Feature 1 | GREEN SHIPS
and direction for every combination of ship speed, true wind speed and true wind direction, thus the FR model takes all potential combinations into account. It is assumed that the rotor is always
rotated at the highest beneficial rotational speed; where that is not possible the rotors are turned off to minimise drag and power consumption. For a full 360degs azimuth the forces are averaged using a conservative approach to identify the effective FR thrust. Te power to drive the rotors is added
to the ship’s electrical load, while the main engine power is reduced due to the lower apparent resistance. In the ship model, the main engine and diesel generators fuel demand is evaluated.
Negative effects Tere are other factors that affect the overall FR performance:
• Increased heeling moment from sway forces
• Extra rudder drag from weather helm. Te sideways (sway) forces on the ship
can become significant when the apparent wind angle is nearly dead ahead or astern. Tis large sideways force when combined with the vertical lever arm of a FR creates a heeling moment on the ship and will thus increase the angle of static heel. However, this was found to have a negligible effect (<1%) to performance, as the static heel does not exceed any roll angle normally experienced by the ship. Another potential effect of large sway
forces is the ability to create unbalanced yaw moments. Tis yaw moment must be countered by applying increased rudder angle (weather helm), which will increase drag and reduce the benefit from the FRs.
Power and savings Figure 3 shows how the FR reduces the effective resistance seen by the propeller. Te benefit is most pronounced at lower
ship speeds. This change in resistance translates into consistent fuel savings for all ship speeds as seen below in Figure 4. With a generic operating profile,
the ship used in this example stands to save up to 10% of its annual total fuel consumption with the installation of two FRs. Tere is good confidence in expecting
30 Figure 4: Fuel consumption rate vs ship speed
better results aſter the application of wind routing and rotor throttling to obtain maximum aerodynamic efficiency.
Conclusions The fuel-saving capability, carbon emission reduction potential and overall seaworthiness of FRs has been proven in several full scale installations to date, with positive results being reported as recently as 2013 with E-Ship 1. FRs represent an opportunity to improve the net efficiency of some ships, both when incorporated into new buildings and when retrofitted.
A FR model has been developed
which allows a conservative assessment of the potential fuel saving benefits, demonstrates appreciable fuel savings, and considers some of the practicalities of their use. NA
Acknowledgements Tis article represents work carried out by BMT Defence Services Ltd with the permission of the Energy Technology Institute’s Heavy Duty Vehicle (HDV) transport programme, whose support in the development of this technology model is gratefully acknowledged.
The Naval Architect January 2015
Figure 3:
Resistance change with FR
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