GRIB file:

Time offset: h  Explain

No of Routes (0 ... 21):
GRIB file gfs20170720103904501.grb (filesize: 80112 byte) contains 32 subset(s) -  1st Valid Date (UTC): 2017-07-20 0000

NOTE: Current default is routing for the Pacific Cup race!

What is this site?

This Cirrugator Easy Routing site offers a router for sailboats, i.e. it tries to predict the fastest route for a given sailboat from a starting to a finish point. This specific version focuses on the Pacific Cup and Transpac races.

The router needs to know the boat properties as provided by the boat's polars, and the weather conditions, which it takes from GRIB files.

This site is a special, simplified version of the router on the main site of Cirrugator, but otherwise identical. It imposes some limits for ease of use, hence its name. Use the Router from the main site if you want the non-limited version. The limits here are:

  • Routing only for the Pacific Cup or the Transpac
  • Only for the full race from Start to Finish
  • Fewer options for tweaking

Usage 1-2-3

  1. Select boat matching yours, or being close to yours
  2. Press Go Button
  3. Wait for response. See below for interpretation

 Click on the top-left text "Cirrugator Easy Routing" to see this start page again.

Routing Options

  • GRIB file: The GRIB files are automatically downloaded by Cirrugator in ~6 hour intervals. The default one shown in the drop-down box is the most recent one available, and is generally the correct one to use.

    Select older ones to study e.g. the impact of recent weather changes on the calculated routes.

  • Time offset: This becomes relevant only when you are about to start the race.

    Let's say you start today at 0900PDT, which is 1600UTC. If the "1st Valid Date" of the GRIB file (found in the bottom line of the above options box) is 1200UTC, then you would enter 16 minus 12 = 4 into the time offset box to account for the fact that at the moment of your start 4 hours have already passed since the 1st valid date. This may not be important, but it will be when you are out at sea and all you have is a 2 day old GRIB file...

  • Boat: Use the dropdown box to choose your boat, or one which comes closest to yours.
    NOTE: If you don't see your boat here, but you have the polars of your boat available, then I can make them available for use in this router. See the "Your Polars" entry below.

    Currently available boats:
  • Settings:
    No of Routes: You can ask the router to calculate several alternative routes at once, though almost always the first one is the relevant one.

    However, the routes may depend sensitively on the starting conditions found in the GRIB file, and in this case the routing might flip to a very different result, like from a "south route" to a "north route". This may be a genuine result or a numerical artefact; it is up to you to determine whether this is relevant or not!

    I recommend routinely trying a higher number of routes to check for anything unusual.

Routing Results

The result of the routing calculation will be displayed in one graph R3, and for each route one graph R2; the latter in combination with detailed numbers for the route.

Note that all data points are mouse-over sensitive! Let the mouse pointer rest on any dot, and values will be displayed.

  • Graph R3: "Passage Overview and Routing using GRIB Forecast (R3)"
    This graph shows a Mercator chart with route(s) and isochrones, overlaid on a wind pattern (red arrows). The black isochrones represent daily intervals, and the gray isochrones intervals of 6 hours. The sidebar of the graph provides some key information.

  • Route Inspection:
    The little table to the right hand side of graph R3 lists the routes by their color name (red, blue, brown, ...), whereby each color name is a clickable link. It brings you to a Route Inspection page, which shows wind pattern graphs for each available GRIB subset. Also drawn in each graph is a mark for your boat at the position where Cirrugator is projecting it to be at the point in time for which this GRIB subset is valid. This is a good way to verify that the position is relevant given the expected wind profile.

  • Graph R2: "Route Analysis vs Longitude (R2)"
    This graph is available for each route calculated, but all but the first are hidden initially. Click on the 'Show / Hide Graph' button to reveal each graph. Note also the 'Show / Hide Numbers' button, which will reveal the numbers plotted in this graph (and a lot more).

    The graph shows boatspeed and both true and apparent wind speed (left Y-axis), as well as true and apparent wind angle (right Y-axis). The blue shading at the top and bottom of the graph relates to the wind angles - the darker blue area represents wind angles greater 160 deg, the light blue angles between 150 and 160deg. Sinc apparent wind angles >160 deg may be difficult to drive over extended periods of time, this shading provides some alert to potentially critical routing.

Your Polars

If you have the polars for a boat not yet listed in this router, and agree to have them shown here, I am willing to implement them in Cirrugator for free. It will allow you to get routings custom made to your boat. Just contact me by email.


Send a mail to



No warranty of any kind is given for the results obtained with this router, not even that it is even fit for the purpose of routing. You must make your own decision on the routes to take!

Be aware of the multitude of error sources: the weather forecast itself as manifested in the GRIB file, the race for most boats is likely to take longer than the forecast can really be trusted, your polars may not quite fit your current boat rigging, and finally this router may have numerical limitations. The impact of all errors may be minimal, but this cannot be stated a priori.


This router takes advantage of the saildocs services to obtain the GRIB files, and this help is gratefully acknowledged.