Boats rotate around three axes - rolling around the fore and aft axis; pitching about the transverse axis; yawing around the vertical axis. It's the Roll Moment of Inertia (RMoI) that should concern us most as it's around the fore and aft axis that a boat is most likely to capsize. This is calculated by multiplying the weight of all the boat's constituent parts by the square of the distance from the boat's Center of Gravity to the part's Center of Gravity - a tedious but necessary task for the designer. The squared term means that the distance of heavy items from the Center of Gravity greatly affect the RMoI, and the greater RMoI the less the boat will react to a gust of wind, or a large wave. So boats with their ballast deep in their keels, their fuel and water tanks as far outboard as possible, and long heavy masts will have greater RMoI's and will be more dynamically stable as a result. Such boats will have long roll periods and will be highly resistant to rapid changes in heel angle. Lost your rig? You'll roll much more! This will be very apparent to a crew unfortunate enough to have lost their rig, as this item is the boat's greatest contributor to the moment of inertia. Without it, the boat's roll period will be very quick and snappy, and the probability of capsize much higher. Makers of grandfather clocks had cause to be grateful for the effects of the rotational moment of inertia. They used it to govern the rate of gain, or loss, of their creations. To correct a 'slow' clock, the pendulum would be shortened slightly thereby reducing the distance to the neutral axis. This decreased the period of oscillation - it would swing faster - and speed up the clock's mechanism. Conversely, for a clock that gained, the pendulum would be increased in length to create the opposite effect. The principal reason for using pendulums in clocks was that for a given length, the period of oscillation remains constant, irrespective of the amplitude. Artwork by And so it is with a boat; if she's rolling gently at anchor, or from gunwale to gunwale in a seaway, the roll period will be the same. So clearly there's a lot more to a sailboat's stability than the Righting Moment alone.
Boats rotate around three axes - rolling around the fore and aft axis; pitching about the transverse axis; yawing around the vertical axis. It's the Roll Moment of Inertia (RMoI) that should concern us most as it's around the fore and aft axis that a boat is most likely to capsize. This is calculated by multiplying the weight of all the boat's constituent parts by the square of the distance from the boat's Center of Gravity to the part's Center of Gravity - a tedious but necessary task for the designer. The squared term means that the distance of heavy items from the Center of Gravity greatly affect the RMoI, and the greater RMoI the less the boat will react to a gust of wind, or a large wave. So boats with their ballast deep in their keels, their fuel and water tanks as far outboard as possible, and long heavy masts will have greater RMoI's and will be more dynamically stable as a result. Such boats will have long roll periods and will be highly resistant to rapid changes in heel angle. Lost your rig? You'll roll much more! This will be very apparent to a crew unfortunate enough to have lost their rig, as this item is the boat's greatest contributor to the moment of inertia. Without it, the boat's roll period will be very quick and snappy, and the probability of capsize much higher. Makers of grandfather clocks had cause to be grateful for the effects of the rotational moment of inertia. They used it to govern the rate of gain, or loss, of their creations. To correct a 'slow' clock, the pendulum would be shortened slightly thereby reducing the distance to the neutral axis. This decreased the period of oscillation - it would swing faster - and speed up the clock's mechanism. Conversely, for a clock that gained, the pendulum would be increased in length to create the opposite effect. The principal reason for using pendulums in clocks was that for a given length, the period of oscillation remains constant, irrespective of the amplitude. Artwork by And so it is with a boat; if she's rolling gently at anchor, or from gunwale to gunwale in a seaway, the roll period will be the same. So clearly there's a lot more to a sailboat's stability than the Righting Moment alone.
Hull-Characteristics.docx