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Advances 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.

Click on "hotspots" below for more on the subject:

Foot 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 leech.

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...

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Heel 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.

The reason 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...

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Jib 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 off" the 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.

Below: Separation caused by the sprit on the inside of an Optimist sail.

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Vortex 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 .

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Think one

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 without stalling.

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Air 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:

If 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 leech.

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 for more...

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