Sailing Against The Wind: Choosing The Right Boat

Sailing against the wind is a tricky manoeuvre that requires a good understanding of physics. It is impossible to sail directly upwind, and sailors must instead employ a technique called tacking, which involves sailing in a zig-zag pattern to reach a destination. This method allows the wind to approach at an angle, rather than head-on, and the wind power collected by the sails is released as a forward thrust. This is because the wind, combined with the shape of the sails, creates a pressure difference that pushes the boat perpendicular to the wind's direction.

The physics of sailing against the wind

Sailing against the wind is made possible by a combination of the shape of the sails, the direction of the wind, and the keel of the boat.

When the wind blows against a boat at an angle, the sail inflates and forms a foil shape. This foil shape creates a pressure difference that pushes the sail perpendicular to the wind direction. This force from the sail's foil shape is then combined with and balanced by other forces, including those of the boat's keel. The keel, a long thin piece that juts down from the bottom of the boat, provides a counter-force that resists the sideways motion of the sailboat, allowing it to move forward.

It is important to note that sailing directly into the wind is not possible, as there is no forward component of the force on the sails in this case. Therefore, boats typically sail at an angle of around 40-45 degrees to the wind and use a technique called "tacking" to zig-zag back and forth and reach their desired destination.

How to tack

Tacking is a sailing manoeuvre used to change a boat's direction through an oncoming wind. It is one of the first skills to learn when sailing.

When a sailing boat is heading in the direction that the wind is coming from, it has to sail in a zig-zag course to ensure the sails work. This is known as "beating to windward" and the point of sail is referred to as "close-hauled", where the boat is sailing as near as possible towards the wind.

To beat to windward, a series of manoeuvres called tacks are required. The boat changes course by turning through the wind, with the sails moving from one side of the boat to the other. The aim is to tack the boat through the wind onto the new course while losing as little speed as possible. This requires coordination from the crew, with the helm steering through the tack and other crew members operating the sheets (control lines).

• The skipper or helm decides it is time to tack and alerts the crew.
• One or two crew members are assigned to each jib sheet winch, depending on the boat's size and the number of crew available.
• The skipper ensures that the new course is clear and everyone is ready.
• The helm calls "Ready About". A crew member then puts two turns of the slack or "lazy" jib sheet around winch (A) on its side of the cockpit and pulls in the slack.
• Another crew member uncleats the "working" jib sheet, keeping it tight on winch (B).
• The helm calls "Lee-oh" and turns the boat towards the wind. As the headsail starts to flap, the crew on the working sheet eases and releases the sheet from winch (B).
• The boat continues to turn through the wind, and the jib blows across to the new side.
• The crew on winch (A) then pulls in the new working sheet. The winch handle is inserted into the top of the winch, and the crew winches in the sheet, trimming the sail for the new course.

It is important to note that the terms "ready about" and "lee-oh" may sound strange to those unfamiliar with sailing, but these instructions help the crew coordinate and act in unison.

Additionally, sailing cockpits can feel crowded when a yacht is beating to windward, and multiple instructions are being given. As the boat goes through each tack, the boom will swing across, and the boat will heel over as the wind fills the sails and blows from a different direction.

By tacking, sailors can reach a point in any direction, regardless of the wind direction.

Keel and its function

The keel is the bottommost structural element of a watercraft, and the main structural member and backbone of a ship or boat. It runs longitudinally along the centre of the bottom of the hull from the bow to the stern. The keel is often the first part of a ship to be constructed, and in British and American shipbuilding traditions, the laying of the keel marks the beginning of a ship's construction.

The keel has two main functions on a sailboat. Firstly, it acts as an underwater foil, minimising the lateral motion of the vessel under sail (leeway). Secondly, it serves as a counterweight to the lateral force of the wind on the sails, which causes the boat to roll to the side (heeling). The keel's broad, flat surface generates a sideways force by displacing water opposite the heeling motion, preventing the boat from tipping over.

The keel also has a hydrodynamic function, interacting with the water to steady the boat and make it easier to steer. This is particularly important for sailing vessels, as without the keel's balancing action, a boat would simply drift downwind.

There are several types of keel, including the full keel, which is more stable but slower and less manoeuvrable, and the fin keel, which is smaller and provides less water resistance and higher speed. Other types include the long fin, bulb fin, and swing keel, as well as hybrid versions with bulbs that carry ballast. Moveable keels may pivot, retract upwards, or swing sideways to allow the boat to sail in a more upright position.

Sailboat design

Sailing against the wind is all about aerodynamics and hydrodynamics. The wind's action on the sails and the creation of forward motion with the help of underwater lateral resistance are the two main components. Sails are similar to aeroplane wings in that they generate lift, but they are oriented vertically.

When the wind comes into contact with the forward end of the curved foil (the luff of the sail), it splits, with one part passing over the downwind (leeward) side and the other over the upwind (windward) side. The wind on the leeward side travels a longer distance due to the sail's curvature, creating a low-pressure area, while the wind on the windward side travels a shorter distance and reaches the aft end faster. This combination creates an aerodynamic lift that pulls or sucks the boat forward.

The pressure in the sails will still push the boat sideways more than pulling it forward. This is where the keel or centreboard below the waterline comes into play, transferring the sideways pressure into forward momentum. The lift created by the sails and the lateral push or hydrodynamic lift of the keel enable upwind sailing.

The ability of a sailboat to sail against the wind depends on its design, the shape of its sails, and the forces they generate. Modern sailboats can sail close-hauled, sometimes even closer than 40 degrees to the wind direction. The boat's speed and course, or angle to the wind, depend on the design of the boat, the shape and condition of the sails, and the wind strength. Most sailboats can achieve an angle of about 30 degrees to the wind direction.

To optimise the boat's performance against the wind, sailors need to understand the "points of sail" and constantly adjust the trim of the sails according to the wind direction and speed. Close-hauled, for example, is roughly 45 degrees off the true breeze, while a beam reach is at 90 degrees. When sailing against the wind, sailors employ techniques such as "tacking," which involves sailing in a zigzag pattern to reach a destination directly upwind.

Apparent wind

The velocity of the apparent wind is the vector sum of the velocity of the headwind (the wind velocity a moving object would experience in still air) and the velocity of the true wind. The headwind is the additive inverse of the object's velocity; therefore, the velocity of the apparent wind can also be defined as a vector sum of the velocity of the true wind minus the velocity of the object.

The angle of the apparent wind moves closer to your front the faster you go. For example, if you put your hand out of the window of a car travelling at 100 kph, with a true wind of 10 kph from the side, your hand will almost feel all the wind coming from the front of the car.

A common mistake when sailing downwind is that the wind 'feels' calmer, and so sailors may be tempted to set more sail than is safe for conditions when turning upwind, where apparent wind will significantly increase.

Frequently asked questions