Dive Deep into the Sailing Niche >> Scuttlebutt Sailing News

In his book The science behind sailing, Dutch scientist Joop Slooff explains in detail the aeromechanics and hydromechanics of sailing, providing comprehensive and scientifically justified descriptions of the reasons boats sail.

Sloof, who also wrote about his involvement with the infamous wing keel design of the 1983 America’s Cup winner Australia II, was inspired to develop a recent report on the trim of the Scuttlebutt sail. Buckle up your belt for this puzzle:


Barry Hayes’ article “Ins and Outs of Inhaulers” is interesting, but misleading, if not inaccurate, in explaining the physics of the flow of interacting mainsails and headsails.

Many theories have been put forward to explain the perceived superiority of a rig with tightly coupled headsails and mainsails. However, it was only in the article +) published by the late Arvel Gentry *) in 1971 that the mechanisms involved were fully understood.

As Gentry argues, the interaction between a mainsail and a foresail is governed by two different mechanisms. The first is the upward / downward movement generated by each of the sails. The second is increased flow velocity near the trailing edge (leech) of the foremast. Figure 1 illustrates what this means.

Figure 1 Mainsail / fore-sail interaction mechanisms.

The flow is shown schematically around a section of lifting sail (solid black line) at a small angle of attack. As discussed in more detail in the airfoils manuals, the presence of a lifting airfoil (sail) causes an upwind upstream and a downwind downstream of the section. In addition, a region with a locally high flow velocity is formed on and above the suction side of the section, just behind the leading edge.

When another sail section (headsail, black dotted line) is placed in front of the first section, it experiences the uplift induced by the first section (mainsail) as a virtual increase in the angle of attack. . The variation of the upwind induced in the direction of the current is experienced by the foremast as a virtual increase in camber, in particular towards its trailing edge.
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+) Gentry, A., ‘The Aerodynamics of Sail Interaction’, paper presented at ‘Ancient Interface III’, 3rd AIAA Symposium on Sailing, November 1971, Redondo Beach, California

Figure 2 Effect of the presence of a mainsail on flow around a foremast.

*) Arvel Gentry, aerodynamicist and boater (like this author), was working at the time as a research scientist at the former Douglas Aircraft Company in Long Beach, California. Its boss, AMO Smith, a well-known fluid dynamician, was to publish, in 1975, a branded article 7.35 on the aerodynamics of aircraft wings with high lift features at the leading edge and trailing edge. Smith’s article describes the mechanisms involved, which are very similar to the mechanisms involved with sails, in a substantial way.
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When the trailing edge of the headsail is positioned in the high flow velocity zone a short distance aft and above the leading edge of the mainsail, which is the case when the sails are lined for “upwind” conditions, even more circulation (resulting in more lift) is required to meet the flow separation condition smooth and tangent to the trailing edge.

In addition, the boundary layer of the foremast experiences a higher “dumping rate” on the trailing edge. That is, it detaches from the trailing edge of the foremast with a higher speed than the free speed.

As shown in figure 2, this implies (cf. Bernoulli’s law) that the pressure at the trailing edge of the foremast is lower (level ‘B’) than without the presence of the mainsail (level ‘A’) . As a positive consequence, the separation of the boundary layer on the upper surface of the foremast is postponed to higher lift levels.

In other words, the foresail can carry more lift when it is close to the mainsail. In summary, it can be said that a headsail in the presence of a mainsail is forced to carry more lift through the uplift induced by the mainsail and that its boundary layer can withstand this level of lift more higher by the higher “discharge speed” / lower pressure. at the trailing edge.

Figure 3 Effect of the presence of a foremast on the flow around a mainsail.

The other side of the picture is that the mainsail is operating in the area of ​​foremast induced downwash. This is experienced by the mainsail as a smaller angle of attack but also as some increase in camber, especially near the leading edge.

Figure 3 shows the effect on mainsail pressure. Due to the reduced perceived incidence and increased camber near the leading edge, the mainsail carries less lift in the presence of the foremast, especially near the leading edge.

An additional aspect is that the foremast acts as a kind of “flow direction device” for the mainsail. Since the flow always starts from the drop of the foremast in the direction of the section slope at the trailing edge, the effective incidence of the mainsail varies little when, for fixed listening angles, the geometric incidence of the whole (apparent wind angle) is varied.

As a result, the lift of the mainsail hardly changes as the apparent wind angle increases or decreases. This means that most of the variation with the angle of attack (apparent wind angle) of the lift and drag of the sail combination takes place on the headsail.

With regard to the description given above, the function of a downhaul or barber hale is to have more control over the width of the gap between the foremast and the mainsail and the sheet angle. foremast. What this, and the effect of the overlap, means for the forces (lift and drag) acting on the respective sails is described in detail in the book, The science behind sailing.


Editor’s Note: With some trepidation, we asked Joop how the amount of overlap between the headsail and mainsail affects the variables, to which he replied:

The increased overlap increases both lift and drag (not surprising of course due to the increased sail area). In terms of driving force, this means that the maximum driving force is increased (important to achieve). However, there is virtually no effect on the driving force / heeling force ratio (important for upwind). See figure on the right.

This means that there is no point in sailing with an overlapping headsail in high wind speeds and at very small apparent wind angles. However, the overlap pays for low to moderate wind speeds and larger apparent wind angles, provided the overlap does not exceed approximately 45%.


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