Institute of Ship Design and Ship Safety

Transcription

1 Institute of and Ship Safety Exercise Rudderdesign and DP Prof. Dr.-Ing. Stefan Krüger Dipl.-Ing.

2 Exercise 12. Februar 2015 Exercise Explain the advantages and disadvanteges of different aspect ratios of rudders. 2. What are the main reasons for placing rudder and propeller? 3. How to choose rudderprofiles? 4. Which DP-classes do you know? 5. Where are they defined? 6. What are the requirements of each DP-class? 7. How are environmental influences are treated? 8. What kind of vessel is using DP-systems for which purpose? 9. What are the advantages and disadvanteges of a diesel-elctric drive? 10. Please state the main advantages of a CPP over a FPP in case of dynamic positioning. 11. What are the advantages and disadvanteges of a configuration with podded drives? 1/4

3 Exercise 12. Februar 2015 Solution Explain the advantages and disadvanteges of different aspect ratios of rudders. Rudder with a big aspect ratio have a high gradient of the lift coefficient. This leads to high steering forces at low angles of attack. Rudder with a small aspect ratio have a high stall angle, but the gradient of the lift coefficient is small. 2. What are the main reasons for placing rudder and propeller? Placement of the propelle: A propeller placed outward produces good forces out of the Hovgaard-effect. But it leads to a big motorbox and this causes problems for the faring of waterlines and buttocks. This can lead to a bad propeller inflow. Placement of the rudder: x-direction: D behind the propeller, depends on the pitch of the propeller. y-direction: rudder in the propeller slipstream, in the shaftline. z-direction: lower and uper edge should be out of the slipstream to avoid cavitation in the gaps. 3. How to choose rudderprofiles? Hollow flank profiles does have a slightly higher resistance, but they provide better stall angles and have a smaller cavitational risk. Thin profiles are good for the propulsion test, becaus of the smaller resistance. In the operation of a ship thin profiles have a clear higher cavitation risk! 4. Which DP-classes do you know? There are the following classes: DP 0, DP 1, DP 2 and DP 3 5. Where are they defined? Originally they are defined by the IMO and they re practiced by the classification societies. 6. What are the requirements of each DP-class? DP 0: manual position control, automatic heading control under specified max. environmental conditions DP 1: automatic position control, automatic heading control under specified max. environmental conditions DP 2: automatic position control, automatic heading control under specified max. environmental conditions during and following any single fault excluding loss of a compartment (two independent computersystems) DP 3: automatic position control, automatic heading control under specified max. environmental conditions during and following any single fault including loss of a compartment due to fire or flood (At least two independent computersystems with a separated backup/systems separated by A60 class division) 7. How are environmental influences are treated? Typically 3 m of deviation in position and 1 in heading are defined in the contract. A more strictly approach is given by the IMCA (International Marine Contractors Association) who require 1.5 m deviation in position and 1 in heading. This are parts of the construction contract and DP-plots are taken into account for comparison. 2/4

4 Exercise 12. Februar 2015 The environmental conditions are defined by a const. current and different windspeeds are taken into account. The seastate is coupled to the windspeed and can be found in the classification documents. A often used comparison case ist the ERN (Environmental Regularity Number). In this number 4 cases are used, but just one allows rudders to be taken into account and the forces are assumed to be directly from the side (90 -case). 8. What kind of vessel is using DP-systems for which purpose? Mostly all ships doing offshore operations, RoRo s and cruiseliners. 9. What are the advantages and disadvanteges of a diesel-elctric drive? Electic drive engines provide a nearly const. torque over the complete rpm-range. Also different electric loads can be shifted (e.g. hotel load). However a diesel-electric system is quiet heavy and has losses in converting the power. 10. Please state the main advantages of a CPP over a FPP in case of dynamic positioning. A CPP is much faster then a FPP, because it must not be reversed and the masses does not have to be accelerated. 11. What are the advantages and disadvanteges of a configuration with podded drives? A pod-configurationprovides a good ERN, but is quiet poor on angles of 45. As a selling argument the idea of providing thrust to all directions is a good option, but in practice this is wrong. Podded drives have a lot of forbidden zones due to propeller-propeller-interaction and propellerhull-interaction. For an example DP-plots of a RoRo-vessel are attached. On the left side a conventional propulsion system is shown and on the right side a podded drive configuration is shown. On the top a typical configuration is shown and on the bottom a modified version for better dynamic positioning is shown. 3/4

5 Exercise 12. Februar /4