Ship Propulsion System Lecture 1Dr: Aly Hassan Elbatran

2

Introduction Ship propulsion system is a part of marine engineering which concerned

by the design and/or selection of main propulsion plant equipments and

machineries.

The main role of this plant is to:-

Produce enough power to overcome the ship resistance

Generate the needed electric power for the various applications

onboard the ship (lighting, control systems, pumps, navigation

equipments, HVAC, etc).

Propulsion system components

3

The main components of a propulsion system are shown on the next diagram:

Prime mover:

The function of the prime mover is to deliver mechanical energy to the propulsor. The prime mover

may be one of the following:

• Diesel engine

• Gas turbine

• Steam turbine

• Electric motor

4

- The diesel engine is the most common prime mover in the merchant marine, mainly due to

its low fuel consumption in comparison with other prime movers.

- Some ship types, such as naval vessels and LNG carriers may have a steam turbine as

propulsion engine. Two kinds of steam plants can be distinguished in marine applications:

Fossil-fired steam plants are frequently found on board naval vessels and LNG

carriers.

Nuclear steam plants are used for Submarines, aircraft carriers and some commercial

ice-breaking vessels especially in Russian arctic areas were provided with nuclear

power plants since these vessels may stay for months in sea.

Propulsion system components

Prime mover

5

- Gas turbines find their application in fast and advanced ship types and naval vessels.

The power to weight ratio of gas turbines is higher than that of diesel engines.

- Electric motors found their way as prime mover in the 90’s; they are used with

electric generation plant combined of an engine (one of the above types) and an

electric generator. They are mainly found in advanced passenger ships, some new

designs of offshore support vessels (OSV) are intended to use electric motors

especially for dynamic positioning applications.

Propulsion System Components

Prime mover

6

Propulsion System Components

Transmission

Transmission is a sub-system of the propulsion system. The transmission’s functions are:

1. To transfer the mechanical energy generated from the prime mover to the propulsor.

2. To transfer the thrust generated by the propulsor to the ship’s hull

It is such as shafts, gearboxes and bearings.

Two types of transmission are used:

Direct: the prime mover is coupled directly, through a shaft to the propulsor (this is the case

with low speed diesel engines).

Geared: the prime mover delivers its energy through a gearbox and a shaft to the propulsor.

The function of the gearbox is to reduce the rotational speed of the engine to match the

desired rotational speed of the propulsor.

7

Propulsion System Components

Proplusor

The propulsor converts the rotating mechanical power delivered by the engine

into translating mechanical power to propel the ship.

The most common propulsor is the propeller. In general, two types of propeller

are distinguished, fixed pitch and controllable pitch propellers. Other types of

propulsors are for example, waterjets and Voith-Schneider propulsors (vertical

axis propeller).

8

Vs

T

RT

Ship

Propeller

The main two forces considered in propulsion system; the resistance of

the water to the ship motion (R) and the thrust developed by the propeller (T).

The Ship Drive Train

9

10

- Brake horsepower (BHP) is the power produced by the

ship’s prime mover.

- Shaft Horsepower (SHP) is the power output the reduction

gears (if installed).

- Delivered Horsepower (DHP) is the power delivered by

the shaft to the propeller.

- Thrust (Developed) Horsepower (THP) is the power

produced by the propeller’s thrust.

- Effective horsepower The horsepower required to move

the ship’s hull at a given speed in the absence of propeller

action.

The Ship Drive Train

11

Shafting efficiency (1% ~ 2%)

ηGB = gearbox efficiency (1% ~ 3%)

Propeller open water efficiency (30% ~ 60%)

EHP = effective horsepower

12

The Ship Drive Train

13

The Ship Drive Train

Once the ship’s effective horsepower has been determined, it is now necessary to

relate EHP to the power produced by the drive train. This is done by relating the power

required to tow the ship through the water (EHP) to the power produced by the

propeller (THP). The ratio of effective horsepower to thrust horsepower is called the

hull efficiency (ηh), and is defined as:

Hull Efficiency

14

The Ship Drive Train

The propulsive efficiency is the ratio of effective horsepower to shaft horsepower,

therefore allowing the designer to make a direct determination of the shaft

horsepower required to be installed in the ship. Common values of propulsive

efficiency typically range from 55% to 75%.

The Ship Drive Train

15

QPC = Quasi-Propulsive Coefficient

The thrust developed by the propeller is linked to the ship resistance by the following formula:

R = T . (1 – t)t = thrust deduction fraction

The thrust deduction fraction is a parameter related to the ship design and it is related to another parameter which is the wake fraction ω.

The Ship Drive Train

16

1. Model testing has determined that a ship has an EHP of 30,000 HP at a

speed of 19 knots. Assuming a propulsive efficiency of 70%, what SHP is

required to be installed to achieve 19 knots?

2. Given the following ship particulars, find the required engine brake power.

Ship speed V=20 knots

Thrust=40 tonnes

Wake fraction ω=0.3

Thrust deduction fraction t=0.6*ω

Quasi-propulsive coefficient QPC=0.68

Transmission efficiency ηt=0.95

Examples