A compass is a device that indicates direction. It is one of the most important instruments for navigation. Magnetic compasses are the most well-known type of compass. They have become so popular that the term “compass” almost always refers a magnetic compass. While the design and construction of this type of compass have changed significantly over the centuries, the concept of how it works has remained the same.
Magnetic compasses consist of a magnetised needle that is allowed to rotate so it lines up with the Earth's magnetic field. The ends point to what are known as magnetic north and magnetic south. Scientists and historians don’t know when the principles behind magnetic compasses were discovered. Ancient Greeks understood magnetism. As early as 2,000 years ago, Chinese scientists may have known that rubbing an ironbar (such as a needle) with a naturally occurring magnet, called a lodestone, would temporarily magnetise the needle so that it would point north and south.
The ship’s pilots are acquainted with the configuration of the coasts; at night they steer by the stars, and in the daytime by the sun. In dark weather, they look at the south-pointing needle. Magnetic compasses are the most well-known type of compass. They have become so popular that the term “compass” almost always refers a magnetic compass. While the design and construction of this type of compass have changed significantly over the centuries, the concept of how it works has remained the same. The magnetic compass was an important advance in navigation because it allowed mariners to determine their direction even if clouds obscured their usual astronomical cues such as the North Star. It uses a magnetic needle that can turn freely so that it always points to the north pole of the Earth’s magnetic field.
The first liquid mariner’s compass believed practicable for limited use was patented by the Englishman Francis Crow in 1813. A gyrocompass is a type of non-magnetic compass which is based on a fast-spinning disc and rotation of the Earth (or another planetary body if used elsewhere in the universe) to automatically find geographical direction. A rudimentary working model of a liquid compass was introduced by Sir Edmund Halley at a meeting of the Royal Society in 1690. However, as early liquid compasses were fairly cumbersome and heavy, and subject to damage, their main advantage was aboard a ship. Protected in a binnacle and normally gimbal-mounted, the liquid inside the compass housing effectively damped shock and vibration, while eliminating excessive swing and grounding of the card caused by the pitch and roll of the vessel.
When installed properly, a good compass serves as a reliable, simple tool for setting and keeping a course. Your Maritime Survey Australia licenced compass adjuster will be able to assist you with the following:
Mounting – If you are installing a compass, the first thing you need to determine is where and how to mount it. Flush-mounted compasses, mounted horizontally on the helm, take up less space and are generally the most stable. If that is not an option on your boat, there are also surfaces, binnacle- and bracket-mounted compasses that don’t require space underneath to install. No matter how you install one, it’s crucial that the compass’s lubber line be parallel with the keel along the centerline to get a proper reading.
Magnetic North – A marine compass will have one set of magnets that is balanced to seek out magnetic north. Magnetic north is not true north — as in the North Pole — but the location of Earth’s north polarisation, near Greenland. The compass rose on nautical charts shows the difference, called variation, between true north and magnetic north for navigation
Corrector Magnets – A quality marine compass will also house a second set of magnets called the corrector magnets. These correct the deviation that can occur from interference caused by your boat’s electronics or other magnetic sources. The correctors are connected to adjustable brass rods installed in the lower portion of the compass housing, enabling your compass adjuster to compensate, or “swing,” your compass card to adjust for deviation.
Gimbals – To protect the compass from jostling around with a boat’s normal pitch and roll, it’s imperative that a marine compass has a built-in gimbal system. The system is mounted on spring-loaded brass pins and kept level with a weighted brass counterbalance.
Cards – The dial, also called the card, is the numbered circular piece you use to read the degree of heading. It rotates on a steel pivot mounted on a sapphire jewel “movement” that helps with smooth turning. Front-reading compasses have a semisphere card, good for eye-level readings, but the flat card style is dampened, thus more stable in rough water. This style also allows you to take bearings relative to your course.
Despite advancements with GPS, a compass is still a valuable tool. You should use every method of navigation available and compare the results to make sure that you are where you think you are.