Presentation on theme: "Chap 7 Resistance and Powering of Ship"— Presentation transcript:
1 Chap 7 Resistance and Powering of Ship Recall Static Equilibrium!What are the forces in the x-axis of the ship?Resistance or DragThrust (Propulsion)We will first look at propulsion, then drag in this chapter.41 knots!
2 Ship Drive Train and Power 7.2 Ship Drive Train SystemEngineReductionGearScrewStrutBearingSealsTHPShaftBHPSHPDHP
3 Ship Drive Train and Power Horse Power in Drive TrainBrake Horse Power (BHP)- Power output at the shaft coming out of the engine beforethe reduction gearsShaft Horse Power (SHP)- Power output after the reduction gears (at shaft)- SHP=BHP - losses in reduction gear
4 Ship Drive Train and Power Delivered Horse Power (DHP)- Power delivered to the propeller- DHP=SHP – losses in shafting, shaft bearings and sealsThrust Horse Power (THP)- Power created by the screw/propeller (ie prop thrust)- THP=DHP – Propeller lossesBHPSHPDHPTHPEHPE/GR/GShaftBearingProp.HullRelative Magnitudes?BHP>SHP>DHP>THP
5 7.3 Effective Horse Power (EHP) EHP : The power required to move the ship hull at a givenspeed in the absence of propeller action (related to resistance)(EHP is not related with Power Train System)EHP can be determined from the towing tank experiments atthe various speeds of the model ship.EHP of the model ship is converted into EHP of the full scaleship by the Froude’s Law.EHP is approximately equal to THP (usually slightly less)Measured EHPVTowing TankTowing carriage
6 Effective Horse Power (EHP) Typical EHP Curve of YPWhat EHP is required for the 12 knot YP cruise?
7 Effective Horse Power (EHP) EfficienciesHull EfficiencyHull efficiency changes due to hull-propeller interactions.Well-designed ship :Poorly-designed ship :Well-designedPoorly-designedFlow is not smooth.THP is reduced.- High THP is neededto get designed speed.
8 Effective Horse Power (EHP) Efficiencies (cont’d)EHPPropeller EfficiencyScrewTHPDHPSHPPropulsive Coefficient (PC) ~ An overall measure of drive train efficiency
9 7.5 Total Hull Resistance Total Hull Resistance (RT) The force that the ship experiences opposite to the motion ofthe ship as it moves.EHP Calculation
10 Total Hull Resistance (cont) Coefficient of Total Hull Resistance- Non-dimensional value of total resistance
11 Total Hull Resistance (cont) Coefficient of Total Hull Resistance (cont’d)Total Resistance of full scale ship can be determined using
12 Total Hull Resistance (cont) Relation of Total Resistance Coefficient and Speed
13 7.6 Components of Total Resistance Viscous Resistance- Resistance due to the viscous stresses that the fluid exertson the hull.( i.e. due to friction of the water against the surface of the ship)- Water viscosity, ship’s velocity, wetted surface area and roughness of the ship generally affect the viscous resistance.Characterized by the non-dimensional Reynold’s Number, Rn
14 Components of Total Resistance Wave-Making Resistance- Resistance caused by waves generated by the motion of the ship- Wave-making resistance is affected by beam to length ratio,displacement, shape of hull, (ship length & speed)Characterized by the non-dimensional Froude Number, FnAir Resistance- Resistance caused by the flow of air over the ship with nowind present- Air resistance is affected by projected area, shape of the shipabove the water line, wind velocity and direction- Typically 4 ~ 8 % of the total resistance
15 Components of Total Hull Resistance Total Resistance and Relative Magnitude of ComponentsAir ResistanceHollowHumpWave-makingResistance (lb)ViscousSpeed (kts)Low speed : Viscous R dominatesHigher speed : Wave-making R dominatesHump (Hollow) : location is function of ship length and speed.
16 Coefficient of Viscous Resistance Viscous Flow around a shipReal ship : Turbulent flow exists from near the bow.Model ship : Studs or sand strips are attached at the bowto create the turbulent flow.
17 Coefficient of Viscous Resistance (cont) Coefficients of Viscous Resistance- Non-dimensional quantity of viscous resistance- It consists of tangential and normal components.normaltangentialflowshipsternbowTangential Component :- Tangential stress is parallel to ship’s hull and causesa net force opposing the motion ; Skin Friction- It is assumed can be obtained from data of flat plates.
18 Coefficient of Viscous Resistance (cont) Semi-empiricalequation
19 Coefficient of Viscous Resistance (cont) Tangential Component (cont’d)- Relation between viscous flow and Reynolds number· Laminar flow : In laminar flow, the fluid flows in layersin an orderly fashion. The layers do not mix transverselybut slide over one another.· Turbulent flow : In turbulent flow, the flow is chaotic andmixed transversely.Laminar FlowTurbulent FlowFlow overflat plate
20 Coefficient of Viscous Resistance (cont) Normal Component- Normal component causes a pressure distribution along theunderwater hull form of ship- A high pressure is formed in the forward direction opposingthe motion and a lower pressure is formed aft.- Normal component generates the eddy behind the hull.- It is affected by hull shape.Fuller shape ship has larger normal component than slendership.large eddyFull shipSlender shipsmall eddy
21 Coefficient of Viscous Resistance (cont) Normal Component (cont’d)- It is calculated by the product of Skin Friction with Form Factor.
22 Summary of Viscous Resistance Coefficient Reducing the Viscous Resistance CoefficientMethod : Increasing L with constant submerged volume1) Form Factor K Normal component KCF Slender hull is favorable. ( Slender hull form will createa smaller pressure difference between bow and stern.)2) Reynolds No. Rn CF KCF
23 Wave-Making Resistance Definition : refer to the previous noteTypical Wave PatternStern divergent waveBow divergent waveBow divergent waveLTransverse waveWave Length
25 Wave-Making Resistance Transverse wave system : Waves perpendicular to wakeIt travels at approximately the same speed as the ship.At slow speed, several crests exist along the ship lengthbecause the wave lengths are smaller than the ship length.As the ship speeds up, the length of the transverse waveincreases.When the transverse wave length approaches the ship length,the wave making resistance increases very rapidly.This is the main reason for the dramatic increase inTotal Resistance as speed increases.
27 Wave-Making Resistance (cont) Divergent Wave System: waves that angle outIt consists of Bow and Stern Waves.Interaction of the bow and stern waves create the Hollow orHump on the resistance curve.Hump : When the bow and stern waves are in phase,the crests are added up so that larger divergent wave systemsare generated.Hollow : When the bow and stern waves are out of phase,the crests match the troughs so that smaller divergent wavesystems are generated.
28 Wave-Making Resistance (cont) Calculation of Wave-Making Resistance Coeff.Wave-making resistance is influenced by:- beam to length ratio- displacement- hull shape- Froude numberThe calculation of the coefficient is too complex for a simple theoretical or empirical equation.(Because mathematical modeling of the flow around shipis very complex since there exists fluid-air boundary,wave-body interaction)Therefore model test in the towing tank and Froude expansionare the best way to calculate the Cw of the real ship.
29 Wave-Making Resistance (cont) Reducing Wave Making Resistance1) Increasing ship length to reduce the transverse wave- Hull speed will increase.- Therefore increment of wave-making resistance of longership will be small until the ship reaches to the hull speed.
30 Wave-Making Resistance (cont) Reducing Wave Making Resistance (cont’d)2) Attaching Bulbous Bow to reduce the bow divergent wave- Bulbous bow generates the second bow waves .- Then the waves interact with the bow wave resulting inideally no waves, practically smaller bow divergent waves.- EX :DDG 51 : 7 % reduction in fuel consumption at cruise speed3% reduction at max speed.design &retrofit cost : less than $30 millionlife cycle fuel cost saving for all the ship : $250 mil.Tankers & Containers : adopting the Bulbous bow3) Stern flaps - helps with launching small boats, too!
32 Coefficient of Total Resistance Coefficient of total hull resistance: the C’s addedCorrelation AllowanceIt accounts for hull resistance due to surface roughness,paint roughness, corrosion, and fouling of the hull surface.It is only used when a full-scale ship prediction of EHP is madefrom model test results.For model,For ship, empirical formulas can be used.
33 Other Type of Resistances Appendage Resistance- Frictional resistance caused by the underwater appendagessuch as rudder, propeller shaft, bilge keels and struts- 224% of the total resistance in naval ship.Steering Resistance- Resistance caused by the rudder motion.- Small in warships but troublesome in sail boatsAdded Resistance- Resistance due to sea waves which will cause the shipmotions (pitching, rolling, heaving, yawing).
34 Other Resistances Increased Resistance in Shallow Water - Resistance caused by shallow water effect- Flow velocities under the hull increases in shallow water.: Increment of frictional resistance due to the velocities: Pressure drop, suction, increment of wetted surface area Increases frictional resistance- The waves created in shallow water take more energy fromthe ship than they do in deep water for the same speed. Increases wave making resistance
35 ? 7.7 Basic Theory Behind Ship Modeling Modeling a ship - Not possible to measure the resistance of the prototype ship- The ship needs to be scaled down to test in the tank butthe scaled ship (model) must behave in exactly same wayas the real ship.- How to scale the prototype ship ?How to make relationships between the prototype and modeldata?- Geometric and Dynamic similarity must be achieved.?DimensionSpeedForceprototypeModelprototype shipmodel ship
36 Basic Theory behind Ship Modeling Geometric Similarity- Geometric similarity exists between model andprototype if the ratios of all characteristic dimensionsin model and prototype are equal.- The ratio of the ship length to the model length is typicallyused to define the scale factor.
37 Basic Theory behind Ship Modeling Dynamic Similarity- Dynamic Similarity exists between model and prototypeif the ratios of all forces in model and prototype are thesame.- Total Resistance : Frictional Resistance+ Wave Making+Others
38 Basic Theory behind Ship Modeling Dynamic Similarity (cont’d)- Both Geometric and Dynamic similarity cannot be achievedat same time in the model test because making both Rn andFn the same for the model and ship is not physically possible.ExampleShip Length=100ft, Ship Speed=10kts, Model Length=10ftModel speed to satisfy both geometric and dynamic similitude?
39 Basic Theory behind Ship Modeling Dynamic Similarity (cont’d)- Choice ?· Make Fn the same for the model.· Have Rn different Incomplete dynamic similarity- However partial dynamic similarity can be achieved bytowing the model at the “corresponding speed”- Due to the partial dynamic similarity, the followingrelations in forces are established.
40 Basic Theory behind Ship Modeling Corresponding SpeedsExample :Ship length = 200 ft, Model length : 10 ftShip speed = 20 kts, Model speed towed ?1kts=1.688 ft/s
41 Chap 7.7 Basic Theory Behind Ship Modeling Modeling Summary1)Measured in tankFroudeExpansion2)3)
45 7.9 Screw Propeller Variable Pitch (the standard prop): - The pitch varies at the radial distance from the hub.- Improves the propeller efficiency.- Blade may be designed to be adjusted to a differentpitch setting when propeller is stopped.Controllable Pitch :- The position of the blades relative to the hub can bechanged while the propeller is rotating.- This will improve the control and ship handling.- Expensive and difficult to design and build
47 Suction FaceLeading EdgeTrailing EdgePressure Face
48 Propeller WalkDue to a difference in the pressure at the top and bottom of the prop (due to boundary layer), the lower part of the prop works harder.This leads to a slight turning moment.Right hand props cause turns to port when moving ahead.
51 7.9 Skewed Screw Propeller Highly Skewed PropellerAdvantagesReduce interaction betweenpropeller and rudder wake.- Reduce vibration and noiseDisadvantagesExpensiveLess efficient operating inreverseDDG51
52 7.9. Propeller Theory Propeller Theory Speed of Advance P Wake Region QPWake RegionThe ship drags the surrounding water so that the wake tofollow the ship with a wake speed (Vw) is generated in thestern.The flow speed at the propeller is,Speed of Advance
53 7.9 Propeller Theory Propeller Efficiency Maximum ~70 % for well-designed propMaximum- For a given T (Thrust),Ao(Diameter ) ; CT ; Prop EffThe larger the diameter of propeller, the better the propeller efficiency
54 Chap 7.9.3 Propeller Cavitation Cavitation : DefinitionThe formation and subsequent collapse of vapor bubbleson propeller blades where pressure has fallen below thevapor pressure of water.- Bernoulli’s Equation can be used to predict pressure.Cavitation occurs on propellers (or rudders) that are heavily loaded, or are experiencing a high thrust loading coefficient.
57 Blade Tip CavitationNavy Model Propeller 5236Flow velocities at the tip are fastest so that pressure drop occurs at the tip first.Sheet CavitationLarge and stable region of cavitation covering the suction face of propeller.
58 Propeller Cavitation Consequences of Cavitation 1) Low propeller efficiency (Thrust reduction)2) Propeller erosion (mechanical erosion as bubbles collapse, up to 180 ton/in² pressure)3) Vibration due to uneven loading4) Cavitation noise due to impulsion by the bubble collapse
59 Propeller Cavitation Preventing Cavitation Remove fouling, nicks and scratch.Increase or decrease the engine RPM smoothly to avoidan abrupt change in thrust.rapid change of rpm high propeller thrust but smallchange in VA larger CT cavitation &low propeller efficiencyKeep appropriate pitch setting for controllable pitch propellerFor submarines, diving to deeper depths will delay or prevent cavitation as hydrostatic pressure increases.
60 Propeller Cavitation Ventilation If a propeller or rudder operates too close to the water surface, surface air or exhaust gases are drawn into the propeller blade due to the localized low pressure around propeller. The prop “digs a hole” in the water.The load on the propeller is reduced by the mixing of air or exhaust gases into the water causing effects similar to those for cavitation.Ventilation often occurs in ships in a very light condition(small draft), in rough seas, or during hard turns.
61 Other forms of propulsion A one horsepower cable-drawn ferry!
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