|
Three dimensional modeling, pressure distribution, stress strain
pictorials, laser cutting, wind tunnel testing, and much more all figure into making a
modern sail for your boat. The following outline will bring you through some of the steps
used in designing sails in the 90’s and beyond.
The first step in any sail design process is to decide on the
geometry of the sail needed to fit the boat. All the outside dimensions need to be
defined. In most cases this is an easy operation and can be accomplished by directly
measuring the boat with a tape measure or consulting a sail plan which is a scale drawing
of the boat. Once the size of the sail is confirmed the interesting analysis starts.
The outside dimensions of the sail are entered into a sail design
program where a 3 dimensional shape is described. In most cases 3-dimensional shapes are
described by horizontal and vertical cross sections of the surface. The key factors in
describing these sections are chord depth %, maximum draft position, entry and exit angle,
and twist ( the amount of deflection the leech has from a straight line between the clew
and head, generally described in degrees). For most boats the optimal sail shape cannot be
applied to the sails because of several obstacles like genoa track locations, spreader
lengths and position, headstay sag and mast bend.
For example, if a sailmaker designs a genoa for a boat that has
long spreaders and does not consider the spreader length when designing the sail it can
not achieve the designed 3-dimensional sail shape the genoa must be trimmed through the
top spreader to do so. Since the spreader cannot move the sail cannot be trimmed in, and
thus the sail sets with more twist than designed. This one little design flaw causes the
sail to set with the following problems, flatter camber than designed, fuller entry to the
sail in the upper portion thus, the sail luffs earlier and, wider exit angle decreases the
power in the sail.
Once the 3-dimensional sail shape is described the sail is ready
for wind tunnel testing. The goal of wind tunnel testing is to optimize the lift and drag
ratios for the boat. These coefficients change with the change in 3-dimensional shape.
Therefore several models may be made to hone in on exact specifications.
|
