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A good hull combines performance and stability in every sense from its shape to the quality of its construction and the durability of its materials.
A well designed hull has a stable self-righting form. 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 Form ==
Thirty-five foot waves are not to be sneered at in any size of boat. On November 9, 1913, a storm on the Great Lakes with 35-ft wave height sank 12 freighters in a single night.<ref>The Detroit News, November 13, 1913, Detroit, MI, USA</ref> The Queen Mary II took a severe pounding from 30-35-ft waves on her maiden voyage in 2004.<ref> CBS News, http://www.cbsnews.com/stories/2004/01/12/world/main592773.shtml</ref> On March 3, 2005, the 72-ft sailing yacht, Team Save the Children, competing in the Global Challenge 2004-2005, became airborne when hit by an exceptionally large wave in the [[:SouthernOceanPassages|south Pacific]]. Today, worldwide, about two large ships sink every month; although most are heavily laden freighters and some are poorly maintained.<ref>The Cargo Letter, Countryman & McDaniel, http://www.cargolaw.com/presentations_casualties.html</ref>
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="50%" border="1">
<tr><th colspan="3">Waterline Length vs.<br />Significant Wave Height (ft)</th></tr>
<tr><td>70</td><td> 38.50</td><td> 24.50</td></tr>
</table>
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.
<ref>Ship-sinking monster waves revealed by ESA satellites, 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>
In 2005, the [http://www.nrl.navy.mil/ Naval Research Laboratory] in Mississippi reported that [https://en.wikipedia.org/wiki/Hurricane_Ivan Hurricane Ivan] created waves of 30-40 m. Such rogue waves will become more common as hurricanes increase in frequency due to global warming.<ref> Hurricanes Whip Up Huge Waves, Philip Ball, Nature, August 4, 2005, http://www.nature.com/news/2005/050801/full/050801-10.html</ref> (Warmer water superheats hurricane cells.)
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>
== Reserve Buoyancy ==
Freeboard, the distance from the waterline to the edge of the highest watertight deck amidships, is a rough measure of reserve buoyancy. Typically buoyancy is lost when the edge of the freeboard meets the water.
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>
== Roll Stability ==
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 superstructure and the easier it is to revert.
Many displacement boats will self-right from 65-70 degrees before they turn turtle. Unlike sail boats very few have positive stability to 130 degrees.
A vessel’s boats’s stability can be divided into two performance categories: initial stability and ultimate stability.
Initial stability defines the angles of heel that are normal to a vessel's operation. This is also the [[#Static Stability|static 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.
On a sail boat, ballast must counteract the lateral forces on the sails. Without this a sailboat will lay down in the water and capsize.
Ballast is usually placed in the keel, which acts as a lever, so you don't need as much weight below as above. The keel is filled with a high density material, such as concrete, iron, or lead. By placing the weight as low as possible in a sailboat (often in a large bulb at the bottom of the keel) the maximum 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 [[#Roll Moment of Inertia]], which can be avoided by distributing weight instead of concentrating it.<ref>http://www.greatharbourtrawlers.com/-stability-vs-ballast.html</ref> Also, removing weight from high up is more effective than adding it lower down.
Adding excessive ballast will make the roll motion more aggressive and less comfortable. Extra ballast will reduce the roll angle but the return will be snappier with a higher roll acceleration and more 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 vesselsand some yachts, holds water to balance the boat. <ref>http://www.bedardyachtdesign.com/articles/the-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 centre of gravity and keep the propeller and rudder submerged.
=== Static Stability ===
==== Centre of Gravity ====
[[File:CentreGravity.png|thumb|left|260px|Centre of gravity where the downward force of gravity equals the weight of the boat]]
The centre of gravity (CG) is the point inside the hull where the downward force 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 CG. A low CG increases 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 bad. Makes you wonder about dinghies on the boat deck.
==== Righting Arm ====
[[File:RightingArmCreated.png|thumb|left|260px|GZ is the righting arm]]
When a boat is upright, the CB is above the CG, on the centreline. As a boat heels, the CB moves to the side in the direction of the heel. The horizontal distance between CG and CB is the righting arm (GZ). Heeling changes the underwater shape of the boat, and begins to move it toward a tipping point. As the edge of the freeboard meets the water, the outboard shift of the CB reduces and eventually changes direction as the boat heels further. This is caused by the change in the underwater hull shape. Obviously as the CB changes direction, the GZ is reduced.
The righting moment (restoring force) is GZ multiplied by displacement (D). The longer the righting arm and/or the heavier the displacement, the greater the restoring forces.
[[File:RightingArmUpsettingMoment.png|thumb|left|260px|More heeling causes righting moment to be upsetting moment]]
As the boat exceeds its range of initial stability, and enters the zone of ultimate stability, the restoring force begins to decrease. This happens due to the changing shape of the immersed hull. As it continues to heel, the CB shifts inboard and the righting moment becomes less and less just when the boat needs more and more to restore it to upright. The boat becomes increasingly unstable. When the CB moves to the opposite side of the CG, the righting moment becomes an upsetting moment. When the boat reaches its Angle of Vanishing Stability it capsizes.
== Roll Acceleration ==
Roll acceleration is the force of gravity (G force) you experience during a roll. High rates of acceleration are very uncomfortable, stress the body, and make it impossible to sleep. Marchaj <ref> Marchaj, Seaworthiness, The Forgotten Factor, chapter 4, "Boat Motions in a Seaway"</ref> has proposed four physiological states: Imperceptible, Tolerable, Threshold of Malaise, and Intolerable. Malaise starts at 0.1 G, Intolerable starts at 0.18 G.
== Roll Moment of Inertia ==
The roll moment of inertia defines the amount of torque (think wave pressure) required to rotate a mass (think roll the boat).<ref>https://en.wikipedia.org/wiki/Roll_moment</ref> Increasing inertia (reducing rolling) is accomplished by spreading out weight aboard rather than having it highly concentrated. Because of the leverage or gyroscopic effect, weight at the perimeter of the boat will have a much higher resistance to changes in motion, increasing dynamic stability.<ref>http://www.kastenmarine.com/beam_vs_ballast.htm</ref>
== Roll Damping Systems ==
Roll-damping systems, as the name implies, are designed 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
=== Active Stabilizers ===
Active stabilizers are another type of roll-damping fin. They have electric or hydraulic motors so that their angle of attack in the water can be adjusted dynamically, a little bit like wing flaps on an airplane. Electro-mechanical sensors and a control system make automatic adjustments to the fins. Actuators can be electric or hydraulic. The plates on the hull must be strengthened where the stabilizers are attached. They should be located close to the pivot point of the hull, typically just aft of the maximum beam. As mentioned before, active stabilizers are more effective on a round bilge hull than on a hard chine hull. They should be located close to the pivot point of the hullAlthough they can dampen rolling motions more than 80%, typically just aft of the maximum beamthey do not increase stability. Unfortunately, they are not considered workable at speeds below 8 knots.<ref>http://www.brayyachtdesign.bc.ca/Stability.pdf</ref>
=== 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 Horn. A ballast stabilizer consists of two interconnected water tanks, one on either side of the centreline. As the boat heels a pump transfers water rapidly between tanks to counterbalance the rolling motion. A variation on this theme is to use sliding weights.
== Maximum Hull Speed ==
Hull Speed = 1.34 * LWL^1/2
[[File:MaximumHullSpeed.jpg|thumb|left|260px|Maximum hull speed occurs when the length of the bow wave equals the waterline length, making the vessel appear to be supported at both ends
– Photo with permission © Djurgardsvarvet http://www.djurgardsvarvet.se/
]]
Maximum hull speed of a displacement boat in knots is 1.34 times the square root of the length of the hull at the water line. Maximum speed is attained when the length of the bow wave is the same as the waterline length. Maximum hull speed is really the maximum efficient hull speed. You can drive a boat faster than its hull speed but it will take increasing gobs of power to do so.
The wetted area (WP) of the boat’s hull is an indicator of friction through the water. WP is very important in a submarine but less so in a surface ship, where wave resistance is more important.
== 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 ==
[Based on sail boats]
