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Selecting the correct bow thruster   |   Considerations   |   One or two propellers?   |   Installation




One or Two Propellers?

Theoretically speaking, a bow thruster with two propellers may show an improvement in performance of 3 or 4%, when compared with a single propeller bow thruster. This only applies on condition that all the components are  perfectly matched and that the two propellers rotate in opposite directions.

 
  However, this law of theory is only valid for the lateral thrust in one direction. The opposite direction will produce a lower performance by the same percentage and, ultimately, nothing is gained. Furthermore, to manufacture a bow thruster with two propellers faces quite a few obstacles, both technically and price wise:
  • Two propellers, rotating in the same direction, will have slightly enhanced performance if the "downstream" propeller turns much faster than the "upstream" one, or if it has a larger pitch. But, given the fact that a bow thruster should operate in two directions, this requirement is completely unattainable, as it would involve the installation of three electric motors and three drive systems. Even if this were the case, the improvement in performance would be negligible.
     
  • In the situation where two propellers are rotating in opposite directions, again, the "downstream" propeller should have a larger pitch. But, if the sense of rotation is reversed, then the "downstream" propeller becomes the "upstream" one, producing the negative effects, described above.
     
  • Logically, the mass inertia of two propellers is much higher than that of one propeller. This is even more so, if the propellers are not made of light-weight plastics. In other words: the required thrust power comes with a considerable delay.
     
  • The more components that are installed, the costlier the product will be and the more sensitive to failure. Furthermore, two propellers in one tunnel are far less accessible than a single one.

  VETUS has come to the conclusion that they will not introduce bow thrusters with two propellers into their range. The performance of VETUS bow thrusters with a single propeller has been calculated and tested with utmost care. Due to the carefully calculated power transmission, the specially constructed electric motors and the exactly dimensioned tunnels of minimum diameter, VETUS bow thrusters provide optimum thrust power per horsepower.

And that is the only thing which really matters with bow thrusters!



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