Naval Architecture

Faculty of Maritime Studies – Marine Engineering Department

Faculty of Maritime Studies – Marine Engineering Department

Lecture 9: Ship Propellers

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SHIP PROPELLERS

IntroductionA rotating propeller sucks water into itself and discharges it in a well defined slipstreams immediately behind the propeller. This action results in an increase in the fluid pressure due to the rotation of the blades.

The effect of this is to increase the velocity of the mass of the water in the slipstream. The change of momentum in this mass provides the propeller thrust. The propeller is turned by the propulsion machinery.

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Screw Propellers

Basic Nomenclature:

Faculty of Maritime Studies – Marine Engineering Department

HUB

ROOT

BLADE TIP

TIP CIRCLE

ROTATION

LEADING EDGE

TRAILINGEDGE

PRESSUREFACE

SUCTIONBACK

PROPELLERDISC

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Screw Propellers

Basic Nomenclature:

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Screw Propellers

Definitions:

• Pitch -Theoretical distance a propeller would move in one

revolution

• Tip Circle- The circle described by the blade tip rotation

• Propeller Disc- The area circumscribed by the propeller’s tip

circle

• Leading Edge- Forward edge of the blade, first to encounter the

water stream

• Trailing Edge- Last part of the blade to encounter the water

stream

• Pressure Face- The high pressure side of the propeller blade (the

back side as the propeller advances)

• Suction Back- The low pressure side of the propeller blade (the

front side as the propeller advances)Faculty of Maritime Studies – Marine Engineering Department

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Propeller Pitch

Faculty of Maritime Studies – Marine Engineering Department

The pitch of a propeller is defined as the distance that the propeller would “drive forward” in one revolution and is denoted “P” .

Dia

met

er

Pitch

Hub

Typically blades are twisted to guarantee constant pitch along the blades from root to tip.

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Propeller Pitch Angle

The pitch angle f relates the pitch length to the circumference of the propeller blade.

tan f = P 2pr

fr𝒑

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Types of Screw Propellers w.r.t Pitch

Generally, Screw Propellers are of two types: Fixed Pitch Propellers (FPP) and Controllable or Variable Pitch Propellers (CPP).

A FPP consists of fixed blades. This means that the position of blades cannot be changed. On the contrary, CPP can move its blade in the desired position by changing the pitch of the blades.

FPP CPP

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In FPP the power generated by the engine and the propulsive forces produced by the propeller cannot be controlled. This leads to high amount of power wastage and increased stresses on the propeller. But in a CPP all these can be prevented by just changing the pitch of the propeller. This function of CPP makes it an integral part of the propulsion system of a ship.

A unique aspect of a CPP is that the propeller rotates in only one direction, unlike FPP. Thus there is no need of a reverse clutch, which is an integral part of FPP for producing reverse thrust, in case the ship needs braking or reversing.

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Faculty of Maritime Studies – Marine Engineering Department

Right and Left Handed Propellers

A right-handed propeller rotates clockwise when propelling a vessel forward, as viewed from the stern of the ship. A left-handed propeller rotates counter-clockwise, as viewed from the stern, when in a forward propulsion mode.

LHP RHP

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Faculty of Maritime Studies – Marine Engineering Department

Right and Left Handed Propellers

When viewing a propeller from astern, the leading edges of the blades will always be farther away from you than the trailing edges. The propeller rotates clockwise, and is right-handed, if the leading edges are on the right. A propeller’s handedness is fixed. A right-handed propeller can never be exchanged with a left handed propeller, and vice versa.

Most single screw vessels have right-handed propellers and clockwise rotating propeller shafts (as viewed from astern).

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Right and Left Handed Propellers

Single propellers tend to naturally push the vessel to one side when going forward (and the opposite side when in reverse), i.e., a right-handed propeller will push the stern to starboard when in forward (and port when in reverse). Since Propellers are not ideally designed for reverse propulsion, this effect is somewhat exaggerated when operating a single-screw vessel in reverse. Twin-screw vessels have counter rotating propellers with identical specifications. The port (left) side propeller is usually left-handed and the starboard (right) side propeller is usually right-handed.

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Skewed Propellers

A marine propeller whose blades are in the form of scimitars, typically with the tip of one blade aligning radially with the root of the following blade is called “Skewed Propeller”.Propeller skew is the single most effective design parameter which has significant influence on reducing propeller induced vibration without sacrificing the efficiency.

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Highly Skewed Propellers

• Reduce interaction between propeller and rudder wake

• Reduce vibration and noise

Advantages:

Disadvantages:

• Expensive• Less efficient when operating in reverse

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Propeller Theory

The ship drags the surrounding water so that the wake to follow the ship with a wake speed (VW) is generated in the stern.The wake speed at the propeller (VW) is less than the ship speed (VS). The difference is called the Speed of Advance.

Q

PWake Region

SV WV

0waterV

Swater VV

VA = VS - VWSpeed of Advance

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Propeller Theory

Propeller efficiency is the ratio between the Thrust Power and the Delivered Power, i.e.,

Propeller Efficiency, hP:

DHP

THPP

TP C

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2

where CT is the Thrust Loading Coefficient, given

by:

oAT

AV

TC 25.0

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Propeller Theory

oAT

AV

TC 25.0

where: T is the Propeller Thrust, (kN) Ao is the Projected Area of Propeller Disc, (m2)

is the Water Density, (Tonne/m3) VA is the Speed in Advance, (m/s)

So, for a given Thrust, T

as Ao , the CT and the

In other words, the larger the propeller diameter, the better the propeller efficiency.

(~70 % for a well-designed propeller)

P

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Propeller Cavitation

The formation and subsequent collapse of vapor

bubbles on propeller blades where pressure has

fallen below the vapor pressure of water.

Cavitation occurs on propellers that are heavily

loaded, or are experiencing a high thrust loading

coefficient.

Definition of Cavitation :

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Propeller Cavitation

Types of Cavitation :

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Faculty of Maritime Studies – Marine Engineering Department

Propeller Cavitation

Types of Cavitation :

Blade Tip CavitationFlow velocities at the tip are fastest so

that pressure drop occurs at the tip first.

Face Sheet CavitationLarge and stable region of cavitation

covering the suction face of propeller.

Consequences of Cavitation:

1. Low propeller efficiency, i.e., thrust reduction

2. Propeller erosion

3. Vibration due to uneven loading

4. Cavitation noise due to impulsion by the

bubble collapse