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SAIL MAKING - Constructing the Sail  
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


Constructing a sail   Luff Curve   Finishing the sail

Constructing a sail

Start by reading the rules for the particular class of yacht.  All specifications are different and you need to be very familiar with them.  Plan the sail on paper taped to your building board or draw directly on to your board tho this can become confusing after a few different sails are drawn.   I use melamine coated MDF as its smooth and easily kept clean and tape thick paper plans to it.  Decide on the number of panels and add those lines to the plan.  I always use horizontal seams (much easier) even though it there may be some slight advantage using seams at right angles to leech particularly with cloth that has directional properties.  Use these plans to transfer the shape for each panel to the sail material allowing at least 10mm extra all round possibly more along the luff as this will have a curve (see next section) up to 15mm or so.  Don't forget to add the extra allowance for overlapping the seams.  Cut all pieces and mark the 40% width (or whatever your board is designed for) on each.  Using the appropriate curvature  for each seam and construct the sail as outlined above.  Now for the good bit.  Tape the head of the sail to the board and hold the tack and clew of the sail with your two hands - amazing a smooth, perfectly shaped 3 dimensional sail appears.  Tension on the clew will show how the leech will stand up or twist off as you vary the tension.  I have to say I was really amazed the first time I did this.  Its just what the sail makers love to show you!!


 I have written a simple program in Microsoft access to plan Marblehead and other sails.  It produces a simplified outline of the sail and calculates the width at the 1/4, 1/2 and 3/4 points, determines the sail area and combines it with the area for the alternate sail (main or jib) to show the rig area.  This is written in access 2003 so should convert readily for later versions. Right click this link (updated July 2013) to download using 'Save Target as' and save zipped files.  Extract all the files into a new folder (e.g. 'Sail Shape') in your documents section.  There are two database files one being the program the other the data.  As well there is a nice little icon you could use on a desktop shortcut and a set of help files accessed from the program in a separate folder.  If you have  Microsoft access - open the file 'Sail Shape.mdb' (not the data file!) and examine the examples I have produced.  You may delete or add to these files.  The data file contains all the stored information so even if I update the program file later it will not interfere with the stored data in 'Sail Shape Data.mdb'


How much Luff Curve

In some ways, adding built -in curvature using seam overlap causes an added complication to the critical question of how much luff curve to add.  Historically, the luff curve was the only means of adding sail curvature.  By placing a section of extra sail cloth along the mast allowed that part of the mainsail to belly out causing a curve to the sail.  This curvature was very much biased towards the leading edge resulting in a very 'blunt' entry and caused back-winding when on the wind.  The only reason it worked was as the wind speed and load on the sail increased, the 'soft' material used for the sail stretched and 'pushed' the depth further back in the sail.  This was fine until the wind increased so much that the curvature was forced so far back that the leech developed a nasty curve to windward. 


effect of luff curve on soft sail cloth


Modern sail cloths exhibit much less stretch than the soft Dacron/terylene or even cotton used in the past. This is even more the case in sails currently used in model yachts as they are much stronger in proportion to their size than those used in full sized yachts and use very low stretch mylar film and films reinforced with carbon fibre etc.  Consequently curvature added by using extra luff curve tends to stay at the luff edge of the sail and not move back as the wind increases to any great extent.  Such a sail then may be quite powerful off the wind but suffers from back-winding when beating thus spoiling its pointing ability.

Why have any luff curve then?

1.    All sail material stretches to some extent so this feature can be used to place some curve in a sail and match it to the expected wind range the sail is to be used for.

2.    The process of building in curvature reduces the projected chord of the sail curve so placing an effective concave curvature along the luff  (and leech for that matter though this is dealt with later) .  A convex curve to the luff is added to compensate for this.

3.    The mast in most rigs takes up a natural curve when tensioned up with the backstay to keep an appropriate tension in the jib stay and more or less tension in the rig varies the amount of curve thus allowing some degree of control over the amount of shape in the sail.

I have found the best way to deal with the luff curve is to attempt to match the curve of the mast when tensioned appropriately and add a little bit - how accurate is that??  It seems that this little bit is the secret that most sail makers keep secret from the rest of us as they have probably determined how much is right by trial and error too.

Clearly the amount of added curve will depend on the mast material and how it is rigged.  A soft mast will need more while a fully rigged and rigid mast will need less.  Most of the sail making articles mentioned earlier use guides like 0.3%-0.75% of the luff length.  Thus amounts to values like 6mm - 15mm of luff curve for a 2m 'A' rig Marblehead mainsail.

The other part of the question is how is this added material distributed over the luff length?  Most agree that the amount is added in a regular curve with the maximum amount added about 60% of the luff length.  A tapered rig which is more flexible towards the top may well have the maximum as high as 75% or even higher.  This is why sail makers ask the question of you when you order a sail about the type of mast material and what means you are going to use to support it - shroudless or shrouds etc (if they don't ask, perhaps you should mention the nature of the mast and its support and ask them about the luff curve.  The response might just make you get a second opinion and a better sail).

There is one big advantage to making your own sails because you are going to have your mast before you make it or have one similar in characteristics.  This way you can set the mast up on a bench on its side with a backstay and forestay tensioned to what you normally use on this sail.  Start with about 2-3 kg backstay tension if you are not sure or enough to give about 12mm for a stiff 2m mast and up to 18mm for a soft one.  Different lengths of masts will require equivalent amounts of bend.  This will enable you to mark the mast curve on the bench (or paper taped to the bench) with a sharp black pen.

The sail with all panels joined can now be laid on the bench over the drawn mast curve.   Now for the tricky bit!!  If you simply transfer the drawn curve to the sail material, the luff curve will end up being too small (see no.2 in 'Why have any luff curve then?').  Instead the sail must be set up in its natural built in curve.  Secure the head and the tack to the bench with tape tensioned slightly.  Raise the clew 10cms or so and tension it so that the sail takes up its natural curve with the sail just meeting the bench at the luff region evenly over its length.  Not only will this show the beautiful regular built in curve but if you had watched carefully you will have noticed the centre section of the sail material move back a little towards the leech as that No.2 action occurs.  Let the sail go and see it move back.  You will need to arrange a temporary sheet or tape to secure the clew in the raised curved sail position and only then can the drawn curve be transferred to the sail cloth.  The sail materials we use are invariable transparent to some extent and the drawn line should be visible under the sail.  It can then be transferred to the sail using the sharp permanent pen.

clew raised to form curved profile ready to mark luff line

If done correctly this allows the luff curve to correctly be adjusted for its built-in curve and the curve of the mast.  Tensioning the rig more or less than the one used to set up the curve on the board will increase or decrease the curve on the sail, powering it up in the lighter winds and flattening it in the heavier ones.

The same principles are used to produce the luff curve on a jib except that I tend to cut the jib luff straight or with a small amount of convex curve (2mm) in the bottom 2/3 of the luff after lifting its clew in the same way.  This gives a slight convex luff curve to the sail if laid flat and sits nicely on a firm forestay.  While many of the articles talk about a slight 's' bend to the jib luff curve  by going slightly concave above the 60% luff height, the amounts mentioned are very small (<2mm) and I have not found it necessary but you could experiment here and let me know if there is any improvement.  (The same effect might be had by slightly decreasing the built-in curvature by a few percent in the top seam but I have not done this either - yet)


Finishing the Sail

Really this is just the fiddly bits at the end and requires patience, little fingers and a few tools rather than anything mind-shatteringly new.  However, there are a few hints worth exploring and it has to be done anyway so why not try to get it looking good.



Once the critical luff line is drawn, I continue my practice of not using any sewing and reinforce the luff with 10mm wide clear Mylar tape.  There is a special sail luff tape available from Radio Yacht Supplies Australia or Radio Sailing Shop amongst other places.  The sail should still be taped down with a gentle tension on the luff.  The tape is laid carefully on the sail material just overlapping the luff line.  This way when the sail is trimmed, the reinforcing is trimmed too and a nice neat edge is produced. Take care not to stretch the tape - use a very light tension just to ensure it lays down without bumps or puckers.  Trimming the luff is done with a very sharp cutting tool.  I have found one in 'Spotlight' with a curved blade which is much better than one with a point as it is less likely to pucker the material.  Alternatively use a 'break off blade' cutter and snap off the blade to reveal a new cutting edge regularly.  You need to take more care with this type of blade to prevent puckering of the sail material so ensure you cut with a low angle between the blade and the material.  Use a metal or plastic guide to assist the cutting.  Always cut with the edge laying over the sail and the blade on  the side to be discarded - cutting errors only effect discarded material then.  I cut over a self-sealing cutting board but board such as MDF will do fine though it will need changing or smoothing regularly.

I have found a remarkable product to assist cutting.  This is 'Clear Grip' (probably a lot of other brands as well) and is a plastic film that adheres to plastic rectangular rulers used by quilters.  It adheres by a static charge it produces on its surface.  When placed on sail material (or quilting cloth) it miraculously holds the material in place allowing cutting to occur without the material moving.  Quilting /haberdashery stores contain a wealth of items for our hobby!  This means I have returned to using 18mm snap off blade cutters to cut the sail cloth.  held at a low angle there is no problem with the cloth puckering and sharpening the blade is as easy as snapping off the used section and beginning with a very sharp cutter.


Measuring the sail prior to Trimming

The sail is now ready for final trimming based on your sail plan.  Careful measurement checked at least twice!!! Define the position of the clew, quarter, half and three quarter points or whatever the rules prescribe.  This is where the widths from the database program come in handy!


Reinforcing the leech and foot and trimming the sail

Once the leech points are marked on the sail, I usually add a thin Mylar reinforcing tape to the leech.  The prevents stretching of the sail and creeping of the seam over the glue from the double sided tape.  Do not try to curve this tape around corners but lay straight.  New pieces are used for each straight section of the leech slightly overlapping the line to be trimmed.  Again it is important to have the sail taped down firmly and to not stretch the tape when sticking it down.  There must be no crinkles or puckered sections in the final sail!  Use a straight edge and the sharp cutter to trim the sail to the marks, cutting through the tape.  This helps to give a clean tidy edge.  I invariably mark the lines ~1mm undersize so the sail measurement are within the defined limits by a very small amount.  Trim the curved part of the leech at the head and clew if appropriate (Marblehead sails define a minimum 900 mm radius for this curve) using a pre-prepared template to aid the cutting.


Head and tack patches

Head and tack reinforcing patches are made using self adhesive sail cloth usually with two overlapping layers a side.  There is little to gain from using other than rectangular pieces at the head and quarter circle outlines for the tack.


Clew reinforcing

In addition to the sticky backed sail cloth I add a quite thick piece of Mylar sail cloth to this critical corner.  I was able to get plenty of of cuts from a local sail maker for this.  It is stuck to the sail using wide double sided tape covering one side.  This firm support really helps the sail set better both keeping the leech firm and spreading the load over the body of the sail.  I use a single layer of reinforcing about a 3-4 mm quarter circle on the clew and cover it with more normal sticky sail cloth.  There may be some advantage in using a layer of this stronger reinforcing at the head as well.



Next add the small stainless steel eyelets (guess where from) to the head, tack and clew.  I add small self adhesive sail cloth reinforcing patches along the luff where the sail ties to the mast will go.  Use a heated sharp metal point to make the holes at the carefully marked points on the luff.  I find about two per panel is sufficient.



Sail Battens are added using the self adhesive material (available from that place again) according to the class rules. This is actually thin fibreglass with a layer of double side sticky tape.  As such they could be constructed easily laying up a single rectangle of glass and resin between two flat pieces of glass or thick plastic covered with release agent.  When set, this is easily cut with scissors and held onto the sail with double sided tape.  I always add some sticky sail cloth to wrap around the leech end of the batten and sail and a small patch placed over the inner end.  This prevents the batten from lifting.


Class insignia, sail numbers and your insignia

Finally add the class insignia, sail numbers and your sail making insignia to your sail to the appropriate place on the sail according to the class rules.


Attaching main to mast

My experience has always been with round masts.  Attaching the sails to this type of mast is most easily done with cord ties such as light spectra.  I use a reef knot BUT one where the cord is wound around twice (left over right - left over right then right over left - right over left).  This knot seems to hold in this type of cord very well with no need for any glue to hold it.  It is also excellent for adjusting the looseness of the loop because there is initially much more 'give' when tightening the knot - pulling on the ends tightens the loop, pulling on the sail  loosens the loop.  When in doubt go for a loose loop.  Too many sails are spoiled by having these loops too tight.  The idea here is to allow the sail to move around the mast.  This has two obvious advantages.  The airfoil created with the sail at the top of a round entry produces more lift with less drag  than one where it comes from the middle and having the luff able to move readily will allow excess luff curve to move forward around the mast and take up a slight excess.  Even errors in judging the luff curve can be  compensated for a littler bit by varying the amount of looseness at different parts of the sail e.g. if too much luff curve then make the bottom and top loops looser and converse for too little luff curve.  I am not too keen on the metal connectors that can be purchased to connect sails for all the reasons given above!

I  wish to add special note about the top attachment (head) of the sail.  As my sails are designed to have this additional curvature at the top which is allowed to twist off to prevent stalling yet still maintain drive it is very important to have a rotating fitting that allows this movement.  Just tying the head with a vertical cord to a backstay crane or similar holds the sail in a for and aft position and does not allow the twist required.  The jib does  not seem to be such a problem as long as the head chord is fairly long.  If this proves a problem reduce the %age draft at the top seam by one.



This sail is treated in almost the same way.  The main difference comes after adding the luff tape.  Rather than the heavy duty luff tape I use the thin one (same as leech tape).  I then make a groove for the forestay using double sided sticky and the special luff material supplied by Radio Yacht Supplies Australia again!  Take care to keep the double sided stick tape away from the very front of the luff otherwise it is very difficult later to get the wire through as it sticks to the glue.  The stainless wire and special loop maker supplied by Radio Yacht Supplies Australia make the forestay easy to make and professional looking.


Steps in making the luff groove

  1. The Mylar reinforcing tape should already be present prior to final trimming of the luff curve.  tape the sail at the tack and head to the working surface to keep it in position but leaving 12mm or so clear to lay the luff tape

  2. Working on one side lay one section of 6mm double sided tape to the Mylar keeping about 1mm in from the front edge. Any adhesive laying in front of the luff will catch on the wire and make it difficult to adjust later.

  3. Lay the luff tape over the double sided tape and secure top and bottom so that half of it hangs over the luff.  Starting at one end remove the top backing strip from the tape a small section at a time from underneath the luff tape ensuring it remains overlapping half its width.

  4. When secured on one side turn the sail over and resecure the sail as before.  Lay a new section of 6mm double sided tape to the top surface again remaining about 1mm in from the luff edge.

  5. Bend the luff tape over the top surface and rub your finger along its length to give it a good crease in its middle.

  6. Prepare your luff wire and lay it in position at the front of the luff taped securely to the bench. Bend the luff tape over the wire in its final position.  The crease you have made before helps a lot here.

  7. Use your four fingers to gently hold the luff tape in position.  You will be able to feel the wire and can make sure it is in position at the same time.  Gently begin removing the backing strip from the tape securing the luff tape in position.  Work slowly a little at a time ensuring the tape and wire are set neatly against the luff of the sail.

  8. It is possible to do step 7 without the wire and feed it in later but you had better be sure you kept the adhesive back from the luff edge all the way up on both sides!  Yes, I am talking from experience here!

  9. The luff tape is made before the sail is trimmed to size so it can be trimmed during the corner reinforcing stage.  Ensure that at least the second of two pieces of corner reinforcing material wraps around the luff to give some strength to the luff tape at these points.