Ocean Cruising – The Physics of Sailing

Sailing Faster Than the Wind – The Physics of Sailing Explained

New York (USA), September 02, 2017

The Physics of Sailing

The driving energy of a sailboat is the difference in relative movement of water speed and wind. On a calm day, wind and water current move in the same direction. Hence, their relative difference is nil. Your sailboat just drifts along on water. When there is a relative difference in wind and water current, energy is generated. Your sailboat utilizes this energy to sail on the water.

The two main components of a sailboat that harness this generated energy are the sails and hull. Wind on the sails and resistance of water on the hull together direct your sailboat. If wind alone were responsible for sailing a boat, your boat would travel at the same speed as the wind and would be able to go directly downwind only. However, this is not the case as often sailboats travel faster than wind speed.

Wind movement and motion of a boat combine to create airflow. This is apparent wind. The sail filled with wind curves. The curved surface causes air to flow faster over the surface of the sail. This creates aerodynamic lift. The hull of a boat resists water force and the keel prevents the boat from being pushed sideways. The altogether effect pushes your boat forward.

The sail can push your sailboat only in the direction of the wind. You cannot sail in any desired direction. Sailboats overcome this hurdle by having obstructions under the water. These include keel and hull. Many often address them as the ‘second sail’.

Underwater flow over hull creates a hydrodynamic force. This force allows movement of a sailboat in any direction other than straight into the wind. The angle of travel of a sailboat fluctuates between 35 degrees to over 80 degrees. Therefore, a sailboat can sail directly over 290 degrees of compass after leaving a tacking angle of 35 degrees on either side of wind.

A sailboat in motion creates its own apparent wind. You can feel this wind onboard. When sailing into the wind, apparent wind is greater than the actual wind. Hence, direction of apparent wind remains ahead of true wind.

As a sailboat surfs forward, proper adjustment of sails generates sufficient aerodynamic lift. When your sailboat goes downwind, sails do not generate aerodynamic lift, as airflow is restricted. The apparent wind is less than the true wind. Together downwind speed remains restricted.

Differences in true wind cause differences in apparent wind. Sails normally have many twists in their designs to accommodate changes in wind directions. Sometimes sails are trimmed because of steep wind gradient. However, these factors remain unpredictable and vary according to prevalent weather.

Simple Tactics to Sail Faster

1. Tacking is an important tactic to harness wind speed. Travel as close to wind direction as possible on one tack and then shift to another tack to go upwind faster.

2. Reaching or sailing perpendicular to wind direction generates greatest lift in forward direction on sails. Although this can cause rolling, turning a boat a little upwind can reduce the rolling effect substantially.

3. Pull in the main sail and jibe in a controlled manner before turning. The main sail opens out slowly on the other side.

Guide to Sailing and Ocean Cruising in a Medium Sized Yacht
The Complete Reference Guide to Sailing and Ocean Cruising in a Medium Sized Yacht

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