Basics of Ship Resistance

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2 Chap 7 Resistance and Powering of Ship Objectives Prediction of Ship’s Power - Ship’s driving system and concept of power - Resistance of ship and its components · frictional resistance · wave-making resistance · others - Froude expansion - Effective horse power calculation Propeller Theory - Propeller components and definitions - Propeller theory - Cavitation

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3 Ship Drive Train and Power Ship Drive Train System

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4 Brake Horse Power (BHP) - Power output at the shaft coming out of the engine before the reduction gears Shaft Horse Power (SHP) - Power output after the reduction gears - SHP=BHP - losses in reduction gear Horse Power in Drive Train Ship Drive Train and Power

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5 Delivered Horse Power (DHP) - Power delivered to the propeller - DHP=SHP – losses in shafting, shaft bearings and seals Thrust Horse Power (THP) - Power created by the screw/propeller - THP=DHP – Propeller losses Relative Magnitudes BHP>SHP>DHP>THP>EHP E/G R/G BHP SHP Shaft Bearing Prop. DHP THP EHP Hull Ship Drive Train and Power

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6 Effective Horse Power (EHP) EHP : The power required to move the ship hull at a given speed in the absence of propeller action (EHP is not related with Power Train System) EHP can be determined from the towing tank experiments at the various speeds of the model ship. EHP of the model ship is converted into EHP of the full scale ship by Froude’s Law. V Towing Tank Towing carriage Measured EHP

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7 Effective Horse Power (EHP) Typical EHP Curve of YP

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8 Effective Horse Power (EHP) Efficiencies Hull Efficiency Hull efficiency changes due to hull-propeller interactions. Well-designed ship : Poorly-designed ship : Flow is not smooth. THP is reduced. - High THP is needed to get designed speed.

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9 Screw Effective Horse Power (EHP) Efficiencies (cont’d) Propeller Efficiency Propulsive Coefficients (PC) SHP DHP THP EHP

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10 Total Hull Resistance Total Hull Resistance (RT) The force that the ship experiences opposite to the motion of the ship as it moves. EHP Calculation

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11 Total Hull Resistance (cont) Coefficient of Total Hull Resistance - Non-dimensional value of total resistance

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12 Total Hull Resistance (cont) Coefficient of Total Hull Resistance (cont’d) Total Resistance of full scale ship can be determined using

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13 Total Hull Resistance (cont) Relation of Total Resistance Coefficient and Speed

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14 Components of Total Resistance Total Resistance Viscous Resistance - Resistance due to the viscous stresses that the fluid exerts on the hull. ( due to friction of the water against the surface of the ship) - Viscosity, ship’s velocity, wetted surface area of ship generally affect the viscous resistance.

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15 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, Froude number (ship length & speed) Air Resistance - Resistance caused by the flow of air over the ship with no wind present - Air resistance is affected by projected area, shape of the ship above the water line, wind velocity and direction - Typically 4 ~ 8 % of the total resistance

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16 Components of Total Hull Resistance Total Resistance and Relative Magnitude of Components Viscous Air Resistance Wave-making Speed (kts) Resistance (lb) Low speed : Viscous R Higher speed : Wave-making R Hump (Hollow) : location is function of ship length and speed. Hump Hollow

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17 Why is a Golf Ball Dimpled? Let’s look at a Baseball (because that’s what I have numbers for) At the velocities of 50 to 130 mph dominant in baseball the air passes over a smooth ball in a highly resistant flow. Turbulent flow does not occur until nearly 200 mph for a smooth ball A rough ball (say one with raised stitches like a baseball) induces turbulent flow A baseball batted 400 feet would only travel 300 feet if it was smooth. A non-dimpled golf ball would really hamper Tiger Woods’ long game

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18 Coefficient of Viscous Resistance Viscous Flow around a ship Real ship : Turbulent flow exists near the bow. Model ship : Studs or sand strips are attached at the bow to create the turbulent flow.

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19 Coefficient of Viscous Resistance (cont) Coefficients of Viscous Resistance - Non-dimensional quantity of viscous resistance - It consists of tangential and normal components. Tangential Component : - Tangential stress is parallel to ship’s hull and causes a net force opposing the motion ; Skin Friction - It is assumed can be obtained from the experimental data of flat plate. flow ship bow stern tangential normal

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20 Coefficient of Viscous Resistance (cont) Semi-empirical equation

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21 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 layers in an orderly fashion. The layers do not mix transversely but slide over one another. · Turbulent flow : In turbulent flow, the flow is chaotic and mixed transversely. Flow over flat plate

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22 Normal Component - Normal component causes a pressure distribution along the underwater hull form of ship - A high pressure is formed in the forward direction opposing the 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 slender ship. Full ship Slender ship large eddy Coefficient of Viscous Resistance (cont) small eddy

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23 Normal Component (cont’d) - It is calculated by the product of Skin Friction with Form Factor. Coefficient of Viscous Resistance (cont)

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24 Summary of Viscous Resistance Coefficient K= Form Factor

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25 Reducing the Viscous Resistance Coeff. Method : Increase L while keeping the submerged volume constant 1) Form Factor K   Normal component KCF   Slender hull is favorable. ( Slender hull form will create a smaller pressure difference between bow and stern.) 2) Reynolds No. Rn   CF   KCF  Summary of Viscous Resistance Coefficient

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26 Wave-Making Resistance Typical Wave Pattern Bow divergent wave Bow divergent wave Transverse wave L Wave Length Stern divergent wave

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28 Wave-Making Resistance Transverse wave System It travels at approximately the same speed as the ship. At slow speed, several crests exist along the ship length because the wave lengths are smaller than the ship length. As the ship speeds up, the length of the transverse wave increases. 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 in Total Resistance as speed increases.

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29 Wave-Making Resistance (cont) Transverse wave System Vs < Hull Speed Vs  Hull Speed Hull Speed : speed at which the transverse wave length equals the ship length. (Wavemaking resistance drastically increases above hull speed)

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30 Divergent Wave System It consists of Bow and Stern Waves. Interaction of the bow and stern waves create the Hollow or Hump on the resistance curve. Hump : When the bow and stern waves are in phase, the crests are added up so that larger divergent wave systems are generated. Hollow : When the bow and stern waves are out of phase, the crests matches the trough so that smaller divergent wave systems are generated. Wave-Making Resistance (cont)

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31 Calculation of Wave-Making Resistance Coeff. Wave-making resistance is affected by - beam to length ratio - displacement - hull shape - Froude number The calculation of the coefficient is far difficult and inaccurate from any theoretical or empirical equation. (Because mathematical modeling of the flow around ship is very complex since there exists fluid-air boundary, wave-body interaction) Therefore model test in the towing tank and Froude expansion are needed to calculate the Cw of the real ship. Wave-Making Resistance (cont)

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32 Reducing Wave Making Resistance 1) Increasing ship length to reduce the transverse wave - Hull speed will increase. - Therefore increment of wave-making resistance of longer ship will be small until the ship reaches to the hull speed. - EX : FFG7 : ship length 408 ft Which ship requires more hull speed 27 KTS horse power at 35 KTS? CVN65 : ship length 1040 ft hull speed 43 KTS Wave-Making Resistance (cont)

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33 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 in ideally no waves, practically smaller bow divergent waves. - EX : DDG 51 : 7 % reduction in fuel consumption at cruise speed 3% reduction at max speed. design &retrofit cost : less than $30 million life cycle fuel cost saving for all the ship : $250 mil. Tankers & Containers : adopting the Bulbous bow Wave-Making Resistance (cont)

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34 Bulbous Bow Wave-Making Resistance (cont)

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35 Coefficient of Total Resistance Coefficient of total hull resistance Correlation Allowance It 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 made from model test results. For model, For ship, empirical formulas can be used.

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36 Other Type of Resistances Appendage Resistance - Frictional resistance caused by the underwater appendages such as rudder, propeller shaft, bilge keels and struts - 224% of the total resistance in naval ship. Steering Resistance - Resistance caused by the rudder motion. - Small in warships but troublesome in sail boats Added Resistance - Resistance due to sea waves which will cause the ship motions (pitching, rolling, heaving, yawing).

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37 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 from the ship than they do in deep water for the same speed.  Increases wave making resistance

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38 Basic Theory Behind Ship Modeling Modeling a ship - It is not possible to measure the resistance of the full-scale ship - The ship needs to be scaled down to test in the tank but the scaled ship (model) must behave in exactly same way as the real ship. - How do we scale the prototype ship ? - Geometric and Dynamic similarity must be achieved. prototype ship model ship

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39 Basic Theory behind Ship Modeling Geometric Similarity - Geometric similarity exists between model and prototype if the ratios of all characteristic dimensions in model and prototype are equal. - The ratio of the ship length to the model length is typically used to define the scale factor.

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40 Basic Theory behind Ship Modeling Dynamic Similarity - Dynamic Similarity exists between model and prototype if the ratios of all forces in model and prototype are the same. - Total Resistance : Frictional Resistance+ Wave Making+Others

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41 Basic Theory behind Ship Modeling Dynamic Similarity (cont’d) - Both Geometric and Dynamic similarity cannot be achieved at same time in the model test because making both Rn and Fn the same for the model and ship is not physically possible. Example Ship Length=100ft, Ship Speed=10kts, Model Length=10ft Model speed to satisfy both geometric and dynamic similitude?

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42 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 by towing the model at the “corresponding speed” - Due to the partial dynamic similarity, the following relations in forces are established.

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43 Basic Theory behind Ship Modeling Corresponding Speeds Example : Ship length = 200 ft, Model length : 10 ft Ship speed = 20 kts, Model speed towed ? 1kt.=1.688 ft/s

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44 Basic Theory behind Ship Modeling Modeling Summary 1) 2) 3)