HNC NAUTICAL SCIENCEHNC NAUTICAL SCIENCEGroup Award Code: G8F5 15Group Award Code: G8F5 15

Unit Code: F0LD 34 Ship StabilityUnit Code: F0LD 34 Ship Stability

Outcome 1 - HydrostaticsOutcome 1 - Hydrostatics

1.2 Basic Stability 1.2 Basic Stability DefinitionsDefinitions

AimsAims

To give the student an understanding of:To give the student an understanding of:

Basic Stability Definitions relating to Basic Stability Definitions relating to Loadlines.Loadlines.

Achimedes Principles 1 & 2Achimedes Principles 1 & 2

ObjectivesObjectives

The Student will be able to:The Student will be able to:define define Draught, Loaded Draught, Draught, Loaded Draught, Waterline Length, Waterline Length, Breadth, Breadth, Volume of Displacement, Volume of Displacement, Displacement, Displacement, Light, Load and Summer Load Displacements, Light, Load and Summer Load Displacements, Deadweight and Summer Deadweight;Deadweight and Summer Deadweight;

ObjectivesObjectives

The Student will be able to:The Student will be able to:

State the first two Archimedes PrinciplesState the first two Archimedes Principles

calculate underwater volume, volume of calculate underwater volume, volume of displacement for a box shaped vessel, displacement for a box shaped vessel,

Calculate the displacement of a box Calculate the displacement of a box shaped vessel.shaped vessel.

Draught (d)Draught (d) Depth of the vessel Depth of the vessel

from keel to waterline.from keel to waterline. The hull is marked at The hull is marked at

the bow, stern and the bow, stern and amidships.amidships.

Marks are 1dm high Marks are 1dm high with a gap of 1dm with a gap of 1dm between each markbetween each mark

The reading is taken The reading is taken from the bottom of the from the bottom of the markmark

40

20

4M

80

60

40

20

3M

80

60

Loadline MarkLoadline Mark

Marked on the hull Marked on the hull amidshipsamidships

Top of Horizontal line Top of Horizontal line indicates deepest indicates deepest loaded condition loaded condition (Summer Zone)(Summer Zone)

Load Draught is the Load Draught is the draught of the vessel draught of the vessel when loaded to the when loaded to the loadline markloadline mark

Loadline MarkLoadline Mark

Loadline Mark is also Loadline Mark is also known as the Plimsoll known as the Plimsoll Line.Line.

Note that the top of Note that the top of the Loadline Mark the Loadline Mark lines up with the top of lines up with the top of the Summer Loadlinethe Summer Loadline

Note also where the Note also where the boot-topping area boot-topping area ends.ends.

Loadline MarkLoadline Mark

Plimsoll Line named Plimsoll Line named after Samuel Plimsoll.after Samuel Plimsoll.

Born in Bristol in Born in Bristol in 1824.1824.

Became involved in Became involved in shipping coal to shipping coal to London.London.

Research into coal Research into coal trade caused him trade caused him concern for the safety concern for the safety of seafarersof seafarers

Loadline MarkLoadline Mark

Became member of Became member of parliament in 1868.parliament in 1868.

1871 Merchant Shipping 1871 Merchant Shipping Act made it an prisonable Act made it an prisonable offence NOT to sail on an offence NOT to sail on an unsafe ship. unsafe ship.

Plimsoll fought this.Plimsoll fought this.

Loadline MarkLoadline Mark 1875 introduced the 1875 introduced the

Unseaworthy Vessels Act.Unseaworthy Vessels Act. 1876 Merchant Shipping 1876 Merchant Shipping

Act was amended to Act was amended to provide for a line to be provide for a line to be painted in the ships side. painted in the ships side.

1877 Merchant Shipping 1877 Merchant Shipping Act was amended again to Act was amended again to impose weight limits for impose weight limits for vessels, engagement of vessels, engagement of seamen, and seafarers seamen, and seafarers accommodation. accommodation.

Loadline MarkLoadline Mark

History Lesson Over!History Lesson Over!

DefinitionsDefinitions

Loaded DraughtLoaded Draught – – the draught when the the draught when the vessel has been vessel has been loaded to the Loadline loaded to the Loadline Mark.Mark.

DefinitionsDefinitions Waterline LengthWaterline Length – the length of the vessel in – the length of the vessel in

metres, measured at that draught and trim, from metres, measured at that draught and trim, from where the waterline cuts the aftermost part of where the waterline cuts the aftermost part of the vessel to where the waterline cuts the the vessel to where the waterline cuts the foremost part of the vessel.foremost part of the vessel.

DefinitionsDefinitions BreadthBreadth– The extreme breadth of the vessel – The extreme breadth of the vessel

measured in metres, measured in metres,

i.e. The breadth of the vessel at it’s widest point.i.e. The breadth of the vessel at it’s widest point.

V/2V/2

Archimedes LawArchimedes Law The The volume of liquid volume of liquid displaced by a floating displaced by a floating

body is equal to the underwater volumebody is equal to the underwater volume

V

W/2W/2

Archimedes LawArchimedes Law The The weight of liquidweight of liquid displaced by a floating displaced by a floating

body is equal to the weight of the bodybody is equal to the weight of the body

W

Volume of DisplacementVolume of Displacement

Equal to the Equal to the underwater volumeunderwater volume

Denoted by the Denoted by the Symbol Symbol

For a box shaped For a box shaped vessel, vessel, = L x B x d= L x B x d

Draught

Breadth

Length

DisplacementDisplacement Displacement (Displacement (ΔΔ) is measured in tonnes is equal to the ) is measured in tonnes is equal to the

weight of the vessel and also the weight of the water weight of the vessel and also the weight of the water displaced.displaced.

Mass (weight)Mass (weight) == ρ ρ VolumeVolume

DisplacementDisplacement ( (ΔΔ)) = = ρ ρ Volume of Displacement (Volume of Displacement ())

Displacement (Displacement (ΔΔ) = Underwater Volume () = Underwater Volume () x Density () x Density (ρρ))

ρρ is density of the liquid in which the vessel is floating is density of the liquid in which the vessel is floating

Example 1Example 1

5m

2m36m

A box shaped vessel 36m x 5m x 4m floats at a draught of 2m in SW.A box shaped vessel 36m x 5m x 4m floats at a draught of 2m in SW. CalculateCalculate

i.i. Volume of displacement (Volume of displacement ())

ii.ii. Displacement (Displacement (ΔΔ))

Example 1Example 1

To Find the Underwater Volume (To Find the Underwater Volume ())

If: If:

Volume = Length x Breadth x DepthVolume = Length x Breadth x Depth

Then:Then:

Underwater Volume = Length x Breadth x DraughtUnderwater Volume = Length x Breadth x Draught

Hence:Hence:

  Volume of displacement (Volume of displacement () ) = L x B x draught (d) = L x B x draught (d)

= 36m x 5m x 2m = 36m x 5m x 2m

  

Volume of displacement Volume of displacement == 360 m 360 m33

Example 1Example 1

To Find the DisplacementTo Find the Displacement

  

If:If:

Displacement = Volume x DensityDisplacement = Volume x Density

  

Then:Then:

Displacement (Displacement () = ) = x x

= 360 m= 360 m33 x 1.025 t/ x 1.025 t/ mm33

  

Displacement = 369 tDisplacement = 369 t

Example 2Example 2

5m

d36m

A box shaped vessel 36m x 5m x 4m has a displacement of 425t. Calculate the A box shaped vessel 36m x 5m x 4m has a displacement of 425t. Calculate the draught at which it will float in FW.draught at which it will float in FW.

Example 2Example 2

To Find the Underwater VolumeTo Find the Underwater Volume

  

Displacement (Displacement () = Volume () = Volume () x Density () x Density ())

    

Displacement (Displacement ()) = Volume ( = Volume () )

Density (Density ())

  

Volume (Volume () = ) = Displacement (Displacement ()) = = 425 t 425 t

Density (Density ()) 1.0t/m1.0t/m33

Underwater Volume (Underwater Volume () =) = 425m425m33

Example 2Example 2

To Find the DraughtTo Find the Draught

= L x B x d= L x B x d

  

    = d= d

L x B L x B

  

d = d = 425 425

36 x 536 x 5

  

Draught = 2.36 mDraught = 2.36 m

More DefinitionsMore Definitions Light Displacement (Light Displacement (ΔΔLTLT)) – weight of hull, – weight of hull,

engines, spare parts, and water in the boilers engines, spare parts, and water in the boilers and condensers to working level.and condensers to working level.

Load Displacement (Load Displacement (ΔΔLDLD)) – the weight of the – the weight of the

hull and everything on board when at a hull and everything on board when at a particular draught.particular draught.

Summer Load Displacement (Summer Load Displacement (ΔΔSUMMERSUMMER)) – the – the

weight of the hull and everything on board when weight of the hull and everything on board when at the Summer Loadline mark.at the Summer Loadline mark.

More DefinitionsMore Definitions

Deadweight (DWT)Deadweight (DWT) – weight of the cargo, fuel, – weight of the cargo, fuel, FW, ballast, stores crew, passengers and FW, ballast, stores crew, passengers and effects. The difference between the Load and effects. The difference between the Load and Light displacements.Light displacements.

DWTDWT == ΔΔ -- ΔΔLTLT

More DefinitionsMore Definitions

Summer Deadweight (DWTSummer Deadweight (DWTSUMMERSUMMER)) – The – The

difference between the Summer Load difference between the Summer Load displacement and Light displacement. The displacement and Light displacement. The weight of the cargo, fuel, FW, ballast, stores weight of the cargo, fuel, FW, ballast, stores crew, passengers and effects. crew, passengers and effects.

DWTDWT == ΔΔSUMMERSUMMER -- ΔΔLTLT