Some of the important ship terms and principle dimentions of Naval architecture and ship construction

Ship Terms

In ship the  narrowing of midship side shell in the region of the upper deck is curved slightly towards the centreline, thus reducing the width of the upper deck and deck above. It improves the appearance of the ship.

A camber is a transverse curvature of the ships deck from the centerline downwards to the side. the camber is used on exposed decks to drive water to the sides of the ship. other decks are also often called camber. most of the modern ships have decks which are flat transversely over the width of the hatch or center tanks and slope down towards the side of the ship.

The vertical distance from the waterline to the top edge of the deck plating at the side of the deck amidships.
Freeboard represents the safety margin showing to what depths a ship may be loaded under various service conditions.      
Example : the type of cargo, the water to be navigated, and season of the year.                  
Purpose of freeboard :
  • Ensure that the load on the ship cannot be beyond her strength.
  • To give the sufficient reserve buoyancy.
  • To keep the deck high enough from warer, to enable the crew to navigate and handle her all weather condition.                                    
The greatest breadth of the ship, measure to the inside of the inside stakes of shell plating.

The greatest breadth of the ship, measured to the outside of the shell plating.

The depth of the ship measured from the underside of the keel to the top of the deck beam at the side of the uppermost continuous deck amidships.

The depth measured from the top of the keel.

The distance from the bottom of the keel to the waterline . The load draught is the maximum draught to which a vessel may be loaded.

The draught measured from the top of the keel to the waterline.

The bottom shell of the ship is sometimes sloped up from the keel to the bilge to facilitate drainage. the rise of floor is small, 150 mm being usual.

The radius of the arc connecting the side of the ship to the bottom at the midship portion of the ship.


The curvature of the deck in a forward and aft direction, rising from the midships to a maximum at the ends. The sheer forward is usually twice that aft sheer. sheer on exposed decks makes a ship more seaworthy by raising the deck at the fore and after ends further from the water and by reducing the volume of water coming on the deck.

If a solid body is immersed in a liquid there is an apparent loss in weight. This loss in weight is the upthrust exerted by the liquid on the body and is equal to the weight of the volume of liquid which the body displaces.

When a ship is floating freely at rest the mass of the ship is equal to the mass volume of water displaced by the ship and is therefore known as the displacement of the ship.

The tonne per centimetre immersion (TPC) of a ship at any given draught is the mass required to increase the mean draught by 1 cm.

T.P.C:  Aw(Waterplane area) × ᵹ /100

The point where a vertical line through a centre of buoyancy of an inclined ship
intersects the vertical line through the centre of gravity when it is floating in equilibrium.
Water plane area coefficient: (Cw): 
Is the ratio of the area of the water plane to the product of length and breadth of the ship.

Midship section area coefficient ( Cm ): 
The ratio of the area of the immersed portion of the midship section to the product of the breadth and the draught.

Block coefficient (Cb): 
Is the ratio of the volume of displacement to the product of breadth and draught.

Prismatic coefficient (Cp): is the ratio of the volume of displacement to the product of the length
and the area of the immersed portion of the midship section.

Wetted surface area: 
The wetted surface area of a ship is the area of the ship's hull which is in contact with the water. This area may be found by putting the transverse girth of the ship, from waterline to waterline, through Simpson's Rule and adding about f per cent to allow for the Longitudinal curvature of the shell. To this area should be added the wetted surface area of appendages such as cruiser stern, rudder and bilge keels.

                                    S = 1.7Ld + ∇/d


                S = c√𝝙VL

Centre of gravity: 
The centre of gravity of an object is the point at which the whole weight of the object may be regarded as acting. If the object is suspended from this point, then it will remain balanced and will not tilt.

Centre of buoyancy: The point through which the total force of buoyancy is considered to act.

Theoretical speed (vt): distance the propeller would advance in unit time if working in an unyielding fluid. Thus if the propeller turns N rev/min. 
=P*N*60/1852 knots.

Metacentric height: distance between C.O.G and transverse metacenter (M).

Pitch of propeller: 
One revolution of the shaft the propeller will move forward a distance

Diameter of propeller
Diameter of the circle or disc cut out by the blade tips.

Pitch ratio: 
It is the face pitch divide by diameter

Water which is in motion at stern of a ship as a result of a ship's movement, the moving water known as wake.

Wake fraction: 
Ratio of the wake speed to the speed of advance.

Speed of advance: 
Speed of ship relative to the wake is termed the speed of advance Va.

Real slip or True slip : 
Difference between theoretical speed and the speed of advance.
Real slip = Vt -Va/vt X100%

Skew : 
Offset of a propeller blade from the vertical in the plane od rotation, it is always a distance in
the direction opposite to rotation.

The difference between the actual distance travelled by a ship and the theoretical distance
given by the product of the propeller pitch and the no. of revolution. It is usually expressed as a percentage and can have a negative value if a current or following wind exists.

Apparent slip: 
The  propeller work in water the ship speed V will normally be less than theoretical speed, or
the difference between the two speed known.

Longitudinal Centre of Flotation: 
It is the point about which the ship will Trim when weight are loaded or discharged if the weight added at L.CF point, trim will not change, only draft change.

Permeability (w): 
Ratio of volume with the space which is assumed to be occupied by water to the total volume of that compartment. Permeability for M/C space: 85%, for accommodation: 95%, for cargo hold
average: 60%

The upthrust exerted by the water on the ship. If the ship float freely the buoyancy is equal to the weight of ship.

Reserve buoyancy: 
It is the potential buoyancy of a ship and depends upon the intact watertight volume above the waterline of ship. If the mass added to ship or buoyancy lost due to bilging the reserve buoyancy is converted into buoyancy by increasing draught.

External hull of a ship consists of bottom shell, side shell and deck which are formed by longitudinal strips plating called strake. Or continue range of plate forming the side of vessel, or metal plate extending ship's hull from stem to stern.

Bilge strake
Strake at the turn of the bilge called.

Stealer strake: 
No.of adjacent strakes fitted at the end of ship called.

Garboard strake: 
Strake adjacent to the keel on each side of ship called.

Sheer strake and its importance: 
It is largest continue strake at the top of the side of vessel main deck. Or uppermost strake of side plating which meet the upper deck. It is 10-20% thicker than other side plating.
when vessel is bending to forces from tension to compression and sheer strake is subjected to maximum compressive and tensile stress. Which is contribute to the strength of the hull.

The stiffners used to strengthening the sides surface of the ship called, without stringer mull shape doesnot formed.

Coffin plate: 
Used to connect stern frame to the flat plate keel.

Shoe plate: 
Used to connect stem to the flat plate keel.
Margin plate: 
At bilges, the tank top may be either continuous straight out to the shell by means of tank margin plate. Which is water tight and set an angle of about 45° to the tank top and meeting the shell almost at right angle.

It is solid wall that extends above the weather deck or any other deck to exposed to weather and fitted for the safety of the crew. Atleast 1 m in height spacing of stays and is not exceed 1.2 m on the forecastle.

Freeing port:
The area of freeing port on each side depend on the length of well deck, the lower edge of the port must be as near to the deck as possible and opening are to be protected by rails spaced approx. 230 mm apart. When hinged flaps are fitted the hings must be of non-corrodible.

The upper edge of a ship's side where the sheer strake meets the deck plating called.

Margin line: 
Is a line drawn at least 76 mm below the upper surface of the bulkhead deck at side. It is the imaginary line, which is drawn 76mm below the uppermost continuous deck. It denotes the limit, up to which ship can flooded/ loaded without sinking. 
For a ship which has a continuous bulkhead deck, the margin line is to be taken as a line drawn not less than 76 mm below the upper surface of the bulkhead deck at side, except that where there is a variation in the thickness of the bulkhead deck at side the upper surface of the deck should be taken at the least thickness of deck at side above the beam. 
If desired however, the upper surface of the deck may be taken at the mean thickness of the deck at side above the beam as calculated for the whole length of the deck, provided that the thickness is no greater than the least thickness plus 50 mm.

Transom space: 
Situated in S.G. room there you can find manhole door near Rudder Trunk this purpose is to inspect Rudder Trunk condition, Lubrication etc.. you can enter inside this place for carried out inspection in Port only and in calm weather or sea.

Buttock line: 
It is equidistant transverse section line from the midship to fwd of the ship, such that they give you the cross section are at various station at all possible draft and trim.
They are mainly used for knowing the light weight displacement at the time of end of construction phase of a ship.

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