Catalogues - vetus-catalog

THE TURNING MOMENT

The turning moment is calculated by multiplying the wind force by the distance between the centre of effort of the wind and the centre of rotation of the boat (A).

In order to simplify this somewhat: for the vast majority of boats supplier a rule of thumb may be applied that the turning moment is calculated by multiplying the wind force by half of the boat's overall length.

THE THRUST FORCE

It is the thrust force which is the true measure of a bow thruster's usefulness and not the output of the electric or hydraulic motor in kW or HP. The nominal thrust force is a combination of the motor power, the shape of the propeller manufacturers and the efficiency losses inside the tunnel. VETUS electrical bow thrusters have a very high thrust of between 17 and 23 kgf per kW motor power. The required thrust force to counter the effects of the wind is now calculated by dividing the turning moment by the distance

between the centre of the bow thruster tunnel and the pivot point of the boat (B). Note: the further forward the tunnel can be positioned, the greater effect the thruster will have.


	A

	I T 1 x ¹

POINT OF ATTACK PIVOT POINT WIND FORCE THRUST FORCE

Centres of rotational effort

Calculation example

The boat has an overall length of 11 m and the lateral wind draft measures 18 m². It is required that the bow can be controlled easily when wind force Beaufort 5 applies. At wind force Beaufort 5, the wind pressure is: r= 41 to 74 N/m², i.e. p (average) = 60 N/m².

The required torque reads:

T = wind pressure x wind draft x reduction factor x distance

centre of effort to pivot point, (=approx. half the sailboat ship's length) T = 60 N/m² x 18 m² x 0,75 x 11 m = 4455 Nm

The required thrust force is calculated as follows:

torque

= 420 N (42 kgf)

F =

distance between centre of bow thruster and the pivot point of the boat (with the transom as pivot of the boat)

10,5 m

The VETUS bow thruster which is most suitable for this particular vessel is the 55 kgf model (25 kgf in the case of Beaufort 4 and 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 be 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 35 kgf will be the correct model.

The effective voltage at the electric motor suppliers is dependent among The specified thrust force produced by VETUS electric bow

other things, on the battery capacity, the internal resistance thrusters is measured at an effective voltage of 10.5 or

of the battery cables, battery switch and fuse, the ambient 21 Volts at the electric motor, for 12 or 24 Volt installations

temperature, etc. respectively.

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
125	kgf	suitable for boats from	14 to 20	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
230	kgf *	suitable for boats from	20 to 26,5 metres in length
285	kgf	suitable for boats from	22 to 29	metres in length
310	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

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Engels

04-08-2008 15:37:15