What is the Purpose of a Bow Thruster?

Bow Thruste with controlWhen manoeuvring a boat at low speed, the required control of the movements of the boat, especially those of the bow, is extremely difficult to achieve. In such cases, a bow thruster is the ideal solution to the problem. Even when the boat is not under way, complete control of the movements of the bow can be achieved by operating the bow thruster. Our selection of bow and stern thrusters.

How to select the correct Bow Thruster

Note: The following information is provided as guidance only, the selection of a bow thruster should always be made by an expert. Please contact us, we will be happy to help with selecting the correct bow thruster for your vessel. Contact us

A Bow thruster delivers a side-directional thrust force. In order to ensure the best performance under all sorts fo 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 position of the tunnel in the bow is of prime importance for the bow thrusters efficiency. The furhter forward the bow thruster is positioned, the greater will be the turning moment. The force applied by the wind onto the boat is determind by the factors, wind speed, angle of wind speed 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 p= ½ ? x V2 (N/m2).? (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 scale of Beaufort. The table shown below will give you an impression of the wind speeds and the resulting wind pressures. It will not be possible to counter completely the adverse effects of the winds of Beaufort 8 or more. However, in such cases, the bow thruster will still lend a strong helping hand.

Wind force Beaufort

Description

Wind speed m/s

Wind pressure 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)

 

The wind draft fo 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 stream-lined, a reduction factor may be applied to the wind draft area, before calculating the wind pressure resulting from the wind speed. The reduction factor is generally set at 0.75.

The torque

Thrust measurementsThe torque is determined by multiplying the wind force by the distance (A) between the centre of effort* of the wind the 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 tahe transom, the torque, caused by the wind force, is calculated by multiplying the wind force by half of the boats length.

* - Depending on the shape of the boats superstructure, the centre of the effort positioned more forward or more aft.

** - The centre of rotation of the boat is 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 centre of the bow thruster and the pivot point of the boat.

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
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
300 kgf * suitable for boats from 22 to 29 metres in length
410 kgf * suitable for boats from 27 to 34 metres in length
550 kgf* suitable for boats from 33 to 43 metres in length

* only available as hydraulically driven bow thruster