Sail to the middle of the line and turn head to wind: the end your bow points more towards is the favoured end, because it is further upwind. You can also compare the compass bearing of the line with the wind direction.
A start line runs between a mark on the committee boat, usually a mast or flag, and a buoy at the other end called the pin. The race committee tries to set it square to the wind, but it is rarely exact, and the wind shifts. The end that is further upwind is the favoured end: a boat starting there is closer to the windward mark than one starting at the other end.
Head-to-wind method. Sail to the middle of the line and luff up until the boat is head to wind, with the sails flapping. Look along the line: the end your bow points more towards is favoured. If the bow points almost exactly at right angles to the line, it is square.
Compass method. Sail along the line and take its compass bearing, from one end to the other. Then point head to wind and note the wind direction. If the wind is exactly 90 degrees to the line, it is square. If it is less than 90 degrees from the bearing towards one end, that end is favoured.
Other checks. Sail along the line on starboard tack, from the committee boat (usually the starboard end) towards the pin. On a square line that is a beam reach. If you have to head up from a beam reach to follow the line, the pin end is favoured; if you have to bear away, the committee boat end is favoured.
Keep checking until the last few minutes, because a wind shift can change the favoured end. A few degrees of bias on a long line can be worth several boat lengths.
Things to weigh against the bias:
- the favoured end is where the crowd gathers, and a poor start there in dirty air is worse than a clean start a little down the line;
- which side of the course you want to go to: if there is more wind or a better tidal stream on one side, start at the end that lets you get there;
- at the pin end on starboard you may be unable to tack onto port until the boats on your windward side have gone.
More in the book: Chapter 15, Your first race