Single handed

Friday, August 19, 2011

Sail Trim and Shape

 Keelboat Course Up Sail Trim Sail Shape Turning The Boat

 The wind is what powers a sailboat.  Both the direction and strength of wind is important in setting the sails and maintaining control of the boat. 
True-wind direction is different from apparent-wind direction.  The true-wind is the direction of the wind which makes the waves.  The true-wind is perpendicular to the waves.  The apparent-wind sails the boat.  When the speed of the boat and the velocity of the wind are close, the difference between the apparent and true-wind is the greatest.  The apparent wind is forward of the true-wind, except when sailing directly into or away from the true-wind.  As one sails faster, the apparent-wind is drawn further forward.   When sailing with the wind the apparent-wind has less of a force that the true-wind.  When sailing against the wind, i.e., in a close reach, the apparent-wind has a greater force than the true wind.
In general, the closer you sail to the wind, the closer the sails are pulled or trimmed to the midline of the boat.  As you sail away from the wind, the sails are progressively let out.  The exact position of the sails are based upon the direction and speed of the apparent-wind.  (The direction of the apparent-wind is determined by the sailboat's tack [or  relationship to the true-wind] and the relationship of the speed of the true-wind to the speed of the boat.)   For the beginner sailor, it is easy to remember the five basic directions of sail (points-of-sail) each of which has its distinct characteristics of speed, heel and sail position. 

Point-of-SailWind Direction
In IronsInto the Wind
 30-40 Deg
Close Reach60 Deg
Beam Reach90 Deg
Broad Reach135 Deg
Running180 Deg

Click to Enlarge !!


It is easy to think of three basic sailing conditions 

Sailing on a Close Reach:  All directions except heading directly into ( in irons ) or away from the wind.  A close reach is one of the fastest points of sail.  When sailing on a close reach, the sails should be bowed (having a draft) and have the appearance similar to an airplane wing.  The apparent-wind travels parallel to the sail. The wind actually pulls the sails similar to an airplane wing with the air passing faster over the longer leeward curvature of the sail.  In this point of sail, the apparent wind is stronger than the true wind and the boat has increased heel.

More Information Go To Physics of Sailing

In a close hauled point of sail the sailboat is trying to sail into the wind.  It is a contest between the boat and the wind and the boat tries to beat the wind.  This point of sail is also called beating.  If the sailboat heels too much from a strong gust of wind, the boat can go slightly into the no-go zone, reducing heel and changing the course a little closer to the desired destination.  This is called feathering.  As the wind dies the boat bears away ( turns away from the wind ) and resumes sailing in a close hauled point of sail.  Sailing efficiently close hauled is also called sailing in the groove.
Sailing on a Broad Reach refers to sailing at an angle with the wind.  In this point of sail, both the jib and mainsail are on the same side of the boat and are filled with wind.   As one turns directly downwind, the mainsail blocks the jib's wind and one is then sailing on a run.
Sailing on a Run refers to sailing with the wind directly behind the boat and the sails let out almost 90 degrees.  One must be careful the spreader (a spar or pole which holds the shroud [sidestay] away from the mast) does not place too much pressure on the mainsail.  Often a spinnaker is set; or the jib is placed on the side opposite to the mainsail (Sailing Wing and Wing).  In a run, the wind should be perpendicular to the sail.  A steady course must be taken to keep each sail full of wind and to prevent an accidental jibe (the mainsail swinging quickly from one side of the boat to the other.  This is very dangerous and can also damage the rigging.  A  strong line called a preventer can be attached from the end of the boom to prevent rig damage and injury during an accidental jibe.   This line runs forward to the bow and then backward being secured in the cockpit.  Never have the boom touching the shrouds.  If you do and an accidental jibe occurs, the boom may snap the shrouds on the opposite side as it forcefully swings around.  Having the crew sit on the same side of the cockpit as the boom will help prevent an accidental jib which may occur if the boat is hit abeam ( on the side ) by a wave or wake.   When sailing wing and wing, a whisker pole can be positioned from the mast to the jib's sheet.  This pole will allow the jib to better fill with wind.  For more about whisker poles  GO TO  Forespar.COM .
Heeling is minimal when sailing on a run and the apparent wind is less than the true wind.  The sails are not trimmed since the wind pushes the sails similar to a parachute.  In a run the wind should be perpendicular to the sail.  The speed of the boat is mainly dependent upon the amount of sail hoisted and, in a hull displacement boat, the length of the hull at the waterline. 

Finally one may turn the boat directly into the wind or In Irons:  Since a sailboat cannot sail directly into the wind, this is usually done to stop the boat.  In this position the wind cannot power the sails.  A sailboat cannot sail directly into the true-wind.  Most boats can only sail 40 degrees to the wind but some boats can sail as close as 30 degrees.

Trimming the Sails on a Close Reach  
When sailing on a reach, the experienced sailor, trims the sails based upon the apparent-wind.   This is important because sails cannot be efficiently positioned based upon the point-of-sail or the direction of the true-wind.  An extreme example is ice-sailing on a broad reach, a sudden strong gust of wind can cause the craft to reach high speeds and cause apparent-wind conditions similar to a close reach. 
On a close reach, one method of testing mainsail and jib position is to let the sail out until it luffs (the leech starts flapping) then pull it in (or trim the sail) until the sail is steady.  It is easier to tell if the mainsail and jib are let out too much, than if the sail is over trimmed (pulled in).  Telltales may also be used as a guide.  Telltales are small pieces of yarn attached to the sail.  Although telltales are helpful when sailing on a reach, they are not useful when running (sailing directly downwind).  In a run, the wind should be perpendicular to the sail.  
Whenever the sails are trimmed using the sheets a change in sail shape occurs.  Other lines and the "fair lead" may have to be adjusted to maintain sail shape.  See section on sail shape.

Jib:  The jib is let out and trimmed using a line called a sheet which is attached to the lower back corner or clew of the sail.  As a general guide, the windward front jib telltale should stream aft (backwards) with an occasional lift, the leeward front telltale should stream aft.  The picture to the right shows the shadow, through the sail, of the jib's leeward telltale and a slight lifting of the jib's windward telltale.  (Often there is a plastic window in this position so one can easily see both telltales.)  In some high wind conditions it may be desirable to have the the windward telltale fluttering.

The picture on the right shows a poorly trimmed jib with the telltales turning downward.  If the windward telltale flutters, sheet in the jib or bear away (turn the boat away from the wind).  If the leeward telltale flutters, let the jib out or head up (turn the boat into the wind).

Sailor's Tip:  If the outside telltale flutters, let the sail out.   If the inside telltale flutters, sheet the sail in.

Mainsail:   Mainsail can be let out and trimmed using two different lines:  The mainsail's sheet and the traveler.  The telltales are on the leech (back edge of the sail) and should be streaming backwards.  If they go in circles or flutter, the sail is not trimmed properly.

The picture on the right shows a poorly trimmed mainsail with a "Luff Bubble" forming just behind the front edge or luff of the sail. 

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