Choosing Your Vetus Thruster

The Influence Of The Wind

The force applied to the boat by the wind is determined by wind speed and direction, and the lateral wind draft area of the boat. When the wind speed rises, the wind pressure increases exponentially. The shape and the dimensions of the boat profile about the waterline (the topsides and superstructure) determine the lateral wind draft area. A streamlined hull and superstructure offer less resistance to the wind and a streamlining reduction factor of 0.75 is applied, when calculating the wind pressure.

Wind Speed Knots	Description	Wind Speed ft/s	Wind Pressure lbs/sq.ft
10 - 16	moderate breeze	17 - 27	0.4 - 1
16 - 22	fresh breeze	27 - 37	1 - 1.9
22 - 28	strong breeze	37 - 47	1.9 - 3.1
28 - 34	near gale	47 - 57	3.1 - 4.6
34 - 40	gale	57 - 67	4.6 - 6.3

The Wind Torque

The torque is determined by multiplying the wind force by the distance between the centre of effort of the wind and the centre of rotation of the boat (A). For the vast majority of power boats and full keel sailboats, an approximate torque is calculated by multiplying the wind force by half of the boat's length.

The Thrust Force

It is the thrust force which determines the effectiveness of a bow thruster and not the output of the electric or hydraulic motor in kW or HP. The nominal thrust force is the result of the power of the motor, the shape of the propeller and the drag inside the tunnel. For direct current electric thrusters, battery size and condition, and cables length and gauge, will also effect the performance of the bow thruster. VETUS electrical bow thrusters have a very high thrust of between 28 and 38 lbs/hp. The required thrust force to counter the turning effect of the wind is calculated by dividing the the wind torque in foot pounds by the distance between the centre of the bow thruster tunnel and pivot point of the boat in feet (B). Note: the further forward the tunnel is positioned in the bow, the greater the effect of the thruster. As a rule of thumb, the stern thrusters must be one type smaller than the bow thrusters. For this calculation example a 77 lbf stern thrusters will be the one to choose.

Calculation Example

The boat has an overall length of 36 ft and the lateral wind draft measures 190 sq.ft. It is required that the bow can still be controlled easily when a wind force of 20 knots applies. At a wind force of 20 knots, the wind pressure is: p = 1.0 to 1.9 lbf/sq.ft i.e.p. (average) 1.45 lbf/sq.ft.
The required torque reads: T = windpressure x wind draft x reduction factor x distance center of effort to pivot point, (= appr. half the ship's length) T = 1.45 lbf/sq.ft x 190 sq/ft x 0.75 x 36/2 ft = 3719 ft.lbs.

The required thrust force is calculated as follows:

F=	torque	=	3719 ft.lbs.	= 107.8 lbs
	distance between center of bow thruster and the pivot of the boat, transom as the pivot		34.5 ft.

The effective voltage at the electric motor is dependent among other things, on the battery capacity, the internal resistance of the battery cables, battery switch and fuse, the ambient temperature, etc. The specified thrust force produced by VETUS electric bow thrusters is measured at an effective voltage of 10.5 or 21 Volts at the electric motor, for 12 or 24 Volt installations respectively.