A Simplified Guide to Small Marine Craft Navigation.

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Chapter 18 (v.1) - Navigational Aids for Sail Powered Craft.

Submitted: May 11, 2017

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Submitted: May 11, 2017



Navigational Aids for Sail Powered Craft.



Sail powered craft, which do not have the same consistency of power-output which gasoline or diesel oil give to motor driven craft, and being considerably slower, strive for greater efficiency and to achieve this very often fit devices for monitoring wind speed and direction.

Such devices could be classed as meteorological instruments, but since their purpose is to assist in the selection of the best course to steer in order to achieve the maximum driving power they may legitimately be classed as navigational aids.

These units are often quite sophisticated, with remote monitoring of measured or computed parameters. Several manufacturers produce wind speed and direction indicators, and several different principles are involved for transmitting the direction indication, although all use a rotating cup head for wind speed measurement.

The instruments described here have excellent presentation and are styled so as to be complementary with other equipment in the maker's ranges of products. This makes an aesthetically pleasing layout relatively easy to achieve, and small consoles are available to simplify cockpit mounting.

The wind speed indicator is simply understood, being a straightforward reading, and is extremely useful for deciding on sail changes, or when to reef.

The wind direction indicator utilizes a two-dial display. One dial is an all-round Coarse Scale display and the other a Fine Scale for use when close-hauled only, the entire scale being occupied by the two sectors from 100 to 500 on either bow. The displays should be sited where the helmsman can see them at all times.

When steering to windward, the helmsman can concentrate on the fine dial, which can be read to one degree. Gradually he will get to know the angle of the apparent wind at which the boat is sailing best.

At 50° from the wind, the fine dial comes against stops and the Coarse Scale is then used. This is even easier to respond to, with an apparent wind diagram on the craft that is always heading to the top of the dial. The coarse indicator can be used when reaching to give indication of wind shifts so that sails can be constantly trimmed, and when running to indicate whether there is any tendency to sail by the lee.

Similar instruments have an anemometer mounted on the masthead that measures the apparent wind speed, while others have a very sensitive wind vane also mounted at the masthead that measures the apparent wind angle.

The wind angle is displayed on two different scales on the same dial, the smaller scale giving 360° indication, and the larger scale, which embraces the whole of the instrument fascia, giving 0° - 60° indication of deviation from a pre-set adjustable datum.

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