CRUISE LINER TECHNOLOGY
A. Pods turned to 180o
B. Full astern - normal mode
C. Full astern - emergency mode
Fig 3. Crash stop simulations showing stopping distance for: A. pods turned to 180deg; B. full astern ordered with slow engine response; and C. full astern ordered with emergency engine response.
required to rotate the pods to 180deg. This power limitation means that less thrust is available to slow the ship.
Engine load management Vessels with podded propulsion often have a number of engine load management programs enabled that are designed to limit shock-loading on the propulsion plant due to rapid or large changes in power demands and to optimise engine utilisation and fuel economy. These load programs effectively limit the rate of response of the propulsion plant and typically enable a faster rate of response at lower revolutions (and hence power) and then more gradual increases or decreases at higher power settings. Fig 2 shows the time taken to achieve full astern revolutions for two typical load management programmes for a diesel-electric propulsion plant. This highlights the large difference in the response rates between normal operating and emergency modes which enables full astern revolutions to be achieved much quicker.
Rotating the pods at speed
If the pods are rotated to 180deg when underway, then dramatic changes to the hydrodynamic inflow to the propellers occur during the rotation. Each propeller's 'angle of attack' to the flow changes and there is a natural reduction in propeller revolutions (similar to that experienced in a turning circle trial) due to the mismatch of propeller load and the available engine torque during the rotation, although this effect is less significant for a diesel-electric prolusion plant.
Once the pods have settled at 180deg there will be a gradual increase in the revolutions to return to the demanded setting. This increase in revolutions will be dictated by the relationship between the propeller and engine torque when operating in reverse flow and more so by the engine load management program that defines the rate of increase of revolutions.
Using the pod controls
In a conventional crash stop, the master or pilot would request emergency astern power and then simply move the telegraph levers to full astern.
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To perform a crash stop by turning the pods to 180deg, he or she would have to undertake several manipulations of the controls to achieve the desired effect. First, the manoeuvring mode has to be selected, then each pod rotated through 180deg once the speed of the ship has decreased sufficiently and then the RPM increased to the desired setting. In an emergency situation the master or pilot is unlikely to want to undertake a series of relatively complex control manipulations. The amount of bridge team coordination required for other tasks (such as preparing the anchors) and the need for a greater spatial awareness in a stressful situation will place far greater demands on their time. Such methods could only work if the onboard computers can perform the series of control manipulations automatically, leaving the master or pilot free to concentrate on the situation unfolding.
It is also worth noting that the emergency response modes usually available allow the very rapid increase and decrease of revolutions for these situations, thus enabling rapid deceleration without the need for multiple control manipulations, as shown in Fig 2.
Net effect
One leading pod manufacturer has suggested that the crash stop can be performed by first reducing the ship’s speed through selection of the manoeuvring mode (giving an immediate reduction in power) and then setting both pods to 35deg outwards and using the increased drag to slow the ship (other methods can be used to slow the ship). Once the ship's speed has sufficiently reduced, both pods can then be fully rotated, enabling ahead revolutions to provide the net astern thrust.
Based on BMT SeaTech experience of ships with 'podded' propulsion, the net result of these actions is that the following limitations would apply when performing a crash stop manoeuvre in this manner:
• a reduction in power available to the propellers due to use of manoeuvring mode
• a delay in fully rotating the pods to face astern until the ship speed has sufficiently reduced
• a transient reduction in power available to the propellers due to the steering angle of the pods during rotation
• significantly extended time to reach the demanded ahead revolutions when the pods are at 180deg, due to the decrease in revolutions during turning and the subsequent application of the load management program.
All of these, in our experience, would lead to an increase in stopping time when compared with the equivalent manoeuvre undertaken by selecting the emergency (crash stop) manoeuvre load program and ordering full astern. This has been confirmed by simulator trials with a leading cruise ship operator using validated mathematical ship models on the PC Rembrandt simulator and comparison with full-scale trials data. Fig 3 presents track plots (on a 100m grid) for these simulated crash stop manoeuvres with a typical modern cruise ship with pods. The three vessel plots represent the suggested crash stop procedure using ahead revolutions and turning the pods to 180deg, a normal crash stop (using astern revolutions) with the normal operating mode and the normal crash stop using the emergency response mode.
It can be seen that the stopping distance of the crash stop with the pods turned to 180deg (plot A) is longer (by approximately 300m) than the same manoeuvre using the emergency response engine mode (plot C). However, turning the pods to 180deg does result in a reduced stopping distance when compared to simply ordering full astern with the 'normal operation' load management program because of the length of time taken to generate full astern revoltuions from full ahead in this mode. BMTSeaTech believes, that whilst these results show that the suggestion of turning the pods to 180deg in a crash stop is a good idea in principle, there are other factors that must be considered for such an approach to be truly effective in operation. The most important of these is the safety of navigation and the freedom of the master or pilot to conduct all other tasks required in emergency situations, without having to coordinate potentially confusing controls.
THE NAVALARCHITECT FEBRUARY 2006
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