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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.