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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 ==
==== 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.
== 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.