Solutions
Two Propellers
Due to the symmetrical layout of this system the course of the ship does not change when switching to reverse.
Steering can be performed very well by differential thrust, and 'MAX' does not need a rudder.
But there is one problem: The Titanic- effect. The Titanic could have been turned away from the iceberg using differential thrust
by reducing the power on one engine, but the priority of the crew was to stop the ship as fast as possible.
So 'MAX' would also meet the iceberg if it had no powerful bow thruster.
Duct- Rudder
Unlike the traditionel rudder, the duct- rudder provides control for ships in forward and reverse.
In straight forward position it looks similar to the Kort Nozzle and could be designed with some similar characteristics.
Unlike the Kort Nozzle there is no need for a tight fit of the propeller and the nozzle.
A big gap between the two has no effect in straight forward position and very little effect during turns.
Due to the stability of its circular shape and the two fixing points on top and on the bottom it can be made very thin, causing only little drag.
The fact that the traditional rudder behind the propeller is omitted (and taken into account the very high water velocity there)
could possibly safe more drag than the duct- rudder produces, so the efficiency of this duct- rudder may be not bad in straight forward position.
This may change in curves, but that is also the case with a conventional rudder.
But the main advantage of this duct- rudder remains: The ship is always controllable, in forward and reverse.
The sideways movement of the stern when a traditional ship goes into revers (see below) can be countered easily.
The Thrust Reverser Rudder
It consists of two halves that move in parallel when acting as a rudder.
In this configuration it looks like a Duct- Rudder and has identical characteristics.
For reversing the two halves close:
For breaking, the engine does not have to stop, only the two halves of the thrust reverser close
Sideways movements of the stern can easily be countered.
The thrust reverser rudder offers a lot of advantages:
- The engine does not need a reversion system and no breaking system.
(Even turbine engines can be installed and connected to the propeller by just a reduction gear)
- Variable pitch propellers can be avoided
- The propeller is protected
- in forward the two halves could be used as an adjustable nozzle
The Gate Rudder
The Japanese patent holders gate rudder idea looks very promising.
It is promoted to enable turning with higher speeds, better efficiency and some other advantages,
but a major benefit (that is mentioned nowhere) is that it seams to provide controllability at reverse!
Contrary to the Duct-Rudder and the Thrust Reversing Rudder, this Gate Rudder system needs no fixing point
or bearing at its lower base, and ship- builders seam to like that.
On the container ship 'SHIGENOBU' this rudder was successfully tested, and efficiency gains
(compared to a conventional rudder) were claimed.
But since the SHIGENOBU is propelled by a variable pitch propeller, a thrust reversing rudder
with a fixed pitch propeller optimized for forward would have improved the efficiency even more!
The Paddle Wheel Thruster
ERIKA
MAX
MORITZ
RAKETE
AAL
WICKIE
A new version of the old fashion paddle wheel, but as can be seen with a major advantage compared to the 'normal' bow- and stern thrusters:
There is no hole through the hull that is either open and not very streamlined or has to be closed by flaps (QM2).
Another big advantage: Most of the tested thrusters ('MAX','MORITZ','RAKETE','Wickie' and 'ODIN') seam to work very well up to cruising speed!
The theory behind these paddle wheels
Two Duct- Rudders and differential thrust
The idea is: Two propellers, two duct- rudders.
In theory that would enable to move the stern to one side but not moving the ship forward or aft
when one propeller runs forward and one backward, cancelling out the forward / backward motion
but adding the sideward motion by the tilt of the rudders (one to the left, the other to the right right).
And it realy works.
ODIN