Sailing Basics

Sailing Basics

Sailing - the Physics Simplified

by Simon White
If wind was the only factor involved in sailing a boat, then a boat would only be able to sail as fast as the wind blows, and would only be able to drift directly downwind. There are actually two main forces involved; the wind on the sails and the resistance of water on the hull and keel.

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. A starboard tack is when the wind is coming over the right hand side of the boat. The boat is said to be on a 'port tack' when the wind comes over the left side of the boat.

Boats cannot sail directly upwind, however, they can make headway in the upwind direction by sailing as close to the wind as possible on one tack, before taking to the opposite tack and repeating the process. 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. The wind can rapidly flip the boom of the main sail from one side of the boat to the other causing danger to the crew. It is best to gybe in a controlled manner by pulling in the main sail (also known as ‘sheeting in’), turning the stern of the boat through the wind and finally letting out the mainsail on the other side.

Reaching – This is when a boat is sailing approximately perpendicular to the direction of the wind. If the angle is exactly 90 degrees, we call it 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. This can be countered by turning the boat upwind slightly.

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