Hull and propeller design gives cruise liner comfort

3 January 2006

Color Fantasy

Color Fantasy, now in service on the Oslo-Kiel route, fulfils DNV Comfort Class I requirements, thanks largely to efficient and open-minded co-operation in hull and propeller design between all parties involved; the yard, owners, classification society, model basin and propeller designers

The ship is the largest cruise liner built to date to have a car deck. A design requirement was that it should meet DNV Comfort I and Clean Class requirements. The challenge was made more interesting by the location of the a la carte restaurant at the stern of the vessel, the choice of twin screw propulsion, and the three different operating modes. The aims were achieved and the methods used were described in a joint Paper presented by Aker Finnyards and Rolls-Royce at a recent Conference.

Color Fantasy is a 75,000gt ship that can carry up to 2,750 passengers and a mix of trucks, trailers, cars and caravans. A feature of the vessel is a 160m long and three deck high arcade; the longest shopping mall yet to be built into any ship. The passenger cabin numbers and space allotted are on a par with high class cruise vessels.

Achieving a very low level of noise and vibration in the passenger and crew areas, even at the service speed of 22 knots, was made more challenging by the location of the Oceanic a la carte restaurant at the extreme stern of the vessel. The restaurant has large panoramic windows and is positioned just above the car decks, the most difficult position from which to eliminate noise and vibration.

At the beginning of the design project, three different machinery alternatives were considered: diesel-mechanical, twin-pod diesel electric, and a hybrid system with diesel mechanical centre line CP propellers and two diesel electric side pods. According to predictions made by Aker Finnyards, pod propulsion had about 7%-10% lower power consumption, and hybrid propulsion 10%-15% lower, but with higher costs and with less service friendliness for the given route, both comparisons relative to traditional twin screw ship with long exposed shaft lines, brackets and rudders. It was felt that the highest Comfort Class design is easiest to achieve with a pod propulsion system and most difficult with a hybrid system. On the other hand, the owner’s experience with proven traditional twin screw propulsion was a factor in the selection process. Also, the owner wanted to operate the new ship in the same way as their earlier vessels, but with different engine operating modes using two, three or four engines connected to the shafts. The choice of proposal was governed by the fact that the shipyard guaranteed compliance with the requirement of Comfort Class both regarding noise and vibration.

The final choice was four medium speed engines coupled in pairs through twin input/single output reduction gears to twin shaft lines and Kamewa CP propellers. Each gearbox has a power take off for a shaft generator, but the shaft generators supply power for the thrusters and are only used for manoeuvring, thus the arrangement does not limit the number of revolutions of the main propeller.
There are three operating modes: cruising, manoeuvring and in port. In cruise mode all four main engines and both propellers run in combinator control. The shaft generators rotate without producing power. In manoeuvring mode when entering and leaving harbour, two main engines and propellers are run in combinator mode. The shaft generators are connected to the other two main engines run at nominal speed and power the thrusters. When in port, the main engines are in standby condition, heated but not running and with clutches open.

The hull design, particularly the aft body, and propeller design were both of importance in achieving the noise and vibration targets, as was the interaction between them. DNVs Comfort Class Conf-V(1) calls for a noise level less than 55dBA in public spaces and 44-49dB in cabins. In vibration terms this means less than 1.5mm/s in the passenger spaces.

Having more than one operating mode increases the challenge for the propeller designer, since different cavitation conditions have to be catered for. Color Fantasy also operates in both deep and shallow water on its regular route. In shallow water, ship resistance and propeller loading is increased. As a consequence, suction side sheet cavitation on the propeller, as well as the cavitating tip vortex strength, will increase.

Normal operating mode at sea uses two main engines per shaft at 85% power. The alternative mode of running one main engine on one shaft and two engines on the other shaft at a total of 67% power gives yet another cavitation condition, since the shaft line with one main engine will operate at a high rpm and low propeller pitch, thus inducing a risk of pressure side cavitation. The task was to find the best compromise between these conflicting requirements.

Apart from the various operating modes, the propeller installation had to meet Ice Class in view of the operating route. Apart from the propeller blades themselves, the installation is designed to meet DNV Ice Class 1A. With the aim of achieving higher hydrodynamic efficiency, the propeller blades are designed to DNV Ice 1B which permits a thinner and more efficient blade section, but they can be changed to 1A if required.

The hull itself embodies a wave damping after body (WDA) developed from the shipyard’s own experience and a combination of CFD and model testing. Aker Finnyards’ early experience was based on Superfast Three and Four and Voyager Class Cruise Liners. More than eight hundred hull forms were analysed using CFD to identify the best possible WDA and bow form for both deep and shallow waters. The result is a hull which greatly decreases wave generation and reduces the required engine power. Incidentally, WDA studies carried out on several vessel types show that with the right design a large cruise liner can save 3%-5% power, a medium speed cruise liner can save 5%-7% while a fast passenger ferry can show a power saving of 7%-15%.

Hull and propeller cannot, however, be considered in isolation. The hull lines and appendages must produce the optimal inflow to the propellers, the propellers must in this flow absorb the power and produce the required thrust at high efficiency without producing any excessive pressure fluctuations on the hull. The contract committed Rolls-Royce to guarantee levels for propeller induced hull pressures in the two principal operating modes, while the propeller open water efficiency and freedom from cavitation erosion are also guaranteed.

Propellers excite the hull in different ways. The low frequency excitation is felt as vibration and the higher frequencies as noise. This excitation can be divided into two different types:

• fluctuating forces and moments transferred from the propellers to the ship via the shaft system (first order blade harmonics)
• pressure fluctuations transferred to the hull through the water which can be divided into pressures generated without cavitation, pressures generated by cavitation on the blades, and pressures generated by the cavitating tip vortex

For a propeller for this type of ship, the latter is the most important factor in noise generation.
One typical propeller design problem - that of having a large enough propeller while still having adequate clearance between propeller blade tip and hull - was avoided in the case of the Color Fantasy where a large clearance to the hull was available. Diameter, number of blades and blade area had to be juggled to find the best compromise between efficiency and freedom from various types of cavitation. The final choice was two Kamewa Ulstein type 144XF5/4 CP propellers of 5.2m dia. Each inward-turning propeller has four blades, rotates at 138rpm, and has an expanded blade area ratio of 0.644.

The hull shape, large clearance and careful design of shaft brackets gave good inflow conditions to the propeller. The propeller hydrodynamic design point is the combination of propeller power, rpm and ship speed at which the propeller is hydrodynamically optimised with regard to cavitation and efficiency. This design point was a balanced compromise between the different operation modes based on Roll-Royce’s long experience of designing propellers for ropax and cruise vessels operating in different modes with stiff noise/vibration requirements. All operating conditions have to be analysed. For example, in the 2+2 ME mode the sheet cavitation on the suction side as well as the strength of the tip vortex has to be kept to a minimum. Sheet cavitation is the main source of pressure pulses, normally at 1st to 4th blade harmonics. These blade harmonics create vibrations in the hull structure at the corresponding frequencies. The suction side tip vortex creates broadband noise on board, normally in the frequency range between the 4th and 8th order of blade frequency, corresponding to 35Hz-70Hz for Color Fantasy. In the single main engine mode, some pressure side cavitation is normally allowed, however, this should be kept to a minimum.

The design was refined in an iterative process. Pressure distribution on the blade surface of the propeller operating in a wake field behind the ship was calculated by potential flow boundary element method. Pressure pulses were calculated according to Holden’s method modified for the Rolls-Royce skew blade propeller design. The strength in the tip vortex was calculated using the tip vortex index method.

Extended model tests were carried out to reach the best possible hull lines and appendages in deep and shallow water. Wake measurements, propulsion and cavitation tests were made. The cavitation pattern on the propeller was video recorded and the propeller-induced hull pressures measured at three different conditions in Marin’s vacuum tank: 85% in deep water, 85% in shallow water and asymmetric engine conditions. A crucial part of the test programme was to study the propeller cavitation, specifically the behaviour of the tip vortex and to investigate possible means of reducing it.

An additional noise reduction feature is a so-called MABS air blowing system. Air is injected along the hull surface above the propellers, reducing noise excitation on the hull. It requires little power to operate and is controlled from the bridge. The advantage is that noise characteristics can be improved under adverse conditions, for example when sailing in shallow water, operating with asymmetric shaft loads or manoeuvring with large rudder angles.

When Color Fantasy was running sea trials, extensive measurements of cavitation, noise, vibration, speed, manoeuvrability and wave making were made. Speed trials were carried out in several different conditions to verify the model test power curves. In practice, the full scale power curves were better than expected from the model tests. Propeller induced hull pressures were measured by ten pressure transducers installed in the hull. Direct observations of cavitation were also made using the borescope technique for video recording and stroboscopic illumination of the propeller.
For several years Aker Finnyards has installed its own continuous measuring system to capture key performance data on its ships during the guarantee period. The system ensures a deeper understanding of the vessel’s operation and gives useful information should anything unexpected occur.

The conclusion of the Paper is that very low levels of noise and vibration can be achieved with a conventional twin screw propulsion system using CP propellers, and that optimum results are dependent on good co-operation between hull and propeller designers, owner and classification society.