Wind usually oscillates with variations both in speed and in direction. An increase in wind speed is called a puff and a decrease in wind speed is a lull. The wind is said to veer when its direction shifts clockwise on the compass and to back when its direction shifts counterclockwise.
The direction the wind is coming from—upwind—is called windward and the opposite direction—downwind—is leeward (rhymes with “clue word”). A turn across or through the wind, so that it blows against the other side of the boat, is called a tack. When the starboard side is windward, the boat is on starboard tack. When the port side is windward, it is on port tack. When running downwind, a tack is usually—but not always—called a gybe.
A wind that moves more forward, toward the bow, is called a header and the boat is said to be headed. A wind that moves aft gives a lift and the boat is lifted. Getting headed while sailing upwind can signal a good time to tack, since a header on one tack becomes a lift on the other.
Points
of Sail
There is true wind—the speed and direction of the wind—and there is apparent wind, which is the combined effect of true wind and boat motion. Apparent wind is what moves the boat, not true wind. Forward motion of the boat shifts the apparent wind forward of the true wind. The boat’s angle to the wind is indicated by the corresponding state of sail trim using terms defined every 30° of arc:
These points of sail are the same on either port or starboard tack. Changing direction by moving down the list, or off the wind, is called falling off. Changing direction by moving up the list, or into the wind, is called heading up. After heading up, while sailing into the wind, you are said to be beating. After falling off, while sailing downwind, you are running.
Close Hauled
or Beating
A properly trimmed sail has a curved shape that acts as an airfoil like the wing of an airplane. Wind hitting the sail from forward splits into two streams. One flows around the convex outside, or leeward surface, of the sail, speeding up as it follows the curve. The other flows across the concave inside, or windward surface, slowing down as it meets the sail.Air pressure is thus less on the leeward surface, giving it a pull, and greater on the windward surface, giving it a push. The combined force of air pressure on the sail—the pull and the push—is roughly perpendicular to the sail itself, not to the wind. Only part of that force points forward; the rest, called the heeling force, pushes the boat sideways, and the keel resists it. Combining the sail force with the keel’s resistance moves the boat forward at an angle to the wind. Most sailboats can point within 40° to 50° off the wind on either tack, creating a no-go zone of 80° to 100° directly upwind where the boat cannot go because it would then be in irons.
It might be worth noting that our explanation of how wind interacts with sails is not strictly correct from an aerodynamics standpoint. It is, instead, a simplified model useful to sailors. More complex models using Bernoulli’s Principle (based on pressure) or Newton’s Law (based on forces) are more accurate and yet less useful when sailing.
Reaching
A beam reach or a broad reach is the fastest point of sail for most boats. Beating while close hauled might feel faster because this point of sail has the most apparent wind. The apparent wind speed while beating is nearly the sum of true wind speed and boat speed. Apparent wind while reaching has only a little more speed than true wind, and boat motion only moves it somewhat forward of true wind in direction without adding much to its speed.This means that when you come off a run and head up into the wind, boat speed and apparent wind will both increase as you move from a broad reach through a beam reach. Then the boat will slow down while the apparent wind continues to rise as you move through a close reach to close hauled. What you are getting close to is the wind, which is why you slowed down.
Running
before
the Wind
Everyone knows that boats cannot sail directly upwind, but fewer appreciate that most boats cannot sail well downwind, either. A boat sailing directly downwind will quickly find itself by the lee, meaning that the wind has shifted around behind the stern onto the same side as the boom, the headsail has emptied and begun to luff, and the boat is about to gybe.Downwind is also generally the slowest point of sail, because only the aft faces of the sails are working; the forward sail surfaces are stalled and give no lift then. Also, the apparent wind that moves the boat is least when sailing downwind, and it rises steadily as you head up into the wind. What makes reaching faster, even though the apparent wind is less than when beating, is that on a reach more of the sail’s force points forward, driving the boat ahead instead of heeling it and pushing it sideways.
VMG
Except when reaching, you must tack at an angle to the wind that maximizes—not boat speed—but rather, progress toward a goal. Called velocity made good (VMG), this is the product of boat speed and the cosine of the angle off the intended direction of travel. Beating upwind, the best close-hauled angle is where speed and direction balance out for maximum VMG.Downwind you can sail deeper—more directly downwind but slower—or tack back and forth at a faster, broader angle. Often broad reaching about 30° off dead downwind, rather than running straight before the wind, gives faster VMG toward a downwind destination because the boat speed gain outweighs the extra distance.
Racers obsess over VMG and have simple instruments that compute and display it. Cruisers can still benefit from knowing how far off the wanted direction of travel to point for highest VMG.
Tacking
and Gybing
Before tacking, the helmsman traditionally warns the crew by shouting, “Ready about.” Crew respond by shouting, “Ready.” The helmsman then shouts, “Hard alee,” and puts the helm down smartly, turning the boat into and through the wind. “Hard alee” refers to a tiller that moves opposite to the windward direction of the turn. On a wheel-steered boat, the helmsman might shout “Tacking,” instead.The instant that the jib begins to luff and backwind the mainsail, crew release the former working sheet and begin to harden up the new working sheet. Keys to a brisk tack are keeping momentum, not turning too far downwind on the new tack, and releasing the jib sheet late in the tack, but not so late that the new working sheet cannot be hardened up smartly before it comes under strain on the new tack.
Going upwind toward a destination, you can tack once or you can tack many times. Since the sails impart no power during a tack, while the turn adds some distance, tacking only once is the fastest course, all else being equal. This assumes constant wind; if the wind veers or backs, as wind often does, it might be better to tack when headed because a header on one tack becomes a lift on the other.
Going downwind, it is again best in most boats to zigzag, tacking from one broad reach to the other. A downwind tack, where the stern passes through the wind, is called a gybe. As when beating, the boom will cross the cockpit, but because the mainsheet is now loose, it will travel much farther and faster than when tacking upwind and can crash across, damaging the gooseneck at the mast, the traveler and other parts of the rig.
When gybing in all but the lightest air, first harden up the mainsheet to center the boom and then ease it after the boom comes across. Avoid sailing by the lee, when the wind crosses the boat astern and blows from the leeward side, lest you gybe accidentally without having time to center the boom.
When running, always be alert for signs that the boat is about to gybe. As it turns directly downwind, the boat will stop heeling and stand upright. The jib will empty and luff as the mainsail begins to block its wind. It is by then probably too late to harden up the mainsheet and you would be wise to duck as the boom crashes across.
Sail Twist
True wind at the top of the mast has higher speed than true wind at the deck, where friction of the air with waves and the water surface slows the wind. Boat motion is the same at every height, of course. Apparent wind, which is the sum of true wind and boat motion, thus has higher speed at the top of a sail than at the bottom. Apparent wind therefore also shifts aft in direction, higher up on the sail, as a consequence of its rise in speed.Another way to think of this change in apparent wind speed and direction, as you move up the sail, is that the top of the sail is in a puff (higher wind speed) and being lifted (more aft wind direction), relative to the bottom, while the bottom of the sail is in a lull, and being headed, relative to the top. This means that the bottom of the sail must be trimmed differently from the top. That difference in sail trim at the top versus the bottom is called sail twist. Many sails have a certain amount of twist sewn into the cloth.
When moving upwind, while apparent wind speed is greatest, the aft shift of apparent wind at the top of the sail is also greatest, and we want to add twist by easing the top of the sail while hardening up the bottom. As we fall off, and apparent wind speed drops, the aft shift of apparent wind at the top of the sail grows smaller, and we want to remove twist by hardening up the top of the sail while easing the bottom.
Proper twist can be set without thinking much about the details. For the jib, while sailing close hauled, first trim the sheet until the luff telltales stream aft. Then head up slowly while watching the windward luff telltales. If the upper ones lift first, move the jib turning block car forward to reduce twist. If the lower ones lift first, move it aft to increase twist. Aim for them to lift together. Unless the turning block car moves easily, adjust the lazy (unloaded) sheet turning block and then tack to test that setting. Recheck the telltales after each adjustment.
For the mainsail, use the leech telltales to judge airflow, aiming to keep the upper one streaming most of the time.
- When close hauled, ease the boom vang, adjust twist mainly with the mainsheet and use the traveler to set the boom angle.
- When reaching, use the mainsheet to set the boom angle and adjust twist mainly with the boom vang.
- Ease the mainsheet when close hauled, or the vang when reaching, to open the upper leech and increase twist. Harden either line up to reduce twist. Recheck the telltales after each adjustment.
Our explanation of twist has implied that, although true wind speed rises as we move up the mast, the true wind direction remains constant at every height. This is not always the case. Sometimes the wind has eddy or shear components that change its direction at various heights. If so, the amount of twist needed might differ on port and starboard tack.
If there seems to be too much twist, check first to ensure that you have eased the topping lift.
Reefing
The force of the wind increases as the square of its speed, making it vital to reduce sail, or reef, well before a rising wind makes it difficult or impossible to do so. The three basic rules of reefing are:- Reef early.
- Reef as if beating even when running.
- When in doubt, take a double reef.
A rising wind tends to keep on rising. Reef before water washes over the toe rail and before you have more than about 10° of weather helm while properly trimmed. On most sloops, take one reef in the mainsail first, and then, as the wind rises, reef the headsail and then take a second reef in the main. A large genoa with lots of overlap might need to be reefed first, before the main.
Reef a roller-furled headsail by partly rolling it in. Hanked-on headsails must instead be swapped down to a smaller size. Mainsails generally have slab or jiffy reefing. To reef such a mainsail:
- Head up directly into the wind.
- Ease the boom vang and mainsheet, then harden up the topping lift to raise the boom.
- Lower the sail until the next reef cringle can slip over the hook on the boom, then harden up the main halyard quite firmly with the winch.
- Haul or winch in the appropriate reefing line to draw the leech cringle down to the boom, ensuring always that the boom can rise so you do not strain the leech.
- Ease the topping lift, then harden up the sheet and the vang.
- Fall off and resume course.
- Tie up the loose bunt of sail only if you can pass the ties above the boom (i.e., there is no boltrope on the foot, or the foot has slots).