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SAIL MAKING - Set the Seam Curvature  
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


Setting the correct Curvature

With a board of the type outlined in the 'Building Board' page the amount of curvature produced in the seam depends on 4 factors

  1. The original curve built into the board as defined by the airfoil copied onto the ribs of the building board.

  2. The length of the board measured from either edge to the join.  I have used 250 mm between the ribs as this seems to be sufficient for supporting the sail sections while not making the board too large.

  3. The thickness of the lifting blocks/shims used to lift the centre of the board.  This determines the angle between the two sections of the board.

  4. The length of the sail panel e.g. the distance from the foot to the first seam

To see a more detailed discussion of the mathematics view the Sail Maths page.


For my sail making I have used the board as defined above by its dimensions and airfoil.  To make it easy to get the correct sail curvature I have produced a spreadsheet which allows the variables to be entered to match your situation.  A section of it is reproduced here to show values I have used on a Marblehead sail.  Its function and features is outlined below.


Calculations to show independence on chord of percentage curvature of seam 
Radius of curve of board 750 mm   Vary the three values on the spreadsheet to    
Width of each side of board 250 mm   match the values for your board and sail    
Length of sail panel   400 mm                  
D' is intermediate calculation showing depth of sail when wrapped around board         
  Curvature of board expressed as percentage of chord when lifted by 1 to 12 mm
Chord (mm) 'D' 1 2 3 4 5 6 7 8 9 10 11 12
450 34.5 3.3% 4.7% 5.7% 6.6% 7.4% 8.1% 8.8% 9.4% 9.9% 10.5% 11.0% 11.5%
400 27.2 3.3% 4.7% 5.7% 6.6% 7.4% 8.1% 8.7% 9.3% 9.9% 10.4% 10.9% 11.4%
350 20.7 3.3% 4.7% 5.7% 6.6% 7.4% 8.1% 8.7% 9.3% 9.9% 10.4% 10.9% 11.4%
300 15.2 3.3% 4.6% 5.7% 6.6% 7.3% 8.0% 8.7% 9.3% 9.9% 10.4% 10.9% 11.4%
250 10.5 3.3% 4.6% 5.7% 6.6% 7.3% 8.0% 8.7% 9.3% 9.8% 10.4% 10.9% 11.4%
200 6.7 3.3% 4.6% 5.7% 6.5% 7.3% 8.0% 8.7% 9.3% 9.8% 10.4% 10.9% 11.3%
150 3.8 3.3% 4.6% 5.7% 6.5% 7.3% 8.0% 8.7% 9.3% 9.8% 10.3% 10.8% 11.3%



Curvature at seams

Determining the amount of curvature and variation up the sail depends a bit on what features you wish for a sail i.e. how flat?, how much drive etc.  These factors are all interdependent so a sail with a large amount of curvature will have excellent acceleration off the wind and will excel in reaches and running but suffer when trying to point.  Conversely one which is cut flat may well point well but lack drive in sloppy lighter wind conditions.  Tensioning the foot can affect the curvature in the bottom half of the sail allowing for some adjustment, but this will not much affect the top half of the sail.  The Cunningham adjustment (luff tension) in a mainsail can also affect curvature but has the disadvantage of forcing the maximum draft forward which can cause back-winding of the main and loss of pointing ability.  So how much?? 

I have updated the spreadsheets I previously used and combined them to include the section which sets your building board parameters and the section which defines the a sail by producing seams with the required curvature and displays the result as a graphic showing the curvature up the sail.  This is often referred to as a mould as it outlines the 3 dimensional shape you are trying to produce in the sail


Click here to download the spreadsheet file  Current File version is 1.1   The file name is Sail Board Chord Depth.xls   I suggest you read the section below before using the file.


This spreadsheet has been developed in Excel 2003 with a significant amount of code (macros if you wish) to automate a variety of actions it performs.  As a result it should work with all versions of excel from 2003 onward.  Because of the code, Excel will warn you of this and probably try to open it with the code disabled.  You MUST ALLOW MACROS (or Allow All Content in later versions) for all its functionality to work.

 A brief outline of the functionality of the spreadsheet is listed below

  • The SS allows users to set the parameters for their Building Board including Board Length and measurements of its curve so calculations relate to your board.

  • It allows sail panel length to be entered for sails which use panels of the same length

  • It allows uses to define a sail definition and display a mould of the sail

  • It calculates the amount of 'lift' needed by your board to produce the curvature you have defined for each seam

  • It allows for sails to be defined where the panel length is not constant but varies up the sail.  This makes it easy to create a diagonal seam across the lowest panel.

  • It allows for the created sail definitions to be stored in the spreadsheet and reviewed or edited later

  • It allows for sail definitions to be exported at any stage to ensure you have a copy of your precious data.

  • It allows for sail definitions to be imported from previously exported data.  This would be useful should an updated version of the file be produced and you wish to import all your work instead of the standard examples included in the file

  • It allows for comments about the sail and it's performance or techniques you used so that later you can refer to them and make adjustments to the sail definitions when producing another sail

  • It can show the effect on the sail mould (and hence on the sail you produce) by varying the foot curvature of the sail.

  • It will prompt you for any changes you make to any sail definition if you make changes and attempt to leave that sail definition

  • It includes a standard default set of values and a set of sample sail definitions I have used to create sails which you can choose to delete or keep or export for storage

  • It includes a descriptive help section should you have questions on functionality

I suggest you create a new folder in your document folder to place the file and produce a shortcut on your desktop to access it.  The exports are created in the same folder as excel files as well with a similar name.
I hope you find this file useful as I have done and would be pleased to receive feedback