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TitleShip Design Procedure Booklet
File Size998.1 KB
Total Pages66
Table of Contents
                            Introduction
Engineering Organisational Structures
THE CHANGING NATURE OF ENGINEERING DESIGN
CURRENT STUDY
	Generic Engineering Design Process
		Fig.1 A typical mechanical engineering design process
		Using computers for design evaluation
			SW Density
			Basic Ratios
				SW Density
			Basic Ship
				SW Density
	Summary
	Steel Weight Approximation Methods
	Steel Weight Estimation by Empirical Formulae
	Example-1: Steel weight calculation
		For Oil Tankers   (Eq. P2-9c)
	Example: W&O weight calculation
	Water Plane Area (WPA) Variation
		As in Eq. P3-10, when fully loaded the volume of displacement is  , and for any other draught ‘H’, the volume of displacement is:
		
General Guidelines
SUMMARY
	Section
Methodology
	Rudders Design
Example
	Rudder Types
	
	References
Example
Solution
	Thrust on the Propeller Blade
	Stopping Distance “S”
	Stopping Time “t”
	Stopping Distance to Length Ratio
                        
Document Text Contents
Page 1

MARINE DESIGN & TECHNOLOGY
ENRME2020



Edited by:

Faculty of
Technology and Environment


A Saajedi
Mechanical & Marine Engineering Programme Leader
School of Engineering


James Parsons Building, Byrom Street, Liverpool, L3 3AF
t: 0151-231-2378 f: 0151-231 2543
e: [email protected] w: www.livjm.ac.uk

2005-2006

mailto:[email protected]
http://www.livjm.ac.uk/

Page 2

ABSTRACT

Page 33

There are a number of methods for obtaining the steel weight of a ship, such as the
following methods:

Weight per meter method,
Cubic number method
Slog-Slog method
Method of Differences
Computational technique (Modern ship building practice)

Example-1: Steel weight calculation
A general cargo ship is 122 m Lbp, by 16.45 m BMld, by 9.2 m DMld with a catalogued
steel weight of 2700 tonnes. A new similar design is being considered having
preliminary dimensions of 131 m length, 17.08 m breadth, and 10.1 m depth. Estimate
steel weight for the new design after correcting for the main dimensions only.

Solution by Differences Method:
Establish the weight ratios per meter L, B, and D for the basic ship:

For Lbp 81.1885.0*
122
2700

t/m Length

For BMld 27.9055.0*
45.16

2700
t/m Breadth

For DMld 04.8830.0*
2.9

2700
t/m Depth

Adjustments for L, B, and D in new design:

For Length: (131-122)*18.81=169.29 tonnes added

For Breadth: (17.08-16.45)*90.27=56.87 tonnes added

For Depth: (10.1-9.2)*88.04=79.24 tonnes added

Total added steel weight = 305.4 tonnes

New ship steel weight shall be about 2700+305.4 = 3005.4 tonnes

It should be noted that the change in dimensions are not always positive, in some
cases changes in one or more dimensions may be negative or even zero.

Tutorial:
A basic vessel has 135 m length, 18.3 m breadth, and 10 m depth with steel weight of
3470 tonnes. A new design for general cargo is being considered having length of
136.8m, B=19.1 m, and D = 9.8m. Estimate the steel weight for new design after
correcting for the main dimensions.



Other corrections for Steel Weight calculation
After modifying for the main dimensions only, further modifications will be required
for slight differences in the steel structures between the basic ship and the new design.
Generally, the modification for the main dimensions only gives the biggest changes in
steel weights. Other modifications shall include the changes between the hull forms
(Cb values at full draught), scantling changes, and the fwd and aft shears of the main
deck

Page 34

Cb Correction
This is carried out as follows:

0.5% for each 0.001 change in the Cb at fully loaded draught.

Example-1a:
Reconsider example-1 where the steel weight for the new design after correcting for
dimensions only was 3005.4 tonnes. Supposing that the Cb values are 0.725 and 0.74
for the basic and new ship respectively, when fully loaded. Calculate the “Form” or
“Cb” correction.

SteelNew
bBasicbNew

b W
CC

CCorrection *%5.0*
001.0

_

234.3005*
100

5.0
*

001.0
725.0740.0

_ bCCorrection tonnes

Scantling Correction
This can be taken as a fraction of the dimensional corrections. It is really a correction
for the difference in proportions of the main dimensions. Feedback from ships already
built indicates that the scantling corrections should be:

ctionDepthCorre

rectionBreadthCor

ectionLenghtCorr

*
2
1

*
4
1

*
3
1

Scantling Correction ratios

Example-1b:
Reconsider example-1 where the length, breadth, and depth corrections were 169.29,
56.87, and 79.24 tonnes respectively. The scantling corrections will be determined as:

)*
2
1

()*
4
1

()*
3
1

( correctioncorrectioncorrection DBLorrectionScantlingC

110)24.79*
2
1

()87.56*
4
1

()29.169*
3
1

(orrectionScantlingC tonnes (approx.)

Shear Correction














Figure P2-1 Shear definition

D

Lbp
Fore Peak Aft Peak

Shear Aft Shear FWD

Page 65

present no advantage for ships with Cb values between 0.625 and 0.725 unless
there are “over driven” according to Watson/Gilfillan criterion;

are again advantageous for Cb values between 0.725 and 0.825, but probably not
for C values over 0.825. b
It is worth emphasising that overall economy may require a balance between
designing for optimum performance fully loaded and in the ballast condition.

A bulbous bow will generally help to reducing pitching, ot on the other hand it is
more likely to cause slamming.


Figure P4-2 Various Bow Line Configurations

There are a number of configurations as shown in figure P4-2. It should be noted that
in most cases the cutting edge of the bow would be cylindrically curved, and in some
cases, it would be shaped sharpened.

Bulbous bows are fitted on ships for the following reasons:

1. To increase the speed for the same power if the ship speed at loaded draught.

2. To reduce vibrations at the fore end of the ship, due to the existence of extra
steel in the structure.

3. Extra steel in the structure shall produce extra strength in the forepeak tank.

4. To reduce pitching due to bow shape effect on the water flow around the bow.

5. The bulbous bow shape can cause the thin ice to be broken ahead of the ship.



Ship Design Procedure Booklet 65

Page 66

Stern Section Shape
The stern lines have to be considered in relation to the following roles:

The accommodation of the propeller(s) with good clearances that will avoid
propeller excited vibration problems;

The provision of good flow to the rudder(s) to ensure both good steering and good
course stability;

The termination of the ships waterlines in a way that minimises separation and
therefore resistance;

The termination of the ships structure in a way that provides the required support
for the propeller(s) and rudder(s) plus necessary space for steering gear, stern mooring
and towage equipment etc. and is economical to construct.

Flow to the Propeller

Where the propeller diameter (D) on a single-screw ship is of normal size in relation
to the draught (H), i.e. D/H is approximately 0.75, the main consideration is ensuring
good flow to the propeller, with a figure of 28 to 30o being about the maximum
acceptable slope of a waterline within the propeller disc area. Keeping to such a figure
tends to force the longitudinal centre of buoyancy (LCB) forward on a full-bodied
ship. Lloyd’s recommends minimum clearances as a fraction of the propeller
diameter for a four-bladed propeller are:

Tip to stern-frame arch =1.00 * K

Stern-frame to leading edge at 0.7R =1.5 * K

Trailing edge to rudder at 0.7R = 0.12

Tip to top of sole piece = 0.03

3.0
*56.2

3050
1.0

2L
PCL

K bWhere & P = power in kW

The recommended clearance for four-bladed propeller on a twin-screw ship, is
1.00*K. Other values are given in the rules for three, five, and six-bladed propellers.

Ship Design Procedure Booklet 66

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