Bow Thrusters

Bow Thrusters Vetus
HOW TO SELECT THE CORRECT BOW THRUSTER?

A bow thruster delivers a side-directional thrust force. In order to ensure the best performance under all sorts of weather and water conditions, the bow thruster should be able to provide a thrust force which is suitable, or rather sufficient for the boat in which it has been installed. The applied moment of the result force - which means to say: the position of the tunnel in the bow - is of prime importance for the bow thruster's efficiency. The further forward the bow thruster is positioned, the greater will be the turning moment.
The force applied by the wind onto the boat is determined by the factors: wind speed, angle of wind attack and the lateral wind-draft area of the boat.

THE WIND PRESSURE
When the wind speed increases, the wind pressure increases quadratically! For wind pressure 'p', the formula reads: p = 1/2  x V² (in N/m²).
p (rho) represents the specific mass of air and 'v' stands for the velocity of the air in m/s. Wind speeds are often gauged in the so-called scale of Beaufort.
The table shown on the right will give you an impression of wind speeds and the resulting wind pressures. It will not be possible to counter completely the adverse effect of winds of Beaufort 8 or more. But, in such cases, the bow thruster will still "lend a strong helping hand".

THE WIND-DRAFT OF THE BOAT
The forces applied by the wind onto the boat can be determined by multiplying the wind pressure by the wind draft area. The wind draft area is determined by the shape and the dimensions of the superstructure. Also the wind angle is playing its part. The worst situation is created if the wind is at 90 degrees to the boat.
However owing to the shape of the superstructure, which is mostly more or less streamlined, a reduction factor may be applied to the wind draft area, before calculating the wind pressure resulting from the wind speed.
This reduction factor may generally be set at 0.75 The lateral wind-draft area

THE TORQUE
The torque is determined by multiplying the wind force by the distance (A) between the center of effort* of the wind and the centre of rotation** of the boat.
Assuming that the wind force is amidships and assuming as well that the boat will tend to turn at the transom, the torque, caused by the wind force, is calculated by multiplying the wind force by half of the boat's length.

*) Dependent on the shape of the boat's superstructure, the centre of effort may be positioned more forward or more aft.
**) The centre of rotation of the boat may be positioned more forward, dependent on the shape of the under-water section.

THE THRUST FORCE
The bow thruster is required to apply a countering thrust force, which is at least equal to the thrust force applied by the wind. The required thrust force of the bow thruster may now be calculated by dividing the torque by the distance (B) between the center of the bow thruster and the pivot point of the boat.

Centres of rotation effort


Wind force    Description                    Wind speed                Wind pressure
Beaufort                                              m/s                             N/m² (kgf/m²)

4                    moderate breeze          5,5 to 7,9                   20 to 40    (2,0 to 4,1)
5                    fresh breeze                 8,0 to 10,7                 41 to 74    (4,2 to 7,5)
6                    strong breeze               10,8 to 13,8               75 to 123  (7,7 to 12,5)
7                    near gale                      13,9 to 17,1               125 to 189  (12,7 to 19,2)
8                    gale                              17,2 to 20,7              191 to 276  (19,4 to 28,2)

Selection table thrust force - boat length
25 kgf suitable for boats from 5,5 to 8,5 metres in length
35 kgf suitable for boats from 6,5 to 10 metres in length
55 kgf suitable for boats from 8,5 to 12,5 metres in length
60 kgf suitable for boats from 9 to 13 metres in length
75 kgf suitable for boats from 10,5 to 15 metres in length
95 kgf suitable for boats from 12 to 17 metres in length
125 kgf suitable for boats from 14 to 20 metres in length
160 kgf suitable for boats from 16,5 to 22 metres in length
220 kgf suitable for boats from 19,5 to 26 metres in length