Section 5
Page 47
The effective span coefficient is then further modified to reflect the fact that as the sails are eased at wider apparent wind angles the effective span is reduced as the sealing of the jib and the hull is lost and the sail interactions become less favourable.
cheffUpwind = eff _ span _ corr × (0.8 + 0.2 × be) cheff Downwind = cheff _ max_ spi × (1.0 + 0.1 × be)
[41]
The term be varies from 1 to zero as apparent wind angle widens from 30 to 90 degrees (Figure 23). 1.2
1
cheff
0.8
0.6
0.4
0.2
0 10
20
30
40
50
60
70
80
90
Apparent Wind Angle - deg Figure 23.
Variation of Effective span factor with Apparent wind angle
Finally the effective height “heff” is calculated from the product of “cheff” and the the highest point of the sail plan “b” above the water surface. This is either the mainsail head (P+BAS) or jib head (IG). If the jib head is higher than the mainsail head then the average is taken. heff = cheff (b + HBI ) [42] The efficiency coefficient “CE” is comprised of the induced drag coefficient and the parasitic drag coefficient that is proportional to lift squared. CE = KPP + SailArea [43] π × heff Finally at each apparent wind angle the total lift and drag coefficient for the sails can be calculated from the lift, and drag coefficients and the “efficiency coefficient” (CE). Cd Sails = Cd Parasite + CE × Cl 2 × FLAT 2 [44] CL = FLAT × Cl MAX The FLAT parameter characterizes a reduction in sail camber such that the lift is proportionally reduced from the maximum lift available. Thus flat = 0.9 means 90% of the maximum lift is being used. What this means in practice is shown in Figure 24, in “full power” conditions (FLAT=1) the available aerodynamic force is determined by the maximum Cl and associated Cd. The total Cd at max Cl is affected by Cdparasite and by the effective rig height that determines the induced drag component.