Ship Stability, Basic Stability Definitions

HNC NAUTICAL SCIENCEGroup Award Code: G8F5 15

Unit Code: F0LD 34 Ship Stability Outcome 1 - Hydrostatics

1.2 Basic Stability Definitions

AimsTo give the student an understanding of:

Basic Stability Definitions relating to Loadlines. Achimedes Principles 1 & 2

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

ObjectivesThe Student will be able to:

State the first two Archimedes Principles calculate underwater volume, volume of displacement for a box shaped vessel, Calculate the displacement of a box shaped vessel.

Draught (d)

Depth of the vessel from keel to waterline. The hull is marked at the bow, stern and amidships. Marks are 1dm high with a gap of 1dm between each mark The reading is taken from the bottom of the mark

40 20 4M 80 60 40 20 3M 80 60

Loadline Mark

Marked on the hull amidships Top of Horizontal line indicates deepest loaded condition (Summer Zone) Load Draught is the draught of the vessel when loaded to the loadline mark

Loadline Mark

Loadline Mark is also known as the Plimsoll Line. Note that the top of the Loadline Mark lines up with the top of the Summer Loadline Note also where the boot-topping area ends.

Loadline Mark

Plimsoll Line named after Samuel Plimsoll. Born in Bristol in 1824. Became involved in shipping coal to London. Research into coal trade caused him concern for the safety of seafarers

Loadline Mark

Became member of parliament in 1868. 1871 Merchant Shipping Act made it an prisonable offence NOT to sail on an unsafe ship. Plimsoll fought this.

Loadline Mark

1875 introduced the Unseaworthy Vessels Act. 1876 Merchant Shipping Act was amended to provide for a line to be painted in the ships side. 1877 Merchant Shipping Act was amended again to impose weight limits for vessels, engagement of seamen, and seafarers accommodation.

Loadline Mark

History Lesson Over!

Definitions

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

Definitions

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

Definitions

Breadth The extreme breadth of the vessel measured in metres, i.e. The breadth of the vessel at its widest point.

Archimedes Law

The volume of liquid displaced by a floating body is equal to the underwater volume

V/2 V

V/2

Archimedes Law

The weight of liquid displaced by a floating body is equal to the weight of the bodyWW/2 W/2

Volume of Displacement

Equal to the underwater volume Denoted by the Symbol For a box shaped vessel, = L x B x dDraught Breadth Length

Displacement

Displacement () is measured in tonnes is equal to the weight of the vessel and also the weight of the water displaced. Mass (weight) Volume Displacement () Volume of Displacement () =

=

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

is density of the liquid in which the vessel is floating

Example 1

A box shaped vessel 36m x 5m x 4m floats at a draught of 2m in SW. Calculate i. Volume of displacement () ii. Displacement ()

2m 5m

36m

Example 1To Find the Underwater Volume () If: Volume = Length x Breadth x Depth Then: Underwater Volume = Length x Breadth x Draught Hence: Volume of displacement ( ) = L x B x draught (d) = 36m x 5m x 2m Volume of displacement = 360 m3

Example 1To Find the Displacement If: Displacement = Volume x Density Then: Displacement () = x = 360 m3 x 1.025 t/ m3 Displacement = 369 t

Example 2

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

d 5m

36m

Example 2To Find the Underwater Volume Displacement () = Volume ( ) x Density () Displacement () = Volume ( ) Density () Volume ( ) = Displacement () Density () = = 425 t 1.0t/m3

Underwater Volume ( )

425m3

Example 2To Find the Draught L x B d = L x B xd = d

=

425 36 x 5

Draught = 2.36 m

More Definitions

Light Displacement (LT) weight of hull, engines, spare parts, and water in the boilers and condensers to working level. Load Displacement (LD) the weight of the hull and everything on board when at a particular draught. Summer Load Displacement (SUMMER) the weight of the hull and everything on board when at the Summer Loadline mark.

More Definitions

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

More Definitions

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

DWT

=

SUMMER

-

LT