Technology guidelines for efficient design and …

1 IntroductionThe number of vessels equipped with controllable pitch propellers (CPP)has steadily grown in recent decades. Today fixed pitch propellers (FPP) areused for vessels where simplicity and operation at sea are dominant. Thisincludes tankers, bulk carriers and vessels are normally equipped with 2-stroke diesel are also a number of vessels where the application of fixed pitchpropellers in combination with a 4-stroke engine and a gearbox hasbecome the standard. Examples of such vessels are container feeders,offshore vessels, dredgers and requirements include the need to use maximum engine power inall operating conditions, along with good manoeuvring capability and fullyautomatic control systems. There is also a continuing demand for increasedpower and ship reliable and efficient machinery, including fixed and controllablepitch propellers, has always been the prime target for W rtsil which haslong recognized these trends and we are continuously updating ourpropulsion systems to meet these in our product line include.

2 NThe most powerful CPP in the worldnThe heaviest FPP in the worldnAdvanced hydrodynamic designsnCompact hub designsnIntegrated control systems with field bus technologynHigh-efficiency article reviews the main design criteria of FPP and CPP installations forvarious applications, focusing on the common design criteria for bothconcepts and the special considerations concerning controllable also address typical developments on the hydrodynamic side aswell as the benefit of combining the engine propeller and gearbox. Inaddition we describe the special application of the Efficiency Rudders aswell as typical cases for the application of in propeller designThe ship speed of vessels has been increased in recent years. Thisdevelopment applies to large container vessels (container carrying capacityhas risen to 8000 TEU), large dredgers (hopper capacity has increased to22,000 m3) and large RoPax vessels (ship speeds and power have gone upto 30 knots and 15 MW).

3 Concurrent with this trend the design of propellers for these vessels hasbecome more difficult and more stringent requirements have beenforwarded to the supplier of the propulsion shipbuilding contract reviews requirements concerning the shipspeed, the level of vibration on board, the level of noise given the vessel sload carrying capacity (deadweight or number of containers or passengers)and the ship speed. In general the requirements placed on the propulsioninstallation can be translated into direct requirements for the propulsioninstallation. The link is as follows:Ship building contract:Propulsion installation:Ship speed for a given power Propulsive efficiencyVibration limitPropeller induced pressure pulses on the hullInboard noise levelsType and extent of cavitation on the propellerBalancing boundary conditionsIt is a general rule when designing propellers to aim for the highest possiblelevel of propeller efficiency while keeping vibration and noise and hencecavitation at the lowest possible level.

4 This leads to conflicting boundaryconditions. Less cavitation, for example, results in a large blade area ratio,whereas trying to obtain a high propeller efficiency requires the ship has its own dedicated design propulsion system whichguarantees the best operational performance. Its propeller, therefore, mustkeep a subtle balance between several extremes, resulting in a compromisethat depends on the experience of the propeller designer and the correctuse of the design tools at his - W rtsil 1-2004 The Ship Power SupplierFig. 1 Propulsion installation for a typical Ropax guidelines for efficient designand operation of ship propulsorsby Teus van Beek, Propulsor Technology , W rtsil Propulsion Netherlands peed MCR [knots]010000200003000040000500006000070 00080000 Power per shaftline [kW]Total power per ship [kW]Fig. 2 Power per shaft versus ship speed for RoPax reduce the boundary limitations as much as possible W rtsil hasdeveloped blade sections that combine large cavitation-free operation withgood structural characteristics and low drag properties.

5 The result is anoptimized design with higher efficiency. In addition the propeller designsystem used and developed by W rtsil consists of interactive design andanalysis modules as shown in the propeller design can only be initiated after the design criteria havebeen selected. The design criteria used in our propeller design systemconsist of information on the type of ship, its mission profile, and possiblelimitations regarding propeller diameter, efficiency, ship speed, cavitationbehaviour and propeller-generated pressure pulses on the ship s criteria are to be considered as the boundary conditions orconstraints for any propeller design . They normally have their greatest impacton the balance between the design and off- design properties of the speed and propeller diameter are closely related. For a givendiameter, a low shaft speed is beneficial from the efficiency point of viewbut it also leads to a relatively high shaft torque and subsequently largeshafts, hubs and gearboxes.

6 A balance must be found betweenhydrodynamic performance and the total cost of the propeller 2 and 3 show the relation between propeller power and shipspeed, and alternatively the power density (the power divided by thepropeller disc area) versus ship speed. These diagrams show a clear trendtypical for most vessels. When the ship speed increases, so does thepower density on the propeller and therefore the difficulty in designing apropulsion installation a first estimateGenerally speaking, the largest propeller diameter gives the highestpropulsive efficiency. However, the diameter behind the ship is normallylimited by the draught of the vessel and the tip a first estimate the propeller diameter based on the power andrevolution rate can be selected using the following formula. This formula isbased on a series of propellers and the optimum selection of diameter andnumber of revolutions given the best propeller efficiency:NPDopt= 10153whereNopt= optimum revolution rate (rpm)P = propulsion power (kW)D = propeller diameter (m)This simple but reliable formula also makes it possible to check the tipspeed of the propeller:VNDPD tipopt= = the tip speed is related to the power density of the the results for various applications shows that, given thepower density, the tip speed normally falls within +/- 20% of the calculatedfigure.

7 The tip speed together with the inflow to the propeller is a dominantfactor in the design of propellers, especially with respect to cavitationperformance (described in more detail below).The propeller efficiency is mainly determined by the given propellerdiameter and required thrust. Based on momentum theory a relation can bederived to obtain the ideal propeller efficiency:hhpropidealTC=-=++-0 1752110 12422rpandT = propeller thrust [N]r= water density [kg/m3]V = the advance velocity [m/s] of the propeller, V = VS.(1-w),where VSis the ship speed [m/s] and w is the wake formula is a first estimate of the propulsive efficiency. Without thededuction of the formula would assume a propeller with an infinitenumber of blade and no frictional or rotational losses. In practice the bladenumber reduces the efficiency and friction of the blades as they are drawnthrough the water and also the finite blade number.

8 In total this adds up toabout a propeller with a larger wetted surface this effect is larger. Theformula clearly shows the positive effect of selecting a large propellerdiameter for a given required thrust. Normally this thrust is based on thethrust required to drive a certain size of vessel at the ship speed required bythe simplified relations can be derived for vibration. Depending onthe tip speed and the tip clearance, the distance between the propellerblade tip and the hull, the propeller vibration is felt every time a bladepasses the presence of cavitation on the propeller blade tip strongly amplifiesthe pressure pulse on the hull. In general the pressure pulses on the hull:nare inversely proportional to the tip clearancenincrease with increased power densitynincrease with increased inflow disturbance from the inflow at the propeller blade tip is much less then the ship speed; forsingle-screw vessels the inflow is as low as 50% of the ship speed.

9 Thisimplies that the blade profile of the propeller varies strongly in the angle ofattack during one designMany vessels today are equipped with controllable pitch propellers,especially vessels with an installed power of less than 10,000 kW. Theengines driving these propellers cannot be reversed. The application ofshaft-driven generators for the efficient generation of electricity or for heavymanoeuvring requirements favours the application of CP pitch propellers are applied when the vessel mainly operates atsea. Simplicity counts. However, there are limits to the operation of fixedpitch propellers. For instance the 2-stroke engine cannot operate below acertain number of revolutions, which will restrict the low-speed operation ofthe vessel. Controllable pitch propellers also make it possible for the dieselengines to absorb the full power in both bollard and free-running the pre- design stage, the main parameters are determined in closeco-operation with the yard and the owner.

10 Diameter, shaft speed and hubsize are selected. In most cases a preliminary propeller design is made and1-2004 Marine News - speed MCR [knots]Power density per shaftline [kW/m ]2 Fig. 3 Power density versus ship speed for with the yard and the owner. The design conditions andoff- design modes are carefully agreed and registered. This is especiallyimportant for dredgers, passenger and RoPax from model tests for the propeller design are:nSelf-propulsion test with stock propeller (and nozzle if applicable)nOpen-water test with stock propeller (and nozzle if applicable)nWake field 4 shows the effect of the wake distribution and the direction ofrotation on the cavitation performance of highly loaded controllable pitchpropellers. Depending on the wake peak and thus the quality of the wake,significant differences exist in the cavitation propeller geometry is based on the following parameters, which wediscuss in detail below:nChord lengthnPitch and cambernSkew and rakenProfile unloadingThe propeller design has an unloaded tip.