Naval Ship's Intact Stability

Thought there are many aspects of naval architecture including: hydrostatics, hydrodynamics, flotation and stability, structures, arrangements, construction, science and craft, and the individual naval architect; the focus of this blog is the intact stability of naval ships.

There are many aspects of Intact Stability explained throughout the blog. This blog will detail the 3 types of equilibrium in naval ships, and things that affect transverse stability.

Intact Stability is the stability of the surface of the ship when the intactness of the hull is maintained. This information does not relate to any scenario where the water tank is damaged or freely flooded by seawater.

A fundamental concept of a floating body is Equilibrium. The 3 types of equilibrium depend on the relation between center of gravity and center of buoyancy.

Stable equilibrium is “when vertical position “G” is lower than transverse metacenter “M.”” The center of buoyancy “B” shifts and become “B1” when this happens. The distance between weight and buoyancy is the moment that brings the ship back to its original position, this is called the righting moment.

Neutral equilibrium is the most dangerous situation possible, avoid at all costs. “It occurs when the vertical position of “CG, center of gravity, coincides with the transverse metacentre “M””. There is no righting moment and the ship would stay heeled as long as a neutral stability prevails. Here there is no righting moment, which could cause the ship to heel more than desired. There is a risk here of cargo shifting enough to offset the equilibrium, then there would be a rise to unstable equilibrium.

Unstable equilibrium is “when vertical position “G” is higher than the position of transverse metacenter “M.”” After this shift, the ship heels and the center of buoyancy “B” shifts to “B1.” This shift makes the righting moment negative, creating a further heel and there is a possibility of the deck being immersed and then the ship is predicted to capsize.

Metacentric height is one of the most vital parameters in a ships stability. If its metacentric height is more than 0, then it is stable; equal to 0, it is neutrally stable; and less than 0 it is unstable.

Transverse stability can be affected by beam winds, lifting of weight by the sides, high speed turning maneuvers, grounding, tension on mooring lines, movement of weight athwartship, and water trapped on deck.

• Beam winds affects the part of the ship that is above the waterline. The resistance on the top of the boat acts against the resistance in the underwater part of the boat.

• Lifting of weight by the sides is when the weight or cargos on the side of ships get lifted up and out by a deck crane. THis causes a heeling moment and then a shift of center of gravity.
• High speed turning maneuvers is when the ship makes a turn, the “centrifugal force acts horizontally on the center of gravity of the ship, in a direction opposite” of the turn. The sharper the turn the more centrifugal force generated which makes more angle of heel.
• Grounding is when one side of the underwater part of the ship makes contact with the ground. Depending on what side hits the ground the opposite side would heel.
• Tension on mooring lines is when mooring lines are too tense, or the ship drift away from the mooring point, the ship can heel.
• Movement of weight athwartship is the movement weight athwartship, in transverse direction, will alter the position of the centre of gravity of the ship.The ship will heel to a point at which new centre of buoyancy is at such a position such that weight and buoyancy act through the same line.
• Water trapped on deck is affected when the sea water on the deck shifts periodically and creates a cyclic heeling moment due to the continuous change in the center of gravity.

The wreckage of the RMS Titanic and the Costa Concordia are just two examples of shipwrecks that have deeply affected the thought process that goes into designing massive ships such as naval vessels.

The RMS Titanic was a British passenger liner that sank in the North Atlantic Ocean in the early hours of 15 April 1912, after colliding with an iceberg during its maiden voyage from Southampton to New York City.

The Costa Concordia was a Concordia-class cruise ship built in 2004 by the Fincantieri's Sestri Ponente yards in Italy and operated from 2005 until 2012 by Costa Crociere. It capsized after striking an underwater rock off Isola del Giglio.

Chakraborty, Soumya. “Ship Stability – Understanding Intact Stability of Ships.” Marine in Sight, Naval Architectire, 27 Oct. 2017, www.marineinsight.com/naval-architecture/intact-stability-of-surface-ships/.

“Naval Architecture.” Wikipedia, Wikimedia Foundation, 16 May 2018, en.wikipedia.org/wiki/Naval_architecture.