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.
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!