Polar Diagrams and Performance Predictions
Performance Package for Endeavour 40 and 43
The United States Sailing Association has analyzed the speed potential of over 700 offshore production boats. USSA then developed a Performance Package for each of these boats including true and apparent wind polar diagrams showing the speed potential on each point of sail.
As of January 1999, packages were available for the E40 and E43. The members price was $170 and non-members was $190. When calling, you must be able to supply the following 5 items: 1) yacht name, 2) sail number, 3) class, 4) prop type, and 5) percentage of overlap.
Call USSA at 1-800-877-2451 for general information. To order, call 1-401-683-0800.
What's Your Boat's Speed Potential?
To develop some of the information you would get from USSA and determine your own boat's best sailing angles for various courses, pick a day with relatively steady wind. Enter the location of a convenient mark in your GPS. Our objective is to optimize your VMG.
What is VMG?
VMG is velocity-made-good, sometimes referred to as speed-made-good. Here is a simple example of VMG. Your boat is going at five knots, according to the knotmeter against a current of two knots. Your boat's VMG, or speed over the ground, is three knots. You turn the boat around and your knotmeter still reads five knots, but the current is now pushing you along at an extra two knots, so your VMG is seven knots.
Today's GPS measures VMG easily and automatically, taking into consideration all the factors, including leeway and current, to measure your true progress (your VMG) toward your next waypoint. For a full discussion of VMG and polar charts, see Chapter 20 of Sail Power, by Wallace Ross. This book was written before the advent of the GPS. Still, it's a good discussion of sails and how to make them work for you.
Don't try to use a LORAN for this exercise. LORANs don't update their data quickly enough. LORAN is great for cruising and daysailing. For racing, if you want navigation instrumentation, buy a GPS. We will talk about more complete instrumentation in another column.
Sail a mile or more to leeward. Set the GPS GOTO function to that mark. Scroll to the GPS screen that reads VMG. Now start beating toward the mark. Try various course angles, sheet tensions, and genoa lead and traveler settings, adjusting your sail trim to suit.
Dedicate a crewmember to monitoring the GPS, recording the course, sail trim, and wind speed, and the VMG resulting from each change. You will soon see which combination of course and sail trim results in your boat's best speed made good to weather in that particular wind/wave combination. Your speed potential will probably vary quite a bit with different wind/wave combinations and course angles. Does your boat do better while pinching or footing? Within a beat, pinching is sailing a little closer to the wind, while footing is sailing a little less close to the wind?
Do the same thing downwind, to find the best gybing angles for your boat and sail combination. Get a mile or two directly upwind of a GPS waypointed mark. Start sailing directly to that mark with your usual downwind sail combination. Note your speed, then come up 10 degrees and check the GPS for the change in VMG. Even though you are sailing slightly away from the mark, the chances are your VMG has improved, meaning that you are moving more quickly toward your destination, even though you are traveling more distance and will have to gybe to make the mark. Come up some more and again note the change in VMG. Keep experimenting with VMG and make notes of what angles work best.
Reprinted from the America Online Sailing Forum Newsletter
GSTDPeterO is Peter O. Allen, Sr., a retired association executive from Rochester, New York. He has sailed and raced dinghies and various racer/cruisers for more than 30 years. He is a US Sailing Certified Club Race Officer. He and his wife currently sail Canto, a '68 Pearson Wanderer, as well as a Laser. He has been designated as the principal race officer for the Sunfish Pan Am Games Trials, to be held in Rochester July 9-11.
Date: March 3, 1999
From: Laura Deere ldeere@swri.edu
The PHRF adjusted for our boat on Canyon Lake is a 236. It is about right for the other boats that race in our fleet. I don't know how well we would stand up in other areas. We normally would race against a Irwin 28 PHRF 222, a Tayana 36 PHRF 227, a Valient 30 PHRF 222, and a few other smaller boats. It all depends on the wind. Our boat seems to race best in 20-25 mph winds. If it isn't blowing we aren't going.
