CP Hydrodynamics

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Controllable Hydrodynamic

Text of CP Hydrodynamics

  • 3Contents

    Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

    Testing of Propellers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

    Calculation Methods for Propellers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

    Series Propellers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

    Lifting Line Designed Propellers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

    Lifting Surface Designed Propellers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

    Surface Panel Designed Propellers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

    Hydrodynamic Design of CP Propellers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

    Design Conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

    Main Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

    Tank Test Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

    Design Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

    Blade Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

    Model Test and Full Scale Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

    Cases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

    Twin Screw 3990 GT Cruise Vessel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

    Single Screw 6000 DWT Chemical Tanker . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

    Single Screw 16000 DWT Tanker . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

    Single Screw 5100 DWT Chemical Tanker . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

    Twin Screw Supply Vessel AHTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

    Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

    References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

    Hydrodynamics of Ship Propellers

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  • 5Designing propellers for ships hasalways been a challenge due to thecomplexity of all the factors involved.These factors are not only related tothe propeller itself but also to the hulland the propulsion system whichmust work together as integratedsystems in an optimised and reliableway.

    Introduction

    A century ago, Sir Charles Parsondesigned the worlds first steam turbine powered vessel named Turbinia. The vessel incorporated anumber of innovative design featuresand broke the speed record at thattime. At Turbinias first sea-trials in1894, it showed a disappointing shipsspeed of only 19.5 knots despitetrying different propeller designs onthe single screw vessel. After intensive research Parson reali-sed that the speed problem arosefrom the formation of vapour bubblesaround the propeller blades. Thisobservation was later to be calledcavitation and today it still plays asignificant role in designing propellers.

    To study the phenomenon of cavita-tion, Parson built the worlds firstcavitation tunnel in which a number ofsmall model propellers were tested.Based on his experiments the Turbi-nia was redesigned by distributingthe power between three shafts andusing much improved blade shapes.He finally succeeded in setting thespeed record with a staggering 34knots, thereby outperforming eventhe Royal Navys torpedo gun boatswhich could achieve a speed of 30knots - a speed considered to beimpressive at that time.

    When faced with the problem ofcavitation, Parson built his own pri-mitive cavitation tunnel and probablywithout knowing it, he contributed tothe understanding of propellers byinventing one of the most powerfultools for analysing of cavitation.

    Over the years a number of toolshave been added to the toolbox -most of which are of the analytictype - but the most important one isprobably the development achievedby the appearance of increasinglyfaster computers over the last twodecades.The design tools for propellers haveevolved in two different directions,one being the empirical/testing andthe other the analytical/calculating.Through the last century they havesupported each other well and bothhave contributed to the understan-ding of the propeller and the condi-tions in which it works.

    The Turbinia steaming at sea onone of her runs. The vessel wasfinally modified to achieve theimpressive 34 knots.

  • 6Testing of Propellers

    Today the testing of not only the pro-peller but also the hull takes place atwell established and recognisedhydrodynamic institutions around theworld.The testing is performed by towingand propelling 6 to 10 m long hullmodels through a 200 to 300 m longmodel tank. In two seperate tests theresistance and the power needed atdifferent ship speeds are measured.In the initial stage of the testing, thepropeller used is selected among thelarge number of existing propellers,which the institute has in its posses-sion. This propeller is usually desig-nated a stock propeller. Later theactual propeller designed for the vessel can be manufactured in modelscale and fitted on the ship model toverify its performance efficiency.

    An important part of the testing, seenfrom a propeller designers point ofview, is the measurement of the wakefield, which will give the inflow veloci-ties to the propeller at any radial andcircumferencial position. The wake

    field is obtained by substituting thepropeller with a pitot probe, rotatedaround the propeller shaft. The probemeasures the pressure which canlater be converted into the threevelocity components (axial, tangentialand radial).

    A hull model being towed in themodel tank for determination of resistance and power.

  • 7When observing a propeller bladerotating behind a ships hull, one willdiscover that the inflow velocitiesand pressure will change dependingon the blades angular position.Especially for single screw full bodyships the twelve oclock position cancause the pressure to drop under thesaturation pressure eventually leadingto cavitation.

    One aspect that cannot be tested inthe long model tank is the cavitationbehaviour of the propeller. This is dueto difference in pressures betweenmodel and full scale and the subse-quent cavitation test is consequentlyperformed in a cavitation tunnel,where the pressure in model scalecan be adjusted to match the fullscale one.The propeller model is placed in thetunnel at the upper measurementsection and driven by its own motor.The water in the closed loop tunnel iscirculated by an impeller situated inthe lower part of the tunnel.

    Only at very few institutions is it pos-sible to place the whole ship modelin the cavitation tunnel and as a consequence the correct wake fieldneeds to be modelled by placing adummy model upstream of the pro-peller. It requires considerable skillfrom the personnel at the institutionsto achieve the same wake field in thecavitation tunnel as was measured inthe model tank. Therefore carefulmeasurements must be carried out toverify that this is the case.

    Today, cavitation tests are not carriedout for the same reasons as in theday of Parson, who discovered apronounced drop in propeller effici-ency. Parsons initial propeller wasprobably what today would becharacterized as a super cavitatingpropeller, fully surrounded by air bubbles, which reduces the efficiencydrastically and creates loud noiseand vibrations in the aft body of theship.

    Propellers for merchant ships oftoday are much more limited in theirextent of cavitation on the bladesresulting in only a marginal drop inefficiency, but the noise and vibrationproblem still remains.

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    54

    1

    4 m

    10.5 m

    7 m

    Cavitation tunnel with:

    1. Test section

    2. Thrust and torque

    dynamometer

    3. Propeller motor

    4. Axial flow impeller

    5. Impeller motor

  • 8Cavitation will be present on mostpropellers of todays merchant ves-sels especially when operating atmaximum power. Compared to theearlier hull designs the ones of todayare much more full bodied (high blockcoefficient ) which unfortunately willcause that the wake field experiencedby the propeller b