HNC NAUTICAL SCIENCE Group Award Award Code: G8F5 15
Unit Code: F0LD 34 Ship Stability Outcome 1 - Hydrostatics
1.2 Basic Stability Definitions
Aims To give the student an understanding of:
Basic Stability Definitions relating to Loadlines. Achimedes Principles Principles 1 & 2
Objectives The 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;
Objectives The 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 pris onable priso nable 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 amended to provide for a line to be painted in the ships side. 1877 Merchant Shipping Act was amended amended again again to impose weight limits for vessels, engagement of seamen, and seafarers accommodation.
Loadline Mark
History Lesson Over!
Definitions
Loaded Draught Draught – – the draught when the vessel has been loaded to the Loadline Mark.
Definitions
Waterline Length 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 waterline cuts cuts the 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 it’s widest point.
Archimedes Law
The volume of liquid displaced by a floating body is equal to the underwater volume
V/2
V/2 V
Archimedes Law
The weight of liquid displaced by a floating body is equal to the weight of the body W W/2
W/2
Volume of Displacement
Equal to the underwater volume Denoted by the Symbol For a box shaped vessel, = L x B x d Length
Draught
Breadth
Displacement
Displacement ( Δ) is measured in tonnes is equal to the weight of the vessel and also the weight of the water displaced.
Mass (weight) (weight) Volume
=
ρ
Displacement Displacement ( Δ Δ) Volume of Displacement ()
=
ρ
Displacement ( Δ) = Underwater Volume (
) x Density ( ρ)
ρ 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 S W. Calculate i. ii.
Volume of displacement () Displacement ( Δ)
36m
2m 5m
Example 1 To 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 1 To Find the Displacement Displacement If: Displacement = Volume x Density Then: Displacement (∆) = ∇ x ρ = 360 m3 x 1.025 t/ m3 Displacement 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.
36m
d 5m
Example 2 To Find the Underwater Volume Displacement (∆) = Volume (∇ ) x Density (ρ) Displacement (∆) = Volume (∇ ) Displacement Density (ρ) Volume (∇ )
= Displacement Displacement (∆) Density (ρ)
Underwater Volume (∇ )
=
=
425m3
425 t 1.0t/m3
Example 2 To Find the Draught Draught ∇
= L x B xd = d
∇
L x B d
=
425 36 x 5
Draught = 2.36 m Draught
More Definitions
Light Displacement (ΔLT) – Light – weight of hull, engines, spare parts, and water in the boilers and condensers to working level.
Load Displacement Displacement (ΔLD) – – the weight of the hull and everything on board when at a particular draught.
Summer Load Displacement Displacement (ΔSUMMER) – – the weight of the hull and everything on board when at the Summer Loadline mark.
More Definitions
Deadweight (DWT) – Deadweight – weight of the cargo, fuel, FW, ballast, stores crew, passengers and effects. The difference between the Load and Light displacements.
DWT
=
Δ
-
ΔLT
More Definitions
Summer Deadweight 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
=
ΔSUMMER
-
ΔLT
