Load Lines | Rudder & Propeller |

**Ship Construction**

*Ship Stresses*

**Shear
force and bending moments**

When a section such as a beam is carrying
a load there is a tendency for some parts to be pushed upwards and for other
parts to move downwards, this tendency is termed Shearing.

The Shear force at a point or station is
the vertical force at that point. The shear force at a station may also defined
as being the total load on either the left hand side or the right hand side of
the station; load being defined as the difference between the down and the
upward forces, or for a ship the weight would be the downward force and the
buoyancy would be the upward thrust or force.

The longitudinal stresses imposed by the
weight and buoyancy distribution may give rise to longitudinal shearing
stresses. The maximum shearing stress occurs at the neutral axis and a minimum
at the deck and keel. Vertical shearing stresses may also occur.

**Bending
Moment**

The beam, which we have been considering,
would also have a tendency to bend and the bending moment measures this
tendency.

Its size depends upon the amount of the
load as well as how the load is placed together with the method of support.

Bending moments are calculated in the same
way as ordinary moments that is multiplying force by
distance, and so they are expressed in weight – length units.

As with the calculation of shear force the
bending moment at a station is obtained by considering moments either to the
left or to the right of the station.

**Hogging
and sagging **

Hogging – When a beam is loaded or other wise
is subjected to external forces such that the beam bends with the ends curving
downwards it is termed as hogging stress.

For a ship improper loading as well as in
a seaway when riding the crest of a wave the unsupported ends of the ship would
have a tendency similar to the beam above.

Sagging – In this case the beam is loaded
or other wise subjected to external forces making the beam bend in such a way
that the ends curve upwards, this is termed as sagging.

Similar with a
ship if improper loaded or when riding the trough of a wave – with crests at
both ends then the ship is termed to be sagging.

For Hogging the ship ends to curve
downwards would mean that the weight/ load amidships is much less than at the
end holds/ tanks.

For Sagging the
ship would have been loaded in such a manner that a greater percentage of the
load is around the midship area.

In a seaway the hogging and the sagging
stresses are amplified when riding the crests and falling into the troughs. Thus especially for large ships there are two conditions in the
stability software – Sea Condition and Harbour condition.

A ship loaded while set in the harbour
condition may allow loading with hogging/ sagging stresses reaching a high
level, when this state of loading is transferred to a Sea condition in the
software the results would be catastrophic since now the wave motions have also
been incorporated.

Thus planning a loading should always be
in the Sea Condition.

Discharging in port may be planned in the
Harbour Condition.

Hogging and sagging cause compressive and
tensile stresses on the ship beam – notably on the deck and the keel structure.

**Water
pressure and Thrust**

Pressure is force per unit area and water
pressure is dependent on the head of the water column affecting the point of
the measurement of the pressure.

Let us assume an area of 1sq.m. then this area of water up to a depth of 1 m below the
surface would have a volume of 1sq.m. x 1m = 1cbm and
the weight of this volume would be 1cbm x density of the water = 1MT (assuming that
it is FW) or 1000kgf, therefore the pressure exerted by this mass would be
1000kgf/sq.m.

Similarly if now the depth of measurement
is increased to 3m then the volume of this area subtending up to the 3m mark
would be 1sq.m x 3 = 3cbm and the weight of the water would be 3MT or 3000kgf
and the pressure exerted would be 3000kgf/sq.m.

If now the liquid had not been FW but any
other then the weight would be found by multiplying the volume by the density
of the liquid. And thus the pressure exerted would be found.

If we now increase the area of the square
of water plane would it make a difference in the pressure?

Let us consider a area of 2000sq.m then
the volume of this water at a depth of 1 m would be 2000cbm and the weight
would be 2000MT (consider FW) and the pressure exerted would be 2000,000kgf/
2000sq.m which would give us again 1000kgf/sqm, thus the pressure is
independent of the area of the water plane.

Thrust however is different,
thrust is taken to be the total weight of the liquid over an area. Thus for the
previous example the thrust would be 2000 tonnes.

Thus the thrust is given by: the area of
the water plane x pressure head x density of the liquid.

Thrust always acts at right angles to the
immersed surface and for any depth the thrust in any of the directions is the
same. The pressure head which is used in the above calculation of thrust is the
depth of the geometrical centre of the area below the surface of the liquid.

For a ship the thrust on the ship side
changes as the depth increases, however the bottom is affected uniformly for a
set depth.

Centre of pressure of an area is the point
on the area where the thrust could be considered to act. It is taken that the
centre of pressure is at 2/3rds the depth below the surface for ordinary
vertical bulkheads and at half the depth in the case of collision bulkheads.

**Racking
stress and its causes**

In a seaway as a ship rolls from one side
to the other the different areas of the ship have motion
which are dependent on the nature of the subject area. The accelerations
are thus not similar due to the various masses of the different sections
(although joined together). These accelerations on the ships structure are
liable to cause distortion in the transverse section. The greatest effect is
under light ship conditions.

**Local
Stresses**

**Panting**

This is a stress, which occurs at the ends
of a vessel due to variations in water pressure on the shell plating as the
vessel pitches in a seaway. The effect is accentuated at the bow when making
headway.

**Pounding:**

Heavy pitching assisted by heaving as the
whole vessel is lifted in a seaway and again as the vessel slams down on the
water is known as pounding or slamming. This may subject the forepart to severe
blows from the sea. The greatest effect is experienced in the light ship
condition.

**Stresses
caused by localized loading**

Localized heavy loads may give rise to
localized distortion of the transverse section.

Such local loads may be the machinery
(Main engine) in the engine room or the loading of concentrated ore in the
holds.

**Shearing
force Curve**

The following example shown is for an old
tanker in the ballast condition.

The compartments loaded are the Fpk tank,
WB tanks 2P and 2S, WB tank 3C and other miscellaneous tanks in the after
section of the tanker.

The SF is calculated as per the manual
with the multipliers having been set by the shipyard and approved by the
classification society.

If we are to assume that the ship is a
beam then the loads are at the fore end – midship region and the after section
which has the accommodation as well as the ER.

The SF curve is reproduced and the maximum
occur at frames 54 and between 68 to 72, this
corresponds to the area on the ship – mid 4C and between 2C (aft to mid
region). Note that the signs have changed between the frames 54 and 68 with a
point between frames 59 to 63 (3C mid to aft) registering 0 value.

**Bending
Moment Curve**

The following example shown is for a old tanker in the ballast condition.

The compartments loaded are the Fpk tank,
WB tanks 2P and 2S, WB tank 3C and other miscellaneous tanks in the after
section of the tanker.

The BM is calculated as per the manual
with the multipliers having been set by the shipyard and approved by the
classification society.

If we are to assume that the ship is a
beam then the loads are at the fore end – midship region and the after section
which has the accommodation as well as the ER.

The BM curve is reproduced and the maximum
occur at frames 59 and 76, this corresponds to the area on the ship – 4C
forward bulkhead and 2C forward bulkhead. Note that the signs have changed
twice.