Ship Propulsion Systems

1. Ship Propulsion Systems OE 524 Spring 2013 Courtesy: Justin Lorio

2. Considerations • Propulsion, stopping, backing, maneuvering • Cost • Power required for sustained operations and endurance • Space and weight • Prime movers have significant impact on design of vessel • Power profile • What percentage of time is ship operating at rated profile • Ship Resistance • Type of propulsor and #(Props, jets, podded propulsors • Establish propulsion plant power rating • Usually 80%-90% of its max continuous power rating • Selection of main propulsion plant • Space and arrangement requirements • How much space does the machinery need to take from the ship • Propulsion machinery configuration • Arrangement of prime mover with shafting/gearing/transmission ...

3. Propulsors • Standard Propeller • Cycloidal Propeller • Water Jets • Paddle Wheel • Human Powered devices • Magnetohydrodynamic

4. Standard Propeller • Fixed Pitch • Reliable • Needs another form of reversing (transmission or reversing engine) • Controllable Pitch • Blades rotate • More flexibility in control of vessel • No need for reversing mechanism • Complex mechanical design

5. Single Screw • Simple • Less moving parts • Ease of engine control • Max propulsive efficiency • Larger diameter propeller more efficiency • Less drag • Can also couple two or more prime movers to a single shaft • Usually for ships operating in full displacement or semi-displacement mode • There are exceptions • Poor maneuvering • No redundancy • Many ports require escort tug

6. Twin Screw • Extremely high powered ships • Practical limits of single screw • Cavitation - reduce loading on individual propeller blades • Blade loading on single prop too high and may cause structural problems • Increased maneuverability with differential thrust • Redundancy in case of mechanical failure of battle damage • Better propeller loading for shallow draft vessels that are diameter limited • Using too little propeller blade area could lead to • poor efficiency • cavitation • vibration • poor engine/propeller matching causing over-revving of engine

7. Contra-Rotating Propellers • Two propellers on the same axis rotating in opposite directions • Forward prop larger diameter 5 blade, aft prop smaller diameter 4 blade • Increased efficiency due to recovery of slip stream rotational energy • Complicated mechanical system leads to mechanical problems • High drag

8. Ducted Propeller • Consists of a propeller in a duct which has an airfoil cross-section • Excellent for bollard pull condition (contributes up to 50% of total thrust) • Great for low speed towing applications (tugs, supply vessels) • Can be steerable • Loses effect with increasing ship speed • Increased drag

9. Azimuthing Drive • Propeller rotates in Z-axis eliminating need for rudder • Precise thrust control • Excellent maneuverability • Can be used in pusher or tractor mode • Tractor mode higher efficiency • Z-Drive • Mechanical drive from on board mover to propeller • Common on tugs and supply vessels • L-Drive • Electric drive motor on top of unit in hull • Common on tugs and supply vessels • Azipod • Electric motor is in the pod • Seen on newer cruise ships and icebreakers • Lower drag than other azis

10. Surface Piercing Propellers • Specifically for high speed vessels where cavitation is an issue for fully submerged propellers (usually above 40 kts) • Very high efficiency at high speeds • Poor off design performance • Poor reversing properties • Generate large amounts of spray • Used mainly by Racing Powerboats, high speed yachts and small high speed military

11. CycloidaIPropellers • Apply thrust precisely in any direction • Excellent maneuverability • No need for a rudder • Fairly reliable even with complex mechanical system • Good for low speed (-15 kts) • High drag • Common on tugs and supply vessels

12. Water Jets • Less efficient than conventional submerged props at lower speeds, but more efficient at higher speeds (20-SOkts) • Greater speed range than conventional props before cavitation starts • Better off design performance than surface piercing propellers • Drive unit usually heavier than prop drive system • Very common on fast ferries

13. Other Propulsors • Human powered devices • Oars, paddles, poles, pulling • Paddle Wheel • Magnetohydrodynamic • Exploits physical properties of electromagnetic field to pump seawater through a fore-aft tube producing thrust • Wind powered • Sails, kites, wings

14. Prime Movers • Gasoline • Diesel • Gas Turbine • Steam Turbine • Human • Wind

15. Gasoline • Internal Combustion • High horsepower • Low weight • High fuel Consumption • Two stroke • Ignition at top of every stroke • Transmits energy into crankshaft in part of down stroke • Exhaust gasses expelled in rest of down stroke • Fuel/air mixture comes in on up stroke • Smaller and lighter than 4 stroke • more emissions than 4 stroke • 4-stroke • Ignition at top of every other stroke • Power transmission through all of down stroke • 1st upstroke expels exhaust • 2nd down stroke intakes fuel/air mixture • 2nd upstroke compresses fuel/air mixture • Heavier than 2 stroke but more fuel efficient and lower emissions

16. Diesel • Internal Combustion • Same principal and 2 and 4 stroke gasoline engines except the fuel is sprayed into the cylinder near the end of the compression stroke rather than being first mixed with air • Ignition is caused by compression as opposed to spark plug • Generally higher torque output than gas • 3 main categories • Slow speed, Medium Speed, High Speed

17. Slow Speed Diesel • 75-250 rpm • Specifically for ship propulsion • Directly coupled to prop shaft (eliminates reduction gear) • One engine per prop • Mounted level directly inline with prop • Very large and very heavy • Used in big ships • 4-12 cylinders • Power -3,000kW-100,000 kW (1kW = 1.34 HP)

18. Medium Speed Diesel • 400-900 rpm • Usually adapted from locomotive engines • Need gearing or electric drive • Inline 4-10 cylinders in line or up to 24 in V configuration • Power ranges from -1,000 kw to 25,000 kW

19. High Speed Diesel • 1000-2400 rpm • Smaller and lighter • Smaller and higher speed boats • Patrol boats, supply boats, fishing boats • Same cylinders as Med-Speed • Power range -~10 kW-4,000kW

20. Gas Turbine • Heavy-duty gas turbines (HDGT)(40,000kW) • Aircraft-derivative units (ADGT) (smaller and lighter, higher power density, SOOOOkW) • Very high power to weight ratio • Small in size • Low emissions • High fuel consumption and sensitive to fuel quality • High maintenance cost • Output at high rpm, significant reduction gear needed for prop drive • Common on military ships combined with diesels (CODAG) • Diesels for loitering/cruise • Turbine boost for flank speed or sprint speed • Also used on some fast ferries

21. Steam Turbine • Fuel runs a boiler, escaping steam turns a turbine • Coal fueled {60,000kW), oil fired {60,000kW) or nuclear • Common on LNG's and warships

22. Transmissions • Direct connected slow speed diesel • Reversing transmission • Reduction gear (single, double)

23. Losses • Usually figure about 50% power delivered into the water • Propulsive coefficient = 0.5 • Better is achievable

24. Auxiliary Propulsors • Bow thruster (drop down, tunnel) • Props mounted in tunnel

25. Propulsor-Prime Mover Combos • Slow-medium speed engines with fixed pitch props • Higher speed with water jets or surface piercing props • If the motor or shaft cant reverse the prop must be variable pitch • High speed engines and complicating shafting/mechanicals don’t mix • i.e. a gas turbine cycloidal prop does not work