SAIL MAKING - Set the Seam Curvature
by
Ben Morris
(last edited 08/06/2021)
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
-
The
original curve built into the board as defined by the airfoil copied onto
the ribs of the building board.
-
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.
-
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.
-
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 |
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Radius of curve of board |
750 |
mm |
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Vary the three values on the spreadsheet to |
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Width of each side of board |
250 |
mm |
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match the values for your board and sail |
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Length of sail panel |
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400 |
mm |
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D' is intermediate calculation showing depth of sail when wrapped around
board |
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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
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It allows uses to define a sail definition and
display a mould of the sail
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It calculates the amount of 'lift' needed by
your board to produce the curvature you have defined for each seam
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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.
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It allows for the created sail definitions to be
stored in the spreadsheet and reviewed or edited later
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It allows for sail definitions to be exported at
any stage to ensure you have a copy of your precious data.
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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
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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
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It can show the effect on the sail mould (and
hence on the sail you produce) by varying the foot curvature of the sail.
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It will prompt you for any changes you make to
any sail definition if you make changes and attempt to leave that sail
definition
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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
STIRLING MARBLEHEAD YACHT RACING CLUB
Inc.
