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More detail:
Why Instruments? (PDF)
Why Instruments?
Sailing instruments do not replace your senses, they enhance and validate them. They also provide information about things you cannot sense directly, e.g. true wind and current. They are vital for comparing performance with target speed, calculating time to laylines and many other items crucial to increasing your probability of winning.
True wind
Boatspeed is certainly an important item to keep tabs on, but for racing, nothing beats true wind speed and direction. (What is true wind?)
Onboard the boat, you can sense the apparent wind
and
boatspeed, but not the true wind. Integrated instruments use
trigonometry to calculate true wind speed, angle and direction.
Apparent wind includes boatspeed and heading, so it
changes
depending on current performance, e.g. during a tack. On the other
hand, true wind is God’s work and won’t change
depending on
what you’re doing at the moment (except maybe praying).
- True wind is the boat’s engine. Knowing it lets you get the most out of your boat and sails – e.g. polars and targets.
- Knowing the true wind, your polars and the bearing of the next leg lets you predict apparent wind on the next leg.
- Keeping tabs on wind direction lets you see even small wind shifts, so you can get onto the lifted tack and onto the favored side of the fleet.
There are a lot of very inexpensive products that use GPS alone to try to monitor performance. A good example is the Velocitek unit. These things can show you your speed (over ground) and have nice post-mortem playback features. However, they don't sense wind, so they infer it from tacking angle. There are many subtle and not-so-subtle issues with this. Rather than going in to detail, here is a white paper on the subject.
Paddle/compass vs. GPS for performance monitoring