The magnetic compass is the oldest instrument for navigation and has been a vital tool for navigators at sea for centuries.

1. Impeller Logs And Compasses
- Magnetic Compass
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By Jonr Jony - http://magneticcompass.net/
For Measuring distance at sea, the old type of log that gave us the knot as unit of speed has long since
given way to more sophisticated mechanical and electronic devices.One of the oldest is the Walker log.
This uses a torpedo-shaped spinner a few inches long towed behind the boat on a length of braided line.
As it moves through the water, spiral fins on the torpedo make it spin, twisting the line. The on-board
end of the line is hooked on to the back of the log instrument, where it turns a shaft connected to a
reduction gear box. This in turn moves the hands on a series of dials, rather like those of an old
fashioned gas meter, to give Direct reading of the distance the spinner has moved through the water.
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Advantages of the Walker log are its rugged simplicity and the ease with which weed or debris can be
cleared from the pinner. Its disadvantages are that its display has to be mounted right at the back of the
boat; that the log line (usually 30 or 60 feet in length) has to be streamed before the log can be used,
and recovered before entering harbour; it tends to under-read at very low speeds; and at speeds over
about ten knots the spinner is inclined to jump out of the water and skitter along the surface.

2. There are definite techniques for streaming and recovering a mechanical trailing log, intended to reduce
the risk of the line tangling.
To stream the log, first attach the on-board end to the hook on the back of the display unit. Then,
keeping the spinner in hand, feed out all the line to form a long U-shaped loop astern before dropping
the spinner overboard, well off to one side of the loop. Some owners like to hold on to the line just
astern of the display unit for a few seconds, just to absorb the snatch as the load comes on to the
line.When recovering the log, speed is essential, especially if the boat is moving fast. Unclip the inboard
end from the hook on the back of the display, and drop it overboard, allowing it to trail out astern while
you pull in the log line. Then holding the spinner, gather in the line, coiling it as you go. Trailing the line
astern like this allows any kinks to unravel.
The electrical trailing log is superficially similar to a Walker log, inasmuch as it uses a spinner towed
astern of the boat on a long line. In this case, however, the spinner is in two parts, and the 'log line' is an
electrical cable. The front part of the spinner is attached to the cable and only the rear part is free to
rotate. As it does so, an electronic sensor in the front part makes and breaks an electrical circuit, so the
on-board display unit receives a short pulse of electricity each time the spinner rotates. These pulses are
counted electronically and are presented as a digital display of speed and distance run.

3. The advantages and disadvantages of this type of log are much the same as for the mechanical Walker
log except that it is dependent on electrical power from internal dry batteries, which in return allows the
display unit to be mounted almost anywhere on board, and that because the line itself is not twisting, it.
On cruising boats, hull-mounted logs are by far the most popular type, though in principle they are much
the same as the electrical trailing log: a rotating impeller sends a stream of electrical impulses to a
display unit mounted in the cockpit or near the chart table.
The impeller - which can be either a miniature version of the trailing log's spinner, or a paddle wheel an
inch or so in diameter - is mounted in a fitting called a transducer, which either protrudes through the
bottom of the boat or hangs down below the transom.The disadvantages of this system are that an
impeller so close to the hull can be affected by the water flow around the hull itself, and that it is
difficult and potentially dangerous to withdraw the transducer to clear weed or debris from it at sea.
The reason in-hull logs are so popular is primarily the convenience of not having to stream and recover
30 feet or more of log line at the beginning and end of each passage.
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