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I think it is advisable to take every opportunity to try to provide guidelines and good practice for a better experience for all beginners. The “Top sailors” may help a lot with this as well, and by doing that, they will enhance SL Sailing, with better trained sailors, better races, and finally better competitions. I do not position myself in the top-sailor category, but nevertheless I try to provide an educational aspect to everything I do. This is simply to return the favor to the community, which has brought me so much, and still does today. Since the glorious days of the SYC regatta (on SLCN), I published a slideshow “How to race SYC Olympic”, ditto for “Hotlaps”; each Fizz Cup has been accompanied by an extensive training program to learn to sail with Fizz2 then with Fizz3. Today, the Sailing Academy takes over, always in the same vein, more and more training initiatives flourish in SL and that’s great.
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The SL Sailing community has, I think, everything to gain by facilitating the learning of new incomers, helping them to sail better and better, and giving them all possible assistance to compete without shame with our best skippers. Some beginners do not even dare attend certain events, for simple fear of “interfering”. A true beginner has so much to learn: how to handle the boat, in some cases understanding gestures, get basic sailing knowledge, then a few basic rules, understand the process of races (starter…) etc.. By helping them on all necessary points we will contribute to greater and better participation. I do not mean “raising” the level at any price, but simply to be able to “smooth out” some gaps.
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I have always enjoyed watching the spectacle of races in which the tactical elements and use of rules outweigh the skills in handling the boat. That’s all I want for the 2010 ACA-SL CUP: great entertainment and a fabulous show.
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This article is therefore in this lineage. I have the opportunity here to try to demystify some of the “barbarians” terms in the jargon of experienced skippers: polar, TWS, TWA, VMC, VMG, VTD, VPP, OTA, SOG, AWA, AWS, DTG, TAE, BRG, COG, CMG, ….and also RTFM. OMG! The subject is vast, and the Sailing Academy will soon deliver a complete training on all these subjects.
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In this introductory article, I propose by relying on an example, to demystify some notions of the “VMG“. To do this, I will rely in part on a course of the next ACA-SL CUP, and her majesty the ACA33 2.52 herself.
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Some paragraphs of this article have been put into practice by a group with which I work/train, and several minutes have been gained by most beginners and even some by already experienced skippers. Any beginner, extrapolating a bit on the basics that I try to provide here should be able to make the race named “Course A” for the ACA-SL CUP, in about 10’30 in normal conditions.
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This item will be decomposed into two parts:
- 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”.)
After this (too) long introduction, let’s enter the heart of the matter, laying some groundwork for true beginners.
The basics:
- 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.
The “polar” is generally a graphical representation (or table), of the performance (speed) of a particular boat at different Points of sail and for different true wind speeds. For a given Point of sail, and a given true wind speed the polar will tell you how fast your boat will go (or potentially could go). In SL, Jane Fossett has made a specialty of measuring polar, particularly for Trudeau’s boats.
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Here is an example of typical polar:
A polar diagram shows the potential speed on various points of sail. The circles (radius going from the center point) are how one gauges the speed of the boat. The lines radiating out from the center point show the point of sail. The 12:00 o’clock position shows as if you were trying to sail directly into the wind. This can not happen, so the curved line starts some degrees off. These degrees show from close hauled (1:00 pm on the clock or so) to dead down wind … 6:00 pm on the clock. The degrees of radiating lines show the True Wind Angle (TWA). Doubling the degrees from where the curved line starts, gives a good idea of how many degrees the boat “through Tacks” and gives data Which you can compare to other boats.
The curved line shows the speed of the boat, at that point of sail. Look to the nearest circle and calculate actual boat speed out from there. Multiple curved lines will show various sail combinations and or wind speeds (TWS: True Wind Speed). Sometimes but not always, you will have posted heeling angles as well, which can be helpful to show wind speed and sail combinations … for when it’s time to reef the sail.
It is a visual way to show a lot of data on one page, instead of having to write it all out!
In this example, we learn :
- 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.
A polar helps make estimates of potential speed, and is usually based on point of sail and TRUE wind speeds, and not on apparent wind, as it can sometimes be found. A polar using apparent wind can not help to estimate the potential speed of the boat (even if sails are trimmed to the Apparent wind (not implemented yet on the ACA33 2.52), the boat performance is best characterized with reference to the true wind). I’d let the experts debate this point.
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Here is another example, which for a given wind, shows the successful use of the spinnaker downwind (curve in blue), and when it is appropriate to use it (here beyond 95 °, but it could be safer beyond 100 °, more stable, no risk to collapse).
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Note: It is fairly easy to draw polars for boats in SL, including those that let you set your own wind (use “/1 set wind …. ” chat command on the ACA33 for example). Set your wind, choose a particular point of sail, adjust your sails perfectly as you were taught, and note your speed …
For example, here are some values for a given true wind speed of 21.3 knots (TWS) on ACA33 v2.52:
| TWA (true wind angle) | Boat Speed |
| 22 ° | 13.7 knots |
| 35 ° | 15.7 knots |
| 60 ° | 17.9 knots |
| 120 ° | 19 knots |
Ok, all that is nice, but
- how may you profit from this information in competition?
- How will it help you?
Well, maybe to win, but at least not to finish dead last …. 
Let’s go a little farther.
Thus we see that the boat will go at different speeds depending on her point of sail for a given wind speed. So far, not a big surprise.
Suppose, as is the very basic principle of sailing, that you want to go from point A to point B.
Let us be more precise: the point A is your starting point, and point B is your arrival point or target (or waypoint, or Race Mark …).
Let’s start with the case where point B is in the axis of the wind, it means the wind comes from B to A. Unless you use the motor, your boat will not allow you to go straight into the wind from A to B. You can also paddle, but you will need a little more time…
You’ll quickly find and understand, and it appears graphically on a polar, if you go from A to B in a zigzag pattern with a low point of sail, say 22 ° with the ACA v2.52, your theoretical speed to be around 13.7 knots. If you navigate to a point of sail of 35 °, you go faster, at about 15.7 knots. However, as your point of sail is higher, you will sail farther (more distance), going far away from the shortest route to your target. May I remind you that the goal in racing is:
- 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 …
This is where the concept of VMG “Velocity-Made-Good” appears, as illustrated by the diagram below:
Explanation of this diagram:
- 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.
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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…
So:
Question 1:
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 |
| 22 ° | 13.7 | ? | ? | ? |
| 28/29 ° | 14.7 | ? | ? | ? |
| 35 ° | 15.7 | ? | ? | ? |
| 60 ° | 17.9 | ? | ? | ? |
The higher the VMG, the fastest the route (has your calculator a “Cos” key?)
Question 2:
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 1: straight line = 1400 meters)
(assumption 2: you are an expert in Trigonometry…)
(assumption 2: you are an expert in Trigonometry…)
Question 3:
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…
Very nice writeup… It is worth noting that many casual sailors only use VMG going upwind, whereas in RL it is really also very useful going downwind.
Thank you! I have struggled with VMG through two other websites and your the first one to explain it to a NON-mathematician. Tomorrow I begin the “VMG Applied” article.