SAIL MAKING - Constructing the Sail
by
Ben Morris
(last edited 08/06/2021)
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.
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.
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.
Mainsail
Luff
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.
Eyelets
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.
Battens
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.
Jib
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
-
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
-
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.
-
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.
-
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.
-
Bend the luff tape over the top surface and rub
your finger along its length to give it a good crease in its middle.
-
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.
-
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.
-
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!
-
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.
STIRLING MARBLEHEAD YACHT RACING CLUB
Inc.
