Why
thrusters?
It is
extremely difficult to control the movements of a boat as
it is slowing to a halt, especially in the bow area. Bow thrusters
can correct this situation by applying a lateral thrust at
the bow of the boat. Even when the boat lies dead in the water,
it is possible to maintain complete control of the movements
of the ship's bow by operating the bow thruster. Stern thrusters
allow the helmsman the opportunity to fine tune his approach
for perfect dockage.
Selecting
the correct bow thruster  Considerations
 One
or two propellers?  Installation
Selecting
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 sufficient
for the boat in which it has been installed. The applied
moment of the thrust force, i.e. 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 = 1/2p × V² (l bf/sq.ft)
p (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
WindDraft 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
Torque
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.
Wind
Speed  Wind Pressure Chart
Wind
Speed
knots 
Description 
Wind
Speed
ft/s 
Wind
pressure lbf/sq.ft. 
10
 16
16  22
22  28
28  34
34  40 
moderate
breeze
fresh breeze
strong breeze
near breeze
gale 
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 
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:
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.



Selecting
the correct bow thruster  Considerations
 One
or two propellers?  Installation
Copyright
© 2004 Joe Molinaro's East Coast Bow Thrusters; technical information
and graphics copyright © 2004
