Supplementary Information for Instructors The following is more advanced theory information, which is not necessarily intended to be taught to intermediate level students. This is a resource for instructors who may have to answer some questions from technically minded students.
When wind flows by both sides of a sail in the ‘pull mode,’ both linear air flow and circulation flow occur. The combination of linear and circulation flow results in:
E High-speed flow (low pressure) on the leeward side of the sail. Low-speed flow (high pressure) on the windward side of the sail.
E A change in flow direction from when it approaches and leaves the sail.
Low Pressure
LIFT (Pull)
High Pressure
Sail Shape and Performance Concepts The ability to make informed decisions about sail shape is one of the most important aspects of controlling a sailboat. Two key elements go into defining sail shape: twist and draft.
Twist Twist describes the curvature of the mainsail and jib leeches, most noticeably near the top. Twist is commonly controlled by sheet tension, jib fairlead position, boom vang, or main sheet bridle height. More tension on all of these controls, More tension on all of these controls cause the leech to curl to windward and decrease twist. The optimal setting on most boats is when the top batten is parallel with the boom and the leech curve is even from the head to the clew of the sail. Wind pressure on the sail changes upwind and downwind, so the boom vang will need adjustment if a boat changes course relative to the wind to keep optimal twist.
Draft Draft describes the depth and location of the curvature in a sail. The draft is controlled by the outhaul, cunningham/downhaul, sprit tension, and halyard tension. More tension on any of these controls reduces depth and moves the curvature forward. Easing these sail controls increases depth and moves the curvature aft.
38 Table of Contents
High Speed Airflow
Low Speed Airflow
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