[this file contains lots of graphics and may load slowly
- sorry for the inconvenience]
 |
Since
autumn -94, WB-Sails has applied a rigorous scientific
method to their Finn-sail development. The results
are excellent: In less than a year we have come
up with a sail/rig combination that has won 7 out
of the 11 races sailed in the two pre-olympic regattas
at Savannah this summer and fall. |
The sail development system consist of:
- a masthead camera fitted to the top of the mast
- a computer program to analyse sail shape from photos
(ShapePlot)
- a computer program to predict sail force (MacSail)
- a computer program to calculate mast bend and its
effect on sail shape (TrimSim)
Initially, several sails of different make were photographed,
before the first prototype sail was built. The MastCam
film is developed onto a CD-ROM, which allows us to read
the shapes directly into the computer for an accurate
analysis. The analyzed shapes are read into a high-end
aerodynamic program MacSail, which predicts the driving
force and the heeling moment of the rig very accurately.
MacSail can pinpoint flaws in sail shape, such as flow
separation or luffing, and allows us to perfect the shape.
Of course, simultaneously with the theoretical work,
most valuable practical information about the behavior
of the sail was obtained from the two champion sailors,
Jali Mäkilä from Finland and Fredrik Lööf
from Sweden. The input from the sailor is even more
important when it comes to judging the stiffness &
bending of the mast.
Finn TrimSim
The Finn class having recently allowed carbon fiber in
the rig, it was necessary to develop a new mast. For this
purpose we wrote a new program, Finn TrimSim, which calculates
the mast bend when material characteristics and mast cross
dimensions are given in ten sections at 60 cm intervals.
TrimSim allows us to tell accurately how much material
needs to be added to achieve a certain mast bend or, on
the other hand, how the mast bend effects the sail fullness
and shape.
Wind tunnel tests
WB-Sails also did a series of wind tunnel tests to study
the importance of the mast shape on sail force. The Finn
class rule allows a small wing mast - it proved clearly
superior to the traditional oval or round spar, so a wing
mast was produced. Sailing, it showed definitely faster
than the round mast with similar bending characteristics.
Part of the advantage comes purely from the area that
the mast adds but which is not measured by the rule. Most
of the efficiency, though, comes from a smoother airflow
on the windward side. On the leeward side the airflow
is slightly more disturbed, but this doesn't seem to bother
too much. All in all, with a correctly shaped wing mast
lift can be increased more than 5% with no effect to the
drag. On the downside, the wing mast is slightly heavier,
more difficult to achieve the correct bending characteristics
with, and much more expensive to produce than a round
mast.
The same scientific development tools - MastCam, TrimSim
and MacSail, are applied to the 470- and Europe-class
as well. They have also been used in sail development
for other one-design classes, such as the Dragon or
the H-boat, as well as the Maxi-yacht Nicorette.
The Sail Design Spiral
- 1 The creation of a new sail design
starts with a MastCam session.
- 2 MastCam photos are analyzed
with ShapePlot, to record the flying shape
of existing sails.
- 3 The flying shape is read into MacSail, to calculate sail forces. Design
flaws are corrected and shape is optimized.
- 4 Sail forces are input to the VPP, to verify performance with the new,
altered shape.
- 5 New sails are designed in the
computer ...
- 6 cut with the Laser and sewn
together.
- 1 ... The spiral starts again:
new sails are photographed for computer analysis.
This way, sail shape is improved systematically. By combining
computer simulation to the real world via MastCam, development
work progresses towards an optimal shape much faster than
with the traditional trial-error method.
ShapePlot caption of a 470
main sail
Sail shape is picked up into the computer by mouse-clicking
tape markers on the sail scan (above). The program then
gives a detailed analysis of the sail (below).
Copyright © 1995 WB-Sails Ltd. All rights
reserved.
WB-Sails Ltd Oy
email: Mikko@wb-sails.fi
|
