SAIL MAKING - Basic Seam Setting
by
Ben Morris
(last edited 08/06/2021)
This information represents a short cut to setting
the seams without dredging through the maths of the 'Set The Curvature' page by
defining a board lift at each seam in a sail as a value in mms. It assumes
that the board has been made with similar curves and size to the one described
and that the seam width - i.e. the vertical distance between the seams is about
350mm - 450mm. Even if these conditions are not met the ratio of the amount
of board lift still applies.
These values have been derived by applying my maths
to the seam production, by a trial and error process to see the effect on the
sail shape and by investigating the seams generated by a variety of formal sail
design programs. It would appear that my simplified cylindrical model does
cause the middle of the leech to fall away slightly due to the triangular nature
of the sail and the need to allow some twist in the sail and still maintain
shape. Consequently the values here suggest that the middle seams require
a little more curvature than previously thought.
The examples here are based on Marblehead and IOM
sails but apply equally well to any RC sail of 2-4 panels.
They also assume that the foot of the sail has
significant camber of 7%.
Typical 5 panel sail - board lift
| Seam No |
|
| Seam 1 |
1.5 mm |
| Seam 2 |
0.5 mm |
| Seam 3 |
1.0 mm |
| Seam 4 |
2.2 mm |
Typical 4 panel sail - board lift
| Seam No |
|
| Seam 1 (Foot) |
1.5 mm |
| Seam 2 |
1.1 mm |
| Seam 3 |
2.2 mm |
Typical 3 panel sail - board lift
| Seam No |
|
| Seam 1 (Foot) |
1.7 mm |
| Seam 2 |
2.2 mm |
These values produce medium fullness sails with
reduced draft at the head. Values can be reduced or increased in the
same proportion to produce sail which are slightly fuller or flatter. They
assume a foot curvature o about 7% and a board of similar configuration to mine.
For producing lifts of various values I use plywood
from modelling shops which have sheets of 1.5mm, and 0.4mm thickness. In
addition the 50micron sail cloth is .05mm so two such sheets are 0.1mm thick.
STIRLING MARBLEHEAD YACHT RACING CLUB
Inc.
