How a sailboat turns wind into motion.

This page explains the forces at work when a boat sails. It draws mainly on a 2008 Physics Today feature by physicist Bryon D. Anderson of Kent State University and on the American Sailing Association's explanation of lift. Where they and other sources differ, both versions are given.

The sail is a wing

When wind meets a curved sail at an angle, called the angle of attack, the pressure on the windward side rises and the pressure on the leeward side falls. That pressure difference is lift. ASA describes the air on the leeward (outer) surface moving faster, which is what lowers the pressure there.

ASA also points out that the resulting force acts mostly at right angles to the sail. Part of it pushes the boat forward and part of it pushes the boat sideways and makes it heel. Much of sailing is about using the forward part and controlling the rest.

Not the "longer path" story

A common explanation says air going over the curved side has farther to travel and must speed up to meet the air on the other side. Physics Today calls this equal-transit explanation incomplete, because a perfectly symmetric wing still makes lift when it is set at an angle to the flow. The article cites NASA material that describes lift as the result of the wing turning the airflow.

Too little angle, too much angle

ASA describes two failure modes. If the angle of attack is too small, the sail luffs: its leading edge flutters and it makes no useful force. If the angle is too large, the airflow separates from the sail and it stalls. Good trim sits between the two.

Telltales, short ribbons fixed to the sail, show which side of that line you are on. ASA says telltales streaming straight back mean the flow is smooth and attached, while fluttering or drooping telltales mean the flow is poor. The sail trim page covers how sailors use them.

Two sails working together

On a boat with a jib and a mainsail, the two sails affect each other. ASA explains that the jib bends the incoming air toward the mainsail (an effect called upwash), and that the air passing through the gap between them, known as the slot, helps the mainsail make more lift without stalling.

Sail shape matters too. Physics Today says tall, narrow sails are favored for upwind sailing because they give the most lift relative to the losses from vortices at the sail's ends.

The keel is a wing in the water

Something has to resist the sideways push from the sails. Wikipedia (a secondary source) notes that on a sailboat the word keel covers the underwater area that resists leeway, the sideways slip of the boat. Physics Today explains that the boat moves at a slight angle to the direction it is pointing, so water meets the keel at an angle and the keel makes its own sideways lift. That lift balances the side force from the sails, and the forward parts of the combined forces drive the boat ahead.

ASA adds that the keel needs water flowing past it fast enough to work, and that marine growth on the keel reduces its effectiveness. Physics Today notes that deep, narrow keels are hydrodynamically efficient, and that wings fitted to the bottom of a keel reduce induced drag.

Drag

Everything that resists the boat's motion is drag. Physics Today lists the main sources on a sailboat:

• skin friction between the hull and the water
• wave-making as the hull pushes water aside
• form drag from the shape of the hull
• induced drag from vortices shed by the sail and keel
• turbulent airflow over the sail, which also cuts lift

The same article models the force a sail makes as proportional to the sail area multiplied by the square of the apparent wind speed, using coefficients measured by experiment. In that model, doubling the apparent wind speed quadruples the force.

Apparent wind

The wind you feel on a moving boat is not the same as the wind on shore. ASA defines apparent wind as the true wind combined with the wind created by the boat's own motion. It is the wind that actually fills the sails, it changes constantly, and it is what trim decisions respond to. Physics Today likewise says sail lift and drag are worked out relative to the apparent wind.

Wikipedia (a secondary source) describes how apparent wind shifts with the boat's heading:

• On a stopped boat, apparent wind equals true wind.
• Sailing close-hauled, the apparent wind comes from farther forward than the true wind.
• Sailing downwind, the apparent wind can be slower than the true wind.

Sailtrain, a UK training site, gives a worked example: a boat on a beam reach in 10 knots of true wind, moving at 6 knots, feels about 12 knots of apparent wind coming from about 60 degrees, farther forward than the true wind. That is why sails need to be trimmed in as a boat speeds up. Sailtrain also notes that telltales, wind vanes and basic wind instruments all show apparent wind. An instrument can only display true wind if it is also fed the boat's speed. Try the effect yourself on the apparent wind calculator.

Faster than the wind

Physics Today says a boat sailing dead downwind can never go faster than the wind. Boats sailing across the wind can exceed it. The article cites iceboats passing 150 km/h in a 50 km/h wind, and a hydrofoil craft passing 40 knots in winds roughly half that speed.

Wikipedia adds that iceboats have so little drag that their apparent wind stays close to their direction of travel, so their sails stay sheeted in on most headings.

Sources differ on which heading is fastest. Physics Today says boats are fastest with the wind abeam, around a beam reach, and that sailing straight downwind is not the fastest point of sail. Wikipedia says speed is typically highest on a beam reach, but that many high-performance craft are fastest on a broad reach. The headings are explained on points of sail.

Heel and balance

Because the sail force pushes sideways as well as forward, a boat leans, or heels. The USCG Auxiliary says modern boats sail best with little or no heel, and that heeling too much suggests the sails are set too tight.

Heel also affects steering. Weather helm, as the PBO glossary defines it, is when you have to hold the tiller toward the windward side to keep a straight course because the boat keeps trying to turn into the wind. Lee helm is the opposite; PBO calls it acceptable in light air but unwanted in stronger wind. North Sails coach Skip Dieball explains balance as lining up the sails' center of effort with the hull's center of lateral resistance, aims for a little weather helm, and says extra heel increases it. The steering and reefing and depowering pages go further.