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Woorabinda Lake - Stirling South Australia

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SAIL MAKING - Sail Sape  
by Ben Morris (last edited 21/07/2013)
 

Shape in Sails Building Board Making Seams Set the Seam Curvature Making a Sail Sail Material Diagonal Seams etc Back to Intro page Setting the Sails The Claudio Tool Measuring Procedures

 

Sail Shape    Sail Twist    Sail Design    Sail Curvature 

Built in Shape

Built in shape is essential if the leech of the sail is to hold up and allow draught or curvature right to the head.  Single panel sails fail miserably here and invariably allow the top half of the sail to fall off and provide little drive.  This is often evident by a diagonal crease across the sail.  The degree of curvature suggested by various texts varies but values between 6% and 14% seem to be the range which most sail-makers use.

 

In addition it would seem that the curvature should increase as the chord decreases so that for a normal sail the foot might have a curvature of say 6% increasing to say 10% at the head.  There are very good reasons for this.

 

Twist in Sails

The main reason for this is the need to keep all of the sail working and not stalling which results in drag.  Telltales on the leech of a mainsail will reveal this by curving around the sail and blowing forward on the windward side.  The interaction of the jib in forcing the wind to a higher angle of attack lower in the mainsail accentuates the need to have twist

Twist in the sail is also required because of the wind gradient up the sail.  Because of the friction between the water surface and the air, the velocity of the wind increases the further we are from the surface.  The head of the sail is in a greater wind speed than the foot..  This is particularly significant in lighter winds.  As the wind increases the apparent wind moves outboard requiring twist to keep its angle of attack to the wind the same and so prevent stalling

 Aerodynamic theory predicts that with a shorter chord as found near the head, the sail is more likely to stall (read about Reynolds number) so to overcome this more twist is needed whereby the top part of the sail is twisted further away from the centre line of the boat.  If close hauled the foot may make an angle of 5 degrees to the line of the boat but the top of the sail may make an angle of 20degrees.

This certainly prevents the top of the sail stalling but it stops it working too as its chord line may well be close to the wind direction.  To avoid this more draught or curvature is added giving more 'lift' to increase the drive so all parts of the sail are working effectively. 

 

Where to Start - Sail Design

Like all good theories there seems to be a lot of trial and error in achieving the desired outcome.  So it is with sail making.  I suppose there are so many variables in achieving an efficient sail that the only way I found to to start was to decide on a set of specifications that seem to be an integration of all the best aspects of sails you can read about or observe.  Then vary them to see what works and what doesn't.

To begin with all of the sails I made were for Marblehead yachts so many of the variables are fixed.  For an 'A' rig, maximum luff height, amount and position of roach on the leech and consequently foot measurement are basically determined within a fairly narrow range.  The sail dimensions at the 1/4, 1/2 and 3/4 positions are also more or less determined as well.  There are a maximum of 4 battens evenly spread up the leech so it made sense to me then to make a sail with five panels with a batten at each panel join.  I chose this in favour of the other common arrangement where only 3 battens are used at the 1/4, 1/2 and 3/4 points and only 4 panels used for the sail.  My thinking was that only three panel seams would not allow for as smooth a curve to be generated lengthwise as four seams.  For a jib with it's shorter luff, I use a four panel sail for a Marblehead with three battens on the 1/3, 1/2 and 3/4 positions matching the seams.  One metre design almost defines the panels as four and three for main and jib and allows battens at the seams.

 

Decide on the type of Curvature

I started by deciding on what type of curve to use and the point of maximum draft.  Having a background knowledge of airfoils commonly used in model gliders I decided that a parabolic curve would generate more lift than a circular section.  This has the slight disadvantage of having a greater angle of attack to the incoming wind but this is certainly no issue with wings where the aerofoil is angled much closer to the wind direction than a yacht sail.  I searched the NACA airfoils for one of about 15% thickness and max thickness of near 35-40% of chord.  Then I changed the front curve to a extend to a point to match the curve of a sail as they  do not have a solid section with such a rounded entry.  (Perhaps the mast can serve as this)

 

  • Profile NACA 642-015 (642-015A has max thickness at 40%)

  • Max Thickness 15% so for ½ is 7.5% (6.85% including extra 2cm)

  • Thickness at 35% on airfoil so on flat curve with 2cms extra length is 40%

  • Chord used is 450mm but becomes 470mm by continuing curve to origin

 

I have since decided to use some of the information on Lester Gilbert's site and created a parabolic curve with the maximum draft at 43%  of the chord. I think this is a better compromise with the maximum draft a little further back to enable a slightly sharper entry.  This should give better pointing without detracting from its power off the wind.  See the spreadsheet for details.