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Learn how to sail faster, make the right calls and win races.
On-board instruments present modern sailors with a wealth of information. This book explains what the numbers really mean, and turns this information into racing results. By mastering your instruments you can make the right calls everytime and know for certain when to tack, which shift to look out for and how the ...
Learn how to sail faster, make the right calls and win races.
On-board instruments present modern sailors with a wealth of information. This book explains what the numbers really mean, and turns this information into racing results. By mastering your instruments you can make the right calls everytime and know for certain when to tack, which shift to look out for and how the tide can work with or against you. With colour diagrams throughout, this instructional guide turns information into excellence.
Accessible to those new to racing, it also has a depth of information that will transform the performance of even professional sailors. Cruising sailors will also benefit from understanding how to get the most from their instruments.
Position Fixing System
Accurately fixing your position in relation to both the nearest land and the race course was, for a long time, the essence of being a good navigator. Knowing where you were as you hurtled towards a lee-shore finish line in thirty-five knots – with the 1.5 oz up, and a night whose blackness is only relieved by the luminescence of the white horses – was no joke before the advent of electronic position fixing systems. In the days of the Global Positioning System (GPS) the problem is almost, but not quite, trivial. The skills of good position fixing now mainly consist of finding the power switch.
Once you’ve got the GPS installed and powered up, as a navigator, you then want latitude and longitude, and some idea of its accuracy combined with a relatively easy means of converting this into something more useful; such as cross track error or speed over the ground. The advice in this book is, of necessity, general; the detail of what you need to know can only come from the manual. Whatever they say about it being the last resort, you should read it. But what we are really interested in here is how to use this information to help you win races, and this is the topic of the final section of the chapter.
THE GLOBAL POSITIONING SYSTEM
The Global Positioning System, or GPS as it is more commonly known, has become the system of choice for all sailors, replacing the likes of Decca, Loran and Omega. It provides continuous, global, all-weather navigation in three dimensions with high accuracy and great simplicity for the end user. Its accuracy is such that it can be used for much more than just position. Starting techniques and two-boat tuning have changed because distances – to the line and between boats – can be measured more accurately than they can be assessed by eye.
So how does it work? A receiver aboard the yacht uses timed radio signals to calculate distance from several transmitting beacons. The position of the beacons is known, and the consequent lines of position are used to triangulate the yacht’s position. The radio beacons are aboard (see Diagram 1.1) a constellation of 24 satellites, which orbit the earth at a height of 10,900 miles. A minimum of four should be visible at any one time, which allows us to calculate position in three dimensions. Trigonometry tells us that each measurement places us on a sphere, and three of them intersecting will place us at two possible points. One of these solutions will be ridiculous, nowhere near the earth for instance, so it can be dismissed. The other point will be our position in three dimensions, latitude, longitude and altitude. To see why we need the fourth satellite we must look at how the distance measurements are made.
The distance measurement uses a concept called pseudo-random codes. A pseudorandom code is a succession of noughts and ones, apparently at random, but which actually repeats itself over a period of time. The satellite transmits these pseudo-random code messages, and the receiver listens and compares it to its own internally generated version of the code. All the satellites work on the same two frequencies and they are identified by having their own codes. The underlying assumption is that the code was issued from both sources simultaneously and so by comparing the codes, the receiver can calculate the time that the signal took to reach it.
The critical aspect of all this is the timing, due to the speed light travels the tiniest timing error will lead to quite big distance errors. So when we say the codes are issued simultaneously from the satellite and the receiver, it must be simultaneous. This requires the most accurate possible clocks on the satellite and the receiver. Putting $100,000 atomic clocks on 24 satellites is one thing, but if every user set required one the potential market would shrink somewhat! The solution is to make an extra distance measurement.
We know that the clock error will be consistent to all three measurements. If this is the case then if we introduce an extra measurement the lines of position will not meet at a point (Diagram 1.2). We tell our receiver that if this happens then it must assume it has a clock error and adjust all its measurements by the same amount until they do meet at a point.
Chapter 1 Position Fixing Systems 1
Some Thoughts 1
The Global Positioning System 2
Using the GPS 6
Chapter 2 Instrument Systems 15
Stand-Alone Equipment versus Integrated Systems 15
The Wind Triangle and some Nomenclature 16
Chapter 3 Setting Up an Instrument System 23
Some Comments on the Calibration Process 23
Calibration of the Compass 25
Calibration of the Boat Speed 26
Calibration of the Apparent Wind Speed and Angle 27
Calibration of the Depth 34
Calibration of the Heel Angle Gauge 35
Calibration of Leeway 35
Calibration of the True or Sailing Wind 37
Don’t Panic!! 45
Damping – High or Low? 46
Chapter 4 Some Instrument Techniques 49
Start Lines and Wind Shifts 49
Heel Angle or Wind Speed? 54
Chapter 5 Polar Tables and Where They Come From 57
What are Polar Tables? 57
Polar Table Accuracy 59
Velocity Prediction Programmes 60
Velocity Made Good and Target Boat Speeds 61
Data Collection and Performance Analysis 64
Chapter 6 Instrument Techniques using the Polar Table 71
Next Leg Calculations 71
Velocity Made Good to the Course: VMC 73
Weather and Current Routing 80
Appendix A A Quick Guide to Calibration 83
Appendix B General Maintenance 89
Appendix C Calibration Records 91
Posted May 8, 2013
Posted May 19, 2013
Posted May 22, 2013