In the case of a catamaran, which has no keel to weigh the boat down, slimmer and more hydro dynamically optimized hulls, this theoretical hull speed rarely applies. The large bow wave that is produced by a fat hull shape will hinder the boat from surpassing its hull speed which is calculated as 1.3 x square of its waterline length.

The heeling monohull will produce more water drag and also by necessity has a wider hull beam, which will suck the boat into a wave pattern, limiting its speed, something that Archimedes already noticed 2200 years ago. Further the attachment points on modern fin keeled monohulls are comparatively small and inefficient in relation to the task of keeping an extremely heavy structure attached. This factor further indicates that the ballast of a monohull is just the starting point of a downward weight spiral and a keel actually increases the total displacement of a monohull by more than just the weight of the ballast. The weight of the keel is just the beginning of a vicious cycle, because the structure supporting the keel has to be even stronger (and heavier) to prevent the keel from falling off. This means that the only reason for the existence of tons of lead, is the necessity to keep its carrier upright. Depending on the type of monohull, the weight of the ballast can often constitute 30%-40% of the total displacement of the boat. On a monohull it is the offset of the Center of Gravity (CG) of the keel from the Center of Buoyancy (CB) when heeled, which will determine its stability.

The more a monohull heels, the more the Righting Moment (RM) and Righting Arm (RA) will increase, counteracting the overturning moment. Usually this ballast is in form of a weighted fin, often with a bulb attached to the bottom. In order to keep a monohull upright she will need ballast, which will resist the heeling force of the wind. Monohulls: StabilityĬatamarans and monohulls deal with stability in fundamentally completely different ways.

Sailboat stability index