Last updated: January 7, 2018
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A 1979 Chrysler 26 Sailboat
SHIP'S LOG FOR THE YEAR 2007
2007 projects and additions:
Polar Plots - Part 2
Polar plots are created using polar coordinates (hence the name) with true wind angle and a vector of actual boat speed. Polar plots are used to determine whether or not the boat is sailing at its maximum speed or velocity made good (VMG) by showing how fast a boat should go at a given wind speed and angle.
Target boat speed is the optimum upwind or downwind point on the polar plot that will get you upwind or downwind the fastest. The polar plot indicates what the target boat speed should be to maximize boat performance (speed); determine whether you are sailing too high (pinching) or too low (footing); aid in sail trimming; and to compensate for changing sea conditions, wind velocities and wind shifts.
The best target boat speed is developed through experience by comparing your boat speed with other boats as well as by constructing your own set of polar plots complete with notes on sail selection, lead position, halyard tension and sea conditions. The best target boat speed is the best speed for sailing upwind or downwind that minimizes the time required to travel towards the upwind or downwind direction thus maximizing performance. Having an understanding of your boat's polar plots is valuable to help understand just how significant the difference in optimum wind angle can be from one wind speed to another.
When you are sailing too fast head up until the boat speed approaches the target speed then bear off to hit the target boat speed at the proper wind angle. When you are sailing too slow bear off to accelerate to the target boat speed, then head up to hit the target speed at the proper wind angle. Adjust the sails only after reaching the target speed and wind angle.
The following method was used to get Mike Andersen's 2006 sailing track data from the Garmin Foretrex 201 into a useable polar diagram. The Garmin binary coordinate data was converted to GPX (GPS Exchange Format) XML data format using GPSBabel. The GPX data were then imported into two excellent applications. The data were first analyzed in GpsActionReplay (GpsAR), a Java program developed by Frenchman Yann Mathet for windsurfing. GpsAR was used to examine the data for bad track points and the data spikes were removed by simultaneously looking at the TrackPoints data table and the Polars-VMG graph. The data were then submitted to the web-based OceanPilot GPS Data Analysis for Sailing and Windsurfing application. The data exported to both GpsAR and OceanPilot contain only X and Y coordinate data and the applications themselves calculate data such as boat speed, wind angle, VMG, etc.
The OceanPilot polar plot provides the speed achieved for each course angle from 0º to 360º. The graph represents speeds for each angle to the wind, 0º at the top, 180º at the bottom, 90º both on the left and right. Concentric circles indicate speed and VMG values. For a given angle, two sets of values are plotted. The outside plot appearing in red is the polar speed curve which represents the boat speed (BSP). The inside plot appearing in black is the VMG speed curve. The document "How to read a polar VGM diagram" is provided on the OceanPilot website.
Looking at the red BSP line, you can see for the 90º angle that the boat speed is approximately 6.3 knots for the starboard and port tacks and the VMG is 0 knots in both cases since the boat is perpendicular to the wind and no progress is being made to windward or leeward. For our sailing conditions the polar VMG plot suggests that the best upwind VMG will be 2.91 knots at a BSP of 4.52 knots at a true wind angle of around 50 degrees for both tacks. The apparent wind angle (the wind as measure on the boat) will be a tighter angle. Downwind the best VMG will be 5.73 knots at a BSP of 6.02 knots when the true wind angle is around 160 degrees. Polar plots are very informative and provide lots of data within one diagram.
Polar plots can be developed by combining the range of wind speed and sail configuration. As an example, boat speed can be graphed for both true and apparent wind speeds. These polar diagrams are then used to determine the optimum course to sail, given existing wind conditions and sail configuration. In these example diagrams the upper curves are for sailing without a spinnaker and the lower curves are for sailing with a spinnaker. The curves are discontinuous because the sail configuration changes for sailing upwind or downwind.
This year we will be using SailCruiser, a PC marine navigation software. SailCruiser accounts for sailboat tacking using wind, speed, and direction and is able to calculate accurate ETA during tacking legs. The program provides a clear display of the optimal tacking route and shows the Tacking Time To Destination. Furthermore, Polar Plots can be entered for individual boat performance. For racers this program would be an invaluable aid, showing tacking routes and TTD around an entire course. For the recreational sailor, SailCruiser provides many useful tools including ETA on journeys.
The February 2007 newsletter of Good Old Boat, featured an article called "Downsizing, upsizing, rightsizing ?", by Karen Larson. I've chatted with Ron and Susie Hatton, of Sacramento, California and they have use their Chrysler 26 as a live aboard spending time on the east coast which entailed 7000 miles of trailoring. This is the third big trip they've made with the boat including the Sea of Cortez, Mexico, the San Juan Islands and Gulf Islands in Canada, and Lake Mead in Nevada. Please email Ron & Susie for more information on extended cruising with a Chrysler 26.
The July/August 2007 issue of Good Old Boat, pages 35-39, featured an article called "The space-age sailor: Understanding satellites and their benefits", by Don Launer.
The July/August 2007 issue of Good Old Boat, pages 40-41, also featured an article called "Wind Terminology: Here are the terms that define the wind", by Don Launer.