W60 Compass

W60 Compass

There are items that belong solely on boats, uniquely nautical in appearance, always purely functional yet having a beauty that functional equipment always has. One such item is the ships compass.

We always provide a magnetic compass to our vessels, even though the vessel may also carry a Fluxgate or other such electronic compass. What if, as with anything that relies on a wire running to it, it fails? How can a mariner obtain a bearing to find his position in this circumstance?

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Steel and iron have the effect of “pulling” at a magnetic compass, creating serious, uncorrectable deviations.

In a steel vessel the simplest way to solve this is to locate the compass outside the steel structure. In this case we install a reflector binnacle compass. This has what appears as a periscope at the helm that contains a mirror reflecting the ships course to the helmsman. It is a simple device.

This arrangement should possibly be adopted in any vessel where the wheel house contains an array of electronic equipment which also has an influence on a compass.

Were we have a fly-bridge, as the W60 does; we have to fabricate using a material that will not affect the compass. The options are GRP or stainless steel. We have used stainless steel. While this may seem the expensive option, stainless steel in eighth plate (1/8 inch) being a common size, is little more costly than mild steel.

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In the photo above we can see the compass card suspended in alcohol; not water. The higher specific gravity of alcohol slows down the movement of the card when it is subject to vessel rolling and turning.

The development of the compass, as we know it today for navigation, has taken place over the past 600 years. Although the magnet or lode stone was discovered long before by the Chinese who did use a form of compass that had South rather than North as a reference.

It was the Italians, or more specifically the Venetians and Genoese, both great trading and shipping centres, who advanced the compass as we know it today.

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On closer inspection of the compass card we can see that our vessel, sitting on the building blocks, is pointing approx north, remembering that the compass has yet to be adjusted. This is not a fluke, the vessel, any vessel, has its own magnetic field, so we build the vessel with a North/South aspect to keep the two magnetic fields, as much as possible, in sync. This means that when it comes to adjusting the compass the deviation to be corrected will be less. Traditionally shipyards are set out with a North/South aspect for this very reason.

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Of course there have been advancements on the compass that the Venetians and Genoese started with. As iron began to be used in shipbuilding, particularly as fastenings in wooden ships, ways had to be found to counter its effect on the compasses magnetic field.

While no doubt many have contributed to its development, the first real advancement was by Matthew Flinders, Royal Navy Captain, cartographer and explorer; the first European to circumnavigate Australia.

Captain Flinders found that by hanging a soft iron bar in front of the compass negated the effect of other ironware used in the vessel. This has come to be known as the “Flinders bar” and today compasses are supplied with several which are contained in the wooden tube fixed to the front of the binnacle.

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A further development are the two rectangular boxes seen each side of the compass. These began as two simple iron balls, adjustable so as to correct the compass. They are the work of Lord Kelvin, scientist and yachtsman who was also responsible for the first reliable depth sounding devices. Today the iron balls take the form of flat bars that lay horizontally.

The magnetic compass should not be considered a redundant feature as it now appears to be, far from it as it provides redundancy when electronic alternatives fail. Every ocean going vessel should be equipped with this most basic piece of equipment as a reliable course can be steered and when a recognizable landmark is found the vessels position and course can be plotted.

Next month after sea trials we will be “swinging” the compass; a job performed by a registered compass adjuster. We will look at this in detail, along with taking a bearing using the asmuthing ring.

Last Updated (Monday, 02 January 2012 22:06)

 

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