Ship Propellers

• A component of a ship-propulsion power plant

• Converts engine torque into propulsive force or thrust

• The advantages of a screw propeller:

light weight, flexibility of application,

good efficiency at high rotative speed, and relative insensitivity to ship motion.

Screw Propeller

Paddle Wheels

• Wheel that carries paddles or “floats” at its periphery

• Rotates about a transverse axis of the ship well above the waterline

• The paddles accelerate the water and experience a reactive thrust that is transmitted to the ship

• Paddle wheel arrangements: Side Wheel & Stern Wheel

• Side Wheelers – effective breadth higher

• Paddle wheels are of two types: Fixed paddles and Feathering paddles

• In the fixed paddle wheel, the paddles are rigidly attached to the wheel along radial lines (larger dia.)

• In the feathering paddle wheel, the paddles are pivoted at the periphery of the wheel (More efficient, heavier, costlier, higher maintenance for linkages)

• The immersion of the paddles varies from 0.1 to 0.8 times the height of the paddle

• Thrust and torque are proportional to the width of the paddle for a given height at constant wheel diameter and rpm, and for a constant ship speed

Paddle Wheel

Feathering Paddle Wheel

Fixed and Feathering Paddle Wheels

Controllable Pitch Propeller

Controllable Pitch Propeller

• The blades are not made integral with the boss but are mounted on separate spindles perpendicular to the propeller shaft axis

• Thus pitch of the propeller blades can be changed

• Astern thrust can be made without changing the revolution rate or reversing the direction of revolution of the propeller

CPP - Advantages• The full power of the machinery can be utilised

in all loading conditions • Provide better acceleration, stopping and

manoeuvring characteristics• Optimum use of propulsion plant at different

operating conditions• Non-reversing propulsion machinery can be

used • speed of the ship may be varied without altering

the speed of the main engine • Produce a higher astern thrust and at a higher

efficiency

CPP - Disadvantages

• The pitch control mechanism is very complicated • High initial and maintenance costs

• Large propeller boss• More vulnerable to damage • Optimum at the design pitch• Blade area limited to enable the pitch to be

reversed and hence requires thicker blade• Thus greater cavitation and more noise • Efficiency at its design point is lower than that of

an equivalent fixed pitch propeller

SCHOTTEL Controllable Pitch Propeller

Nozzle Propeller

Nozzle Propellers

• A screw propeller surrounded by nozzle

• Two types – Accelerating & Decelerating

• Accelerating ducts are used in heavily loaded propellers

• The small clearance between the propeller blade tips and the duct suppresses the trailing free vortices shed by the blades

• Duct develops thrust and also drag, drag substantially less than thrust

• Thrust of propeller + duct > an equivalent open propeller (i.e. one without a duct) whereas the torque is smaller.

• The efficiency of the ducted propeller is therefore greater than that of the open propeller

• This may also be explained by the reduced kinetic energy losses in the slipstream due to the suppression of the trailing vortices and the reduction of the slipstream contraction.

• A decelerating duct decreases the inflow velocity into the propeller

• Therefore the pressure at the propeller location increases and delays cavitation

• The duct generated thrust is negative (pointing aft)

• The efficiency of a ducted propeller with a decelerating duct is lower than that of an equivalent open propeller, but its cavitation properties are superior.

• Decelerating ducts are usually used for high speed vessels where cavitation and underwater noise reduction are vital.

Ducted Propeller

Accelerating and decelerating nozzles

SCHOTTEL Rudder-Propeller (SRP)

Supercavitating Propellers

Typical Blade Sections

Supercavitating Propellers

• Use when unacceptable levels of cavitation cannot be avoided

• Used in ships in which high engine powers, ship speeds and propeller rpms are combined with small propeller diameters and low depths of immersion

• For a supercavitating propeller to work efficiently a proper combination of the advance coefficient J and the cavitation number σ are essential

Recommended field of application for supercavitating propellersZone 1: Best region SCPZone 2: Marginal region with some cavitation on all propellersZone 3: Best region for conventional propellersZone 4: Region of low efficiency for all propellers

Problems with Supercavitating propellers• Propeller blade strength (use titanium alloys, copper-

beryllium alloys and special stainless steels of high strength and hardness)

• Their performance in off-design conditions (SCP works well only at SC condition. It has to pass through a low speed range in which it will not cavitate fully and hence work at low efficiency. As a result, unless there is a very large power margin, the vessel may never get past its low speed range.)

• Solutions: (Promote cavitation development on propeller back by using trip wire behind LE, pump air through an opening in the blade, use two propellers - conventional subcavitating propellers for low speed operation and supercavitating propellers for high speed operation)

Cavity generation at low speeds

Propeller Cavitation

Contra-Rotating Propeller

Contra-rotating Propellers

• Consists of two propellers rotating in opposite directions on coaxial shafts, one propeller being placed close behind the other

• The aim is to reduce the rotational energy losses in the slipstream

• The thrust load is distributed between two propellers so that the efficiency is higher than with an equivalent single propeller, and the propeller diameter and blade area ratio can be reduced

• Disadvantages: Greater weight, complexity of the gearing and coaxial shafts, sealing of the shafting against the ingress of water from outside is also a major problem

• Improvements in efficiency of up to 15 percent can be obtained compared to single screws

• The mechanical complications of fitting contra-rotating propellers are not justified by the increases in efficiency that can be achieved

HYBRID - CONTRA-ROTATING PROPULSION SYSTEM

Why use hybrid contra-rotating propellers?

• Better propulsive efficiency (5~15% increase - recover the slipstream rotational energy of the forward propeller)

• Reduced cavitation (lower propeller loading, propeller diameter can be increased, blade area less and reduce cavitation)

• Reduced hull resistance (When comparing with a traditional twin-screw arrangement we observe that the resistance of the appendages such as propeller,shafts, brackets, skegs, rudders etc. is absent, stern shape improved)

• Improved maneuvering (~ 2x rudder forces)

Tandem Propellers Two propellers on the same shaft and turning in the same direction

SCHOTTEL Combi Drive (SCD)

• Total thrust is divided between the two propellers

• Use when a high thrust requirement is combined with a restricted propeller diameter

• The propellers are usually of the same diameter and have the same number of blades

• At normal thrust loadings, tandem propellers have no significant advantage over equivalent single propellers.

• At high loadings, tandem propellers have higher efficiencies than single propellers and lower propeller induced vibration.

• Have higher rotational energy losses

• Greater weight and cost

• Semi-tandem propellers - the blades of the two propellers are skewed in opposite directions at the inner radii while at the outer radii the blades are in the same line. This arrangement is designed to reduce the effects of non-uniform inflow to the propellers

Podded Propeller

Arrangements/Features

• The propeller is supported in a streamlined body of revolution (pod) by a vertical strut extending downward from the hull of the ship.

• The propeller is driven through a shaft from inside the hull through bevel gears contained within the pod

• The propeller with its pod and supporting strut can be rotated about a vertical axis through 360 degrees by a separate mechanism so that the propeller thrust can be directed at any angle in a horizontal plane (Azimuth Pods)

• In recent years, the Z-drive has been replaced by an electric motor housed within the pod, and the power range has been continuously extended

ADVANTAGES:• Excellent manoeuvrability

• Very good backing performance

• Good speed control over the complete range

• The use of non-reversing machinery

DISADVANTAGES:• Podded propeller units are available only upto a limited

power• The complicated Z-drive and azimuthing mechanism are

complicated

• Possibility of interference between the podded propeller strut and the hull, or between the different podded propeller units, which are often used in pairs

Cycloidal Propellers

Arrangements and Types

• A cycloidal propeller consists of a number of spade like blades fitted to a disc usually set flush with the ship hull

• The disc is made to revolve about a vertical or nearly vertical axis while the blades are made to rotate about their own individual axes through a mechanical linkage system

• Kirsten-Boeing propeller (1928) - the individual blades make half a rotation about their own axes for every revolution of the disc about the propeller axis

• Voith-Schneider propeller (1931) - the blades make one rotation about their own axes for every revolution of the disc

Ship with Twin Cycloidal Propellers

Action of Kristen-Boeing Propeller

Kirsten-Boeing propeller:

• Each blade revolves about the centre O of the propeller while rotating about its own axis A in a manner such that the blade is aligned along the line joining A and a point C

• The resultant of the velocity of advance VA of the propeller and the tangential velocity ωR of the blade produces a hydrodynamic force F which has components parallel and perpendicular to OC

• By varying the position of the point C the thrust can be directed at any angle to the velocity of advance of the propeller

• Each blade undergoes half a rotation about its own axis A for each complete revolution about the centre O

• The distance (eccentricity) is fixed and the magnitude of the thrust can only be varied for a given speed of advance by varying the angular velocity ω = 2πn of the propeller

• The eccentricity of a vertical axis propeller is analogous to the pitch of a screw propeller. The Kirsten-Boeing propeller thus behaves like a fixed pitch screw propeller.

Voith-Schneider Propeller - Actions

Voith-Schneider propeller:• The action of the is similar to that of the Kirsten-Boeing

propeller except that each blade makes a complete rotation about its own axis A for each revolution about the propeller axis O

• The eccentricity OC can be adjusted between zero and a value less than R, so that the magnitude of the resultant thrust of the propeller can be controlled by varying the distance OC

• The direction of the thrust can be controlled by varying the angle between OC and the velocity of advance VA

• VSP with its controllable eccentricity is thus analogous to a controllable pitch screw propeller

Voith-Schneider Propeller Arrangement

VSP Features:• VSP may be driven by an electric motor mounted above the

disc on the vertical axis of the propeller, or through a horizontal shaft and a right-angled bevel gear drive by a diesel engine or electric motor

• The tangential velocity of the blades is usually in the range of 10-20 m per sec, and it may be necessary to provide reduction gearing between the engine and the propeller

• The power transmission also includes elements such as flexible couplings to deal with misalignments, vibration and torque fluctuations

• The blades of a VSP may be made of stainless steel to withstand cavitation erosion and corrosion

• The efficiency of a VSP is quite low. So VSPs are fitted only in those ships in which exceptional manoeuvrability is required

• The direction and magnitude of thrust can be varied without changing the speed or direction of revolution of the engine

• Rudder and steering gear are eliminated

• Hull form of the ship more simplified

• The complex mechanism requiring high maintenance is a disadvantage

• A cycloidal propeller rotating about a transverse horizontal axis fitted at the stern of the ship is known as a “Whale Tail” propeller

The ‘hubless rim-driven propeller’ is a tightly integrated system combining electrical, mechanical and hydrodynamic elements.

There is no conventional hub at the center.

The electric motor takes the form of a thin ring mounted in the tunnel.

The stator carries the permanent magnet rotor in water-lubricated bearings and the rotor is fitted with propeller blades that point inwards. The housing of the stator serves as the nozzle, increasing the flow through the propeller.

Hubless Rim-Driven Propeller

The Ring Motor Assembly

The propeller is electrically driven with a permanent magnet motor.

The stator consists of motor windings.

The rotor has a number of permanent magnets

Rotor

Stator

The advantages of hubless rim-driven propeller

Better Hydrodynamic Performance The thickness of the propeller blade can be reduced, and the distribution of the

radial thickness and the radial load distribution can be modified to enhance the propeller's efficiency

Since it has no hub, central shaft and supporting struts, the water inflow to the propeller is more uniform and undisturbed.

The turbulence is less compared to other conventional propellers. .

Reduced Cavitation Noise Level Cavitation noise is mainly generated at the tip vortices by the pressure differential between the pressure and suction side of the propeller blade.

In hubless rim-driven propellers the blades are attached to rotor ring rather than to the central hub, eliminating the clearance and block the contact between the pressure and suction side of the propeller blade, thereby preventing the generation of the tip vortices.

Free propeller tips in the centre have a far lower speed and are therefore less prone to cavitation.

No Oil Lubrication RequiredThe water flowing between the stator and rotor itself would help to cool the

motor bearings which reduces the maintenance costs.

Reduced Risk of Entangling Since there is no central shaft, support stays, rudder etc. the risk of mooring lines, ropes etc to be caught on the propulsor is less

Easy to RepairThe device can be mountable and dismountable under water so that it can

be removed for servicing if required without dry docking the vessel.

Better Shallow Water NavigationPodded propellers could offer advantages for vessels operating in shallow waters, allowing inland and coastal vessels to keep their propeller fully submerged regardless of load.

Maximum Operational Freedom

The propeller can be made such that it can be freely rotated over 360 deg.

Waterjet Propulsion1. SWIVELLING NOZZLE, 2. REVERSING SCOOP3. STEERING AND REVERSING MECHANISM4. STATOR VANES, 5. IMPELLER, 6. INSPECTION HATCH7. IMPELLER SHAFT, 8. WATER INLET

Waterjet PropulsionConsists of a pump inside the ship which draws water from outside, imparts an acceleration to it and discharges it in a jet above the waterline at the stern. This jet reaction providing the thrust to propel the ship

Advantages:

• There are no appendages and hence there is a reduction in resistance

• Waterjet propulsion can be used in shallow water without any limitation on the size of the pump

• Improved manoeuvrability, stopping and backing ability are obtained

• There is no need to reverse the main engine, i.e. no reversing gear is required in the propulsion plant

• The torque of the waterjet unit is constant over the complete speed range, i.e. full power can be maintained at low speeds without overloading the engine

• The speed of the ship from full ahead to full astern can be controlled without altering the rpm of the engine

• A higher static thrust can be obtained permitting high acceleration to full speed

• There is less noise and vibration

Disadvantages:• The waterjet propulsion unit occupies considerable

space inside the ship, and the water passing through causes a significant decrease in buoyancy

• It is necessary to provide a grating at the water inlet to prevent debris from getting in and damaging the pump. This grating decreases the efficiency of the system, particularly as it gets clogged

• Waterjet propulsion is less efficient than conventional screw propulsion at moderate speeds

• But for high speed craft, waterjets may have a higher efficiency

• Waterjet propulsion should be considered for ships of moderate size having speeds exceeding 25 knots

Magnetohydrodynamic drive

• An electric current is passed through seawater in the presence of an intense magnetic field. Pushing the water out the back accelerates the vehicle.

• MHD is attractive because it has no moving parts, which means that a good design might be silent, reliable, efficient, and inexpensive.

• The major problem with MHD is that with current technologies it is more expensive than a propeller driven by an engine. The extra expense is from the large generator that must be driven by an engine. Such a large generator is not required when an engine directly drives a propeller.

• The first working prototype, the Yamato 1, was completed in Japan in 1991 and was successfully run

Yamato 1

Bow Thrusters