- With reference to the motion of a vessel in the water:
(a) list the SIX degrees of freedom, describing the meaning of EACH term;(6)
(b) sketch the position of a bilge keel, describing how it reduces vessel motion.(4)
(a) Six Degrees of Freedom (6 marks)
The motion of a vessel at sea is described by six degrees of freedom:
- Surge – linear movement of the vessel forward or backward along its longitudinal axis.
- Sway – linear movement of the vessel sideways (port to starboard) along its transverse axis.
- Heave – linear movement of the vessel up and down along its vertical axis.
- Roll – rotational motion about the longitudinal axis (vessel tilting port and starboard).
- Pitch – rotational motion about the transverse axis (bow up and down).
- Yaw – rotational motion about the vertical axis (bow swings left or right).
(b) Bilge Keel – Position & Effect (4 marks)
Position:
- A long, narrow fin fitted externally at the turn of the bilge (junction of bottom and side shell).
- Runs longitudinally over the midship half-length of the vessel.
- Ends are tapered to avoid damage during docking or grounding.
How it reduces motion:
- When the vessel rolls, the bilge keel resists water flow across its surface, creating hydrodynamic resistance.
- This damps rolling motion, reducing roll amplitude and making the vessel more stable.
- Most effective at the bilge because this is the area of maximum water flow disturbance during roll.
Sketch (exam-style):
- Cross-section of hull showing bilge keel fitted at bilge turn.
- Arrows illustrating rolling motion and resistance forces on bilge keel.