KNOWING Everything you need to know about the shipboard compass

The compass is the instrument that can indicate along which direction we are traveling relative to magnetic north.

A LITTLEHISTORY
The first people to know about the compass were Chinese navigators; they used a simple magnetized needle that was influenced by magnetic north; then legend has it that in the 1300s the Amalfitan Flavio Gioia (who, by the way, never existed) invented the rose (actually devised by the Greeks).

The first liquid spherical compass is from the early 1900s, by Englishman White. It is therefore an instrument that, in its essence, has not undergone major transformations. It uses the Earth’s magnetism to work: it is known that a calamity attracts soft iron objects; similarly, a magnetized needle is drawn toward the magnetic north, as if it were a pole of a giant calamity the size of the Earth.

The magnetic compass essentially consists of a special vessel (the mortar) enclosed by a domed glass, containing a special liquid in which are immersed one or more magnetized needles, fixed on a graduated rose resting at its center on a pivot; the latter is in turn integral with a gimbal suspension (internal or external) that ensures the continuous horizontal positioning of the rose, whatever the heeling of the yacht.

leggi anche

Veneziani Yachting alongside the Blue Carbon Sailing Team: a partnership for sustainable sailing

THE ROSE.
The rose is divided into degrees from O to 360 (north to east, Figure 32) and usually varies in diameter between 50 and 150 millimeters. In the best-kept models a hard stone (sapphire or ruby) is placed in its center to eliminate as much as possible any friction that might occur in the system that binds the rose on the pivot pin.

Rose and magnets (i.e., the moving crew of the compass) should not be affected by the movements of the boat, but should rotate freely to indicate magnetic north as accurately as possible.

For a more definite reading, degrees are plotted and numbered every 5 or even, on larger diameter compasses, every 1 or 2 degrees. Cardinal points are internationally referred to as N (North), E (East), S (South) and W (West or West). Sometimes the North may also be marked with a symbol, while the background color of the rose is almost always black or, more rarely, red or white.

THE LINE OF FAITH
On the mortar (which is bonded to the boat) is drawn the line of faith, a line that always indicates the longitudinal axis of the yacht (of course if care was taken during assembly to collimate this line on the boat).

With it, one can determine by how far the boat’s direction of motion (i.e., the stern-axis) moves away from magnetic north. Depending on whether the lubber line is drawn at the forward or aft end of the compass (i.e., faces north or south when the boat is sailing 0°), the compasses result in direct or reverse readings.

In the first case, which is the simplest and most common, the helmsman to maintain the course simply has to follow the directions given by the magnetized needle, bringing the boat to the right if the value of the course angle to be followed is shifted to the right relative to the line of faith and vice versa otherwise.

leggi anche

VIDEO Discovering the secrets of Harken winches and equipment

With reverse-reading compasses, e.g., wall compasses, in which by necessity the lubber line is drawn toward the stern, however, the reverse reasoning must be done: if it is the lubber line that is to the right of the value of the heading angle you wish to hold, then you must rotate the hull clockwise until the heading and lubber line coincide, and vice versa.

THE MORTGAGE
It is the vessel, the spherical “box” that contains the actual instrument. Inside the mortar is placed a liquid that can have various compositions, based on distilled water and alcohol, glycerin or petroleum. It serves to slow down and dampen all movements and vibrations on the rose, thus to promote easy reading (depending on the degree of dampening of oscillations, one can distinguish compasses for fast motorboats or for sailboats). Its transparency must not alter over time; for this purpose, the instrument is always provided with a special compass cover so as to diminish the action of sunlight. Regarding changes in temperature, resulting in expansion of the liquid, there is an expansion membrane inside the mortar so as to compensate for the increase in pressure or volume.

It may happen that you have to refill the liquid when you observe bubbles forming; this is not a serious damage (unless it is too frequent) and can be easily remedied through the special “filler cap” located at the bottom, provided you have a small amount of spare liquid.

The upper protection, as we have already mentioned, consists of a transparent flat or hemispherical domed glass (in this case it also acts as a magnifying glass due to the dioptric effect of the liquid). The glass ensures good rose reading, with no image distortion even under severe skidding conditions.

Finally, the mortar is attached to the yacht with a column mount called a binnacle, including the night lighting system and any magnetic offsets. Illumination is an important feature: the light should be amber and glare-free, spread over the entire surface of the rose, but without being too strong, otherwise it would disfavor pupil adjustment for adequate night vision of maneuvers.

THE CHOICE OF COMPASS
The compass must take into account some important requirements: the size of the boat and the material from which it is made; the location where it will be installed (different is a wall-mounted compass, which is excellent for small cabin cruisers, than a compass to be installed on the rudder wheel mount, which is typical of larger displacement yachts).

Of course, it is essential that there are no magnetic or ferrous masses in the vicinity, as well as electrical circuits or instruments capable of influencing the magnetic field, for a radius of at least 70-100 centimeters around the compass (in this regard, remember never to leave cameras, portable radios, knives or other seemingly harmless things nearby), on pain of abnormal needle indication.

The compass should be installed in a position that is easy for the helmsman to read, so not too far away and with the lubber line (whether it is on the front or back side, depending on model with direct or indirect reading) always placed frontally.

Maintenance is very low; besides remembering to cover it when not in use, in case salt encrustation is observed, a good washing with fresh water and careful drying of all surfaces is sufficient.