- angle of attack - the angle the foil faces to the flow of water over it
(this varies on rudders as you steer the boat).
- aspect ratio - the comparison of the length to the width of the planform
of your rudder or centerboard. High aspect ratio foils are long and
skinny while low aspect ratios are short and squat. Boats with higher
maximum speeds are more apt to use high aspect ratio foils (think of
the fin on a sailboard as the ultimate example of this) while slower boats
are more suited to lower aspect ratio foils.
- chord - the distance from the leading edge to the trailing edge.
- ellipse - a 'stretched' circle (oval). An important shape in
- foil section - the cross-sectional view of your rudder or
centerboard. (Similar to the cross-section of an aircraft wing
[airfoil] but nearly always symmetrical from side to side).
- NACA 4 digit series (NACA0008, NACA0010, etc) - the most popular
foil sections used in sailboats. The first 2 digits ('00') indicate
that the section is symmetrical, and the last 2 digits ('08', '10' etc)
define the maximum width of the section as a percentage of the length of the
chord. In other words, a NACA0008 foil is 8% as thick as the foil is
wide. The maximum thickness on these foils is found at a
distance 30% back from the leading edge.
- planform - the outline of your rudder or centerboard when viewed from the
- spline - a curved line that is defined by a number of points that it
passes through (or close to) - think of flexing a thin piece of wood around
those points. There are several different mathematical forms of
splines with different properties.
First of all, the obvious: the planform you design must fit within your
fleet's class rules. Some fleets offer complete freedom in this, while
others may force you to fit within a few mm tolerance of their official
The simplest planform is the simple rectangle, and will give good performance
while also being easy to fabricate.
A performance increase can be had by going with an elliptical tip - this can
be done with either an elliptical leading edge or trailing edge while keeping
the opposite edge straight (or the tip can be fully elliptical). The
performance increase comes from the increased lift and reduced drag from this
shape - with a rectangular tip, water can 'escape' by going around the bottom of
the foil rather than across the chord, while with an ellipse** its always a
shorter path across the chord than it is around the bottom.