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post #8 of Old 05-09-2003
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Factors influencing boat balance

Geeeez Loueeeez, You ask a serious question and you get Looney Toons.

To start with, there are two ways to think about balance. The first looks at all of the component forces acting on a boat and the second is the method used in designing a boat. At any given time there is the real instantaneous balance that is the resolution of all forces acting on the boat. These include the solid-state forces of the sails and underbody plus those non-solid state forces such as induced by wave action and changes in windspeed and direction. If you resolve all of the forces of drive and side force with all of the forces of drag, heeling, leeway, steering moments, lift, and righting moment from the hull and keel, and asymmetry of rig and hull when heeled over, AND if the boat is going in a straight line these forces are in balance.

Of course there is such a wide range of variables to this dynamic balance that it is next to impossible to actually calculate the actual forces at work to sufficient detail to accurately predict the balance of any particular boat at any specific instant.

In sailing a boat we are constantly altering the balance of the boat as we trim or ease the sails, or power the sails up or down. We alter balance by controlling heel, altering fore and aft trim (bow down more weather helm- stern down less). In a general sense beamier and more flat bottomed boats do tend to get more assymetrical as they heel than narrower or rounder bottomed boats. To a great extent modern hull modeling has greatly reduced this heel induced imbalance over earlier designs for beamier boats.

When a boat is actually being designed, balance is calculated on a static basis and adjusted empirically. In other words the center of lateral plane is calculated from the static profile of the hull and keel. The center of effort is calculated from the static profile of the sail plan without adjustments for roach or genoas or spinnakers and the two centers are then checked for alignment. On a design that is actually in balance, the center of effort viewed statically is generally forward of the center of lateral resistance. This distance is called the ''Lead'' (pronounced like to ''lead'' a horse rather than lead like the metal). The amount of lead that a designer gives a boat comes from that designers experience in designing similar types of boats. The lead does not always work out right. In most cases the error results in weather helm, which is better than the other choice a lee helm. One classic case of the lead being worked out wrong is the J-24, which to this day typically has a lee helm.


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