The Physics of Sailing Simplified

Two main forces are involved in sailing a boat; the wind on the sails and the resistance of the water on the hull. If wind was the only factor, then a boat would only be able to sail directly down wind, and would not be able to go any faster than the wind was blowing.

The physics of sailing is a little more complicated than the wind simply pushing the boat along. Sailing boats move forward because of the flow of wind over the sails and the action of water resistance on the hull. Quite often sailing boats will actually travel faster than the wind.

The movement of the wind and the motion of the boat combine to create an air flow known as the ‘apparent wind’. The curved surface of a filled sail causes this air flow to move faster over the leading surface of the sail than the other side creating lift much like an aeroplane wing. The resistance of the water on the hull and keel prevents the boat being pushed sideways and so the boat moves forwards.

Depending on the efficiency of the rig, it is possible for a sailboat to sail directly over 290 degrees of the compass. In other words the boat can sail in any direction apart from 35 degrees either side of the direction of the wind (often called the ‘no go zone’). Other boats with less efficient rigs will have a larger no go zone perhaps of about 45 - 55 degrees either side of the upwind direction.

Points of Sail

Some common terms for a sailing boat’s motion relative to the wind:

Tacking – Turning the bow (or front) of the boat through the wind so that the wind comes over the opposite side of the boat. When the wind is coming over the right hand side of the boat, we say the boat is on starboard tack. When the wind comes over the left side of the boat it is on a port tack.

Although a boat cannot sail directly into the wind, travelling for a distance as close as possible to the wind on one tack and then tacking to the opposite tack enables a boat to make headway in the upwind direction. When a boat is sailing as close as possible to the wind direction, it is termed ‘beating’ or sailing ‘close hauled’.

Running – Sailing a boat within about 30 degrees either side of the downwind direction. Turning the stern (or rear end) of the boat through the wind is called gybing. This can be dangerous as the wind can flip the boom of the main sail rapidly from one side to the other. It is best to gybe in a controlled manner by pulling in the main sail (also known as ‘sheeting in’), then turning the boat, and finally letting the main sail out on the other side slowly.

Reaching – This is when a boat is sailing approximately perpendicular to the direction of the wind. When the angle is exactly 90 degrees, it is termed a ‘beam reach’ as the wind is coming over the beam (or side) of the boat. Sailing in a direction about halfway between a beam reach and close hauled is called a ‘close reach’, whilst sailing a little downwind from a beam reach is called a broad reach.

For most modern sailing boats, reaching is the fastest way to travel as the direction of the wind generates the most lift in the forward direction on the sails. However, often this can take the course of the boat parallel to the waves causing excessive rolling. Turning the boat slightly upwind can alleviate this.