264:". Its effectiveness can be as good as that of stabiliser fins. However, that depends on the ship speed (higher is better) and various ship design aspects such as position, size and quality of the rudder positioning system (behaves as fast as a stabiliser fin). Also important is how quickly the ship will respond to rudder motions with roll motions (quick is better) and rate of turn (slow is better). Despite the high costs of high-quality steering gear and strengthening of the ship's stern, this stabilisation option offers better economics than stabiliser fins. It requires fewer installations, is less vulnerable and it causes less drag. Even better, the required high-quality components provide excellent steering properties also for those periods when roll reduction is not required and a significant reduction of underwater noise. Known navy ships with this stabilisation solution are F124 (Germany), M-fregat and LCF (both of Dutch Navy).
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operations, and the design sea states are usually taken into account. The diagram at the right shows the center of gravity is well above the center of buoyancy, yet the ship remains stable. The ship is stable because as it begins to heel, one side of the hull begins to rise from the water and the other side begins to submerge. This causes the center of buoyancy to shift toward the side that is lower in the water. The job of the naval architect is to make sure that the center of buoyancy shifts outboard of the center of gravity as the ship heels. A line drawn from the center of buoyancy in a slightly heeled condition vertically will intersect the centerline at a point called the metacenter. As long as the metacenter is further above the keel than the center of gravity, the ship is stable in an upright condition.
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In case a ship is underway, a fast rudder change will not only initiate a heading change, but it will also cause the ship to roll. For some ships such as frigates, this effect is so large that it can be used by a control algorithm to simultaneously steer the ship while reducing its roll motions. Such
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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; by replicating stable designs, serious problems were usually avoided. Ships today still use this process of adaptation and variation; however,
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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 calculus-based methods of determining stability, particularly Pierre
Bouguer's introduction of the concept of the metacenter in
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A gyroscope has three axes: a spin axis, an input axis, and an output axis. The spin axis is the axis about which the flywheel is spinning and is vertical for a boat gyro. The input axis is the axis about which input torques are applied. For a boat, the principal input axis is the longitudinal axis
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is a long, often V-shaped metal fin 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). Rarely, a ship may have more than one bilge keel per side. Bilge keels increase hydrodynamic resistance when a vessel rolls, limiting
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For merchant vessels, and increasingly for passenger vessels, the damage stability calculations are of a probabilistic nature. That is, instead of assessing the ship for one compartment failure, a situation where two or even up to three compartments are flooded will be assessed as well. This is a
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multiplied by spin speed) is the key quantity. In modern designs, the output axis torque can be used to control the angle of the stabilizer fins (see above) to counteract the roll of the boat so that only a small gyroscope is needed. The idea for gyro controlling a ship's fin stabilizers was first
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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
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of the gyro's flywheel is a measure of the extent to which the flywheel will continue to rotate about its axis unless acted upon by an external torque. The higher the angular momentum, the greater the resisting force of the gyro to external torque (in this case more ability to cancel boat roll).
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gyros were mounted in the forward part of the ship. While it proved successful in drastically reducing roll in the westbound trips, the system had to be disconnected on the eastbound leg for safety reasons. This was because with a following sea (and the deep slow rolls this generated) the vessel
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conventions (SOLAS). 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
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Intact stability calculations are relatively straightforward and involve taking all the centers of mass of objects on the vessel which are then computed/calculated to identify the center of gravity of the vessel, and the center of buoyancy of the hull. Cargo arrangements and loadings, crane
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prior to 1860. Before this, a hull breach in any part of a vessel could flood its entire length. Transverse bulkheads, while expensive, increase the likelihood of ship survival in the event of hull damage, by limiting flooding to the breached compartments they separate from undamaged ones.
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While the typical "active fin" stabilizer effectively counteracts roll for ships underway, some modern active fin systems can reduce roll when vessels are not underway. Referred to as zero-speed, or
Stabilization at Rest, these systems work by moving specially designed fins with sufficient
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Ship stability diagram showing center of gravity (G), center of buoyancy (B), and metacenter (M) with ship upright and heeled over to one side. As long as the load of a ship remains stable, G is fixed. For small angles M can also be considered to be fixed, while B moves as the ship
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Stability is also reduced in flooding when, for example, an empty tank is filled with seawater. The lost buoyancy of the tank results in that section of the ship lowering into the water slightly. This creates a list unless the tank is on the centerline of the vessel.
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reduce the roll a vessel experiences while underway or, more recently, while at rest. They extend beyond the vessel's hull below the waterline and alter their angle of attack depending on heel angle and the vessel's rate-of-roll, operating similarly to airplane
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In stability calculations, when a tank is filled, 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.
528:, 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.
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Damage stability calculations are much more complicated than intact stability. Software utilizing numerical methods are typically employed because the areas and volumes can quickly become tedious and long to compute using other methods.
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to slow the rate of water transfer from the tank's port side to its starboard side. It is designed so that a larger amount of water is trapped on the vessel's higher side. It is intended to have an effect counter to that of the
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When the boat rolls, the rotation acts as an input to the gyro, causing the gyro to generate rotation around its output axis such that the spin axis rotates to align itself with the input axis. This output rotation is called
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Angular momentum is the measure of effectiveness for a gyro stabilizer, analogous to horsepower ratings on a diesel engine or kilowatts on a generator. In specifications for gyro stabilizers, the total angular momentum
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may be employed on vessels to reduce rolling, either by the force required to submerge buoyant floats or by hydrodynamic foils. In some cases, these outriggers are of sufficient size to classify the vessel as a
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Within this framework different countries establish requirements that must be met. For U.S.-flagged vessels, blueprints and stability calculations are checked against the U.S. Code of
Federal Regulations and
480:. This assumes the ship remains 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 lower side. This results in a
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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 center of gravity and actually increasing the
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concept in which the chance that a compartment is damaged is combined with the consequences for the ship, resulting in a damage stability index number that has to comply with certain regulations.
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of the boat since that is the axis around which the boat rolls. The principal output axis is the transverse (athwartship) axis about which the gyro rotates or precesses in reaction to an input.
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were first used to control a ship's roll in the late 1920s and early 1930s for warships and then passenger liners. The most ambitious use of large gyros to control a ship's roll was on an
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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
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108:. Today, most ships have means to equalize water in sections port and starboard (cross flooding), which helps limit structural stresses and changes to the ship's heel and/or trim.
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When fins are not retractable, they constitute fixed appendages to the hull, possibly extending the beam or draft envelope and requiring attention for additional hull clearance.
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stability computers that calculate this distance on the fly based on the cargo or crew loading. There are many commercially available computer programs used for this task.
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scientist, Dr
Alexanderson. He proposed a gyro to control the current to the electric motors on the stabilizer fins, with the actuating instructions being generated by
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International
Convention on Load Lines does not cite active stability systems as a method of ensuring stability. The hull must be stable without active systems.
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Ship stability, as it pertains to naval architecture, has been taken into account for hundreds of years. Historically, ship stability calculations relied on
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and ship design that deals with how a ship behaves at sea, both in still water and in waves, whether intact or damaged. Stability calculations focus on
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Ship stability illustration explaining the stable and unstable dynamics of buoyancy (B), center of buoyancy (CB), center of gravity (CG), and weight (W)
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Add-on stability systems are designed to reduce the effects of waves and wind gusts. They do not increase a vessel's stability in calm seas. The
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tended to 'hang' with the system turned on, and the inertia it generated made it harder for the vessel to right herself from heavy rolls.
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Longitudinal bulkheads have a similar purpose, but damaged stability effects must be taken into account to eliminate excessive
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Depending upon the class of vessel either a stability letter or stability booklet is required to be carried on board.
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Active stability systems, found on many vessels, require energy to be applied to the system in the form of pumps,
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attached to the side of the vessel or tanks in which fluid is pumped around to counteract the vessel's motion.
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vessel, and is to be placed in the area of the ship where it would cause the most damage to vessel stability.
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The first stabilizing gyroscope to be fitted on a ship,
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ABS Rules for
Building and Classing Steel Vessels 2007
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International
Convention for the Safety of Life at Sea
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Ship response to disturbance from an upright condition
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Damage stability (Stability in the damaged condition)
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Intact stability for ships at sea is governed by the
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659:"Fins Purposed For Big Liners To Prevent Rolling"
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244:frequently use this type of stabilizing system.
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634:by D.K. Brown, Chatham Publishing (June 1997)
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394:. Unsourced material may be challenged and
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724:Title 46 U.S. Code of Federal Regulations
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414:Learn how and when to remove this message
88:has allowed much more analytical design.
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306:Gyro stabilizers consist of a spinning
91:Transverse and longitudinal waterproof
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461:International Code on Intact Stability
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677:. International Maritime Organization
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392:adding citations to reliable sources
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260:a system is usually referred to as "
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118:International Maritime Organization
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644:"Italian Liner To Defy The Waves"
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612:Stabilization while not underway
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262:Rudder Roll Stabilisation System
356:Calculated stability conditions
268:Gyroscopic internal stabilizers
173:are interior tanks fitted with
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696:46 CFR Ch. I (10–1–99 Edition)
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776:Seamanship (seafaring) topics
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504:In order to be acceptable to
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82:computational fluid dynamics
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841:Ship-to-ship cargo transfer
632:From Warrior to Dreadnought
596: – German sailing ship
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522:Korean Register of Shipping
514:American Bureau of Shipping
318:on the hull structure. The
314:that imposes boat-righting
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876:Man overboard rescue turn
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542:United States Coast Guard
518:Lloyd's Register of Ships
426:When a hull is designed,
255:Rudder Roll Stabilisation
506:classification societies
112:Add-on stability systems
675:"Intact Stability Code"
298:, in which three large
708:Resolution MSC.267(85)
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432:classification society
428:stability calculations
350:thyratron vacuum tubes
344:proposed in 1932 by a
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86:fluid and ship motions
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291:passenger liner, the
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388:improve this section
278:USS Henderson (AP-1)
146:the amount of roll.
62:Newcastle University
564:Free surface effect
459:(IMO) standard the
180:free surface effect
95:were introduced in
42:centers of buoyancy
826:Naval architecture
729:2007-07-01 at the
526:Det Norske Veritas
500:Required stability
478:metacentric height
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207:electric actuators
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38:centers of gravity
34:naval architecture
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662:Popular Mechanics
647:Popular Mechanics
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209:. They include
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679:. Retrieved
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594:Pamir (ship)
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861:Dry-docking
698:govinfo.gov
681:29 February
606:SS Eastland
58:towing tank
896:Sea anchor
791:Navigation
783:Seamanship
619:References
333:precession
312:precession
285:Gyroscopes
155:Outriggers
150:Outriggers
143:bilge keel
129:Bilge keel
72:the 1740s
891:Anchoring
582:Mary Rose
558:Capsizing
375:does not
191:Paravanes
186:Paravanes
93:bulkheads
911:Category
866:Ropework
806:Pilotage
727:Archived
552:See also
308:flywheel
234:ailerons
160:trimaran
97:ironclad
886:Mooring
881:Buoyage
816:Sailing
396:removed
381:sources
289:Italian
280:(1917).
227:Active
175:baffles
106:heeling
48:History
447:heels.
316:torque
300:Sperry
242:yachts
871:Knots
205:, or
683:2024
524:and
482:list
379:any
377:cite
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293:SS
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