Ship stability

Ship stability

Ship stability is an area of Naval Architecture and ship design that deals with how a ship behaves at sea, both in still water and in waves. Considerations are made as to the center of gravity and center of buoyancy of vessels and how they interact.


Ship stability is a complicated aspect of naval architecture which has existed in some form or another for hundreds of years. Historically, ship stability calculations for ships relied on rule-of-thumb calculations, often tied to a specific system of measurement. Some of these very old equations continue to be used in naval architecture books today, however the advent of the ship model basin allows much more complex analysis.

Master shipbuilders of the past used a system of adaptive and variant design. Ships were often copied from one generation to the next with only minor changes being made, and by doing this, serious problems were not often encountered. Ships today still use the process of adaptation and variation that has been used for hundreds of years, however computational fluid dynamics, ship model testing and a better overall understanding of fluid and ship motions has allowed much more in-depth analysis.

Transverse and longitudinal bulkheads were introduced in ironclad designs between 1860 and the 1880s, anti-collision bulkheads having been made compulsory in British steam merchant ships prior to 1860 [From Warrior to Dreadnought DK Brown ] . Prior to this, a hull breach in any part of a vessel could flood the entire length of the ship. Transverse bulkheads, while expensive, usually increase the likelihood of ship survival in the event of damage. It does so by isolating seawater to smaller sections protected by bulkheads. Longitudinal bulkheads were intended to do the same, however, the damaged stability effects must be taken into account to eliminate excessive heeling. Today, most ships have means to equalize the water in sections port and starboard (counter flooding), which helps to limit the stresses experienced by the structure, and also alter the heel and/or trim of the ship.

Add-on Stability Systems

These systems are designed to reduce the effects of waves or wind gusts. They do not increase the stability of the vessel in a calm sea. The IMO International Convention on Load Lines does not mention active stability systems as a method of ensuring stability. The hull must be stable without active systems.

Passive Systems

Bilge Keel

A bilge keel is a long fin of metal, often in a "V" shape, welded along the length of the ship at the turn of the bilge. Bilge keels are employed in pairs (one for each side of the ship). A ship may have more than one bilge keel per side, but this is rare. Bilge keels increase the hydrodynamic resistance when a vessel rolls, thus limiting the amount of roll a vessel has to endure.


Outriggers may be employed on certain vessels to reduce rolling. Rolling is reduced either by the force required to submerge buoyant floats or by hydrodynamic foils. In some cases these outriggers may be of sufficient size to classify the vessel as a trimaran, however on other vessels they may simply be referred to as stabilizers.

Antiroll Tanks

Antiroll tanks are tanks within the vessel fitted with baffles intended to slow the rate of water transfer from the port side of the tank to the starboard side. The tank is designed such that a larger amount of water is trapped on the higher side of the vessel. This is intended to have an effect completely opposite to that of the free surface effect.


Paravanes may be employed by slow moving vessels (such as fishing vessels) to increase stability.

Active Stability Systems

Many vessels are fitted with active stability systems. Active stability systems are defined by the need to input energy to the system in the form of a pump, hydraulic piston, or electric actuator. These systems include stabilizer fins attached to the side of the vessel, or tanks in which fluid is pumped around to counteract the motion of the vessel.

tabilizer Fins

Active fin stabilizers are normally used to reduce the roll that a vessel experiences while under way. The fins extend beyond the hull of the vessel below the waterline, and alter their angle of attack depending upon heel angle of the vessel. They operate in a very similar way to airplane wings. Cruise ship frequently use this type of stabilizer system because the high cost of incorporating it into the vessel can be justified. Pleasure yachts down to 15M LOA will increasingly choose active fin stabilization as the cost/benefit ratios are perceived to improve.This system may have any of the following disadvantages:
* Because the fins may be retractable, they may take up valuable space in the engine compartment.
* When fins are not retractable, they constitute fixed appendages to the hull, possibly extending the beam or draft envelope; at a minimum, requiring attention for additional hull clearances.
* Altering the angle of attack requires the vessel to use fuel in supplying the power required to do so. However the power expended for fin motion may be offset by power recovered through more stable tracking on course. Power saved by following a more accurate course may be difficult to quantify.
* The fin and actuator mechanism is expensive to manufacture and fit into the vessel, especially when compared to a bilge keel.

While the typical "active fin" stabilizer will effectively counteract roll for ships under way, some active fin systems have been shown capable of reducing roll motion when vessels are not under way. Referred to as Stabilization while not under way or Stabilization at Rest, these systems work by moving fins of special design, with the requisite acceleration and impulse timing to create effective roll cancellation energy.

Calculated stability conditions

When a hull is designed, stability calculations are performed for the intact and damaged states of the vessel. Ships are usually designed to slightly exceed the stability requirements (below), as they are usually tested for this by a classification society.

Intact Stability

Intact stability calculations are relatively straightforward and involve taking all the centers of mass of objects on the vessel and the center of buoyancy of the hull. Cargo arrangements and loadings, crane operations, and the design sea states are usually taken into account.

Damaged Stability

Damaged stability calculations are much more complicated than intact stability. Finite element analysis is often employed because the areas and volumes can quickly become tedious and long to compute using other methods.

The loss of stability from flooding may be due in part to the free surface effect. Water accumulating in the hull usually drains to the bilges, lowering the centre of gravity and actually increasing the metacentric height. This assumes the ship remains completely stationary and upright. However, once the ship is inclined to any degree (a wave strikes it for example), the fluid in the bilge moves to the low side. This results in a list.

Stability is also lost due to flooding when, for example, an empty tank is holed and filled with seawater. The lost buoyancy of the tank results in that section of the ship lowers into the water slightly. This creates a list unless the tank is on the centerline of the vessel.

In stability calculations, when a tank is holed, its contents are assumed to be lost and replaced by seawater. If these contents are lighter than seawater, (light oil for example) then buoyancy is lost and the section lowers slightly in the water accordingly.

For merchant vessels, and increasingly for passenger vessels, the damage stability calculations are of a probabalistic nature.This is a concept in which the change that a compartment is damaged is combined with the consequences for the ship, resulting in a damage stabillity index number that has to comply with certain regulations.

Required Stability

In order to be acceptable to classification societies such as the American Bureau of Shipping, Lloyd's Register of Ships, and Det Norske Veritas, the blueprints of the ship must be provided for independent review by the classification society. Calculations must also be provided which follow a structure outlined in the regulations for the country in which the ship intends to be flagged.

For U.S. flagged vessels, blueprints and stability calculations are checked against the U.S. Code of Federal Regulations (CFR) and SOLAS conventions. Ships are required to be stable in the conditions to which they are designed for, in both undamaged and damaged states. The extent of damage required to design for is included in the regulations. The assumed hole is calculated as fractions of the length and breadth of the vessel, and is to be placed in the area of the ship where it would cause the most damage to vessel stability.

In addition, U.S. Coast Guard rules apply to vessels operating in U.S. ports and in U.S. waters. Generally these Coast Guard rules concern a minimum metacentric height or a minimum righting moment. Because different countries may have different requirements for the minimum metacentric height, most ships are now fitted with stability computers that calculate this distance on the fly based on the cargo or crew loading. "Cargomax" is a popular computer program used for this task.

See also

* Stability conditions (watercraft)
* Stabilizer (ship)
* Free surface effect
* Metacentric height
* Stabilization at zero speed


* [| Title 46 U.S. Code of Federal Regulations]
* [ ABS Rules for Building and Classing Steel Vessels 2007]
* [ Overview of a few common Roll Attenuation Strategies]

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