Propeller and Engine

Resistance in calm The resistance was discussed extensively in Chapter 5 (see Fig 5.4 in and rough weather particular). Since we are now interested in the upright case wc can forget about the heel and induced resistance, and if the hull is not too fouled we can also forget about the roughness drag. What is left in calm water is then the friction and the residuary resistance. How the friction is computed was explained in detail in Fig 5.8 and the residuary resistance calculation was presented in Figs 5.18 and 5.19. However, the formulae of the latter figures are quite complex and we could do with a more approximate estimate for the present case. As was pointed out in Chapter 5, the residuary resistance, in percentage of the displacement, is more or less the same for all yachts at a given relative speed (Froude number), and we have plotted this approximate relation in Fig 9.1. From Figs 5.8 and 9.1 the reader can thus obtain an estimate of the resistance in calm weather.

Fig 9.1 Estimation of residuary resistance

In rough weather we also have the added resistance in waves (mentioned in Chapter 5), and the windage (discussed in Chapter 7). Let us start with the latter.

. Fig 9.2 gives the appropriate formulae for calculating the windage of the hull, mast and rig separately. In principle they have already been given in Chapter 7, but they are repeated here for clarity and some missing coefficients are also included. The frontal area of the hull and superstructure may be taken simply as the maximum beam times the freeboard forward, and the drag coefficient is assumed to be 0.5. Often, somewhat higher values are used, but considering the fact that the w 7 O

windspeed at the level of the hull is significantly smaller than at 10 m height, where observations are made, this should be accurate enough.

0.05

0.04

0.03

0.02

0.01

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