# HOW TO SELECT THE RIGHT SIDE POWER UNIT THRUSTER

Bow Thrusters are normally chosen according to how much lateral push they need to supply. Such push or thrust is based on the following parameters: Boat’s Lateral Wind Area: in other words all that is exposed above the waterline which is affected by lateral wind pressure. In practical terms the exposed surface area is reduced by about 20- 25% to take into account the fact that the lateral surfaces concerned ( superstructure, topsides) are not completely flat nor perpendicular and therefore in some way enhance the wind to slide off exercising less pressure. - Position of the Bow Thruster. Determines the degree of leverage with which the Bow Thruster pushes to one side or the other. The more the Bow Thruster is installed in the bow the more the torque obtained delivers greater capacity to rotate, further facilitating manoeuvrability ( while remembering the precautions mentioned in this article) - Wind Strength – This is, as probably known to all, expressed as per the Beaufort scale ranging from force 1 to 10. For example in a force 5 wind speed varies between 8 to 10.7 metres per second. From this indicative range of wind speed applying the following formula p = 1/ 2 rv where v is velocity and r is standard air density equal to 1.23 kgm/ m3, we can obtain the matching pressure range which will be comprised between 41 e 74 N/ m2. Let’s now take a practical example as follows: Supposing we have a 10 metre long boat with an exposed lateral wind area equal to 12m2. We wish to install a Bow Thruster which can easily manoeuvre in a force 5. First we need to calculate necessary torque: MT = FV x SL x A x 0.8

Where FV = estimated wind velocity chosen as equal to 60 N/ m2 SL = exposed lateral wind area equal to 12 m2 A = distance between the wind’s pressure centre and the Pivot Point ( which is situated at practically the extreme stern of the boat). This distance is nearly equal to half the boat’s size ( 5m). 0.8 = is the applicable coefficient given for wind pressure on non flat surfaces.

Putting these values into the formula we obtain: MT = 60 x 12 x 5 x 0.8 = 2880 N/ m2 To calculate the thrust needed by the Bow Thruster we divide needed torque obtained MT by the length of the leverage of the Bow Thruster which is the B distance between the Bow Thruster and the Pivot Point. The distance is assumed as being of 9 metres.

Therefore: THRUST = MT / B = 2880 / 9 = 320 N ( 33 Kgf)

Therefore we‘ ll choose a Bow Thruster with a thrust of at least 33 Kgf or more.