in sail aerodynamics - part two
The wide spread
use of lift and drag is another heritage
from aircrafts. When it comes to sailboat
aerodynamics, drive and heel are more appropriate
and descriptive, after all the sailboat
is driven by the wind, unlike the airplane
which is lifted in the air. Drive is in
the direction of the motion of the boat,
heel tends to lean the boat over, while
the directions of lift and drag don't have
such an obvious link to boat performance.
At apparent wind angles over 90 degrees
drag even adds to the drive of a sailboat,
so when it comes to sailboat aerodynamics
drag has a very different role from airplanes.
For the underwater hull, lift and drag
are most appropriate: drag acts directly
opposite to drive, and lift directly opposite
to heel. Heeling moment is an important
factor when it comes to sailboats, while
the equivalent rolling moment is fairly
unimportant in the case of airplanes.
on "hotspots" below for more
on the subject:
vortex & leeward side separation
When you bear away from the wind and
don't ease the sheet, separation takes place
through a different mechanism. There is
always a vortex present at the foot of the
jib (similarly under the boom of the mainsail),
where the sail meets with the deck. This
is independent of the fact whether the foot
touches the deck or not, although a gap between
the foot and the deck does accentuate the
strength of the vortex.
When you bear away, airflow
on the leeward
side behind the jib first detaches at
the very tack, streaming up the luff before
bending backwards into the foot vortex.
A triangular area of stalled air is spreading
up from the foot of the genoa or the jib,
increasing in extent as you turn away from
the wind. This disturbance causes a similar
streak of separation on the mainsail, too,
extending from above the tack towards the
The conventional belief, borrowed from
airplanes, that the triangular jib is a "tip-staller",
is untrue. The jib is so twisted,
and the narrow head so close to the mainsail,
that flow remains attached in the head
while separation spreads from the tack
to the clew and then up the leech.
Massive flow separation on
the leeward side of a modern IRC boat,
and a Star jib, at angles close to stall. Blue
colors show areas of flow separation,
re3d colors mean accelerated flow (pressure
drop or suction).
Click on the pictures for more...
effects on flow pattern
Heel has an important effect on air
flow over the sails. Partially the reason is
that heel reduces the sails' angle of attack.
This can be understood if you think that you
would heel the boat over 90 degrees, then the
Windex would inevitably fly straight back,
indicating a zero angle.
Heel influences flow also otherwise - separation
behind the jib tack is greatly reduced,
and so is the vortex under the the boom,
as the windward rail rises in front of
the sails when the boat is heeled. Air
flow is bent more and more upwards on the
windward side of the sails, as heel increases.
Heel effects: Above, flow separating
at the foot at zero heel. Below, no separation
at all at 20 degrees of heel.
could be on one hand that heel reduces
the effective angle of attack, and on the
other hand that the deck rises in front
the jib foot, and the leeward side of the
jib gets closer to the sea surface.
Click on the picture for more...
head vortex on the main
As always at the tips of airfoils,
there is a vortex forming behind the leech
of the jib towards the head. When this vortex
travels behind the leeward side of the main,
it bends streamlines above itself away from
the surface of the main, effectively "ripping
flow from the main surface at the hounds
level and a little above it. This can be
seen on the simulation as a triangular, disturbed
air area. Above this area, closer to the
top the flow can still be attached.
Lower fractional rigs (7/8-rigs) suffer
more from this than masthead or high fractional
rigs. This could be one reason that 9/10-rigs
have grown in popularity lately.
Above: A large separation area, shown in
blue, in the leech of the mainsail. The vortex
from the head of the jib bends the flow loose
of the mainsail surface just above the hounds.
Separation caused by the sprit on the inside
of an Optimist sail.
Click on the picture for more...
behind the mast
On the leeward side of the main, right
behind the mast we have the dreaded "separation
bubble" caused by the mast - largely
the culprit for the bad reputation of the
mast. It turns out this is not a separation
bubble in the meaning of the word, but rather
a separation vortex with similarities to
that on the inside of the jib.
The vortex on the leeward side of the
main is confined to a relatively narrow
area behind the mast. The air sucked in
below the tack of the sail rises
towards the top of the mast. The flow most
always reattaches on the surface of the
main relatively soon behind the mast.
In case of a fractional
rig, the vortex is intensified by a new
stream of air behind the hounds, creating
an effective suction on the front side
of the top mast. Thus the mast seems to
benefit from what the mainsail loses in
the separation area at the leeward side
of its luff.
Above: A narrow whirl of dead air creeping
up the backside of the mast.
Below: Separation vortex behind a
classic 6mR mast .
Click on the picture for more...
To understand how the mainsail and
the jib work together, it may be useful
to think of them as one wing with
a slot in the middle. The jib forms
the front part of the wing (leading
edge in aircraft terminology) and the
mainsail forms the back part (trailing
edge). The jib is the curved, smooth
shaped front part of the wing, the
main is the more straight, flat part.
When you adjust the main sheet or traveler,
you adjust the camber of the whole
wing in a very smooth way, around the
hinge that the mast forms.
When you keep in mind that pressure
always acts perpendicular to the surface,
it is easy to see why the jib usually
is in charge of most of the forward
driving force, while the main causes
most only heel. When sailing upwind
most of the surface of the jib is oriented
in the direction of the motion, while most
of the surface of the main is oriented
perpendicular to it, the leech often even
pulling back. But it would be wrong to
think that the jib is more important for
that. The mainsail is bending the air in
front of the jib, allowing the boat to
point higher than with the jib alone, and
also helping the jib bear more loading
in the slot - accelerated or not?
The old question about the slot effect seems
to have a different answer depending on how
you define "slot", and also how
you define to accelerate:
you consider the slot as the area between the
mast and forestay, in front of the mast, then
the air is slowing down. But if you define
the slot as the area between the mainsail and
the genoa behind the mast (like I would), then
air is definitely accelerating in the slot,
as witnessed by the colors turning from yellow
to orange and red, when the air moves from
the mast towards the leech of the genoa. You
can try this yourself: When you stand of the
fore-deck, the wind is lame, but when you
walk back into the slot between the main an
the genoa, you really feel the wind blowing
in your face by the time you reach the genoa
This acceleration is
relative, however, the airspeed is not faster
in the slot area than in case the mainsail
were alone, without any jib at all. The main
effect of the genoa or jib is to slow down
the air speed in the front part of the main,
no Venturi-effect there.
Click on the picture