- As a first step (this article) we will see some basic definitions, and why it is good to know them,
- In the next chapter, we will put into these ideas into practice on the course at the ACA-SL CUP. (You will have some homework to do as well…“Teach them to fish and you feed them for life”.)
- Each sailboat has her own characteristics in terms of design, rigging, behavior, equipment and performance.
- In terms of performance, data that is commonly used, is called the “polar“. In RL, the designers of racing yachts often provide a “VPP“ (Velocity Prediction Program) which, taking into account the characteristics of the vessel, will permit for different wind speeds and Points of Sail, to determine the speed of the boat. The result is called a “polar”, usually refined by real tests on the water. This practice (to provide VPP) is unfortunately not in use in SL.
It is a visual way to show a lot of data on one page, instead of having to write it all out!
- For this boat, the minimum point of sail is about 30 degrees (however she will sail at a low speed with this true wind angle).
- With a wind of 30 knots (green), the boat should go about 10 knots at 45 °, 20 knots at 90° and 27 knots at 140 °. This polar perfectly shows the “no go” zones below 30 ° upwind, and beyond 150 ° downwind, points at which performance decline considerably.
|TWA (true wind angle)||Boat Speed|
|22 °||13.7 knots|
|35 °||15.7 knots|
|60 °||17.9 knots|
|120 °||19 knots|
- how may you profit from this information in competition?
- How will it help you?
Let’s go a little farther.
- Not to go as fast as possible
- Neither sail the shortest route,
- but to find the ideal compromise between distance and speed to arrive as quickly as possible, and if possible before the others, to your target, usually a buoy, cunningly placed right in the axis of the wind by wicked organizers …
- We want to sail into the wind, going upwind from A to B..
- Draw a red route. Suppose the polar says: with a 30° angle to the wind, estimated speed is 5 kn. I trace this route for an hour and then I tack (-30° to the wind), to reach the shortest way to my target after an hour.
- We now trace the green route for a wind angle of 45 °. The polar gives me a speed of 8 kn.
- We finally trace the Blue Route for a wind angle of 60 ° to the wind, the polar gives me a speed of 10 Kn.
- It notes that after two hours the green route, although longer than the red edge me closer to my target. By contrast, blue, even longer, bring me too far from the target (but is still a better choice than red).
- The best compromise (speed/distance) between the 3 routes, is thus the one in green.
VMG is the projection of your speed on the wind axis. In the diagram, the VMG are given along the direct route to the target.
- Vs is the speed of the boat, and A angle upwind.
- In this example VMG 45: 8 • cos (45) = 5.66 Kn
Ok, maybe easier, if your scientific calculator is not waterproof, you can also use the polar graph to estimate the VMG, as shown in the picture below. Simply draw a straight horizontal line to the polar vertical axis, it will tell you VMG:
Easy, isn’t it?
In real life (and sailing a Fizz or ACC in SL), tidal streams/currents have also to be taken in account to determine the best optimal route.
It happens sometimes that the VMG is also graphically represented on the polar graph, as in the picture on left.
Ok, but I still have no idea of what to do with that !!!
Okay, here we go… Back to the “Course A“, for the ACA-SL CUP.
- You are starting from the startline at Pacific.
- Your target: the buoy located at Flotsam.
- The true wind is set to 225 °.
- The buoy is in the wind axis (not exactly but well…see below)
- Distance in a straight line is approximately 1400 meters.
Note: I say here that the buoy is in the wind axis, to avoid introducing more complex concepts, including VMC “Velocity-Made-On-Course” (or VTD: Velocity To Destination), but you can learn all this at the Sailing Academy. Many speak wrongly of VMG instead of VMC, including in RL where some manufacturers (GPS, Sailing SW) are fueling confusion. For VMC, the reference is no longer the axis of the wind but axis to the active waypoint, is to say the waypoint towards which you sail, typically using a GPS (or dedicated scripted tools in SL, such as Navigator), and often the next race mark in a regatta. The VMC is a corrected ground speed using drift and all the effects of the current. Another thing is also to learn to select the fastest course to sail upwind and downwind in an oscillating breeze, the “Wally”, and how to use a polar graph to estimate the VMC…
Oh, btw, it is time for me to have a break,
and time for you for some homeworks…
What is the optimal true wind angle from the Pacific line, to arrive as quickly as possible to the buoy at Flotsam (assumed in the axis of wind)? To help you, here is a table of the polar of the ACA33, with the conversion knots to meters per second (with TWS = 21.3 knots):
|TWA (True Wind Angle)||BS (Boat Speed) in knots||BS in m/s||VMG in knots||VMG in m/s|
Once you had determined the best VMG,
- what is the actual distance you will do to reach Flotsam,
- and how many seconds will you need to get there?
(assumption 2: you are an expert in Trigonometry…)
After answering the first two questions, for a perfect race, what is the exact spot where you will tack to join Flotsam in 2 tacks?
Turn on your calculators, it will be solved in the next article …
I urge competitors to the SL-ACA to check themselves their polar, as I could voluntarily introduce some errors … and practice will never replace the theory.
In the next article to come soon, we will also discuss some points of strategies for a better start in fleet races …pfff…