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=== Static Stability ===
Static stability determines the angle of heel under constant wind or wave conditions. Factors that increase static stability are heavy displacement, low centre of gravity, and a centre of buoyancy that shifts outboard quickly when the boat heels. Boats with wider beams exhibit more static stability (stiffness) and less dynamic stability.
=== Dynamic Stability ===
Dynamic stability determines the roll in response to a transient wind gust or violent wave that is shifting the performance into the zone of ultimate stability, i.e., instability. Heavy displacement and a narrow beam improve dynamic stability somewhat. A wider beam catches the wave early, giving it more leverage and time to act on the hull. Once a boat is inverted, the increased static stability associated with a wider beam becomes a liability since it keeps the boat inverted for a longer period of time.
From this, we can see that a lower CG is better. The lower the CG, the longer is the initial righting arm (GZ), giving the boat a quick roll and snappy response. The higher the CG, the lower is the righting moment (RM) and the slower the roll. Carrying ballast and other weighty items as low as possible lowers the centre of gravity. Keeping superstructure weight to a minimum and not storing heavy items on deck will also help. Adding ballast to the flybridge to slow the roll, as some people have advocated, is a very bad idea.
A good amount of freeboard will improve both the maximum righting moment and the limit of positive stability. Too much freeboard will make the boat tippy by raising the CG. Adding ballast to make the boat stiffer reduces the freeboard and reduces the zone of positive stability. Adding ballast to the flybridge, as recommended by one magazine, is absolutely crazy.
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A well designed hull has a stable self-righting form. It incorporates 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 ==
Hulls can be rounded (round bilge) or designed with a hard chine. A chine is the line of intersection between the sides and bottom of a flat-bottomed boat. A radius chine has a V-bottom, rising to a second chine that starts the sides, yielding a shape closer to a rounded bilge. A rounded hull looks nicer than a slab side, and intuitively should sit better in the water.
Past the bow, the hull should shift from a gentle to a rapid increase in beam. If this is done right, the boat will sail in a pool of calm water. The bow generates a positive pressure wave, which is cancelled out by the negative wave caused by the rapid increase in hull form. Such a boat will have a low Prismatic Coefficient ([[#Prismatic Coefficient|also see below]]).
== Length on Water Line ==
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]].)
== 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.
== Roll Stability ==
There are several kinds of roll or heeling stability: ballast, form, static and 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.
The righting moment is a force generated by the righting arm (GZ). The righting arm is the transverse distance between the centre of gravity (CG) and the centre of buoyancy (CB).<ref>A Best Practices Guide to Vessel Stability, Guiding Fishermen Safely Into the Future, Second Edition, United States Coast Guard, http://www.uscg.mil/hq/g-m/cfvs/</ref> Hopefully this will become clearer as you read on.
=== Static Stability ===
Static stability determines the angle of heel under constant wind or wave conditions. Factors that increase static stability are heavy displacement, low centre of gravity, and a centre of buoyancy that shifts outboard quickly when the boat heels. Boats with wider beams exhibit more static stability (stiffness) and less dynamic stability.
=== Dynamic Stability ===
Dynamic stability determines the roll in response to a transient wind gust or violent wave that is shifting the performance into the zone of ultimate stability, i.e., instability. Heavy displacement and a narrow beam improve dynamic stability somewhat. A wider beam catches the wave early, giving it more leverage and time to act on the hull. Once a boat is inverted, the increased static stability associated with a wider beam becomes a liability since it keeps the boat inverted for a longer period of time.
From this, we can see that a lower CG is better. The lower the CG, the longer is the initial righting arm (GZ), giving the boat a quick roll and snappy response. The higher the CG, the lower is the righting moment (RM) and the slower the roll. Carrying ballast and other weighty items as low as possible lowers the centre of gravity. Keeping superstructure weight to a minimum and not storing heavy items on deck will also help. Adding ballast to the flybridge to slow the roll, as some people have advocated, is a very bad idea.
A good amount of freeboard will improve both the maximum righting moment and the limit of positive stability. Too much freeboard will make the boat tippy by raising the CG. Adding ballast to make the boat stiffer reduces the freeboard and reduces the zone of positive stability. Adding ballast to the flybridge, as recommended by one magazine, is absolutely crazy.
=== What Affects Static and Dynamic Stability ===
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 Period ==