Vetus How To Size Bow Thrusters
HOW TO SIZE 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 thrust 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 efficiency. The force applied by the wind onto the boat is determined by the factors: wind speed, angle of wind attack and lateral wind draft area of the boat.
The Wind Pressure
When the wind speed increases, the wind pressure increases quadraticallly!
For wind pressure 'P', the formula reads: P = 1/2ρ× V² (lbf/sq.ft)
ρ (rho) represents the specific mass of air and 'V' stands for the
velocity of the air in ft/s.
The table below 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 40 knots 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, due 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
Wind Speed Knots
|Description||Wind Speed ft/s||Wind pressure lbf/sq.ft.|
|10 - 16
16 - 22
22 - 28
28 - 34
34 - 40
|17 - 27
27 - 37
37 - 47
47 - 57
57 - 67
|0.40 - 1
1 - 1.9
1.9 - 3.1
3.1 - 4.6
4.6 - 6.3
The torque is determined by multiplying the wind force by the distance (A) between the center of effort* of the wind and the center 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 upon the shape of the boat's superstructure, the center of effort may be positioned more forward or more aft.
** - The center of rotation of the boat may be positioned more forward, dependent upon the shape of the underwater 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.
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 ft/2 = 3719 ft.lbs.
The required Thrust Force is calculated as follows:
The VETUS bow thruster which is most suitable for this particular vessel is the 121 lbf model.
Always bear in mind that the effective performance of a bow thruster will vary with each particular boat, as the displacement, the shape of the underwater section and the positioning of the bow thruster will always be variable factor.
Want to select a bow thruster? click here All about Vetus bow thrusters