An Accident at Sea can ruin your whole day!


By Allison Peter

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.


is 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.


is the up-and-down motion of the bow and stern around the boat's center of flotation (an athwartships axis aft of amidships).


is the up-and-down motion of the entire boat, a lifting and dropping in response to wave action.


is the jarring outcome of pitching, a result of flat areas in hull shape impacting with water.


is 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.


is 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.


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.


is the boat's all-tip weight. Light-displace-ment boats will give a quicker, jerkier motion than a like boat of moderate displacement.

Lateral plane

is 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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