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Keelboat Course Up Sail Trim Sail Shape Turning The Boat
 

  

  

 

   
Sail shape when sailing on a reach

The Mainsail (pronounced "main-sul"):   For maximum efficiency a sail should have a curve, or draft.   The larger the draft or curvature of the sail usually creates a more powerful sail.  In moderate wind conditions, the draft should be approximately 45% of the way back from the luff (front edge of the sail), much like a wing of an airplane.  In light winds, the mainsail  should be set for maximum efficiency.  The larger the draft or curvature of the sail usually creates a more powerful sail.

However, in stronger winds it may be desirable to flatten the mainsail.  Flattening the curvature of the sail will make it less aerodynamic and reduce the "pull" on the leeward side of the sail.  This can be done by tightening the backstay (flattens the upper 2/3 of the sail), and by tightening the outhaul of the mainsail (flattens the lower 1/3 of the sail).

Tightening the backstay will bend the top of the mast backward and the mid-portion of the mast forward.  This will flatten the upper two thirds of the mainsail and depower the sail.  However, this also moves the draft or maximum curvature of the sail aft (toward the stern).    Setting the Cunningham will tighten the sail's luff (front edge of the sail) and move the draft forward and back to its correct position approximately 45% of the way back from the luff (front edge of the sail).

    
Picture of the Cunningham set with a line (left-hand picture).  Non-set Cunningham (right-hand picture).
 

 

Tightening the outhaul will flatten the foot of the mainsail and depower the sail.  Loosening the outhaul will increase the draft of the mainsail and power-up the sail.   One must be careful not to cup the sail with too little tension on the foot of the sail by having the outhaul to loose. 

Tightening the boom vang will reduce the twist of the mainsail.  Loosening the boom vang will tend to twist the sail, causing the superior portion of the sail to be let out in relation to the lower portion.  Adjusting the boom vang will compensate for a change in sail shape as the mainsail is let out or eased.  It can also be used to compensate for an increase in wind speed and change in apparent wind direction which is present aloft (towards the top of the mast).  Twisting of a sail is a fine adjustment which is guided by the behavior of the telltales on the mainsail's leech (back edge of the sail).  If the boom is over the water, one may want to first trim the lower part of the mainsail by using the mainsail's sheet , then loosen the boom vang until the aft (top) portion of the mainsail or telltales start to luff then tighten the boom vang until the luffing stops and the telltales are streaming backward.   
 

Once the proper shape of the mainsail has been achieved, any changes in the tension of the boom  vang or the mainsail's sheet will change the shape of the sail.  The mainsail's sheet not only determines the position of the boom in relationship to the deck but also places a downward pull on the leech (aft or back edge) of the mainsail.   If one lets the mainsail out by loosing it's sheet, decrease tension on the main's leech may occur, resulting in an undesirable twist of the sail.   If one wishes to let out the mainsail with little change in sail shape, the "traveler" should be used.  ( By also adjusting the mainsail's sheet, the sail can be let out without any change in shape ).  The traveler is a track with an adjustable car to which the mainsail's sheet is attached.  By sliding the traveler's car to port or starboard the position of the boom is changed without changing the length of the mainsail's sheet.

Many beginning sailors will only use the mainsail's sheet to trim the sail.   However, for maintaining proper sail shape the mainsail's sheet, traveler and boom vang should all be used.  All three lines will change the sail's twist and trim (position from the sailboat's midline) to some degree.  When the boom is near midline, being over the sailboat, the traveler can be used to ease or trim the sail and the main's sheet's used to control sail twist. ( In this position, the sheet's main pull is downward )  As the boom is let out over water, the mainsail's sheet can be used to trim or ease the sail and the boom vang to control sail twist.  ( In this position, the sheet's main pull is horizontal. ) 

  

The Jib:  There are three ways of modifying jib shape. 

The first is with the jib sheet.  In moderate winds, a rule of thumb is to have the maximum sail draft (deepest part of the curve in the sail) about 45% back from the sail's luff (front edge).  One can also ease the jib ( or let the jib out ) and trim it (pull it in), stopping just as it ceases luffing.  It is important not to have the jib to tight or cupped.  If this happens, significant power may be lost from not only the jib but also the mainsail. 

The second is to move the jib sheet's block ( fair lead ) aft ( towards the stern or back of the boat ) or forward.   Moving the block aft will place more tension on the foot (bottom edge) of the sail as the jib's sheet is tightened.  This will flatten the foot or bottom of the sail and twist the top of the sail, thus, depowering the jib.  Moving the bock forward will place more tension on the top of the leech (back edge) as the jib's sheet is tightened.  This will result in an increased draft of the jib and untwisting of the sail.  This will power up the jib.  If the jib luffs at the top of the leech first, the fair lead is to far aft.  If the jib luffs at the bottom of the leech first, the fair lead is to far forward.

The third way is to straighten the forestay (or headstay) which the luff (front edge) of the jib is attached to.  This will straighten the front of the jib and improve efficiency.  This is done by tightening the backstay.  One should note that when the backstay is tightened, the mainsail's sheet and boom vang may also have to be adjusted or the mainsail's shape will change. 

Copyright 2004      Web Page last updated 04/02/2005