Misc. Stability theory Flashcards
SQA AND HND (5 cards)
Explain with the aid of a sketch why a vessel will heel when turning.
As a vessel turns, a centripetal force acts
inwards towards the center of the turn, approximately through the center of buoyancy of the vessel. Also, an equal and
opposite force called the centrifugal force
acts at and through the center of gravity, out
of the turn. This creates a couple which generates a heeling moment out of the turn. The vessel will heel about the center of flotation due to the heeling moment caused by the centrifugal and centripetal forces
During a sea passage a vessel develops an angle of loll, Describe how this effect can be minimized, other than through improving the GM.
Heel angle in a turn is a function of a ship’s speed squared, and of turn radius. The heeling angle can be reduced by increasing the radius of turn and by reducing the
speed of turn.
explain why the fixed and free trim curve are the same up to the angle of deck edge immersion for an offshore supply vessel.
“Up to the angle of deck edge immersion, the waterplane area and shape of an offshore supply vessel change very little with heel. As a result, the longitudinal position of buoyancy and trimming moment remain nearly the same whether trim is fixed or free, so the curves coincide. Beyond this point, asymmetry increases and the curves diverge.”
Describe the dangers of icing accretion on decks and superstructure to the
stability of a vessel.
Increase in vessel mass.
Reduced freeboard, Reduced angle of deck edge immersion.
Increased top-weight. increased KG. Reduced GM.
Reduction in GZ,
impact increasing with roll angle.
The reduction in GM causes a reduction in the initial gradient (slope) of the GZ
curve.
A greater build-up of ice on one side of a vessel will cause the vessel’s center of gravity to move off-center resulting
in list.
a general reduction in GZ, and an initial negative GZ.
List induced / GZ reduced / Deck edge immersion angle reduced / range reduced / angle and value of peak GZ
reduced / Dynamic stability reduced / GM reduced.
The net effect of ice accretion, particularly if asymmetric, is reduced stability by all measures at small, medium and
large roll angles
