The Sailcut CAD Handbook

Sailcut is a software for designing boat sails and developing then into flat panels. Sails can be either 4 sided sails like for old timer gaff rig or 3 sided sails like jibs or main sails for Marconi rig.

The first version of Sailcut was developed in 1978 and used by Robert Lainé for making the sails of his IOR 1/4 ton named "Flying Sheep III". Sailcut has been available on the web since 1994 and is used by many professional and amateur sail makers for offshore racing, cruising and recently for model yacht.

Sailcut uses a unique mathematical definition of the surface of the sail which ensure that the sail profile is smooth and aerodynamic.

Sailcut CAD is written with portability in mind. As such it is written in C++ and uses the Qt library from Trolltech for the graphical user interface. Sailcut CAD uses OpenGL to display the 3D view of the sail. Sailcut CAD is known to compile and run on GNU/Linux, Microsoft Windows and MacOS/X.

You can download the latest version of Sailcut CAD from the project's home page at http://sailcut.sourceforge.net/. Sailcut CAD is made available both in binary (compiled for you) and in source code form.

Sailcut should compile on a wide variety of systems provided you have a C++ compiler and Trolltech's Qt4 toolkit. If you wish to make use of Sailcut CAD's 3D view, you will need OpenGL support on your system.

As of release 0.6.5, Sailcut CAD uses different extensions for each file type instead of ending all files with ".xml". If you wish to open sails created with a previous version of Sailcut CAD you should rename your sail definition file so that it ends with ".saildef". When opening the resulting file, all data except for the mould will be preserved. Redefine your sail mould, then save the file.

The program is tailored to design either triangular or quadrangular boat sails. A classical triangular sail is essentially a quadrangular sail with a very small top edge.

The surface of the sail is generated from a single set of equations defining the profile of the sail at all levels. The profiles rest on the edges of the sails which are defined by their length and the amount of round (also called roach) in each side and the twist of the sail. The "Dimensions" window is divided into a number of boxes which group the parameters defining the sail.

You can use the "Compute" button to compute and display ancillary data. This can result in some text box color bein changed. If the color is red then the value exceed the upper limit and if yellow it is below the lower limit and the value itself will be changed to the accptable limit.

When you have finished entering the dimensions, press OK to display the sail in 3D.

4.1.1. "Rig geometry" box

The first step is to select the type of sail you are going to work on, and enter the data defining the rig geometry and sail plan (see Figure 1, “Sailcut plan definition” for sail plan definition).

Select the type of sail by pressing the corresponding Radio Button:

• Jib for any sail which will be set on a stay,

• Mainsail for any sail set on a mast,

• Wing for any type of kite symetrical about the foot.

The rig data are used for displaying the sails in their proper relative position with the "rig viewer".

Figure 1. Sailcut plan definition

4.1.2. "Sail dimensions" box

This is where the dimensions of the sail are entered.

On a main sail the minimum value for the gaff length (headboard) is constrained to 5 mm. Value smaller than that will default back to 5 mm. The gaff angle is constrained to values such that the angle between the gaff and the straight leech is at most 90 degrees.

Positive round (roach) of the luff, foot, leech and gaff extend the sail outside of the straight line.

Negative round is equivalent to hollowing that edge of the sail.

The position of the round or roach is expressed in percentage of the side length starting from the lower or most leftward end of that edge.

Figure 2. Sailcut edges definition

Dimensions and angles defining the sail plan are expressed in millimetre and degrees.

Length of the sail sides and diagonal are the 3D straight line distance between the corners of the sail.

The actual length on the finished sail lais on the floor can be slightly longer depending on the shape of the sail. For example, the foot length entered in the screen below is 3600 mm. If the foot camber is null then that will be the actual distance between clew and tack (straight foot) of the finished sail. If a 10% camber is entered for the foot depth, then the actual foot will be the length of the arc which has 10% camber, that is 2.7% longer than the straight line foot length.

Having entered the sail main dimensions you can press on the "Compute" button to obtain additional informations on the sail, like the X-Y coordinates of the corners of the sail, the perpendicular length LP measured from the clew to the luff as well as IRC racing rules width.

The X-Y coordinates of the sail corners are usefull to quickly adjust the data entered. For example if you find that the clew height (Y) is way below or above the height of the tack when you would like it to be leveled, then you can substract or add the difference to the leech length.

4.1.3. "Layout" box

Click on the radio button corresponding to the desired layout of the sail. The layout of the panels does not affect the shape of the sail which is defined by its dimensions and its mould. Except for the Radial cut layout, the number of panels is determined by the cloth width and seam width entered in the "Cloth" box.

• The most commonly used layout is the "Crosscut". The panels are laid perpendicular to the straight line joining the peak to the clew of the sail.

• The "Twist foot" layout is similar to the cross cut except that the lower panels are rotated such that they do not intersect the foot of the sail.

• The "Horizontal cut" layout lay the seams in the horizontal plane. This option can be used to visualise the profile of the sail at various levels and to output files with the 3D coordinates of the sails for use by CFD tools.

• The "Vertical cut" layout places the panels parrallel to the straight line joining the peak to the clew of the sail. This is the favorite layout for the old timer's main sail.

• The "Mitre cut" layout is the favorite for the old timer's genoa. The sail is divided in two parts by a line joining the clew to the mid point on the luff and the panels organised to be perpendicular to the foot in the lower part of the sail and perpendicular to the leech in its upper part.

• The "Radial cut" is used mostly for competition as the cloth is mostly aligned with the directions of maximum strain. When using the Radial cut option it is important to understand the definition of the number of sections, number of radial gores and number of luff gores (see Figure 3, “Radial cut gores definition”).

Figure 3. Radial cut gores definition

4.1.5. "Cloth" box

Enter there the width of cloth used, the width of the seams between adjacent panels, the width of material to be added to the leech to make the leech hem and the width of material for the other edges hems.

Figure 4, “Sailcut seams and hems definition” describes the location of the various hems and seam width. Sailcut will compute the panels such that they fit within the declared cloth width including the seam and hems width as appropriate.

Figure 4. Sailcut seams and hems definition

The depth of the sail can be entered at three levels located at the bottom (foot) the middle (maximum depth level) and near the top.

The vertical position of the maximum depth profile is controlled by the vertical slide bar to the right of the left vertical frame.

The luff shape and the leech shape can be adjusted for the Top profile and Middle profile only. The foot profile is always an arc of circle.

In order to avoid that the leech makes a hook in the upper part of the sail when the wind increases it is recommended that the Top profile luff shape value be higher than that of the middle profile and that the leech shape value at the top be lower than the middle value.

This viewer is used to display several sails on the same rig.

The File->Add sail menu entry is used to purge the viewer.

The File->Add sail menu entry is used to add sails already created and saved with Sailcut.

Once a sail is added to the rig viewer the sail information frame appears below and it is possible to translate the sail in the 3 directions by adding X-Y-Z displacement values. If you have misplaced a sail use the Reload button to recover the initial sail. You can also use the Remove button to eliminate a sail. 2 slides allows you to ratate the rig in azimuth and elevation and view the ig from any vantage point.

The File->Save menu entry is used to save a rig with a combination of sails.

Rigs which have been saved can be later opened as an entity with File->Open menu entry.

Note that the rig viewer window must be closed to allow you to return to the main screen of Sailcut.

It is possible to zoom, pan and rotate the sail in the view window:

The developed sail is display by clicking on the "Development" tab from Sailcut CAD's main window. This presents you with a view of the developed (flat) panels of the sail. The view controls are the same as those of the main window. The blue line represents the edge of the finished panel (draw line) and the red line represents the outer edge taking into account the seam and hems width allowance (cut line).

You can export the points which define the edges of the developed panels with the draw and cut lines to the following file formats:

Once you have customised you sail, you can save it to a file (File->Save or File->Save As) and load it (File->Open) next time you want to work on it. Both the sail's dimensions and the parameters of the mould are saved.

Sailcut CAD uses XML files to store the sail data. These files are plain text so they can easily be viewed using your favourite text editor.

On top of Sailcut CAD's native file format it is possible to export the points of the edges of the panels that make up a sail. You export the three dimensional sail to the following file formats:

The File menu offers various printout possibilities:

The printout scaling is such that the sail drawing and the largest developed panel automatically fit in one page. For printing panels to a precise scale it is preferable to export the developed sail in a DXF file and use a CAD package to print the panels.

Figure 5. Developed panels drawing

Sailcut software was initially written in 1978 in Basic language on a computer with 1.6 KB of memory, one line text screen and a small 32 columns text printer. Hence the necessity to keep the surface formulation simple for designing the sails which I built and used on my IOR ¼ton.

This short cycle: "design => manufacturing => utilisation => modification", without commercial constraints linked to sailmakers work habits has allowed me to converge quickly on a compact and robust way of describing the sail surface. The method is valid for classical triangular sails and also for quadrangular sails used on old timers and modern rigs with very large headboard. Later on, the use of Sailcut by professional sailmakers has necessitated the addition of graphic interface to the kernel of Sailcut, but that is an other story... Since 1993 the Microsoft Visual Basic version of Sailcut is available at http://www.sailcut.com and since 2003, the source code of Sailcut re-written in C++ is available at http://sailcut.sourceforge.net. For protection of the intellectual rights, the name Sailcut is a registered trademark, but the author maintains free and unrestricted access to Sailcut.

A sail is a complex surface which sailmakers have historically defined by notions like depth at various height and position of the point of maximum depth along the local cord of the profile. This method of defining the surface of the sail by control points allows for an easy comparison of the shape between the intended design and reality. Unfortunately a large number of different surfaces can pass through these few control points. Then notions like the slope at the leading edge and trailing edge of the profiles were introduced to help sailmaker get a better control of the shape of the sail profile. Using interpolation between basic control points, with or without constraints on the tangents at the extremities of the profiles, to determine the depth of the sail in all points were too demanding for old days personal computer processing capability.

From the beginning of my racing activities, I was interested in the aerodynamic of sails. The books "Theory of wings sections" by Ira H. Abbott and Albert E. von Doenhoff, and "Sailing Theory and Practice" by C.A. Marchaj convinced me that the distribution of camber along the profile was the determining factor in the quality of a sail profile. I was very sceptical about the definition of a profile by its depth, the position of its maximum depth along the cord and segments of cubic or quadratic curves on either side. Rather than trying to reproduce existing sail shape based on depth measurements, I looked for a law of distribution of its camber giving a reasonably aerodynamic profile on the complete sail surface. The first attempt was to model directly the distribution of camber, however that required to process simultaneously the first and second derivative of the surface many points of the sail surface, far too much work for my small computer. At the time I was racing on the North Sea often in relatively heavy weather for my ¼ ton and I wanted sail profiles with a high peak of pressure very far forward to fight against the tendency of the depth to move backward as the cloth stretched in increasing winds. I finally selected a simple equation defining only the second derivative of the profile and giving a monotonic decrease of its value along the cord of the profile. Experience showed that with the then available cloth, the leech was sometime falling to leeward in the upper part of the sail. I then introduced a second term in the equation to be able to control the minimum value of the second derivative at the leech. This equation is therefore controlled by only two parameters.

The coordinate system used is such that the plane X-Y contains the tack, the clew and the head of the luff. The X axis is horizontal and orientated positively from tack toward clew. The Y axis is vertical orientated upward and the Z axis (depth) is perpendicular to the X-Y plane. Profiles are defined by the intersection of the surface of the sail with an horizontal plane parallel to Z-X plane. The depth Z of any point of a given profile is a function of the local X ordinate normalised to the profile local cord as shown in Figure 6, “Sailcut coordinate system”.

Figure 6. Sailcut coordinate system

The following equation is used to describe the second derivative of the profile function of X:

Z''= K*[-A*(1-X)^AV - AR*X]

After a first integration it gives the slope of the profile:

Z'= K*[A*(1 - X)^(AV + 1) / (AV+1) - AR/2*X^2 + C]

Finally after a second integration the equation giving the depth at any point is:

Z = K*[-A*(1-X)^(AV+2) / (( AV+2)*(AV+1)) - AR/6*X^3 + C*X + B]

To meet the profile end conditions (X=0, Z=0) and (X=1, Z=0) the constants B and C are:

B = A / ((AV + 2) * (AV + 1))
C = AR / 6 - B

The maximum depth is obtained when the slope Z' is equal to zero, this allow to calculate K such that the depth at that point is the one desired.

The factors AV and AR give a measure of the camber at the leading edge (AV) and trailing edge (AR). Together with the maximum depth value these factors are sufficient to describe the profile of the sail at any height.

The factor A defines different families of profiles with a different distribution of fullness fore/aft. In practice A = 1 give good profiles for sails used in light conditions. I prefer to use sail profiles with more fullness forward and a flatter leech as obtained with the factor A = 1 + AV / 4. This is the factor used in Sailcut and it give a good range of utilisation of the sails.

The following table give an example of profile data obtained with the above equations.

AV = 5.00
AR = 0.02
K = 2.94
A = 2.250
B = 0.054
C =-0.050
curvature = z" / (1+ z'*z')^3/2

Having defined a single equation for all profiles it is a matter of varying the maximum depth and the factors AV and AR as function of the height of the profile to generate the complete surface of the sail. The profile at foot level being always an arc of circle, the factors AV and AR are equal to zero and only the depth of the foot is entered by the user. A profile called "mid profile" is located around the middle of the height and the factors AV and AR are set such that the profile has the required shape. A third control profile defined as for the "mid profile" is located at the top of the sail. For all other profiles the depth value is interpolated by a quadratic equation and the factors AV and AR are interpolated linearly between between the foot, the middle and top values.

In total 3 values of depth, 2 pairs of factor (AV, AR) and the vertical position of the "mid profile" are used to define the basic mould of the sail.

Note that in Sailcut software the value displayed for the luff factor is equal to the AV ceofficient while the leech factor displayed is 50 time the AR coefficient used in the above equations such that the users can use more friendly range of data than second and third decimal figures.

The above basic mould is not sufficient to define a real sail. Indeed the luff, gaff, leech and foot of the sail are never straight and further more the sail profiles are always twisted from the foot to the top of the sail. I use the distance from the point of maximum round to the straight line and two arcs of parabola rejoining the adjacent corners to define the real edges of the sail. The profiles defined by the sail mould described above are resting on the real edges of the sail. The twist of the sail is finally obtained by applying to each profile a rotation around the leading edge end point.

It is to be noted that this method of modelling the surface of sails gives shapes without bumps or hollows and guarantees that there is no inversion of camber in the profiles. The method is applicable to triangular and quadrangular sails and Sailcut is commonly used for designing old timers gaff sails.

Sailcut CAD is constantly under development and feedback from users is very welcome! If you think you found bug, visit Sailcut's homepage , you will find instructions in the "Reporting a Bug" section.

You can help us improve Sailcut even if you are not a programmer! Simply using Sailcut and reporting any bugs you might find is of considerable help to us. We are also looking for people to help keep translations up to date and to produce new translations. If you are interested in translating Sailcut into your native language, visit the Sailcut CAD homepage and send an email to the development mailing list!

If you have some knowledge of C++ and are interested in making Sailcut CAD a better program, visit the Sailcut CAD homepage where you will find both snapshots of the Sailcut CAD code and how to access to the CVS repository. Once you have had a chance to familiarise yourself with the code, contact us via the forums or our mailing lists!

This section describes the structure of the file generated by Sailcut CAD using the menu File->Export development->to TXT sail. The extension of a text sail file is ".txt".

A sail is made of a number of panels, each panel has 4 basic sides : left, top, right, bottom which are joined by a drawing line. The origin is at the bottom left corner of a rectangle surrounding the panel. These four basic sides define the net area of the panel after assembly in the sail.

Around the basic panel there is provision for stiching the panels and sail edge hems, the outer side of the panel is defined by four sides named cutLeft, cutTop, cutRight, cutBottom and the material is cut along these sides.

Depending on whether or not you have added material for hems around the sail some of these sides may be identical to the basic sides of the panel.

Test main sail cross cut (flat)   // name of the sail
===== CPanel : 0 ====           // begining of panel 0
== CPanelLabel : name ==        // marker for panel label name
0
== CPanelLabel : height ==      // marker for label height
5
== CPanelLabel : color ==       // marker for label color
1
== CPanelLabel : origin ==      // marker for label origin coordinates
427.717 764.064 0               // X Y Z coordinates (Z is always =0)
== CPanelLabel : direction ==   // marker for label orientation
168.204 -57.8975        0
== CSide : left ==              // begin left side of panel
#0      92.5718 886.006 0       // X Y Z coordinates of point 0
#1      92.5718 886.006 0       // X Y Z coordinates of point 1
...
== CSide : top ==               // begin top side
#0      92.5718 886.006 0       // X Y Z coordinates of point 0
#1      262.77  886.006 0
...
== CSide : right ==             // begin right side
#0      3533.09 25.5986 0
#1      3526.2  169.113 0
...
== CSide : bottom ==            // begin bottom side
#0      92.5718 886.006 0
#1      259.921 823.943 0
...
== CSide : cutLeft ==           // begin left cut line
#0      0       899.006 0       // X Y Z coordinates of point 0
    of left cut line 
#1      0       899.006 0
...
== CSide : cutTop ==            // begin top cut line
#0      1150.25 899.006 0
#1      262.77  899.006 0
...
== CSide : cutRight ==          // begin right cut line
#0      3574.36 0       0
#1      3566.15 171.031 0
...
== CSide : cutBottom ==         // begin bottom cut line
#0      0       899.006 0
#1      252.966 805.191 0
...
===== CPanel : 1 ====           // beginning of panel 1
== CPanelLabel : name ==        // marker for panel label name
1
== CPanelLabel : height ==
5
== CPanelLabel : color ==
1
== CPanelLabel : origin ==
889.341 2.64113 0
== CPanelLabel : direction ==
170.396 0.562482        0
== CSide : left ==
#0      548.746 0.388633        0
#1      367.439 68.706  0
...
== CSide : top ==
#0      203.679 871.331 0
#1      393.052 872.078 0
...

This section describes the structure of the file generated by Sailcut CAD using the menu File->Export 3D sail->to TXT sail. The extension of a text sail file is ".txt".

A 3D sail is made of a number of panels, each panel has 4 basic sides : left, top, right, bottom which are joined by a drawing line.

Test main sail cross cut (3D)   // name of the sail
===== CPanel : 0 ====           // begining of panel 0
== CPanelLabel : name ==        // marker for panel label name
0
== CPanelLabel : height ==      // marker for label height
5
== CPanelLabel : color ==       // marker for label color
1
== CPanelLabel : origin ==      // marker for label origin coordinates
427.717 764.064 0               // X Y Z coordinates (Z is always =0)
== CPanelLabel : direction ==   // marker for label orientation
168.204 -57.8975        0
== CSide : left ==              // begin left side of panel
#0      92.5718 886.006 0       // X Y Z coordinates of point 0
#1      92.5718 886.006 0       // X Y Z coordinates of point 1
...
== CSide : top ==               // begin top side
#0      92.5718 886.006 0       // X Y Z coordinates of point 0
#1      262.77  886.006 0
...
== CSide : right ==             // begin right side
#0      3533.09 25.5986 0
#1      3526.2  169.113 0
...
== CSide : bottom ==            // begin bottom side
#0      92.5718 886.006 0
#1      259.921 823.943 0
...
===== CPanel : 1 ====           // beginning of panel 1
== CPanelLabel : name ==        // marker for panel label name
1
== CPanelLabel : height ==
5
== CPanelLabel : color ==
1
== CPanelLabel : origin ==
889.341 2.64113 0
== CPanelLabel : direction ==
170.396 0.562482        0
== CSide : left ==
#0      548.746 0.388633        0
#1      367.439 68.706  0
...
== CSide : top ==
#0      203.679 871.331 0
#1      393.052 872.078 0
...

This describe the structure of the file generated by Sailcut using the menu File->Export development->to XML sail or File->Export 3D sail->to XML sail. The extension of an XML sail file is ".sail3d".

A sail is made of a number of panels Each panel has 4 basic sides : left, top, right, bottom which are joined by a drawing line. The origin is at the bottom left corner of a rectangle surrounding the panel. These four basic sides define the net area of the panel after assembly in the sail. Around the basic panel there is provision for stiching the panels and sail edge hems, the outer side of the panel is defined by four sides named cutLeft, cutTop, cutRight, cutBottom and the material is cut along these sides. Depending on whether or not you have added material for hems around the sail some of these sides may be identical to the basic sides of the panel.

<!DOCTYPE Sailcut >
<CSailDoc>                          // header begin file
<CSail name="sail" >                // begin sail + name of sail
<vector size="10" name="panel" >    // indicate that the sail is made
                                       of 10 panels
<CPanel name="0" >                  // begin panel 0
<CSide name="left" >                // begin of left side of the panel
<vector size="7" name="point" >     // number of points on left side is 7
<CPoint3d name="0" >                // first point = 0
<real value="92.5718" name="x" />   // first point coordinate X
<real value="886.006" name="y" />   // first point coordinate Y
<real value="0" name="z" />         // coordinate Z is always 0 
        // for a developped panel
</CPoint3d>                         // end of first point
<CPoint3d name="1" >                // second point = 1                             
<real value="92.5718" name="x" />                
<real value="886.006" name="y" />
<real value="0" name="z" />
</CPoint3d>                         // end of second point 
 ...
</vector>                           // end of list of left side points
</CSide>                            // end of left side 
<CSide name="top" >                 // begin top side
<vector size="21" name="point" >    // number of points on top side is 21
<CPoint3d name="0" >                // first point = 0
<real value="92.5718" name="x" />
<real value="886.006" name="y" />
<real value="0" name="z" />
</CPoint3d>
<CPoint3d name="1" >                // second point = 1
<real value="262.77" name="x" />
<real value="886.006" name="y" />
<real value="0" name="z" />
</CPoint3d>
 ...
</vector>                           // end list of points of top side
</CSide>                            // end top side
<CSide name="right" >               // begin right side
 ...
</CSide>                            // end right side
 
<CSide name="bottom" >              // begin bottom side
 ...
</CSide>                            // end bottom side

<int value="1" name="hasHems" />    // header indicating that the panel 
                                       has hems cloth around the edges
<CSide name="cutLeft" >             // begin left side cut line
<vector size="7" name="point" >     // left side has 7 points 
<CPoint3d name="0" >                // first point = 0
<real value="0" name="x" />
<real value="899.006" name="y" />
<real value="0" name="z" />
</CPoint3d>                         // end first point
 ...
</vector>                           // end list of left side points
</CSide>                            // end left side cut line
<CSide name="cutTop" >              // begin top cut line
 ...
</CSide>                            // end top cut line
<CSide name="cutRight" >            // begin right cut line
 ...
</CSide>                            // end right cut line
<CSide name="cutBottom" >           // begin bottom cut line
 ...
</CSide>                            // end bottom cut line
</CPanel>                           // end of first panel
<CPanel name="1" >                  // begin second panel = 1
<CSide name="left" >                // begin left side
<vector size="7" name="point" >
<CPoint3d name="0" >
<real value="548.746" name="x" />
<real value="0.388633" name="y" />
<real value="0" name="z" />
</CPoint3d>
 ...
</vector>
</CSide>                            // end left side
 ...
 ...
</CSail>                            // end sail
</CSailDoc>                         // end file