Thursday, June 10, 2010

Polars and routing

America’s Cup winning navigator Peter Isler shares a nugget on how to tune a boat's polars to improve routing information:

Garbage in - Garbage out. That saying certainly applicable in the world of weather routing, where software takes precious few inputs as “givens” and then produces an optimum route that appears so nice and tidy on the computer. But the whole process is fraught with peril. The boat’s polars, the weather “GRIB” files, and even the settings of various dials on the software itself all affect the end result. When coming up with a strategy for the crew on deck, the ocean racing navigator must consider the multiple variables that affect the calculation of this so called ‘optimum route’, tweak them to the best of his ability and then interpret the result with careful analysis.

A good place to start is the boat’s polars. These usually start their life as the output of a computer program in the designer’s or handicapper’s office and on most boats that’s how they stay. But before you do any weather routing (or polar editing) you need to know to what wind speed the polars are referenced. Most designers work with polars that are referenced to the meteorological standard of wind speed at a height of 10 meters. But wind speed varies with height and if your wind instruments are located at the top of your mast, you (or your designer) will need to convert the ‘10m polars’ to ‘masthead polars’. The process involves using a conversion formula which unfortunately is not always ‘perfect’.

In fact nothing in the whole routing process is perfect, so one should not expect perfection in the outcome. But careful preparation and a thorough understanding of the common routing pitfalls can greatly improve the results.

MORE INFO: On June 16, in Newport, RI, Peter Isler is giving a seminar on using Expedition software in the Newport to Bermuda Race. Details at: http://www.customoffshore.com

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