Seakindliness describes a boat's motion. It's different from sea-worthiness, which defines whether or not a boat is fit to sail. A seakindly boat is easy on its crew and reduces human fatigue, a decisive factor in safety at sea. Your boat is in constant motion. Each movement-roll, pitch, heave, yaw, or sway-is a response to wave action. The combination of these motions is perceived as seakindliness or lack thereof. Rollis a side-to-side rotation about a fore-and-aft axis (usually near the waterline). A boat that rolls fast, often, and deeply means discomfort.Pitchis the up-and-down motion of the bow and stern around the boat's center of flotation (an athwartships axis aft of amidships).Heaveis the up-and-down motion of the entire boat, a lifting and dropping in response to wave action.Poundingis the jarring outcome of pitching, a result of flat areas in hull shape impacting with water.Swayis a side-to-side movement in which wave action moves the boat to either side of the course line. The heavier the boat and the more underbody area, the less tendency to sway.Yawis a back-and-forth weaving in the boat's course in response to waves. Boats with high directional stability tend to yaw less. These design characteristics affect a boat's motion at sea.Hull midship section.A wineglass-sectioned hull shape will have a slower and easier motion at sea than that of a hard-bilged hull shape.Beam.Relatively wide beam (high beam-to-length ratio) contributes to stability but can mean a faster, more sudden roll and a quicker heave. A relatively narrow beam (low beam-to-length ratio) can have a more moderate response to heave and a slower, easier roll.Displacementis the boat's all-tip weight. Light-displace-ment boats will give a quicker, jerkier motion than a like boat of moderate displacement.Lateral planeis the underwater profile's area. A boat with minimum lateral plane (shallow, canoe body, minimum-area fin keel) will move rapidly in a seaway. A boat with generous lateral plane and heavier displacement gives a more comfortable ride.Moments of inertia,defined as side-to-side (roll) and fore-and-aft (pitch), are measures of how far weights (like ballast and mast) are from the boat's center of gravity. A boat with a low moment of inertia will pitch less but more quickly; a boat with a high moment of inertia will pitch more slowly but more deeply. |
This article was found in the SAIL 1997 Sailboat Buyers Guide. Their Web Address is: www.sailbuyersguide.com
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