Vessel Stability and Construction Flashcards

What is the effect of a 'negative GM' on vessel behaviour?

Answer: The vessel is lolling — it has no tendency to return to upright and will roll to a permanent list or capsize if disturbed

A negative GM means the metacentre is below the centre of gravity. The vessel has no initial restoring moment and will spontaneously list to one side ('loll'). Any additional disturbance may cause capsize. This is an extremely dangerous condition requiring immediate corrective action.

What is the 'downflooding angle' and why is it a critical stability criterion?

Answer: The heel angle at which water first enters an opening to the vessel's interior (hatch, ventilator, door) — once exceeded, progressive flooding begins and stability deteriorates rapidly

Downflooding angle is the angle of heel at which an unprotected opening (deck hatch, ventilation inlet, cockpit drain) first becomes submerged. Beyond this angle, progressive flooding begins — water enters faster than buoyancy is lost, accelerating capsize.

How does hull form affect stability — specifically, what is the advantage of a wide, shallow hull versus a deep, narrow hull?

Answer: A wide, shallow hull has high initial stability (large GM) but limited range of stability; a deep narrow hull has lower initial stability but a wider range of positive stability

Wide, shallow hulls (high beam-to-draft ratio) have large waterplane areas giving high GM and initial stability — they resist small heels strongly. But their stability typically vanishes at lower heel angles. Deep, narrow hulls have lower initial stability but maintain positive stability to higher angles.

What is the purpose of watertight compartmentalisation in vessel construction?

Answer: To limit flooding to one compartment when the hull is breached, maintaining sufficient buoyancy and stability for the vessel to survive and remain afloat

Watertight compartmentalisation divides the hull into separate sections separated by watertight bulkheads. If one compartment is breached and floods, the remaining watertight compartments provide enough buoyancy to keep the vessel afloat — this is the 'damage stability' concept.

What is 'dynamic stability' and how does it differ from 'static stability'?

Answer: Static stability is the righting moment at a fixed heel angle; dynamic stability is the work done by the righting lever over a range of heel angles, representing the vessel's ability to absorb energy from waves

Static stability is the righting moment (GZ) at any one heel angle. Dynamic stability is the area under the GZ curve — representing energy. A vessel with large dynamic stability can absorb more heeling energy from a wave without capsizing, even if its static GZ at any angle is similar to a less dynamically stable vessel.

What construction feature is a 'sheer strake' and what function does it serve?

Answer: The uppermost continuous plank or plate forming the top edge of the hull — it is highly stressed and contributes significantly to the longitudinal strength of the vessel

The sheer strake is the top plank (wood) or plate (steel/aluminum) of the hull side, running the full length at deck level. As the top chord of the hull's longitudinal girder, it carries significant tensile stress in hogging and compressive stress in sagging — making it a critical structural member.