##### 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

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