How do you determine your tacking angles on the racecourse?
Dan Dickison responds:
Thanks for the question. This is something that many tacticians and racing sailors in general take for granted, but it's always worth knowing that you've got a good understanding of the concept.
My personal preference is to get out on the racecourse early and look at the compass numbers as you sail upwind trying to adjust to the conditions. (Conditions have an influence on tacking angles as you'll see.) Your compass numbers will tell what your tacking angles are, so it's important to pay attention. If you're out sailing in flat water and the wind is blowing about 12 knots (and you're on a reasonably performance-oriented kind of sailboat), it's fairly safe to assume that your boat will tack through about 90 to 100 degrees, if not better than that. But this is when you can use the compass to quantify this assumption and know farily precisely what the tacking angles are. Later on when you're racing, you'll be able to make a quick judgement regarding just how far to go to tack on a layline for the weather mark.
Of course every boat has slightly different tacking angles depending upon the vessel and the conditions. A Melges 24 will tack through roughly 90 degrees in flat water and 10 knots of breeze. But if the breeze drops significantly or if the sea state gets more choppy, that angle will broaden. This holds true for almost every displacement sailboat except perhaps America's Cup boats—these behemoths manage a fairly narrow tacking angle in most conditions.
Some racing sailors simply estimate their tacking angles in their head due to years of experience and others take a more sophisticated approach by using a hand-held compass, a compass mounted on the boat, or a product like Accu Tack, which is simply a placard marked with degrees of heading which is placed on the deck and oriented with the lengthwise axis of the boat. Of course other sailors up the ante even more by using electronic equipment to help them determine the tacking angles, but I feel that it's important to maintain a seat-of-the-pants understanding of just where the boat will be pointed on the next tack. That way, even if your equipment craps out on you, you'll have confidence about calling laylines and crossing situations with other boats.
Here's hoping that this information is helpful to you. For a more detailed description of this concept, have a look at one of several good books available ("Winning in One Designs" by Dave Perry, or "Championship Tactics" by Gary Jobson and Tom Whidden).