Technical Notes
Triangular Sail Issues
“In order to decrease the induced drag of a triangular sail, we should more nearly approach elliptic loading, and this would mean making the upper half of the sail more heavily loaded. With this end in view, it would be necessary to have higher angles of incidence near the sail top than near the boom. However, with current sail control arrangements, this is very difficult, if not impossible, to achieve. For an ordinary, highly tapered, triangular sail, such a setting is impossible.
“It appears that by their very nature, triangular sails penalize light crews greatly in conditions when the 'spilling' wind technique cannot be avoided.
“Where the close-hauled course is not the most important performance feature and the maximum available lift coefficient becomes of preeminent value, low aspect ratio gaff-headed or even square sails may prove true.
“It appears that by their very nature, triangular sails penalize light crews greatly in conditions when the 'spilling' wind technique cannot be avoided.
“Where the close-hauled course is not the most important performance feature and the maximum available lift coefficient becomes of preeminent value, low aspect ratio gaff-headed or even square sails may prove true.
Bermudan Rig
“The Bermudan rig, consisting of triangular sails, having a maximum chord at the foot and tapering almost uniformly to a point at the sail head, although very simple from a practical point of view is, from the standpoint of aerodynamics, inferior. Very severe tapers may lead to aerodynamic characteristics much inferior to those of rectangular sails such as, for instance, that incorporated in the rig of Pen Duick III shown in Fig. 2.122.
“If the potential possibilities in developing more power with relatively low heeling moment are appreciated, the long passage ocean racing of 'Around the World' type might well lead to an entirely different concept of offshore racing rig - something along the Pen Duick III line (Fig. 2.122) with lower aspect ratio sail of various planforms.
'For all its aberrations, the evidence of senses is essentially to be relied upon, provided we observe nature as a child does - without prejudices and preconceptions, but with that clear and candid vision which adults lose and scientists must strive to regain' - (Sir Peter Medawar)
Elliptical Platform
“By allowing a certain amount of washout, to which every sail has a natural tendency, a relatively close approximation to the elliptical loading may, in the case of a rectangular sail, be easily achieved. These characteristics of a rectangular planform are exactly opposite to those of a triangular one.
“... another way of approximating an elliptic planform [is] by means of curved spars. The same effect can also be achieved by employing more efficient platforms than triangular, such as rectangular and trapezoidal shown in Figure 2.122
“... another way of approximating an elliptic planform [is] by means of curved spars. The same effect can also be achieved by employing more efficient platforms than triangular, such as rectangular and trapezoidal shown in Figure 2.122
Sailing Rules
“... It is rather a pity that gaff-headed sails have become almost completely ousted from the sailing scene. Certainly, the rating rules have in this respect a more profound effect on the shape of sails than the aerodynamic requirements or wind in all its moods.
“The penalty incurred, for example, by the sail measurement system on the width of the headboard of the mainsail or the length of its top batten is so high that it virtually precludes any attempt to improve the aerodynamic effectiveness of the modern tall rig.
“Those curious prohibitions, which, after years of enforcement, became part of sailing tradition, effectively discouraged ocean racing people from making experiments with unorthodox rigs which could have led to the development of less tall but more efficient rigs. So triangular sails prevail.
“The penalty incurred, for example, by the sail measurement system on the width of the headboard of the mainsail or the length of its top batten is so high that it virtually precludes any attempt to improve the aerodynamic effectiveness of the modern tall rig.
“Those curious prohibitions, which, after years of enforcement, became part of sailing tradition, effectively discouraged ocean racing people from making experiments with unorthodox rigs which could have led to the development of less tall but more efficient rigs. So triangular sails prevail.
Rectangular Foils
“In practice, foils have a definite span and two or perhaps one free tip (as in the case of a spade rudder or fin keel attached to the bottom of the hull). No matter how the foils are mounted, vertically on the hull like sails or fin keels, or more or less horizontally like hydrofoils or wings, fluid in motion follows the universal inclination to flow from high pressure to low pressure regions by every available path.
“The examination of figure 2.131 reveals, for example, that the rectangular foil, as compared to triangular ones, has a more uniform distribution of the local lift coefficient, c1, and its tip parts, operating at lower effective incidence angles, produce a lower c1 than the remaining part of the foil.
“The examination of figure 2.131 reveals, for example, that the rectangular foil, as compared to triangular ones, has a more uniform distribution of the local lift coefficient, c1, and its tip parts, operating at lower effective incidence angles, produce a lower c1 than the remaining part of the foil.
Foil Flow Pattern
“Examination of the flow pattern round any foil of finite span shows that at the foil tips the air or water tends to flow round the end from the underneath (or windward) surface where the pressure is higher than the ambient pressure, to the upper (or leeward) surface where the pressure is lower. The result of this is threefold:
- The foil surface near the tip is much less efficient at producing lift.
- This decrease in lift is accompanied by an increase in drag.
- An additional disturbing air movement developing towards the tips modifies the direction of the oncoming flow near the foil, hence the effective angle of incidence along the foil span changes, as do lift and drag.
Lanchester and Endplates
“When the endplates are removed, the flow will tend to spill over the free ends, as indicated in the right part of Fig. 2.84 B, i.e., from the side where positive pressure exists to the suction side distinguished by negative pressure. Such a flow wipes out the pressure differences at the tips and reduces them over the entire span of the foil.
Lanchester clearly understood this phenomenon when, in 1897, he secured patent No 3608 covering the use of end plates, called by him 'capping plates', at the wing tips to minimize the pressure losses there, six years before the Wright brothers' flight.
In the patent specification, Lanchester describes the capping plates' action to stimulate as far as possible the condition of a foil operating two-dimensionally in order to minimize the dissipation of pressure. This sail A was demonstrated to be superior to a similar sail without end plate B, since the total force Fta was inclined forward of Ftb. This is equivalent to saying that the induced drag was less by the amount delta D, resulting in sail A having about 15 - 20 percent more driving force than sail B. Triangular sails, as we shall see, are thus notable examples of the worst planform from a purely aerodynamic point of view.
Lanchester clearly understood this phenomenon when, in 1897, he secured patent No 3608 covering the use of end plates, called by him 'capping plates', at the wing tips to minimize the pressure losses there, six years before the Wright brothers' flight.
In the patent specification, Lanchester describes the capping plates' action to stimulate as far as possible the condition of a foil operating two-dimensionally in order to minimize the dissipation of pressure. This sail A was demonstrated to be superior to a similar sail without end plate B, since the total force Fta was inclined forward of Ftb. This is equivalent to saying that the induced drag was less by the amount delta D, resulting in sail A having about 15 - 20 percent more driving force than sail B. Triangular sails, as we shall see, are thus notable examples of the worst planform from a purely aerodynamic point of view.