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Leisure yachts normally fully depend on the on-board man power to perform tasks such as sail trimming, rig adjustment, steering and sail changing. As sailboats get larger, with sails and rigs increasing in size, it becomes impossible to manhandle them without some mechanical assistance. Therefore, a wide variety of mechanical devices have evolved to allow to control highly-loaded systems with relative small forces as can be performed by human muscular strength.

Winches and block-and-tackles are the most common devices to perform this task. These devices apply the physical principle of mechanical leverage to increase force and torque at the expense of a longer hauling path and ultimately lower speed. The ratio of load to effort is called mechanical advantage and this ratio is inverse proportional to the velocity ratio (complying to the law of the conservation of energy).

The average forces a man may exert are:

  • pull directly downwards from above the head: a force equivalent to the body weight.
  • pull horizontally with both hands standing up with feet braced: about 35 daN.
  • pull horizontally with one hand standing upright: about 25 daN.
  • pull comfortably in any position: about 15 daN.

The loads for trimming sails on an 40ft yacht in a blow on the other hand may easily exceed 1000 daN, illustrating the required ratio of mechanical advantage.

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