take your polars offshore

8/8/2019 Take Your Polars Offshore

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Joseph Comeau

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JUNE 3, 2008

Pro navigator Stan Honey explains how using your boat's polars offshore can get you down the track faster. "From the Experts" in our June2008 issue

by Stan Honey

In the April '08 issue we covered how to use your boat's polar data and target boatspeeds to

get to the next mark more efficiently. This month we'll look at applying the same concepts toan offshore race, where the marks are farther down the course and there are a few more

variables to contend with.

Let's assume you've done your prep work and have your target cheat sheet posted in the

cockpit and at the nav station. The trimmers and helmsmen are all on the same page.

Everyone's goal is to get to the finish as fast as possible. Your tactician or navigator will have

already entered target waypoints into his routing software to best take advantage of projectedand current weather conditions.

A common offshore strategy is to maximize your boatspeed to that next "mark." We call this

velocity made good to course (VMC). As it was for your buoy race, the polar curve you've

gotten from your designer, or with US SAILING's Performance Package, is a useful graphicaltool. You can make a transparency of the polar plot, center it on the boat position on your

chart, and align its vertical axis with the wind direction. If you then create a line perpendicular

to the rhumb line to the next mark, shift the line until it is tangent to your polar curve. This

will show you the wind angle that gives the greatest vector component in that direction. As shown in the diagram at right, this is easy to do with a pair of

drafting triangles.

In our example, the rhumbline direction is 60 degrees. The solid black line is perpendicular to 60 and tangent to the polar curve at approximately 69 degrees.This is your heading to maximize VMC. You will not be heading directly to the mark, but somewhat further off the wind. The tactical expectation is that with

all the wind shifts likely to occur over the course of this leg, opportunities will arise to compensate for not heading straight to your mark.

Calculat ion of optimum routes

The most prevalent offshore use of polars is in conjunction with gridded binary weather data (GRIB files) to compute the optimum course for your boat.

Software packages commonly used for this application include Deckman for Windows, Expedition, and Maxsea.

The polar file used with your routing software should reflect the boat's actual average performance, including the fact that in light air offshore there is often a

sloppy sea state, and in heavy air the boat is often sea-state limited and cannot be pushed above a seamanlike speed upwind in the typical sea-state associated

with heavy air. Further, the boat's average performance includes the full mix of helming skills, not just the performance of the best helmsman. So the polars

you use for routing will end up being a separate file from your target polars. The routing polars need to be achievable. If you run a route on a GRIB file, and it

turns out the first four days of the GRIB file forecast are accurate, in four days the boat actually needs to be exactly where the router calculated it would be. If your polars consistently overestimate your boat's performance, and you don't actually get to where the router calculates you will be at various times in the

future, then the calculated route will not be relevant for your boat, and the routing software will be "playing" shifts you will never experience. There are,

however, many pitfalls in computing optimum routes from polars. Lets look at them individually.

The weather forecasts (GRIB files) are not perfect. A typical GRIB file from the U.S. Global Forecast System (GFS) will contain forecast data extending out 15

days. The GFS data will be good for the first four days or so, but is less accurate further out in time. The routing algorithms don't consider this decreasing

certainty of the forecasts with extended forecast time. So, if you take a 15-day GFS GRIB file, and compute a transoceanic passage, you may find the router

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JUNE 10, 2010

Better Starts in Current (/experts/better-starts-in-current)by (/experts/better-starts-in-current) Andrew Campbell (/no-index/credit/andrew-

campbell)

When there's current flowing across the starting line, there areopportunities aplenty to score the perfect start. "From the Experts" inour June 2010 issue

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JUNE 08, 2010

Get It Over With (/experts/get-it-over-with)by (/experts/get-it-over-with) Stuart Walker (/no-index/credit/stuart-walker)

When trepidation is holding you back from winnig, it's time to setaside your fears, embrace confidence, and watch your results more.From the Experts in our June 2010 issue

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Gear Shifting For Waves (/experts/gear-shifting-for-waves)by (/experts/gear-shifting-for-waves) Andy Horton and Dave Powlison (/no-index

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Waves don't always line up with the wind. When this happens, yourapproach to each tack must differ. "From the Experts" from our June2010 issue

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