Performance

THE WEIGHT OF THE VAST MAJORITY OF YACHTS IS TOO IMPORTANT TO ALLOW SPEED SAILING.

It is therefore, pointless to fit them with carbon or hi-tech materials (unless seeking to impress…) as the shape of their hulls and their excessive weight will never allow them to perform above their designed displacement speed,rarely more than 5 or 6 knots (see Velocity Prediction Program VPPX).

PERFORMANCE / SPEED: WHAT DOES IT MEAN?

"There is an existing technical definition of the force of a yacht, it bears a direct influence on its speed."

The speed of a yacht is the result of the equation between the propulsive forces and the forces opposing the movement. This resistance is formed by the drag due to air friction on the sails, the windage of the boat and the friction of water on the hull and appendices (the wetted surface).

The forming of waves is to be taken into account in this braking

As the hull moves through the water it sets into movement a system of “waves”. This movement firstly creates a bow wave followed by a trough then another wave. The horizontal distance between the crests of the two waves is called the wavelength. This wavelength increases with the speed of the boat to reach a maximum equivalent to the water line of the boat.

The speed in knots of a wave system is expressed according to the following formula:

• V = (square root of L)) X R
• L=length of wavelength and R is a multiplying factor. If the wavelength is expressed in metres, R = 2.43. If the wavelength is expressed in feet, then R = 1.34.

Since the advancing yacht is caught between its bow wave and its stern wave, we understand that this same formula serves to determine the speed of the boat’s hull. So the value of “L” used will be
the length of the water line and we will use the same values for the multiplier “R”. Normal displacement hulls are not capable of exceedind this speed.

• e.g. : Yacht with a 15m water line: V = 3.873 x 2.43 i.e. 9.40 knots maximum

We are referring to maximum speed here and not to average speed. In reality this would be reduced in the case of “standard” type yachts. For these boats the true average speed would be approximately  5 – 6 knots.

Only light yachts with planing hulls that are well canvased are spared this reality and can thus reach much greater speeds.