# Principles of Yacht Design

Fig 5.8 Calculation of the frictional resistance

0.008

0.007

0.006

0.005

0.004

0.003

0.002

0.075

Reynolds number: Rn = V = velocity of yacht in m/s For hulls: L = 0.7-L,

For keels and rudders: L = mean chord [ keel: 1.45 m ] [ rudder: 0.5 m ] v = kinematic viscosity

[ hull: 408.6 N ] [ keel: 93.6 N ] [ rudder: 34.0 N ]

p = water density [ salt water at 20 'C » 1025 kg/m ]

2 2 2 Sw= wetted surface [ hull: 25.2 m ] [ keel: 4.4 m ] [ rudder: 1.3 m ]

For total frictional resistance — add the contributions from hull, keel and rudder. [ 536.2 N ]

always be obtained by multiplying by the so-called dynamic pressure 0.5 • p • V2 and a representative area, here normally the wetted surface Sw.

The values computed for the YD-40 are at 3.5 m/s, or 6.8 knots, the same speed as in Fig 5.4. By adding the contributions from the hull, keel and rudder the total friction is obtained as 536 N, also given in the bar of Fig 5.4.

Viscous pressure Fig 5.9 shows a typical pressure distribution on the hull at a given resistance depth, ie along a certain waterline. It is seen thai the bow and stem pressures are higher than in the undisturbed water at this depth, while