Check your Keel
Do you have proper
keel shape?
by Paul Bogataj
Are you
fast upwind, but slower downwind? Perhaps it is the other way around. There are
many factors that influence a Star boat's performance. One of these is the
section shape of the keel. A keel optimized for upwind sailing is not much
different than a keel optimized for downwind sailing. The proper keel shape
balances upwind and downwind performance, but it is useful to know what the
extremes look like.
When a
boat is sailing upwind, the keel is producing a sideways force to windward to
offset the force of the sails pulling to leeward. This equilibrium allows the
boat to sail forward and results in the boat and keel moving through the water
at a slight angle. This leeway angle is self‑governing in order to
provide only the force necessary. If more force is required, the leeway angle
will increase to satisfy the requirement.
The
other factor that influences the amount of sideforce produced is speed. At a
constant leeway angle, the amount of force generated is proportional to the
speed of the boat squared. So, if the boat were going twice as fast, the force
would be four times greater. Since changes in speed result in different amounts
of sideforce, the leeway angle automatically varies to maintain its equilibrium
with the sail’s force. Hence, if the boat is going faster, the leeway angle
will decrease.
In
order to design a keel section for upwind sailing, it is important to know how
much lift the keel needs to produce. It is then possible to analyze potential
sections using a computational fluid dynamic (CFD) method to determine an
optimum shape for those conditions. The performance of the keel at upwind
sailing conditions can be compared with its performance at downwind sailing
conditions (basically zero leeway angle) to determine the most appropriate
shape. The other condition of interest is maneuvering, when the boat is going
slower than normal and trying to accelerate with the full amount of sail force.
Examples of this occur when tacking and starting, and result in much higher
leeway angles.
The CFD
method computes the characteristics of the flow around, and in particular, very
close to, a keel section. The water close to the keel's surface that is
disturbed by friction with the keel is called the boundary layer. Some of this
water moves along with the keel and acts to disturb additional water just off
the keel's surface in a shearing manner. At a small distance (1/8" to
1/4") away from the keel's surface, the water is undisturbed by friction
and passes at an undiminished velocity. But, the water affected inside the
boundary layer requires energy and causes drag. By determining the details of
the flow along the keel section and how the water is influenced by friction
with the surface, the drag of the section is calculated with the CFD method.
Three
sections are presented in Figure 1 that have varying fullness forward, causing
differing sharpness to their leading edges. The performance predictions for
these are shown in Figure 2. Drag was computed and is plotted at various lift
levels for each shape.
Notice
that it is possible to design a section (labeled “downwind keel”) that has very
low drag near zero lift, which would be fast for downwind sailing.
Unfortunately, this section has higher drag at the lift required for upwind
sailing and significantly higher drag in highly loaded, slow‑speed
maneuvering situations. It is also possible to design a section (labeled
“upwind keel”) that has low drag at the high‑lift conditions, but it
suffers severely at the low lift conditions of downwind sailing.
Through
application of the CFD method, a section (labeled “compromise keel”) was
designed that maintains low drag at upwind sailing lift conditions, and
balances the compromises in performance downwind and when highly loaded. The
downwind drag is acceptably low and the maneuvering drag is not unacceptably
high, yielding a shape in between the two extremes. The printed shapes can be
transferred to templates to check the shape of your own keel's leading edge.
Finding it toward one of the extremes may explain your one‑sided
performance upwind or downwind.
