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Choosing the Right Compass - Danforth

A magnetic compass is a relatively simple piece of navigational equipment. Its sole function is to point to Magnetic North. Properly installed and adjusted, it will reliably guide you home, even in the event that your electronic navigational equipment becomes disabled. It should be considered as a valuable piece of safety gear.

compass card apparent size

Selecting the right compass for your boat is an easy process. There are usually a few correct compass choices for any given boat, based upon the boat size, design, speed and of course, your personal preference.

Modern compasses are primarily defined by their size, card style, mounting style, color, and whether they are gimbaled. "Size" is defined by the "apparent" card diameter, that is, how big the card appears under the magnifying lens effect of the transparent plastic compass dome (See Fig. 1). Common sizes are 2", 3", 3-3/8", 3-3/4", 4-1/2", 5", 5-1/2". In general, the larger the compass, the larger the numbers will appear. Hence, the easier the headings will be to read. Another benefit of a larger compass is that it will be more stable in a heavy seaway.

There are three different card (or dial) styles, each with its advantages. The most common style historically is the "flat" card (See Fig. 2A). Modern flat cards are actually dished-shaped like a saucer. The purpose of the turned-up edge is to improve readability at low angles of viewing. Headings are read from the top, forward edge, farthest from the helmsman. Unlike the front-reading style compass card, a quick glance at a flat card provides instant directional orientation. "South is this way and East is that way."

compass flat card

Flat card compasses are the correct choice when the helmsman's line of sight to the compass is from above, as with sailboat pedestal and center console powerboat installations. A major advantage with the flat card style is that the numbers and graduation lines will appear much larger, compared to the other card styles. This is because the flat card is a greater distance from the transparent compass dome than with other card configurations. The magnification effect is therefore greater. As a result, a 3" compass with a flat card can be easier to read than a 3-3/4" compass with a front-read or dual-read card.

compass front reading card

By far, the most common card style currently is front-reading. See Fig. 2B. Front-reading cards are dome-shaped or conical-shaped with the headings read from the card skirt (vertical edge) closest to the helmsman. This style of card is recommended when the compass is mounted at, or just below, the helmsman's line of sight, when a flat card could not be easily read at such a low angle of viewing. Most compasses under 3" in size are front-reading and are most commonly found on powerboats up to about 25'.

There are two drawbacks to front-reading cards. The magnification of the card is only slight. Hence, compared to a flat card compass of the same size, the numbers will appear much smaller and will be more difficult to read. The other drawback is the confusion sometimes created when one attempts to orient oneself directionally by looking at a front-reading card. If, for example, the current heading is "S" (South), the "W" (West) card designation is to the left of the "S" on the card. By turning the boat left however, you would be heading East. With a flat card it is much simpler. If the heading is "S", the "W" card designation is to the right of the "S" on the card. By turning right toward the "W", you would be heading West.

compass dual reading card

The third and most recent card style to evolve is the dual-reading card. This card style combines the elements of both the flat-reading and front-reading card systems. See Fig. 2C. Depending upon the manufacturer, the "flat" graphics may be complete (graduations every 5o) or only partial, which is far less useful for steering. (All Danforth dual-reading cards have complete graphics). The "front-reading" graphics are always complete with this style of card. Compasses with dual-reading cards are the best choice when the helmsman's viewing angle may vary greatly (standing v.s. sitting). For example, when the helmsman may stand or sit while underway, changing the viewing angle from above to level with the line of sight. Dual-reading compasses are common found on mid to large size powerboats where the compass is mounted on top of the console at eye level for a seated helmsman.

Compass mounting options are flush mount, bracket mount, binnacle mount and bulkhead mount. Flush mounting is the preferred method for Boat Builders (See Fig. 3A). Although a clearance hole needs to be cut in the mounting surface, which increases the time required to install the compass, the finished installation is "cleaner" in appearance and gives a more integrated look that is less like an add-on or afterthought. Drawbacks with a flush-mounted compass, in addition to the need to cut a hole in the console, include the requirement for a flat mounting surface that is level within a few degrees and has adequate clearance underneath to accommodate the bottom of the compass. Flush mounting also requires more console space than does a bracket mounting.

Flush mount compass

Bracket mounted compasses provide for the easiest and most versatile installation (See Fig. 3B). Requiring only two screws to affix the compass, the bracket's small "footprint" allows it to be easily mounted in confined areas, where console space is at a minimum. Another benefit is that the bracket can be rotated for mounting on surfaces canted fore and aft, from horizontal to vertical.

Bracket mount compass Binnacle mount compass

Binnacle mounting is most common on sailboats, where the compass, mounted in a cylinder, is affixed to the top of the steering pedestal (See Fig. 3C). Binnacle mounting is sometimes referred to as "deck mounting", when the compass cylinder is mounted to a console or deck. Another variation to binnacle mounting is "surface mounting". Surface mounting usually refers to smaller compasses which are removable from their bases that have been affixed onto the surface of the console. The binnacle mounted compass and its variants are all held to the mounting surface with a number of screws, usually two to four. One drawback to this style of compass is that it occupies a lot of mounting real-estate, much more than with bracket mounting and sometimes more than flush mounting.

Bulkhead mounting is the same as flush mounting except that the compass is installed on a vertical surface (See Fig. 3D). A variation of bulkhead mounting is "panel mounting", where a small compass is mounted in the gauge panel. This style of compass can usually be adjusted to accommodate surfaces inclined up to 30°.

Bulkhead mount compass

Gone are the days when the statement "You can have any color compass you want, as long as it is black" has relevent meaning. Modern compasses are now available in several color configurations. Although a black housing with a black card remains popular, white and gray housings are readily available, as are white, blue and red cards.

With respect to "gimbaling", it is better to have it than not. Gimbaled compasses are usually more expensive than non-gimbaled ones but have a couple of important advantages. They are more stable and more readable. Gimbaling allows the compass card to remain level over a very wide range of roll, pitch and yaw angles. Gimbaling also ensures that the lubberline (index pin) remains perpendicular to, and in proper alignment with, the card for accurate readings. Gimbaled compasses are essential for sailboat installations and recommended for powerboat installations as well. Generally, gimbaling is the "norm" in compasses over 3-3/4" in size.

Ready to make your compass choice? Go to the Compass Selection Guide on page 5. Simply find your boat style and size at the top of the chart, then follow that column down to find the compass model choices recommended for your boat. Each of these compasses can be found in the catalog on the page listed beside the compass model number.

Danforth Compass Catalog

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