Load Lines Rudder & Propeller      


Ship Construction


Ship Dimensions and Form

General Cargo Vessel

These types of ships in general are built with longitudinal framing at the decks and in the double bottoms. Transverse framing is at the sides.



The transverse strength is given by fitting transverses at the deck and plate floors are fitted in the double bottoms.

Longitudinal framing is not usual in general cargo vessels due to the high broken stowage involved. Also deep transverses then have to be fitted about 3.7 metres to give the ship transverse strength.

Bilge wells are fitted with a cubic capacity of 0.17 cbm. Nowadays ceiling on top of tank tops are generally not fitted as such the plating is increased by 2mm. However where ceiling is fitted they should be removable in sections. The ceiling where fitted should have a clear space for drainage at least of 12.5mm.

Cargo battens are fitted to the sides and to the turn of the bilges – size of 50mm thick and spacing between rows of 230mm.


Shown above is a centre line bulkhead in the lower hold and in the tween deck. This extends from the transverse watertight bulkhead to the hatch coamings.


These ships may have two or more longitudinal bulkheads – today with double hull concept at least 3 but normally 4.

The bottom and deck are also framed longitudinally and so are the sides and the sides of the longitudinal bulkheads.

The length of a tank is not to exceed 0.2L. As the size of the tanker grows transverse wash bulkhead are fitted at about mid length of the tank. These are for size of tanks over 0.1L or 15m whichever is more.

Centre line was bulk heads are fitted where the breadth exceeds the dimensions as laid out in the Rules for different size of tanks.

Cofferdams are provided both forward of the oil carrying space as well as in from of the ER bulkhead. Generally the pumproom is located within the cofferdam aft. Some ships have a forward pump room located in the forward cofferdam.

The cofferdams are to be at least 760mm in length

Some smaller ships have a combined transverse and longitudinal framing system.

In lieu of bulwarks these ships are to have open rails on deck.

Cargo tanks are tested by a head of water in the cargo tank – 2.45m above the highest point of the tank.

Generally a system of staggered test is undertaken. Alternate tanks are filled and the empty tanks is inspected. Once all the empty tanks are inspected, the filled tanks are empties and the reverse tanks are filled and the other alternates inspected.

Inspecting of the tank welding are done by rafting within a tank.





Bulk Carriers:

These ships are characterised by their ability to carry cargo in bulk. If carrying grain and other lighter cargo all the holds are filled.

However if heavy cargo such as iron ore is carried then alternate holds are filled and to the designed loads only.


The vessel may be constructed on the combined system, longitudinal framing together with transverse framing which are fitted at the sides. The longitudinal framing is fitted in the double bottoms, the deck and the bottoms of the wing tanks.

The wing tanks may be utilised to carry cargo as well as remain empty. They carry ballast water during the ballast passage.

Transverse webs are fitted at in the wing tanks at intervals as laid out in the Rules. And side stringers are fitted at about 1/3rd and 2/3rd the depth of the tanks.




Combination Carriers:

These ships are capable of carrying ore as well as oil in bulk.

Transverse bulkheads are usually of the cofferdam type with all the stiffening on the inside.

There is a rise of floor of the inner bottom which facilitates drainage to the drain well arranged on the centre line. The pipelines run through a duct keel. The duct keep entrance in the pumproom has a oil and gas tight door.


On the top the hatch covers are mainly the side rolling Macgregor type.

The hatch breadth is usually about 50% of the breadth of the beam. The main disadvantage of this type of ship is the stability – since they are not built with a longitudinal partition in the centre the free surface effect is enormous and this necessitates overall loading complexities.


Together with this is the sloshing effect which tend to damage the fitting inside.

The stability book would give the loading levels as well as the loading stability requirements as per the Rules.



Longitudinal framing is used throughout the main body length of the ship. Transverse framing is used on the fore part and the after part.


The ships are built having a cellular construction at the sides. Strong longitudinal box girders are formed port and starboard by the upper deck – the second deck – top of the shell plating and top of the longitudinal bulkhead. The upper deck and the sheer strake form the box girder. These girders also provide stiffness against racking stresses and used as water ballast tank spaces.


A form of bulkhead is fitted at intervals, centre to centre with water tight bulkheads being fitted as required by the Rules. The bulkhead gives support to the double bottom structure.

The container guides consist of angle bars about 150mm x 150mm x 14mm thick connected to vertical webs and adjoining structure spaced 2.6m apart. The bottom of the guides is bolted to brackets welded to the tank top and beams. The brackets are welded to doubling plates, which are welded to the tank top.

Ro – Ro

Roll on Roll off ships have generally two ramps at either end of the ship to facilitate the loading of vehicles.

The main characteristic of these types of ships is the clear decks un interrupted by transverse bulkheads. Deck heights are sufficient to accommodate the various types of vehicles carried.


The lower decks may be used for carriage of cars while the upper may be used for the carriage of bigger vehicles.

Transverse strength is maintained by fitting deep closely spaced web frames in conjunction with deep beams. These are usually fitted every 4th frame and about 3 m apart.

The lower decks which are divided by watertight bulkheads have hydraulically operated sliding bulkhead doors which are opened while working cargo in port.

The deck thickness is increased to take the concentrated loads; a reduction in the spacing of the longitudinals with an increase in size. A centre line row of pillars is fitted.

Ramps are fitted at the bow and at the stern to facilitate the loading and discharging of vehicles. The separate decks are reached by fixed and sometimes hydraulically operated foldable operated ramps.

A service car is provided within the ship to transfer the lashing gear to the different decks.


The stern ramps are generally set at an angle to the ships centre line to ensure that the ship can work cargo in any berth.


The basic construction of these vessels follows the dry cargo vessel in their detail, a large number of decks being fitted.


Each passenger ship is differently built with the naval architects and the classification societies agreeing on the various additions to the various pillars and bulkheads.

However the basic rule and the provisions of SOLAS, MARPOL are complied with.



Midship in way of ER




The purpose of rounding the beam is to ensure a good drainage of the water and also to strengthen the upper deck and the upper flange of the ship girder against longitudinal bending stresses- especially the compression stresses.

Rise Of Floor

This is the distance from the ‘line of floor’ to the horizontal, measured at the ship side. Purpose basically is to allow drainage of the double bottom water/ oil to the centre line suctions.


This is the inward slope of the side plating from the water line to the upper deck – today ships generally do not have a tumblehome.


This is the curvature of the side plating at the forward and gives additional buoyancy and thus helps to prevent the bows from diving too deeply into the water when pitching.

The anchors are also clear when lowered from the flare of a ship.


This is the rise of ships deck fore and aft. This again adds buoyancy to the ends where it is needed during pitching. For calculating the freeboard a correction is applied for the sheer. In modern ship the after sheer has been greatly reduced.


This is the slope, which the forward end has with between the bottom plating and the upper deck. The length between perpendiculars and the length overall difference is mostly due to the rake forward. It helps to cut the water and thus adds to the ships form.

Parallel Middle Body

This is the part of the main body of the ship and it is a box like structure enabling maximum cargo carrying capacity. It also helps in the pushing when tugs are used to assist the vessel in berthing. Cargo stowage is also greatly facilitated.


This part is the fore end of the ship and helps give the box like mid length a ship shaped structure.


The after part similarly to the fore part entrance helps in giving the box like mid length a ship shaped structure and thus the handling of the vessel is enhanced.

“Length” means 96 per cent of the total length on a waterline at 85 per cent of the least moulded depth measured from the top of the keel, or the length from the fore side of the stem to the axis of the rudder stock on that waterline, if that be greater. In ships designed with a rake of keel the waterline on which this length is measured shall be parallel to the designed waterline.


Moulded breadth: is the greatest moulded breadth – measured inside plating.

Breadth (B) is the greatest moulded breadth of the ship at or below the deepest subdivision load line.

Draught (d) is the vertical distance from the moulded baseline at midlength to the waterline in question.

Depth and the draught both are measured from the top of the keel. The depth is measure from the top of the deck beam. If there is a camber then allowance is given as 1/3 rd of the camber.

The rest of the meanings are all self-explanatory.




Forward perpendicular

This is represented by a line, which is perpendicular to the intersection of the designed load water-line with the forward side of the stem.

After perpendicular

A line represents this, which is perpendicular to the intersection of the after edge of the rudderpost with the designed load water line. This is the case for both single and twin-screw ships. For some ships having no rudderpost, the after perpendicular is taken as the centre-line of the rudderstock.

Length between perpendiculars

This is the horizontal distance between the forward and after perpendiculars.

Length on the designed load waterline

This is the length, as measured on the water-line of the ship when floating in still water in the loaded, or designed, condition.

Length overall

This is the length measured from the extreme point forward to the extreme point aft.

Base line

This represents the lowest extremity of the moulded surface of the ship. At the point where the moulded base line cuts the midship section a horizontal line is drawn, and it is this line, which acts as the datum, or base line, for all hydrostatic calculations. This line may, or may not, be parallel to the load water line depending on the type of ship.

Moulded depth

This is the vertical distance between the moulded base line and the top of the beams of the uppermost continuous deck measured at the side amidships.

Moulded beam

This is the maximum beam, or breadth, of the ship measured inside the inner shell strakes of plating, and usually occurs amidships.

Moulded draught

This is the draught measured to any water-line, either forward or aft, using the moulded base line as a datum.

Extreme beam

This is the maximum breadth including all side plating, permanent fenders etc.

Extreme draught

This is obtained by adding to the draught moulded the distance between the moulded base line and a line touching the lowest point of the underside of the keel. This line is continued to the FP and AP, where it is used as the datum for the sets of draught marks.