2. SHIP MOTION
SHIP MOTIONS ARE DEFINED BY THE SIX DEGREE OF FREEDOM THAT A SHIP, BOAT OR ANY
OTHER CRAFT CAN EXPERIENCE

3. TRANSLATION
HEAVE
 The linear vertical (up/down) motion.
SWAY
 The linear lateral (side-to-side) motion.
SURGE
 The linear longitudinal (front/back) motion.

4. ROTATION
ROLL
 When the vessel rotates about the longitudinal (front/back) axis.
PITCH
 When the vessel rotates about the transverse (side-to-side) axis.
YAW
 When the vessel rotates about the vertical (up-down) axis.

5. SOCIAL EFFECT OF PITCHING AND ROLLING
 At 45 degrees rolling, there is no difference between deck (floor) and
the bulkhead (wall).
 Eating was very difficult, things would spill off your plate.
 Sleeping was almost impossible.
 Always tired due to lack of sleep and fighting the motion of the ship.
 The chairs, chained to the deck, slid the several feet in either direction
in keeping with the ship’s motion.

6. CENTRE OF GRAVITY
Archimedes Principle :
‘A body floating or submerged in a fluid is buoyed up by a force equal
to the weight of the water it displaces.’

7. Centre of gravity (G) : all gravity forces as one force acting downward
through ship’s geometric centre.
Centre of buoyancy (B) : all buoyancy forces as one force acting upward
through underwater geometric centre.
Metacentre point (M) : a point where an imaginary vertical line (through the
centre of buoyancy) intersects another
imaginary vertical line (through a new centre of
buoyancy) created after the ship is displaced, or
tilted, in the water.

8.  Metacentre remains directly above the centre of buoyancy regardless of
the tilt of the floating ship.
 When a ship tilts,
 One side displaces more water than the other side.
 The centre of buoyancy moves and is no longer directly under the centre
of gravity.
The transverse metacentre (M) is used when transverse incinations (heel
and list) are considered.
The longitudinal metacentre (M) is used when longitudinal inclinations
(trim) are considered.
Metacentre Height (G M) is the distance between the centre of gravity and
the metacentre.

9. STABILITY AT SMALL ANGLE
 Transverse stability
The vessel’s weight and the force of buoyancy must be equal for the
vessel to float.
If the forces are not on the same vertical line they will form a couple.
A righting couple being formed when the vessel is heeled by the external
force.
The moment of statical stability :
W x G Z (unit : tonnes/metres)

10. CHANGING THE RELATIVES POSITIONS OF B, G & M
There are 3 condition of equilibrium:
 Stable equilibrium
 Neutral equilibrium
 Unstable equilibrium

11. STABLE EQUILIBRIUM
 G M is positive.
 G is below M.
 When external forces are removed vessel will roll back to the initial
upright position.
 The external forces could be due to wind or waves.

12. NEUTRAL EQUILIBRIUM
 K G = K M.
 G M = 0.
 If rolling happened, the ship cannot back to the origin.

13. UNSTABLE EQUILIBRIUM
 G is above M
 The ship will capsize if unstable equilibrium occurs.

14. LAUNCHING
 Involves transferring the ship’s weight from the blocks that have
supported it during construction to the launch ways.
 Launching from a building berth may be endwise, sidewise, or by in-
place floatation.
 Launching from a building dock is performed by flooding the dock to
the depth required to the float the ship.
 Tugs are needed to control the ship after entry into the water.

15. EXAMPLE OF LAUNCHING
 Marine Railways
A cradle of wood or steel on rollers. The ship is placed on the cradle and hauled
out along a fixed inclined track. This process is usually limited to ships
less than 5000 tons.
 Graving Dock
An excavation in the ground that can be flooded when a caisson (gate) is opened.
Once flooded, the ship is floated in, the caisson closed, and the water is pumped
out of the excavation.
 Floating Dry Dock
A structure with sealed wing walls between which the ship is positioned. Water is
pumped out of the wing walls to raise the ship clear of the water line. Water is
pumped back into the wing walls to lower the ship back into the water.