“I can’t change the direction of the wind, but I can adjust my sails to always reach my destination.”
– Jimmy Dean
Although Jimmy Dean was better known as a country singer and purveyor of breakfast sausage, this quote helps demystify the magic of sailing from point A to point B for beginners.
Trimming sails to reach one’s destination are part of the art of sailing. Boats can sail upwind and they can sail downwind, but they cannot sail directly into the wind.
Hoist the mainsail of a catboat at anchor and the sail will luff and flutter just like a burgee on a staff as the boat weathercocks into the wind. If the anchor is raised, the mainsail will continue to luff and the boat will remain ‘in irons’ or ‘in stays’ and slowly drift astern. The boat is trapped in the ‘no-go zone’, a 90 degree pie shaped wedge into the wind.
In order to make way, the bow of the boat needs to be turned away from the true wind by an angle of 45 degrees, to the first point of sail, close hauled.
When close hauled, whether the wind is coming over the port side (port tack) or over the starboard side (starboard tack), the mainsail boom is trimmed in close to the centerline of the boat. As the mainsail is trimmed in and stops luffing, the boat will gather way and begin to sail. If the destination is directly into the wind, the only method of reaching that destination is by tacking or turning the bow through the wind and alternating the close hauled point of sail back and forth between starboard and port tacks. This zigzag course is known as beating to windward.
If the destination causes the wind to be forward of the beam, but further aft off the bow than when sailing close hauled, then the sails are trimmed to the next point of sail, a close reach.
The boom is eased to leeward of the centerline and tacking is no longer necessary to reach the destination and the rhumb line can be sailed instead.
When the wind is coming directly over the beam, the point of sail to trim to is called a beam reach.
The boom is eased even further, about halfway to the shrouds, and this is the fastest point of sail for a monohull.
As the wind moves further aft and comes over the stern quarter of the vessel, the point of sail is called a broad reach.
The boom is eased out from its beam reach position, so the wind and sail are perpendicular to each other.
With the wind dead astern or almost dead astern, a boat is said to be on a run or running.
The boom is eased out to its maximum position near the shrouds, and this is the slowest point of sail for a monohull.
These basic definitions of the five points of sail approximate the trim necessary to allow a boat to go in the direction one wants, but they don’t describe how a boat uses the wind to propel itself.
Understanding that bit of science depends on whether one is sailing upwind (close hauled, close reach and beam reach) or downwind (broad reach and run).
The mainsail is a vertical fabric airfoil supported by a mast and boom.
Envision an aluminum airplane wing standing on end in place of the mast with the wing tip where the head of the sail would be, and the aerodynamics of sailing can be compared to the aerodynamics of flight. An airplane uses lift to fly, caused when the air pressure above the wing is lower than that below the wing. When sailing upwind, all sails use this same physics of lift to pull the boat forward. The leeward side of the sail has lower pressure than the windward side of the sail and aerodynamic lift is generated to propel the sailboat. Whether describing a wing or a sail, Bernoulli’s Principle and its associated equations predict how this pressure differential creates lift.
When sailing downwind, lift is replaced by the aerodynamics of drag, and the boat is propelled forward by push. Think of the square-rigged ships during the ages of exploration and colonization of the New World, they were designed to be pushed across oceans by the invisible hand of the trade winds.