8/3/2019 How to Size the Correct Bow Thruster
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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 primeimportance 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 attackand 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 = 1/2 V
(lbf/sq.ft)
(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 pressureby 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 - Wind Pressure Chart
Wind Speed
knotsDescription Wind Speed
ft/sWind pressure
lbf/sq.ft.
10 - 16
16 - 22
22 - 28
28 - 3434 - 40
moderate breeze
fresh breeze
strong breeze
near breezegale
17 - 27
27 - 37
37 - 47
47 - 5757 - 67
0.40 - 1
1 - 1.9
1.9 - 3.1
3.1 - 4.64.6 - 6.3
The TorqueThe 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 positionedmore forward or more aft.
** - The center of rotation of the boat may be positioned more
forward, dependent upon the shape
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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.
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 ft/2 = 3719
ft.lbs.
The required Thrust Force is calculated as follows:
