Page 171 - Vetus

Calculation example
2)
The boat has an overall length of 36 ft. (11 metre) and the lateral wind draft measures 194 ft. (18 m . It is required that the bow
2
can be controlled easily when wind force Beaufort 5 applies.
At wind force Beaufort 5, the wind pressure is: Rho= 41 to 74 N/m , i.e. Rho (average) = 60 N/m .
2
2
The required torque is
T = wind pressure x wind draft x reduction factor x distance centre of effort to pivot point, (=approx. half the ship’s length)
T = 60 N/m x 18 m x 0,75 x (11 x 0,5) m = 4455 Nm
2
2

The required thrust force is calculated as follows
torque 4455 Nm
F = = = 420 N (42 kgf)
distance between centre of bow thruster and the pivot 10,5 m
point of the boat (with the transom as pivot of the boat)
The most suitable VETUS bow thruster is for this particular vessel is the 99 lbf (45 kgf) (55 lbf (25 kgf) in the case of Beaufort
4 and 156 lbf (75 kgf) in the case of Beaufort 6). 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 remain variable factors.
As a rule of thumb it can be assumed that the stern thruster may be “one model smaller” than the bow thruster model, as it
has been calculated. Therefore, in this case a stern thruster type 77 lbf (35 kgf) will be the correct model. Below is a selection
table of bow thruster models against recommended boat length. Please note that this table is given for general guidance
only and the calculation shown above should be used whenever possible. If you are in any doubt about the best thruster for
your boat, your VETUS support representative will be glad to help you with the decision.

Selection table thrust force - boat length
Metre: 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 ..... 43
Feet: 20 26 33 39 46 52 59 66 72 79 85 92 98 105 112 ..... 141
25 kgf
35 kgf
45 kgf
55 kgf**
60 kgf
75 kgf
95 kgf**
125 kgf
160 kgf**
220 kgf
230 kgf*
285 kgf
310 kgf*
410 kgf*
550 kgf*
* only available as hydraulically driven bow thruster
** available as hydraulically and electrical driven bow thruster

Creators of Boat Systems 171