double bottom structure

double bottom structure of a ship

1. Bottom Structure
Function, role, types of

2. Bottom Structure
‐Keel types
‐Floor types
‐Bilge Keel
‐

3. An overview
In ancient times , originally ships were constructed
with single bottoms, liquid fuels and fresh water being
contained within separately constructed tanks. The
double bottom structure safeguards the ship in the
event of bottom shell damage , when accidental
grounding occurs .
The double bottom also provides liquid tank space at
the floor level of the ship. This double bottom evolved
during the early part of the twentieth century.
Smaller vessels such as tugs, ferries, and cargo ships
of less than 500 gross tonnage have a single bottom
construction. Larger ocean-going vessels, other than
older
tankers, are fitted with some form of double bottom.
It’s an IMO/SOLAS regulated requirement to construct

4. Keel

Keel is located at the centerline and is the
lowest structural member of the ship.

Thus, it is the backbone of the ship which
runs throughout the length of the ship.

The ship when dry docked, Keel rests on the
centerline keel blocks.

Thus , due to these reasons, the keel plate is
thickest for that ship.

At the keel strake level sheer stress is
maximum.

Strake: is a horizontal chain of plates between
two consecutive seams. These are numbered
alphabetically or alphanumerically e.g. A, B,C …
or A1, A2,B1,B2…..

6. Types of KEEL

7. Single Bottom

In smaller ships having single bottoms the
vertical plate open floors are fitted at every
frame space , transversely, and are stiffened at
their upper edge. A center line girder is fitted
and one side girder is fitted each side of the
center line where the beam is less than 10 m.
Where the beam is between 10 and17m two
side girders at each side are fitted and if any
bottomshell panel has a width to length ratio
greater than four, additional continuousor
intercostalstiffeners are fitted. The
continuous center and intercostal side girders
are stiffened at their upper edge and extend as
far forward and aft as possible.

8. Double Bottom

Continuous members: are those structural
members which runs through other crossing
structural members . For instance the bottom
centerline girder runs along the length of the
ship at the center of the keel plate where
transverse floor plates terminate /originate. If
the continuous member is smaller in scantling
than the crossing structure then it pierces
through that structure, e.g. the bottom and inner
bottom longitudinal, piercing through transverse
floors.

Intercostal members: Contrary to the continuous
ones these runs in between two crossing
structures.
For example ,Side keelsons port and starboard
sides are intercostal members, as they run in
between consecutive transverse floors.

10. Double Bottom construction

Two types of double bottom structure :

11. Floors

Floors are the transverse structural members at
every frame of bottom structure of the ship.

Thus it strengthens the bottom plate of the hull.

Floors are essential structural member for all the
types of bottom structures.

Floors are of two types-
1) Solid floor-these may be further subdivided into
i)solid plate floor and ii)Solid plate floor with
manhole.
2) Bracket floors- are made of angle stiffeners
and plates at the tank sides and center girder side.
Below the watertight bhd, watertight plate floors are
fitted. Where the floors are pierced by continuous
bottom longitudinal/inner bottom longitudinal, collar
plate is welded doublecontinously to make the floor
watertight.

12. Solid Floor(with manholes)

For transversely framed double bottom
construction.

13. Bracket Floor

Transversely framed double bottom
construction.

14. Solid Floor(with manhole)

For longitudinally framed double bottom
construction.

15. Bracket Floor

For longitudinally framed double bottom
construction

16. Double Bottom construction
of a Bulk Carrier

17. Engine Seating

18. DB TANK TESTING

19. Cargo Tank testing

20. Scantling

It’s specification of structural angles, bulb plates , channels,
I beams. For a offset bulb plate (OBP), if web= 100,
thickness=10 (no Flange) then scantling is 100x10

For structural angle scantling is defined as web x flange x
thk

There are two types of ordinary angles a) Equal angle-
when
web=flange i.e. say 100x100x10 or 75x75x8 are such equal
angle.
b) Unequal angle- Web and flange are not same e.g.
100x75x10 O.A.

ii) Fabricated sections- are manufactured in the shipyard as
per the production drawings, as per the scantling.
Longitudinal Deck Girder, Hatch side girder , Deck
Transverse-all under deck , Bottom continuous center girder,
side girder- all on the bottom shell , Fabricated frames both
transverse and longitudinal , Stringers, Transverse web
frames – all fitted on the side shell are such fabricated
sections (ref JAG AMEESHA Shell Expansion)

For plate, it’s thickness is the scantling.