Miscellaneous

Construction

Hull Construction

Transversally stiffened

This structure is now virtually obsolete and may not be used on hulls greater than 120m in length


The hull requires a plate floor every 3.05m and a frame every 1m. Hence there are 3 frames for every plate floor. The two frames are attached to the floor angle iron transverse.

For the aft framing of the aft peak tank or the for'd framing of the for'd collision bulkhead the maximum framing pitch is 0.61m. Also for the for'd 0.2l of the ship the maximum spacing of the frame is 700mm (this helps to prevent damage due to slamming).

Underneath the engine seating a plate floor is required every frame.
The keelplate is made from heavier section of plate and has its ends tapered to allow it to be welded onto the normal hull plating



Duct keel construction for transversely framed hull


Longitudinally framed hull (tanker)


The longitudinal framing is much better able to resist buckling when the hull is hogging


Longitudinal framing (Dry Cargo)


Spectacle prop shaft supports



Cast construction


Welded construction


Engine Seating

Flat Bed Plate


There are transverse plate floors at each frame. The thickness of the engine seating is governed by the power, weight,and length of the unit



Small Drop-raised Seat


Large Drop-raised seat


Historically the engine bolts at the after end of the engine were fitted bolts to take the shear of the thrust and the more for'd bolts were loose fit bolts allowing for expansion.

This method has proved unreliable and the more modern practice is to weld lugs on the bedplate and have brackets and fitted chocks



Bedplate location


The holding down arrangement should be arranged to be above any bilge water level to allow for easy access and inspection


Engine Mounting for separate thrust block


Where the thrust is taken in the gearbox casing it is necessary then to have the mounts for the casing as close as possible to the centreline of the shaft so as to ensure little or no bending moment on the casing. The mountings should be suitably extended in a similar fashion to the thrust block arrangement shown above


Bulkheads

There are three basic types of bulkhead, watertight, non watertight and tank.

Different types of bulkheads are designed to carry out different functions.

The watertight bulkhead several important ones;

  1. It divides the ship into watertight compartments giving a buoyancy reserve in the event of hull being breached. The number of compartments is governed by regulation and type of vessel
  2. cargo separation
  3. They restrict the passage of flame
  4. Increased transverse strength, in effect they act like ends of a box
  5. Longitudinal deck girders and deck longitudinal are supported by transverse watertight bulkheads which act as pillars




The number of bulkheads depends upon the length of the ship and the position of the machinery. There must be a collision bulkhead positioned at least 1/20th of the distance from the forward perpendicular. This must be continuous to the uppermost continuous deck.


The stern tube must be enclosed in a watertight compartment formed by the stern frame and the after peak bulkhead which may terminate at the first continuous deck above the waterline. The engineroom must be contained between two watertight bulkheads one of which may be the after peak bulkhead.

Each main hold watertight bulkhead must extend to the uppermost continuous deck unless the freeboard is measured from the second deck in which case the bulkhead can extend to the second deck.


A water tight bulkhead is formed from plates attached to the shell, deck and tank top by means of welding. The bulkheads are designed to withstand a full head water pressure and because of this the thickness of the plating at the bottom of the bulkhead may be greater than that at the top. Vertical stiffeners are positioned 760mm apart except were corrugated bulkheads are used.



Length of ship (m)

Number of bulkheads  

Above

Not exceeding  

Machinery midships

Machinery Aft

90

105

5

5

105

115

6

5

115

125

6

6

125

145

7

6

145

165

8

7

165

190

9

8

190

To be considered individually

Number of bulkheads (cargo ship)

Watertight bulkheads must be tested with a hose at a pressure of 200 Kn/m2 . The test being carried out from the side on which the stiffeners are fitted and the bulkhead must remain watertight.

Water tight bulkheads which are penetrated by pipes, cables etc. must be provided with suitable glands which prevent the passage of water.

Water tight doors


Vertically mounted watertight door



To allow the passage for personnel water tight doors are fitted , openings must be cut only were essential and they should be as small as possible. 1.4m high, 0.7m wide being the usual. Doors should be of mild steel or cast steel, and they may be arranged to close vertically or horizontally.

The closing action must be positive i.e. it must not rely on gravity. Hinged water tight doors may be allowed in passenger ships and in watertight bulkheads above decks which are placed 2.2m or more above the waterline. Similar doors may be fitted in weather decks openings in cargo ships.


Hinged Watertight doors


Hinged water tight doors consist of a heavy section door which when closed seals on a resilient packing mounted in channel bar welded to the door frame.


The door is held firmly in the door frame when closed by the dogging arrangements shown which allow the doors to be opened from either side. Normally six of these dogs are spread equally around the periphery.



Automatic watertight operating gear



Automatic operating gear allows the remote operation of watertight doors. These are fitted on many vessels including passenger ships.

In the event of fire or flooding, operation of switches from bridge/fire control area sends a signal to an oil diverter valve. Oil from a pressurised hydraulic system is sent to a ram moving the door.

The door may also be operated locally by a manual diverter valve. In addition, in the event of loss of system pressure the door may be operated by a local manual hand pump

remote door position indicators are fitted as well as were appropriate alarms to indicate operation.

Bulkhead definitions

Class A

Are divisions forming bulkheads and decks that;

Class B

These are divisions formed by bulkheads, decks, ceilings and lining

Class C

These are divisions constructed of approved non-combustible materials. Combustible veneers are allowed were they meet other criteria

Main vertical zones Divided by Class A bulkheads and not exceeding 40m in length


Stern Frame


A stern frame may be cast or fabricated and its shape is influenced by the type of rudder being used and the profile of the stern. Sternframes also differ between twin and single screw ships, the single screw sternframe having a boss for the propeller shaft. Adequate clearance is essential between propeller blade tips and sternframe in order to minimise the risk of vibration. As blades rotate water immediately ahead of the blades is compressed and at the blade tips this compression can be transmitted to the hull in the form of a series of pulses which set up vibration. Adequate clearance is necessary or alternatively constant clearance, this being provided with ducted propellers such as the Kort nozzle. A rotating propeller exerts a varying force on the sternframe boss and this can result in the transmission of vibration. Rigid construction is necessary to avoid this. The stern post, of substantial section, is carried up inside the hull and opened into a palm end which connects to a floor plate, This stern post is often referred to as the vibration post as its aim is to impart rigidity and so minimise the risk of vibration. Side plating is generally provided with a Rabbet or recess in order that the plating may be fitted flush. The after most keel plate which connects with this region the structure od the ship serves no useful purpose and it is known as the 'deadwood'. This may be removed without ill effect on stability or performance and some sternframes are designed such that the deadwood is not present.


Tank Inspections

The following describes a few of the common defects found in various hull constructions;