Modern propulsion Techniques

Ships and boats have developed alongside mankind. In major wars, and in day to day life, they have become an integral part of modern commercial and military systems.
In 950 BC, the Egyptians used a screw-like device for irrigation purposes. Archimedes (287-212 BC), the first scientist whose work had a lasting effect on ship propulsion is credited with the invention of the screw. His screw pump, created to pump out flooded ships and for supplying water to irrigation ditches, was the forerunner of the screw propeller.
A Propeller is essentially a type of fan which transmits power by converting rotational motion into thrust for propulsion of a vehicle such as an aircraft, ship, or submarine through a mass such as water or air, by rotating two or more twisted blades about a central shaft, in a manner analogous to rotating a screw through a solid. The blades of a propeller act as rotating wings and produce force through application of both Bernoulli’s principle and Newton’s third law, generating a difference in pressure between the forward and rear surfaces of the airfoil-shaped blades and by accelerating a mass of fluid rearward.
A Propeller is the most common propulsor on ships, imparting momentum to a fluid which causes a force to act on the ship. The ideal efficiency of a propeller is that of an actuator disc in an ideal fluid. An actual marine propeller is made up of sections of helicoidal surfaces which act together ’screwing’ through the water (hence the common reference to marine propellers as ”screws”). Three, four, or five blades attached to a boss (hub) are most common in marine propellers.

CONTROLLABLE PITCH PROPELLER

CONTROLLABLE PITCH PROPELLER

Propulsion systems for ships and boats vary from the simple paddle to the largest diesel engines in the world. These systems fall into three categories: human propulsion, sailing, and mechanical propulsion. Human propulsion includes the pole, still widely used in marshy areas, rowing which was used even on large galleys, and pedals. Propulsion by sail generally consists of a sail hoisted on an erect mast, supported by stays and spars and controlled by ropes. Mechanical propulsion systems generally consist of a motor or engine turning a propeller. There are many variations of propeller systems like single screw, twin screw, contra-rotating, controllable-pitch, and nozzle-style propellers etc.

Some Modern Propulsion Techniques

Conventional propellers are still very popular because of their simplicity as well as easy availability and cost/maintenence advantages. However, innovative designs that offers improved performance in terms of propeller efficiency and maneuverability are gaining currency, particularly for smaller and special purpose vessels.
Contra-rotating propellers, also referred to as coaxial contra-rotating propellers, apply the maximum power of a single piston or turboprop engine to drive two propellers that rotate in opposite direction ie. one clockwise and the other anti-clockwise. Contra-rotating propellers are common in some marine transmission systems, in particular for medium to large size planing leisure crafts. Two propellers are arranged one behind the other, and power is transferred from the engine by means a planetary gear transmission.

CONTRA-ROTATING PROPELLERS

The advantages are that:

(a) Propeller-induced heeling moment is compensated (negligible for larger ships)
(b) More power can be transmitted for a given propeller radius and
(c) The propeller efficiency is usually increased.

An Azimuth thruster is a configuration of ship propellers placed in pods that can be rotated in any horizontal direction, making a rudder unnecessary. These give ships better maneuverability than a fixed propeller and rudder system. Primary advantages are electrical efficiency, better use of ship space, and lower maintenance costs. Ships with azimuth thrusters do not need tugs to dock, though they still require tugs to maneuver in difficult places.

A Controllable Pitch Propeller (CPP) or Variable Pitch Propeller is a special type of propeller with blades that can be rotated around their longitudinal axis to change their pitch. If the pitch can be set to negative values, the reversible propeller can also create reverse thrust for braking or going backwards without the need of changing the direction of shaft rotation. CPP for marine propulsion systems have been designed to give the highest propulsive efficiency for any speed and load condition. A correctly sized controllable pitch propeller can be efficient for a wide range of rotational speeds, since pitch can be adjusted to absorb all the power that the engine is capable of producing at nearly any rotational speed. The CPP also contributes to the improved maneuverability of a vessel.
A Bow thruster is a propulsion device built into, or mounted to, the bow of a ship or boat to enhance its maneuverability. Bow thrusters make docking easier, since they allow the Captain to turn the vessel to port or starboard without using the main propulsion mechanism which requires some forward motion for turning. An impeller in the tunnel can create thrust in either direction which makes the ship turn. Most tunnel thrusters are driven by electric motors, but some are hydraulically powered. These bow thrusters, also known as tunnel thrusters, may allow the ship to dock without the assistance of tugboats, saving the costs of such service. For vessels with Dynamic Positioning Systems (DPS) a number of side thrusters are often employed.

BOW THRUSTER

The Voith Schneider propeller (VSP), also known as a cycloidal drive is a specialized marine propulsion system. It is highly maneuverable, being able to change the direction of its thrust almost instantaneously. It is widely used on tugs and ferries. From a circular plate, rotating around a vertical axis, a circular array of vertical blades (in the shape of hydrofoils) protrude out of the stern of the ship toward bottom. Each blade can rotate itself around a vertical axis. The internal gear changes the angle of attack of the blades in sync with the rotation of the plate, so that each blade can provide thrust in any direction, very similar to the collective pitch control and cyclic in a helicopter.

VOITH SCHNEIDER PROPELLER(VSP)

The Kort nozzle is a shrouded, ducted propeller assembly for marine propulsion. The hydrodynamic design of the shroud, which is shaped like a foil, offers advantages for certain conditions over bare propellers. Kort nozzles or ducted propellers can be significantly more efficient than unducted propellers at low speeds, producing greater thrust in a smaller package. Tugboats are the most common application for Kort nozzles as highly loaded propellers on slow moving vessels benefit the most. Kort nozzles may be fixed, with directional control coming from a rudder set in the water flow, or pivoting, where their flow controls the vessel’s steering. Shrouding of this type is also beneficial to navigation in ice fields since it protects the propeller tips to some extent.
An advanced type of propeller used on German Type 212 submarines is called a skewback propeller. As in the scimitar blades used on some aircraft, the blade tips of a skewback propeller are swept back against the direction of rotation. In addition, the blades are tilted rearward along the longitudinal axis, giving the propeller an overall cup-shaped appearance. This design preserves thrust efficiency while reducing cavitation, and thus makes for a quiet, stealthy design.

KORT NOZZLE

An innovative Rolls-Royce propulsion system, promoted for single-screw vessels operating up to 17 knots, improves propulsive efficiency and fuel consumption. The system comprises a ’bulb hubcap twisted rudder’ and uses a combination of features designed to smooth the flow of water from the propeller as it passes over the rudder.
The combination of a tapered hubcap, a bulb on the rudder and a spade rudder with a twisted leading edge profile increases propulsive efficiency, cutting fuel consumption significantly.

ROLLS-ROYCE PROPULSION SYSTEM

Maneuvering

Dynamic positioning (DP) is a computer controlled system to automatically maintain a vessel’s position and heading by using her own propellers and thrusters. Position reference sensors, combined with wind sensors, motion sensors and gyro compasses, provide information to the computer pertaining to the vessel’s position and the magnitude and direction of environmental forces affecting its position.

BILGE KEELS

A Bilge keel is a strake, or small keel or blister, running along much of the length of the hull. It is used to reduce the hull’s tendency to roll. They are typically fitted one on each side, low down on the side of the hull, so as not to increase the draft of the vessel. A bilge keel is often in a ”V” shape, welded along the length of the ship at the turn of the bilge. Bilge keels are employed in pairs (one for each side of the ship). Bilge keels increases hydrodynamic resistance to rolling, making the ship roll less. Bilge keels are passive stability systems.
Ship Stabilizing Gyroscope is a technology developed in the 1800s and early 1900s and used to stabilize roll motions in ocean going ships. However, more recently (since the 1990s) a growing interest in the device has reemerged for low speed roll stabilization of vessels. The ship gyroscopic stabilizer typically operates by constraining the gyroscope’s roll axis and allowing it to ”precess” either in the pitch or the yaw axes. Allowing it to precess as the ship rolls causes its spinning rotor to generate a counteracting roll stabilizing moment to that generated by the waves on the ship’s hull. Its ability to effectively do this is dependent on a range of factors that include its size, weight and angular momentum.

Outriggers may be employed on certain vessels to reduce rolling. Rolling is reduced either by the force required to submerge buoyant floats or by hydrodynamic foils. In some cases these outriggers may be of sufficient size to classify the vessel as a trimaran, however on other vessels they may simply be referred to as stabilizers.

Active Fin Stabilizers are normally used to reduce the roll that a vessel experiences while under way. The fins extend beyond the hull of the vessel below the waterline, and alter their angle of attack depending upon heel angle of the vessel. While the typical ”active fin” stabilizer will effectively counteract roll for ships under way, some active fin systems have been shown capable of reducing roll motion when vessels are not under way. Stabilization at Rest, these systems work by moving fins of special design, with the requisite acceleration and impulse timing to create effective roll cancellation energy.
Engine manufacturers and ship builders are promoting a variety of innovative propulsion arrangements, each offering different combinations of fuel efficiency, environmental benefit, maintainability and better maneuverability. With improvement in technology better propulsion devices and techniques are set to take centre stage in propulsion of future vessels.

ACTIVE FIN STABILIZERS