecasts of ocean currents and tidal streams , just as reliable, are now available, from the Australian company TideTech or European organization MyOcean (²) frequently mentioned on this blog. These predictions to several days for ocean currents is reduced to a few hours for the coastal currents with high resolution, but are still not approaching a sufficient accuracy for local tidal streams.

• Polar speed

Polar speed are generally derived from specialized programs called VPP (³) combined with naval architecture programs. They are most often provided by architects or boatyards. The problem is they are perfectly theoretical, and can be easily degraded depending on the charge of the ship, the state of the sails and the hull cleanliness, the quality of the helmsman (the autopilot so), and one thousand other factors reducing the theoretical targets speed. Routing applications, thus, typically offer the possibility of applying a coefficient of overall reduction of the polar. All of this brings a certain factor of vagueness.

Routing, distance and grid

The implementation of routing requires some common sense. It should be kept a reasonable proportion between the distance and the resolution of weather file used. To exaggerate, a course of 60 nautical miles with a GFS file 0.5 ° (30NM) will never give relevant routing. It goes without saying, but still worth saying. If it only does have such a file, can not reasonably optimize a route with at least 10 grid points, at least 300 nautical miles, but never routing a trip around Belle-île island, as I read it somewhere. It has no sense !

In the examples below I compare precisely routing on a theoretical course 150 NM, with a GRIB GFS 0.5° and a GRIB high resolution WRF France 0.04° (2,4 NM)

With GFS file without current data, routing provides a gain of 5 hours on the direct route, but through an area of strong winds.

The above optimized route shows 23 waypoints for 23 isochronous calculation. It notes that there is between 3 and 4 isochronous between two grid points, the calculation must therefore operate geometric linear interpolation of data and a temporal interpolation between two grid points. Wind and current constant, this provides little impact on the result, but if the variations in strength and direction are important along the way, the result can be significantly distorted.

With the WRF France file, the route is more accurate in following more closely wind changes but does not avoid the area of strong winds.

With high resolution, in approaching more direct route, the gain is more than from 2 hours on arrival. Is routing to be followed ? In this case the direct route, a little less fast but in medium wind conditions, isn't it better ? For my own this is the choice I would do, except indead if I was in regatta. But I am on a cruise and I don't want to tire neither the boat nor stomachs. So always have a very good reason to deviate from the direct route !

Just compare the grid of two files to be convinced of the proportion necessary to keep between the distance and the resolution of the model to use :

Expanding the map shows that a high density of data between two isochronous allows a more precise calculation, especially if the data varies over the course.

I would like to quote Pierre Yves Goemans, a Belgian sailor familiar of navigation areas with strong currents and frequent changes in weather, who wrote on his blog last year:

Should we therefore believe routing across hundreds of miles?
Yes provided that they do not use the results of how more accurate that are data ! The big lesson I withdraw from this first experience, is that one must analyze the results and understand the route options proposed by the tool, in order to make the strategic choice of navigation, taking into account the uncertainties in the data... Such bold option, to the side or offshore, remain valid if the wind turns a bit or has a slightly different strength? Question to which it is not always easy to answer...

Wise reflection that I offer to the meditation of all, and I shall not fail to remember.

(¹) See the entry "Prediction Models" in the Glossary.
(²) Weather4D PRO further improves routing
(³) Velocity Predicting Program

Related article :
ENCs, Routing and sailing shaves stone


3 Replies to "Routing and usage precautions”

  1. As always, Francis, Thank you for your successful lectures on modern navigation technology based on high technologies of current computing introduced in your blogs.

  2. Thanks again Francis for the sum of free information provided in your blogs.

  3. It is always a pleasure to take knowledge of measured and insightful comments of this excellent blog.
    Thank you Francis

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