sail2win sailing team - one design racing FC8 & X99



Downwind Speed

by John Heyes


Finding the quickest route downwind

In reality every boat has a set of angles and wind speeds at which it's downwind VMG (Velocity Made Good), will be much enhanced and in sprit boats this angle can be very specific, with a pronounced increase in performance. The key to getting down to the bottom mark before the other guys is to know at what angle for the current wind speed your boat is fastest and then to adapt your strategy to spend as much of the leg as possible on that 'fast' angle.
Polar Diagram For boats with conventional, symmetric chutes racing in a mixed fleet, the way to find the best downwind VMG is by using a table of target speeds for each wind strength; data which can be taken from the boats polar diagram. At first glance a polar diagram can appear as a complex picture of concentric circles, but it is really only a simple graph with three rather than two axes. Each curve is a plot of boat speed for a particular wind strength and each point on the curve corresponds to a wind strength value. Only half the diagram is drawn usually as the curves should always be symmetrical. The other half of the diagram would show the same data but on the other gybe. Most manufacturer's will supply a copy of the designers polars for the boat or you can get one calculated for your own boat free with an IMS rating from the rating office. You can get racing software that will compute your own polars and target speeds from recorded measurements of actual boat speeds achieved from the boats sensors.
Target SpeedTarget speed is the speed the boat needs to to be going to achieve the maximum velocity made good, either upwind or down. The simplest way to use target speed information is to have a chart of target speeds for every 5 knots of wind stuck up in the back of the cockpit in easy view of the helmsperson and tactician. Once you know the sort of speed the boat should be going in 15 knots it is then just a matter of finding the right groove or wind angle that will provide that target speed. Write the wind angle next to the target speed on the chart to give the helm an idea of where he/she should be headed. Offshore navigators often have this data provided by computer, on rugged marinised PC's so that they can be used on the rail.

If you find you are sailing faster than the target figure suggests first check the matching wind angle from the polars. It is likely that you are just sailing too high, sailing fast but not making the optimum VMG downwind. Heading off a little and dropping a few tenths of a knot will actually get you to the bottom mark quicker.Similarly, if you are struggling to make the target speed, head up to build speed and then check the wind angle to see that you are sailing high enough.


Instruments that can help

As your downwind performance improves and we look for more ways of going faster, the attention shifts to the quality of the information being used. How reliable are the target speeds, is the builders polar diagram accurate for your particular boat and sails, is the speedo, wind angle and wind speed, calibrated accurately ?

Many of the latest instrument systems can provide a bewildering array of useful computed functions such as true wind angle, VMG, COG (course over ground), SOG (speed over ground), calculated from data only sampled from sensors once or so a second. You may think that a constant instrument display of VMG would be the ideal answer to finding the fastest angles downwind . Used intelligently, it can provide a useful aid as long as the user is aware of its limitations and the errors which can creep in from the way in which the function is computed. The fastest instrument systems sample data from the various sensors; boat speed,compass, wind angle, every four seconds and others can be less frequent.The effects of heel angle on a fluxgate compass can cause error of up to 4 degrees for every 10 degrees of heel. The wind cups at the top of the mast require around 0.2 knots of breeze before they begin to rotate which should be allowed for in the calibration as should an allowance for up-wash of air flow up the rig, artificially accelerating the flow experienced at the masthead.

If you are using a VMG read-out in a sprit boat to find the fastest reaching angle, it is vital that the instruments operate fast, otherwise the information you are trying to steer to will be from the previous gust or wind shift. In the same way that the speed of computer operation is measured by its Baud rate, the fastest instrument system operates at 28,800, others at 9,600 and the NMEA system at just 4,800. So if you are looking to invest in a VMG instrument for a sprit boat check out carefully the specifications and how fast and often it will update before you start to trust the information it provides. In the reality of the race course, the average VMG instrument is unlikely to be fast enough to supply you with the quality of data you require. Conventionally rigged boats are better off in my opinion using target speeds and have an eagle-eyed tactician calling the shifts and choosing when to gybe. Never forget that windshifts do not stop downwind. If you gybe the boat on each major windshift you actually sail a shorter, straighter course to the mark.

For sprit boat sailors racing in a handicap fleet, without the benefit of similar asymmetric boats around them to judge the fastest angles, I guess a VMG display would be a useful starting point to learn the best angles to sail, but after a while you will learn to feel what angle the boat likes for each wind strength. It seems to me that the best helms in the Melges 24 class are mainly ex-dinghy sailors who retain a 'seat-of-the-pants' feel for keeping the boat on the plane and for playing the gusts and waves offwind. There is a further point to be aware of when using VMG displays for offshore or round the cans racing; that VMG displayed gives you a velocity made good downwind value and may not necessarily take you directly towards the leeward mark. Zig-zagging downwind with the asymmetric up it can be all too easy to fly past the mark half a mile away if you are just chasing the optimum gybe angles in a shifting breeze. More useful is a VMG display linked into a GPS (Global Positioning System), which can give you an actual VMG to the mark and helps to guide you into the mark. This is where on board tactical software systems or just an on-screen GPS plot can be a great help, providing a visual picture of your track into the mark and even giving such information as time and distance to go to the layline. We found our way to a mark in thick fog in the middle of the English Channel in the Rolex Commodores' Cup offshore race just using a tiny Magellen hand held GPS , which could be easily used on the smallest of sprit boats.

VMG Tabelle X99


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