principles of shiphandling_ basic propeller types

8/18/2019 Principles of Shiphandling_ Basic Propeller Types

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4/1/2016 Principles of shiphandling: Basic Propeller Types

http://principlesofshiphandling.blogspot.in/p/propeller-types.html

Defines Shiphandling and Maneuvering. Also describes the Basic movement of a ship and talks about

the factors and forces affecting shiphandling.

Principles of shiphandling

► Propeller ► Hydraulic ► Ship Engine ► High Speed Ads by Google

Ship handling Pivot point Basic Propeller Types

Basic Propeller Types

Controllable Pitch Propellers

These propellers now have a relatively good track record for

reliability and are becoming increasingly common on a wide range of

tonnage. Unlike the fixed pitch propeller, the blades of these

propellers can be altered, to set whatever pitch is required, across the

whole power band from full ahead to full astern. This is usually

achieved with hydraulic pumps or pistons, the older mechanical

systems being less able to cope with the size and speeds of the

modern vessel. The hydraulic pumps themselves are activated by an

electric motor, which is in turn operated by remote bridge control.This may be either pneumatic or electronic. In small craft and in

some older systems the bridge control may be mechanical, using

cables linked direct to hydraulic rams, but these are becoming

increasingly rare.

To use a controllable pitch (CP) propeller the main engine has to be

clutched in, so that the propeller is continuously turning, usually at

quite high revolutions. As it is neither practicable, nor economical, to

run an engine continuously at excessively high rpm, it is important to

have some sort of combined control over both rpm and pitch, so that

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the pitch for slow speeds is balanced with a reduction in revolutions.

On most ships this is achieved by installing a 'combinator' which

automatically balances engine revolutions against propeller pitch,

thus producing a saving in fuel and better propeller performance.

Fixed or conventional Propellers

These propellers are mounted on a propeller shaft that turns on

“demand” . Speaking for its type, the blades on this type of propeller do not move and the pitch is fixed. The shaft changes the direction of

its rotation to effect a motion either ahead or astern depending on the

type of the blade.

Movements

Ahead movement

To use ahead power, a ship with a CP propeller is not restricted to the

stepped progression, through dead slow ahead to full ahead, that hasfor some, been long associated with fixed pitch propellers. Any

speed can be selected, simply by adjusting the combinator control to

the required setting. It is also possible to set the propeller pitch for

extremely low speeds, so that on these occasions when it is essential

to proceed at very slow speeds, the propeller and rudder are still

active and steerage way can be maintained for a lot longer than is

usual. This is particularly advantageous if compared to the many

ships with fixed pitched propellers, where the speed for dead slow

ahead can sometimes be as high as five or six knots, due to excessive

engine revolutions and/or stalling speed of the engine.

Slowing Down

When 'low speed' or 'stop' are demanded, the blades of the CP

propeller are set with a very fine angle and pitch, to the extent that

when they are viewed from astern, they will appear to open like a fan

and the propeller will look rather like a closed disc or wheel. If the

ship's speed is too high and does not already match the propeller

speed, the flow of water through it will be restricted and a great deal

of turbulence will develop behind the propeller, which will also have

an adverse effect upon the rudder (see illustration below).


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The rudder shielded with the propellers fine pitch setting.

If therefore, the ship's speed is not reduced slowly and progressively,

in much the same way as a large directionally unstable ship, the

rudder will be shielded and the steering may become erratic or poor.

Whilst it is irritating if the steering is poor, it should not be forgotten

that CP propellers are instantly available for corrective 'kicks ahead',in a virtually unlimited supply, and are not liable to the restrictions

that can be experienced with fixed pitch propellers, such as limited

air supplies for starting the engine or delays whilst the engine is put

astern.

Transverse Thrust One of the most common concerns mentioned by

many officers, and quite rightly so, is the uncertainty as to which

way the bow will cant, if at all, when a controllable pitch (CP)

propeller is put astern. This is also something the pilot needs to know

when he comes on board. To answer this question, it is firstnecessary to know which way the propeller is turning when it is

viewed from astern. With the majority of CP propellers it is in an

anti-clockwise direction and they are called left-handed. It is

important however, when informed that a CP ship is left-handed, that

it is not confused with a fixed pitch left-handed ship, because the CP

propeller, it should be remembered, rotates the same way all the

time. When the pitch is set for stern power, it is only the angle of the

blades that has changed and the propeller is still rotating

anticlockwise or left-handed. The effect is now similar to a fixed

pitch right handed propeller working astern. The flow of water through the propeller is directed up onto the starboard quarter and

may be strong enough to thrust the stern to port, so that the bow is

seen to cant or 'kick' to starboard.

It is important to note that the transverse thrust on some ships with a

CP propeller may be weak and unreliable, due to vortices or

turbulence around the propeller blades. This is usually the result of

specific design limitations and might, for example, occur when a CP

propeller is designed to operate at slow speeds, with fine pitch


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settings, but high shaft revolutions. In another example, if the pitch is

altered from ahead to astern, some of the inner or lower sections of

the blades may be set at rather crude angles and this, too, can create

vortices around the propeller. These characteristics can also have a

detrimental effect on the maximum stern power available when

compared to a similar ship with a fixed pitch propeller. It is therefore

advisable to exercise some caution, when anticipating the effects of

stem power on some CP ships.

Stopped and alongside

On some ships, due to inferior design, age or poor maintenance,

when the control stick on the bridge is positioned for stop with zero

pitch, the propeller might not be accurately aligned to the same

setting and some residual thrust may still exist. Although, with

experience, a ship's master can compensate for this error, it is still

imperative to remember, that even though 'stop' has been requested, a

ship with a CP propeller can unexpectedly creep ahead or astern.

This is very important during the period prior to slipping from a

berth, when the engine is started and the shaft is clutched in. If the

ropes are not kept tight, particularly while singling up, the vessel

may start to move along the dockside with serious consequences if

other ships are tied up close ahead or astern. It is also noticeable on

some ships that the CP propeller, which is constantly running with

high revolutions, can pump quite a lot of water up onto the quarter

and along the ship's side, even with zero pitch set. In the case of a left

handed CP propeller this water will be flowing along the starboard

side.

If then, the ship is laying alongside a solid dockside starboard side to

and the ropes are slackened off, this can act like a tide flowing from

astern and push the stern away from the dockside (see below). It can

also make it very difficult to get the stern alongside when berthing,

particularly when coming into a berth stern first and there has been a

need to use stern power as well. This might not be resolved, until

either a stern line is ashore and tight, or the revolutions are reduced.

Creating eddies and currents on zero pitch.


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