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A well designed hull has a stable self-righting form. It incorporates A large range of positive stability is achieved with an optimum vertical centre of gravity combined with the right proportion of beam, freeboard and wide weight distribution.<ref>http://www.kastenmarine.com/beam_vs_ballast.htm</ref> In most yachts stability increases until 45-60 degrees of heel and then slowly diminishes until it vanishes at 90-120 degrees.<ref>http://www.brayyachtdesign.bc.ca/Stability.pdf</ref> The hull can incorporate passive stabilizers like radius chines and bilge keels. A flared bow optimizes performance underway. A double hull, if affordable, offers extra security against penetration by floating objects. A bulbous bow reduces the horsepower and fuel required for a given speed, and slightly increases the top speed in displacement boats of more than 45 ft, while providing a mount for a bow thruster and forward-looking sonar. Active stabilizers are effective but expensive and work better on a round bilge. == Hull Shape Form == 

 A boat’s length on the water line (LWL) affects its resistance to capsizing, and the [[#Maximum Hull Speed|maximum speed ]] of a displacement hull. (A [[BulbousBowGeneral|bulbous bow]] can increase maximum speed.) If the height of a beam abaft wave breaking at the bow or stern exceeds the boat’s length, it won’t be able to motor up it to the top. It is likely to pitchpole, i.e., tumble end over end. Also in general, in heavy weather and high waves or offshore, a longer boat performs better, and has better directional stability. (See [[#Length to Beam Ratio]].)

Rogue, or freak, waves are another matter. They can arise anywhere in any sea condition, in heights from 50 to 100 ft or more, endangering even the largest ships. <ref>French Research Institute for Exploitation of the Sea, http://www.ifremer.fr/metocean/rogue_waves.htm/</ref> <ref>http://www.sciencenews.org/pages/sn_arch/11_23_96/fob2.htm/</ref> <ref>Science News Online, Science News, http://www.science-frontiers.com/sf066/sf066g14.htm/</ref> Until recently they were thought to be rare but most common in the [https://en.wikipedia.org/wiki/Agulhas_Current Agulhas Current] off the Cape of the same name on the southeast coast of South Africa, between Durban and Port St. Johns.<ref>Wikipedia, http://en.wikipedia.org/wiki/Freak_wave/</ref> <ref>SA Sailing Directions Vol 1 page 43, http://www.dynagen.co.za/eugene/freaks.html/</ref> <ref> Freak waves, rogue waves, extreme waves and ocean wave climate, Kristian B. Dysthe, Department of Mathematics, University of Bergen, Norway, et al, http://www.math.uio.no/~karstent/waves/index_en.html/</ref> <ref>Understanding the Freak Wave, Dr Paul Taylor, Department of Engineering Science at Oxford University, et al, http://www.epsrc.ac.uk/ContentLiveArea/Downloads/Adobe%20Portable%20Document%20Format/E-TaylorP.PDF</ref>In early 2016, researchers at MIT said they may have developed a method that would allow ships to have 2-3 minutes of warning before a rogue wave appears.<ref>http://www.popsci.com/now-ships-can-get-advance-warning-monster-rogue-waves</ref> In December 2016 the [http://public.wmo.int/ World Meteorological Organization] officially measured the highest recorded rogue wave.<ref>http://public.wmo.int/en/media/press-release/19-meter-wave-sets-new-record</ref> <table width="8050%" border="1">

<tr><th>Boat<br/>LOA</th>

 But research in 2004 by the [http://www.esa.int/ESA European Space Agency] indicated freak waves are very common, and not always associated with currents like the Agulhas or the [https://en.wikipedia.org/wiki/Gulf_Stream Gulf Stream]. During a three-week period, its MaxWave project using satellite-borne Synthetic Aperture Radar detected 10 massive waves, some nearly 100 ft (30 m). The next phase of the project, WaveAtlas, will analyse two years worth of data to map the location and frequency of freak waves [24.<ref>Ship-sinking monster waves revealed by ESA satellites, 47]ESA Portal, 21 July 2004, July 21, 2004, http://www.esa.int/esaCP/SEMOKQL26WD_index_0.html/</ref> <ref>Predicting Rogue Waves, Technology Review, March 01, 2007, http://www.technologyreview.com/Infotech/18245/</ref>

Even coastal waves can become rogues. Waves have been observed on the Alabama coast as high as 32 m; while coastal 30-ft waves are frequent in Maine.<ref> Vijay Panchang, Texas A&M University at Galveston, www.tamug.edu/mase/wave_file/wave%20%simulations.htm</ref>Rogues have also been observed on the Ottawa River.<ref>http://ottawacitizen.com/news/local-news/egan-alone-on-a-quiet-river-when-came-the-giant-waves-mayday-in-a-canoe</ref> 

Freeboard plus draft is the total height of the hull. A generous freeboard gives lots of headroom inside, and makes it easier to recover from a knockdown. Too much freeboard makes a boat tippy.The combination of extremely wide beam and low freeboard is dangerous.<ref>http://www.brayyachtdesign.bc.ca/Stability.pdf</ref> 

There are several kinds of roll or heeling stability: [[#Ballast Stability|ballast]], [[#Hull Form|form]], [[#Static Stability|static]] and [[#Dynamic Stability|dynamic]]. These are important in determining a boat’s resistance to capsizing from a beam-on wave, and the type of rolling motion. The rolling motion dictates your comfort. Dynamic stability and large angle stability must be considered as equal partners with the boat's static stability.<ref>http://www.kastenmarine.com/beam_vs_ballast.htm</ref> Inherent in the design of every boat is a restoring force from rolling called the righting moment (RM), and a point of instability. A boat capsizes when the force of a wave causes it to heel over to its point of instability, called the Angle of Vanishing Stability. Beyond this point, the boat capsizes and may stay inverted. The wider the beam, the more difficult it will be to revert. The upside down boat sits on the water like a flat-bottomed boat. The deeper the keel, the greater the counterbalancing force to the superstructureand the easier it is to revert.

Initial A boats’s stability defines the angles of heel that are normal to a vessel's operation. This is also the static can be divided into two performance categories: initial stability. This is usually between zero and 15 degrees of heel. A wide-beamed boat heels less (has greater stiffness), and is more comfortable. But a narrower-beamed boat has more ultimate stability. Ultimate stability is the angle of vanishing stability.

The righting moment is Initial stability defines the angles of heel that are normal to a force generated by the righting arm (GZ)vessel's operation. The righting arm This is also the transverse distance between the centre of gravity (CG) and the centre of buoyancy (CB) [28[#Static Stability|static stability]]. Hopefully this will become clearer as you read on.=== Centre of Gravity ===The centre of gravity This is the point inside the hull where the downward force usually between zero and 15 degrees of gravity equals the weight of the boat, i.e., its displacement. It is the midpoint of the mass. Keeping weight low in the hull lowers the CGheel. A low CG increases wide-beamed boat heels less (has greater stiffness), i.e., resistance to heeling and capsizing. That’s why engines are mounted low, ballast is put in the keel; and heavy superstructures or loads on deck are badmore comfortable. Makes you wonder about dinghies on the boat deck.=== Centre of Buoyancy ===The centre of buoyancy is But a counteracting force to gravity. It is the midpoint of the underwater volume of the narrower-beamed boat, ihas more ultimate stability.e., it Ultimate stability is the centre point angle of the geometric shape of the hull. It is on the centre line of the hull, usually amidships with a vertical height just a bit more than half the draftvanishing stability.

The upward thrust of righting moment is a force generated by the CB counteracts righting arm (GZ). The righting arm is the downward thrust transverse distance between the centre of gravity(CG) and the centre of buoyancy (CB). To illustrate this<ref>A Best Practices Guide to Vessel Stability, float a bowl in some water. Put your finger in Guiding Fishermen Safely Into the centre and push downFuture, Second Edition, United States Coast Guard, http://www. The bowl will resist sinking and push backuscg. Your finger is gravity. The resistance mil/hq/g-m/cfvs/</ref> Hopefully this will become clearer as you feel is the buoyancyread on.

Plenty of hull area beneath the waterline lowers the CB. As a boat is more heavily loaded, increasing the draft, the CB moves lower, reducing the righting arm, and the freeboard and ultimate stability are reduced.=== Righting Arm Ballast Stability ===When Ballast is weight added to a boat is upright, below the CB is waterline to counteract the effects of weight above the CG, on waterline. <ref>https://en.wikipedia.org/wiki/Ballast</ref> Without the centrelineballast a vessel witll be very tippy and happier upside down. As  On a sail boat heels, ballast must counteract the CB moves to lateral forces on the side sails. Without this a sailboat will lay down in the direction of water and capsize. Ballast is usually placed in the heelkeel, which acts as a lever, so you don't need as much weight below as above. The horizontal distance between CG and CB keel is the righting arm (GZ)filled with a high density material, such as concrete, iron, or lead. Heeling changes By placing the underwater shape of the boat, and begins to move it toward weight as low as possible in a tipping point. As sailboat (often in a large bulb at the edge bottom of the freeboard meets keel) the watermaximum righting moment can be extracted.<ref>https://en.wikipedia.org/wiki/Sailing_ballast</ref> <ref>http://www.wavetrain.net/boats-a-gear/458-modern-sailboat-design-ballast-stability</ref> However, this will increase the outboard shift [[#Roll Moment of the CB reduces and eventually changes direction as the boat heels further. This is caused Inertia]], which can be avoided by the change in the underwater hull shapedistributing weight instead of concentrating it. Obviously as the CB changes direction<ref>http://www.greatharbourtrawlers.com/-stability-vs-ballast.html</ref> Also, the GZ removing weight from high up is reducedmore effective than adding it lower down.=== Righting Moment ===The righting moment (restoring force) is GZ multiplied by displacement (D)Adding excessive ballast will make the roll motion more aggressive and less comfortable. The longer Extra ballast will reduce the righting arm roll angle but the return will be snappier with a higher roll acceleration andmore conducive to seasickness. <ref>http://www.kastenmarine.com/beam_vs_ballast.htm</ref> Ballast really plays a role at higher angles of heel. Once the heel angle starts to reach or exceed 45 degrees ballast comes into its own.<ref>http://www.brayyachtdesign.bc.ca/Stability.pdf</ref> A ballast tank, found on larger vessels and some yachts, holds water to balance the heavier boat.<ref>http://www.bedardyachtdesign.com/articles/the displacement, -effects-of-water-ballast-on-sailboat-stability/</ref> Water can be pumped from side to side to counteract rolling. On large cargo ships travelling empty water can be pumped in to lower the greater centre of gravity and keep the restoring forcespropeller and rudder submerged.

=== Prismatic Coefficient ===

== Midsection Coefficient==

== WP Area==

== A/B Ratio==A/B, the ratio of the area above the water to the area below the water, is a deprecated measure of stabilitythat is not used by marine architects. It is a gross rule of thumb that is easily misused. Stability can be better predicted using computer programs that consider many factors. A For argument's sake, a lower ratio, say below 2.5, is inherently more stable than a top-heavy boat with a high A/B ratio of say 3.0 or more. == Ballast to Displacement Ratio==

The higher the number, the more resistant a boat is == Displacement to movement, and the more comfortable it is. Obviously bigger boats give a better ride; although the formula favours a narrow beam. Use with caution analysing power boats.Bulbous BowBulbous bows were developed in the 1950’s for large cargo vessels, to improve their penetration of the water, and reduce fuel consumption. The underwater bulb creates a wave 180 degrees out of phase with the original bow wave. This cancels or reduces the bow wave. The first merchant vessel with a bulbous bow was the <i>Yamashiro Maru</i> delivered in November 1963 by the Mitsubishi Heavy-Industries, Ltd. Nagasaki Shipyard in Japan.<ref> Ripples in Time, Bulbous Bow – Introduction of wave-making resistance reduction technology, http://www.nykline.co.jp/english/seascope/200010/</ref> Today all the largest ships, including Nimitz-class aircraft carriers, have bulbs.<ref> Reagan Takes a Bow, http://www.nn.northropgrumman.com/Reagan/About_the_ship/Bow.htm</ref>Length Ratio ==

A bulbous bow will reduce fuel consumption 3% to 15%; increase sea keeping by dampening pitching motions by up to 20%; and increase speed slightly. The greatest benefit to fuel and horsepower will be at speeds over 6 knots, lessening as speed decreases.<ref>Bray Yacht Design and Research, http:Displacement/Length Ratio = D/www(0.brayyachtdesign.bc.ca/</ref> <ref>Nordhavn, http://www.nordhavn.com/design/full/bulbous.htm/</ref> <ref>Cape Horn Yachts, http://capehornyachts.com/</ref>The <i>Yamashiro Maru</i> used 25% less horsepower than a similar vessel with no bulb.<ref>Ripples in Time, Bulbous Bow – Introduction of wave-making resistance reduction technology, http://www.nykline.co.jp/english/seascope/200010/</ref> A bulb slightly increases the length of the waterline (01 * LWL), the determining factor in the top efficient speed of a displacement hull. A smaller boat with a bulb goes slightly faster than the LWL formula suggests.Bulbous bows have no place on a zippy semi-displacement hull. But a full displacement boat, making a long passage at a constant speed, is an ideal candidate for a bulb. Even so, many designers have resisted bulbous bows on boats under 60 ft. But there is no good reason not to have one on displacement boats of 50 ft or even 45 ft if the hull form is suitable<ref>Bray Yacht Design and Research, http://www.brayyachtdesign.bc.ca/</ref> <ref>http://members.shaw.ca/diesel-duck/library/articles/bulbous_bows.htm/</ref> and the cruising speed is 6 kt or more. Designs by DeFever, Nordhavn, Cape Horn and Moloka’I Strait all have bulbs.^3

The key seems Displacement to be hull form. Bulb design length ratio indicates if the boat is a bit of an art. Bulbs should be incorporated into the general hydrodynamic design of the hullheavy (results greater than 300), not appended like crude cigarsmedium (200-300) or light (75-200) cruiser. Generally, bulbs Displacement is in long tons (2240 lb). Note that ranges for sail boats are a modified ellipse shapedifferent (325-400, pointed on the bottom 275-325 and flattened on top200-275 respectively). A bulb should not extend forward D/L of the extremity of the bow. Vertically it should be just below the surface, to create 280-350 indicates a wave in front of the ship that boat is 180 degrees out of phase with the bow wave. This means it creates a wave hollow where normally you would find a wave crestheavy cruiser suited for serious offshore work.<ref> http://members.shaw.ca/diesel-duck/library/articles/bulbous_bows.htm</ref>

One reason bulbs are better suited to larger boats is that they work better at higher speedsCapsize Risk = B/(D/[0. A 45-ft trawler with an LWL of 38 ft has a maximum hull speed of 8.2 kt. Its cruising speed is more likely to be 6 kt, just sufficient to justify a bulb. At lower speeds, bulbs simply increase drag9*64])^0.333

Bulb designs for large ships often incorporate The Capsize Risk is a bow thruster or sonar domeseaworthiness factor derived from the [http://www. Putting sailingusa.info/formula.htm USYRU] analysis of the bow thruster forward as much as possible increases disastrous 1979 FASTNET Race, funded by the steering leverage[www. A watertight hatch gives access to the interior sname.org/ Society of the bulbNaval Architects and Marine Engineers]. With a bulb, you will need a bowsprit Values less than two are good for the anchor, or hawseholes port and starboard, to avoid scraping the bulb with the anchor chainsail boats. No comparable data exists for displacement boats.

== Roll Damping Systems ==Roll-damping systems, as the name implies, are designed Length to reduce the roll of a vessel. Reducing roll increases comfort. Roll-damping systems are passive or active, and can be internal or external. The main types are:* Bilge Keels* Active Stabilizers* Ballast Stabilizers* Paravanes* Flopper StoppersBeam Ratio === Bilge Keels ===Bilge keels are a type of fin attached to the chines of the hull. They serve as passive roll stabilizers, by offering resistance to the water when the trawler rolls. They should be located as far aft as possible, to reduce roll and improve stability.<ref>Cape Horn Yachts, http://capehornyachts.com/</ref> Long low-aspect keels can reduce rolling by 35-55% [43]. (Aspect ratio is the ratio of width to height, e.g., 4:3 is 4 units wide by 3 units high.)

Bilge keels can be made strong enough to support the hull and keep the boat upright when it is accidentally or deliberately grounded. To support grounding, both the keel and bilge keels are engineered three to four times stronger than ABC requires. Failing to engineer the keels adequately can cause bilge plates to crack.<ref>Ship Construction, D. J. Eyres, Butterworth-Heinemann, ISBN 07506807092007<Length/ref> Such keels will offer some protection to accessories attached to the main keel. Bilge keels can also be designed as short angled fins.Beam Ratio = LOA/B

The downside of bilge keels is that they increase drag slightlyA lower Length to Beam (L/B) number indicates a beamier boat. Hopefully the extra drag will be offset by the performance of Boats with a bulbous bow.=== Active Stabilizers ===Active stabilizers are another type of roll-damping finwider beam have better initial stability, and more interior room. They have electric or hydraulic motors so that their angle of attack in the water can be adjusted dynamicallyworse ultimate stability, a little bit like wing flaps on an airplane. Electro-mechanical sensors and a control system make automatic adjustments high inverted stability, meaning it is hard to the finsturn them upright. Actuators can be electric or hydraulic. The plates on the A beamier hull must be strengthened where the stabilizers are attached(L/B ratio below 2. As mentioned before7) has more room inside, active stabilizers are but is less efficient and pounds much more effective on a round bilge hull than on a hard chine hullgoing into head seas. They should be located close to the pivot point of the hull<ref> Downeast Lobster Boats, typically just aft of the maximum beamhttp://www.=== Ballast Stabilizers ===Ballast stabilizers were once common only on large cruise ships but have begun appearing in European yachts and a few large trawlers like Cape Horndowneastboats. com/hulldesign.html </ref> A ballast stabilizer consists of two interconnected water tanksbeamier boat has less roll angle but more [[#Roll Acceleration|roll acceleration]], one on either side of which is the centrelineprimary culprit of seasickness. As the boat heels Even small changes in beam have a pump transfers water rapidly between tanks to counterbalance the rolling motiondramatic effect. A variation on this theme is to use sliding weights<ref>http://www.kastenmarine.com/beam_vs_ballast.htm</ref>

Obviously the pumping systems should have excellent redundancy. You wouldn’t want water ballast on the wrong side of the boat at the wrong time.== Paravanes & Flopper Stoppers ==A discussion of roll damping would be incomplete without mentioning paravanes narrow hull has better directional control and flopper stopperssteers better. Paravanes are long poles extended horizontally For boats from the sides of 30 to 50 feet in length a trawler, hull with winged paravanes that reduce the boat’s rolling inertia when underwayan L/B ratio above 3.0 is more efficient and pounds less into head seas.

Paravanes work the same way a high-wire performer uses a balance pole, or you use your arms when play walking down a beam or curbComfort factor = D/(0. They originated on fishing trawlers, which anyway have booms to set and raise nets65*(0. They can be very effective7*LWL+0.3*LOA)*B^1.33)

At other timesThis Comfort Factor, the booms can bouncedeveloped by Ted Brewer, or even smack predicts the overall comfort of a sail boatwhen it is underway.<ref> Ted Brewer Yacht Design, so people weigh them down with chainhttp://www. They’re also cumbersome to set tedbrewer.com/yachtdesign.html</ref> The formula predicts the speed of the upward and raisedownward motion of the boat as it encounters waves and swells. If Faster motion makes passengers more uncomfortable. The higher the number, the more resistant a boom boat is lost to movement, and the more comfortable it is. Obviously bigger boats give a better ride in bad weathercalm conditions; however, the boat can capsize from formula rightly favours a narrow beam. Less beam means less [[#Roll Acceleration|roll acceleration]]: the imbalancemain cause of seasickness. You might want to consider them only if you’re converting a fishing boat that was designed for themUse with caution analysing power boats.