Sea Technology

JAN 2017

The industry's recognized authority for design, engineering and application of equipment and services in the global ocean community

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Page 45 of 76 January 2017 / st 45 Problems with Polar Targets Polar plots have traditionally been expensive and diffi- cult to obtain. For older boats, it could be difficult to track down polars from the manufacturer. Even if you get the polars from a manufacturer, you'll probably get polar "tar- gets" that give you the boat's "ideal"—not actual—perfor- mance. Computer simulations have been the other main source of theoretical boat speeds on polar plots, i.e., polar targets. For example, U.S. Sailing, the governing body for sailing in the United States, sells theo- retical polar plots made with a computer simulation called a velocity prediction program (VPP). However, a polar plot of theoretical targets doesn't ensure that the theoretical po- lars are valid. To overcome this hurdle, SailTimer Inc., as an indus- try first, made a GPS device that could learn the actual polars for an individual boat. Later, the SailTimer app was also the first mobile app that could learn polars for an in- dividual boat, which, for a racing boat, eliminates the need for a paying sponsor or professional tactician on board. Sailing experts used to tell us that it is impossible to record actual data on a boat's polar plots; these are only theoretical targets that can never be measured. But you really only need wind data and boat speed. Why navigate with theoretical numbers that are generic for a model of boat or made from a complicated comput- er simulation when you can navigate with actual perfor- mance data from your own individual boat? The data are now easily available. S ailboat navigation is at a historic point, and innovation in mobile apps is opening up new navigation methods. An example is the free SailTimer app, which is unique in using polar learning and tacking distances to determine optimal tacks or "laylines". This software was origi- nally developed (in the days before apps) because GPS chartplotters did not account for tacking distances when displaying estimated time of arrival (ETA). This prompted SailTimer Inc. to make the first GPS and, later, the first mobile app that could display opti- mal laylines. The logic of the SailTimer tacking results is very straight- forward. It starts with the tacking distance and the boat speed to determine the tack- ing time for different routes and the optimal tacks. The same as if you are wonder- ing how long it would take to drive a car 10 mi. at 5 mi. per hour; you would divide the distance by the speed: 10/5 = 2 hr. Simple. Since 2012, some GPS chartplotters and other apps have also started to display laylines. They use a tradition- al method dating back to the 1940s, before GPS and com- puters existed. The optimal tacks were traditionally determined us- ing polar "targets" and velocity made good into the wind (VMG-W). They were originally the domain of weather- routing software for crossing oceans. But does it make sense to use a 1940s method in an expensive new GPS chartplotter or multifunction display (MFD)? Sailboat Navigation Evolves SailTimer App Improves Navigation Using Actual Boat Performance Data By Dr. Craig Summers Polar plot showing the unique speed profile for a particular sailboat in all wind directions.

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