SAIL MAKING - Seam Curvature
by
Ben Morris
(last edited 29/04/2010)
Setting the correct Curvature
With a
board of this type 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. I have defined this by determining the radius of curvature
of the airfoil section assuming it is close to a circular curve
-
The
width of the board. I have used the 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 in 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
|
Calculations to show independence on chord of percentage curvature of
seam |
|
Radius of curve of board |
750 |
mm |
|
Vary the three values on the speadsheet to |
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|
|
Width of each side of board |
250 |
mm |
|
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??
As a starting point I suggest the following,
Main (4
seams) Seam 1 -
4% Seam 2 - 1% Seam 3 - 1% Seam 4 -
10%
Jib/Main (3
seams) Seam 1 - 5%
Seam 2 - 1% Seam 3 -10%
A
spreadsheet (Sail
Seam Analysis) details examples and explanation for these values and sets up
a process for varying them to suit your taste.
The last
sections of the
calculation sheet mentioned earlier explains the maths in this spreadsheet.
These
figure produce a sail with slightly more draught than most commercial sails (I
think they are too flat anyway!) so varying the figures one or two up/down will
allow for personal choice. The more sails you make and use, the better
will you be able to refine an optimum set of curvatures. You can see how
most of the curvature of the sail is defined in the bottom and top seam.
This is because the sail between those seams is basically just a section of a
cylinder with no need to increase curvature. In fact as the sail is
usually triangular in shape, the small added curvature helps move the maximum
draft forward and the leech tight. An added bonus of putting additional built-in draft on the
lowest seam is to help reduce the formation of diagonal creases running from the
clew to the middle of the mast.
STIRLING MARBLEHEAD YACHT RACING CLUB
Inc.
