Emma Mærsk, Worlds Largest Container Carrier




Rudder, propeller & bow thruster:

Rudder:

There are many many ways of controlling the direction of travel, and to keep it simple I will completely disregard pleasure vessels ways of doing this, and focus solely on large vessels. Here’s a few configurations:
  • The classic is to have the rudder right behind the propeller, thus redirecting all or some of the thrust.
  • Having the rudder(s) and propeller separated, thus maintaining maximum forward drive at all times. (This configuration is frequently found on submarines.)
  • Being able to turn the propeller, sometimes even 360 degrees, thus having great maneuverability. This has been seen on ferries and supply vessels.
There is one major detail that should be noted when building a model ship: We scale down the size and weight of the ship, thus also the rudder area, and we scale down the speed, but we cannot scale down the viscosity of the water.
That results in the fact, that model rudders are less efficient than their real counterpart, simply because the reduced flow applies less force on the rudder. Plain and simple as that, but what do we do to counter that? Well.. Most modelers over-size the rudder by 10-15 pct. Other modelers stick a piece of transparent material to the rudder, thus expanding the area without it being visible.

Important: As when building any model, the center of weight should be placed in front of the center of length, by no less than 2" (10cm), thus improving turning and rudder response.

The way that the rudder is hinged plays a role as well. Let me illustrate:



Left rudder: Here the rudder is hinged by the leading edge. Only some of the propeller thrust is redirected, so rudder response is a bit slower, but some straight forward thrust is maintained.
Right rudder: Here the rudder is hinged behind the leading edge at a distance equal to half the propeller diameter (propeller radius). All of the propeller thrust is thereby redirected, giving a high and efficient rudder response, but all straight forward thrust is lost for the duration of a turn.

In reality of cause, the ship will not stop, turn, and then go on, it’ll use the forward (or backward) momentum, and keep on sliding through the water. Remembering the ‘detail’ with scaled rudders and water flow, makes me choose the rudder configuration to the right.

The real Emma uses a mix of both, as shown in this picture:



Notice the outline of the moving part right above the waterline, and the position of the vertical shaft around which it turns.

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Propeller:

A word of consideration: The propeller is one of the most visible parts on the outside, which everybody gets close to for a closer view. Therefore, do consider buying the propeller as a ready-made product, rather than making one your self. The price tag is only around $40.. You can get really nice propellers from Pandan Models (UK) which I can highly recommend! Outstanding world wide service, and product range!
(Look for a six bladed prop.)
Also, do observe the max. RPM rating. It is dangerous to exceed this, as a blade braking off travels at high velocity in a random direction, easy taking out an eye. Consider limiting the RPM at the remote, and NEVER stick your fingers close to the prop while the sub is turned on! Something might cause it to spin without warning, and you need all of your fingers in the future, right? The receiver in the ship is a very sensitive thing. Someone miles away might trigger it, someone might touch the remote, or even switch it off before the ship is switched off! (This will cause the receiver in the ship to go apes...)

The Emma Maersk propeller is a 131t. one-piece fixed pitch propeller, diameter: 9.6 meters: (Note that it turns clockwise.)

    

Propellers is a science of their own, and thick books have been written about cavitations, pitch, efficiency, noise levels, materials and speed. In short, the propeller is designed to be most efficient at a very narrow RPM range, set by the shape of the blades, the number of them, and the forces (torque) applied, pitch etc. (This goes for fixed blade propellers only, not prop's with adjustable pitch.)
A risk for every propeller operating at high rotational velocities is cavitations. This occurs when the local pressure, associated with high local velocities in the fluid, is lower than the vapor pressure. When these vapor-filled (not air-filled) cavities in the wake arrive in regions with a higher pressure they collapse violently, causing local shock waves in the water that can erode the nearby surface. This dynamic behavior of large cavities can also generate vibrations in the ship structure. I would recommend a few experiments once the ship is done, setting an upper limit to the main engine RPM in the remote.

More images can be found in the Picture gallery.


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Bow thruster:

A bow thruster is a propeller fitted within a transverse tunnel in the bow, and sometimes also in the stern. That enables the vessel to turn much better, and highly improves maneuvering. The real Emma has got two thrusters in both ends, each with a thrust of 25 tones.
This image shows thrusters for model ships, and they can also be bought for example from Pandan Models (UK)


A bow thruster will of cause use one channel on the remote, and should be assigned to the sideways movement of the main engine throttle stick. On my remote I can mix the rudder- and thruster channel, so that when I turn the rudder using the rudder stick, then the thruster starts up too, thrusting the bow in the upper site direction. If there is no forward movement, it will move the bow only, leaving the stern where it is. When sailing you don’t notice that the thruster is working, and it greatly improved the turning radius of the submarine. For this reason, I have copied the setup to the Emma as well.
However, it is necessary to be able to undo this mix, because when backing and turning using the rudders, then the thruster function should be reversed to match the rudder intensions.

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