Do you want a magnetic or electronic compass?
Magnetic compasses contain a free-spinning compass card, which aligns with the Earth’s magnetic field and points in the direction of magnetic north. Small onboard magnetic fields, caused by ferrous metal (a steel steering wheel or iron engine block), magnets (a speaker or microphone), or current-carrying wires can divert the compass away from magnetic north.
This deviation must be corrected by moving the compass away from the local magnetic interference (if possible) or by adjusting small compensating magnets inside the compass (using a process called swinging the compass). For deviations too large for the compensating magnets, usually greater than 20°, a deviation table may need to be created.
Electronic fluxgate compasses measure the relative strength of magnetic fields passing through two coils of wire. Using sophisticated electronics, these sensors can deduce the direction of the Earth’s magnetic field. They are still affected by local magnetic fields like steel components and speaker magnets, but they can compensate electronically for those fields that do not change in strength or location.
One advantage of electronic compasses is that their sensors may be mounted remotely in many cases. This allows you to place the sensor where it will be least affected by moving magnetic fields and the motion of the boat. Of course, it does increase the complexity of the installation.
What mounting style do you need?
Size and mounting style are the most important criteria when selecting a compass. There are five basic mounting styles:
Flush-mount compasses are set in a cutout in a horizontal surface with half above the surface and half below.
Bracket-mount compasses are highly versatile since the bracket can be mounted on surfaces with different tilt angles. They can also be removed easily for storage or security.
Bulkhead-mount compasses are popular in smaller cruising sailboats (without a binnacle) and on the dashboards of small powerboats, where they are sized and styled to match the other instruments. The aft bulkhead of a sailboat’s cabin provides good visibility in the cockpit, but is sometimes inclined away from vertical. Some compasses can mount at angles of up to 45°, but others require a leveling block to make their internal gimbals work properly
Surface-mount compasses sit on top of a surface and can be easily removed by pressing two buttons. Mounting is easy, as no large holes need to be cut.
Binnacle-mount compasses also mount on a surface or steering pedestal, like those used on wheel-steered sailboats. Powerboat owners select them when there is no room under the mounting surface for a flush-mount compass to protrude.
Riviera Comet 1 flush mount with flat card
Riviera Pegasus bulkhead-mount for sailboats has 45° lubber lines to help estimate tacking angles
Danforth Scout bracket-mount with combination card.
How big should the apparent card diameter be?
We refer to a compass card’s “apparent” size because the dome and fluid inside it magnify the actual card size. Bigger compasses have better performance for several reasons. They are easier to see from a distance or under poor lighting and have larger numerals, markers and lubber lines. With the larger volume of the compass bowl and more mineral oil fluid, they have more effective damping of the motion of the compass card.
How will you read the compass card?
There are two basic types of traditional compasses: direct reading and flat cards.
Direct reading cards: The heading is read from the side of the card nearest the helmsperson. This is more logical for some users, as you don’t have to peer into the compass to see the heading. Direct reading cards do not bisect the compass dome, however, and therefore have more movement in rough conditions.
Flat cards: The heading is read by looking inside the compass dome, on the forward side of the card. These compasses generally have lubberlines (heading references located in the direction the boat is traveling) and additional lines at 45° and 90° to the boat’s heading.
Dual reading cards: Read like both direct and flat cards. These are almost exclusively bulkhead-mounted sailboat compasses, and can be read more easily from the sides than conventional cards.
Digital numeric displays are how electronic compasses display a heading. They often use cardinal and intercardinal (i.e., N, NW, W, SW, S, SE, E, NE) descriptions as well. Some boaters prefer to see their heading displayed spatially on a compass card in the traditional manner, relative to other points on the compass. See our selection of electronic instruments, like the Velocitek ProStart.
What’s inside a compass
The compass dome allows the compass dial to rotate smoothly through heel, pitch and yaw motions. The heavy-duty, optically clear polymer dome also provides clear and accurate magnification of the dial. It is formulated to prevent sun crazing and will stay stable throughout extreme temperature ranges.
Gimbal system: compasses are engineered to operate normally without bottoming out due to pitch and roll. An internal gimbaling system often provides this important function. Gimbals cradle the dial assembly within an axis system bisecting the compass bowl. These ultra-light weight aluminum gimbals and pans are held in place with spring loaded brass pins. A brass counter balance keeps the entire system level.
Compass bowls and housings provide maximum protection and support for the internal compass components. They are made from brass, or high-strength glass reinforced polymers.
Corrector magnets: compasses that are intended for use on boats that could have deviation problems have built-in compensators. These compensators are non-interactive magnets that are balanced and optimally spaced for accurate correction. Compensation instructions are included.
Fluid baffle and roller diaphragm: baffles reduce turbulence in the dampening fluid. Diaphragms allow the fluid to expand or contract with changes in temperature or pressure without forming bubbles.
Compass installation
Selecting a quality compass is only half the job. After all, it is a precision instrument that needs to be installed properly to be effective and accurate. Some basic installation pointers:
- Choose the most visible and comfortable location for the helmsman.
- The compass must be aligned with the fore and aft axis of the boat.
- Route electrical wires away from the compass. Do not mount near instruments containing magnets or near the engine.
- Twist night light wires around one another to cancel out their magnetic field.
- Check for deviation caused by magnetic objects. Deviation is the error caused by iron and steel on a boat (hardware and fittings) that causes the compass to point away from magnetic north. Compasses need to be “swung” to determine the amount of deviation, and corrector magnets, which lie below the compass, can be adjusted by hand to eliminate deviation. However, compass correcting is complicated, and generally requires an expert (a so-called compass adjuster) to do it correctly. Beware: corrector magnets cannot eliminate changing electromagnetic sources, such as electrical wiring and electronics.
COMPASS FLUID TYPE
Most spherical dome compasses built after the 1960s use odorless mineral spirits to damp the movement of the card. If your compass is older, chances are it contains a mix of isopropyl alcohol and water. But no matter what fluid is inside, a bubble is a sign of a developing problem.
Bubbles and low fluid
Today’s compasses are designed to work with a completely fluid-filled dome, therefore a bubble indicates that fluid is missing. Leaks are hard to spot because of the mineral fluid’s tendency to evaporate immediately upon contact with air. Like the tube of a bike tire, a compass can develop a slow leak, in which case you might notice a bubble in the morning when it is still cool, but not in the afternoon, when the compass is heated up and the fluid has expanded. No matter what, a bubble is a sign of trouble and we recommend you have the compass checked out as soon as possible. Get it fixed and have the fluid replenished, otherwise you risk damage to the card.
Are you going to use these binoculars on a boat?
How binoculars are measured
Binoculars are described using two numbers, such as 7 x 50 or 8 x 25. The first number identifies the magnification or power. The second is the diameter of the objective lens (the lens at the front) in millimeters. Magnification power describes how much closer objects appear when you view them. In a 7 x 50 binocular, the image is magnified seven times. Increased magnification reduces the brightness of the image, so as magnification increases, binoculars require increasingly larger objective lenses to maintain brightness. The larger their size, the more light they can gather.
Features of marine binoculars
Typical Magnification: Many binoculars used for birdwatching or other land applications have too much magnification for use on rocking, pitching boats. The more an image is magnified, the harder it is to keep an object in view, so 7X power is the practical magnification limit for small boats (under 15 m). Image stabilizing binoculars allow comfortable viewing with much greater magnification, up to 18 X power, because they automatically compensate for movement.
Waterproof construction: The damp marine environment, combined with changes in temperature, causes interior lenses of non–waterproof binoculars to fog. Waterproof construction, with the interior o-ring sealed and filled or “charged” with dry nitrogen, combined with flotation in the strap, protects your binoculars if they fall overboard.
Rangefinders are handy for taking bearings or determining approximate height of or distance to an object.
Do you need a built-in compass?
Bearing compasses: Built-in compasses, which appear superimposed near the image you see through the lens, let you take bearings from an object that would be invisible with a non-magnified hand-bearing compass. They’re highly recommended for marine use.
Rangefinder reticule: If you know the height of an object, such as a hill or navigation marker (often printed on charts), and can measure the angle to its top using binoculars equipped with a rangefinder reticule, you can calculate your distance from that object.
Do you want image-stabilizing binoculars?
Image-stabilizing (IS) binoculars provide a steady image, even on a bouncing boat. The rolling, pitching and bouncing motion on a boat makes it nearly impossible to keep an image in focus using high-powered binoculars, unless they are Image-Stabilized. IS binoculars automatically compensate for movement to deliver crisp, sharp images, even at magnifications as high as 18-power, which is more than double the magnification typically found in marine binoculars.
Individual focus, center focus or fixed focus?
Binoculars may have independent eyepiece focus to compensate for the differences between eyes and for different distances. In center-focus binoculars, one eyepiece adjusts to accommodate the difference between your eyes. A central focus knob then adjusts both sides simultaneously for distance.
Steiner Binoculars use a fixed-focus system, with a very deep depth of field, called Sport Auto-Focus. With Sports Auto-Focus, once you’ve adjusted your ocular settings for differences in your individual eyes, you won’t need to adjust the binocular again for varying distances. You’ll get a sharp, clear picture from 1 m to infinity. This set-it-and-forget-it system works well when you need to quickly grab your binoculars so you won’t miss the action.
How much were you thinking of spending?
As the saying goes, you get what you pay for. For marine binoculars, the features we’ll examine here illustrate the difference between economy and premium products.
Relative brightness
The relative brightness of a pair of binoculars is a function of the quality of the optics and the ratio of the objective lens diameter divided by the magnification, squared (50 ÷ 7)2. Thus 7 x 50 binoculars have a relative brightness of about 50, while 8 x 23 binoculars have a relative brightness of only 8.2. Objects will be visible in far less light with the 7 x 50 models.
Light transmission efficiency
Cheap glasses may allow only half the light entering the objective lenses to reach your eyes. Our least expensive glasses pass about 75 % of the light. Truly exceptional binoculars, such as top models from Steiner pass more than 93 % to 97 % of light to your eyes, making all objects appear brighter. Quality optics also make the image sharper. Inexpensive glasses may produce astigmatic images that are fuzzy at the edges. Superior glasses are sharp from edge to edge and are less fatiguing to the eyes when used for extended periods.
Lens coatings
When light enters or leaves a piece of glass, about 5 % is reflected back. With as many as 16 air/glass surfaces inside your binoculars, there could be a lot of internal light bouncing around, reducing the brightness, sharpness and contrast of the image. Lenses are coated using one or more thin layers of chemicals (most commonly magnesium fluoride) reducing this internal reflection from 5 % to 1 % or less. But not all coatings are the same. If you look at the outside lens surfaces, quality lens coatings will appear as subtle tints of violet, blue or green. Heavily colored lenses in cheap glasses actually reduce the amount of light transmitted. Also, better binoculars include more layers, with more complex chemical combinations, on more surfaces, to achieve their amazing light transmission efficiency. The choices:
- Coated: one or more surfaces coated with a single layer.
- Fully coated: all air-to-glass surfaces are coated with a single layer.
- Multi-coated: one or more surfaces coated with multiple layers.
- Fully multi-coated: all air-to-glass surfaces are coated with multiple layers.
Field of view
The field of view describes the width of the image you see, measured in feet at the distance of 1000 m. Binoculars offering 385' field of view show the viewer a cone that is 385' wide 1000 m out. Higher-powered image stabilized binoculars offer narrower field of view (200–340') than conventional units (up to 430').
Marine binoculars with dogleg-shaped Porro prisms.Arrows in the diagram (from top to bottom) Eyepiece lenses / Prisms / Objective lenses.
Prisms
Prisms, used to invert and magnify an upside-down image, are either Porro (binoculars with a dog-leg shape) or roof prisms (with straight tube configuration that are easier to hold). There is some disagreement as to which is best, but it’s generally believed that Porro prisms yield superior optical performance. They transmit more light, resulting in brighter images, and provide better depth perception, because their objective lenses are farther apart. However, some roof prisms with phase shift coating provide excellent performance.
Prisms come in two glass types, BK-7 and BAK-4. BK-7 uses boro-silicate glass and BAK-4 use a denser, finer barium crown glass, which eliminates internal light scattering and produces sharper images than BK-7. The higher quality is reflected in the price of the binoculars. Roof prisms are lighter and more compact, but they are more complex and difficult to manufacture, and have more precise tolerances, so they are usually more expensive than Porro prisms.
