vantages and challenges typically found in each one.Marinized vs. non-marinized Desktop computers were designed to sit atop a desk, and laptop computers were designed to be picked up, carried around, and even survive an airplane flight. This means that the internal components of a laptop system are more securely constructed than an equivalent desktop. Most failures of on-board desktop systems are due to the vibrations and shock found on most boats, not by exposure to water, spray, or salty air. This means that, on the average boat, a typical desktop computer is more prone to physical failure than a laptop. It’s possible for rough seas to unseat an expansion card inside a desktop system, with drastic results. Realizing the special factors that exist aboard boats, a small number of companies have been designing both desktop and laptop computers specifically for this harsh environment. These systems are called marinized computers. So you wind up with a choice between a conventional desktop system, a conventional laptop system, or a marinized system. We’ll discuss each of these choices in a little more detail. A conventional desktop system offers the most bang for your buck. For a given amount of money, you’ll get more processing power, more expandability, and cheaper replacement parts in a typical desktop computer than in a laptop computer. While a desktop computer seems like a bulky item for a nav station, most units can be securely mounted in a cabinet or locker with connections to a keyboard, mouse, and screen at the nav station. If you choose to do this, make sure there is adequate ventilation for the computer wherever it’s mounted. Heat can be fatal for a computer, especially in tropical temperatures. Despite the fact that all of the internal circuitry of a computer is based on direct current (DC) electricity, conventional desktop systems are built around an alternating current (AC) power supply module (although the power module inside the computer then converts the AC to low-voltage DC for use in the computer’s circuits). This means you’ll need an inverter or generator in order to power your computer while underway. If you buy the normal CRT monitor that’s offered with most desktop computers, you’ll wind up with a nice bright display but a more significant power drain than a similarly sized flat-panel LCD display found on most laptops. As an example, a 15-inch CRT color monitor draws 1.6 amps at 110 volts AC, not including the computer, while my entire laptop system, including computer, 13-inch LCD screen, and battery charger, draws 1.0 amps at 110 volts AC. A convenient but pricey compromise is to use a flat-panel LCD screen connected to a desktop computer. A laptop computer with a given performance will be more expensive than the corresponding desktop model. Much of the price difference is due to the more complicated packaging, heat dissipation, and more expensive LCD screen. The good news is that you wind up with a system that is less likely to fail due to vibration or shock. A convenient advantage to a laptop system is that you can take it off of the boat. This is important for security reasons, as well as for the ability to plan a voyage at home and enter all of your waypoints while sitting in your living room. When used at the nav station, an efficient and secure method of mounting the computer is by using a hinged arm supporting a platform onto which the laptop is secured by one or two straps. An example of this can be seen in an accompanying photograph. Typically, a laptop system will have one serial port that can be used to communicate with any NMEA-0183-compliant device such as a GPS, knotmeter, or depthfinder. This is the same serial port you would use to interface with your SSB for receiving weatherfax on your computer, or to interface your Inmarsat unit to your computer to send and receive e-mail. Once a computer is on board, it won’t take long before you have more things to plug into it than you have available ports. We’ll cover more on connectivity below. The component that is most sensitive to rough handling is the disk drive. This little unit, typically 2.5 to 3.5 inches wide, spins a stack of disks at anywhere from 3,600 to 10,000 rpm inside a sealed case. As a disk spins, a very small magnetizing head travels back and forth across the surface of the disk. Today’s drives typically hold 20 to 30 billion bits (two to four billion bytes) of information. In order to fit all of this information in such a small space, the drives are manufactured to incredibly precise specifications. A common two-gigabyte disk drive has a head that travels at about 55 miles per hour relative to the spinning disk. The head is suspended on a cushion of air five millionths of an inch above the surface of the disk. If the head were actually to come in contact with the disk surface, it would likely scratch off the magnetic coating that stores all of the data. This impact with the disk is called a head crash, and it results in a loss of data and permanent disk damage. Now picture this disk drive on a boat in rough seas. While being battered around by the ocean, the drive head is trying very hard to maintain a distance of 5 millionths of an inch above the disk’s surface. Obviously, a drive needs to be incredibly well designed in order to be able to withstand this environment for any length of time. Not surprisingly, then, one area that receives a lot of attention in marinized computers is the disk drive. Just about any quality marinized computer will have a drive that’s shock-mounted. This helps to lessen the forces transmitted to the head and disk surface, minimizing the possibility of a head crash. There are different degrees of shock mounting, varying from using rubber pads outside the disk drive’s case to encasing the entire drive in a shock-absorbing gel.There is wide variation in what constitutes a marinized computer system. Unfortunately, there are no industry standards regarding this designation. Some systems merely use a few extra rubber gaskets and a corrosion-inhibiting spray on an off-the-shelf computer, while other systems are designed from the ground up specifically for the marine environment. The protective, corrosion-inhibiting coating sounds like a nice feature, but in practice corrosion is rarely a problem on most circuit boards. A more likely problem is an unexpected dose of water from an open porthole or a wave breaking through an open companionway. As protection against a reasonable amount of water (e.g., using the computer in the rain), some marinized computers will have a waterproof membrane built into the keyboard. In this case each key presses down on the membrane, which in turn presses down on the actual electronic switch safely located below the membrane. Alternatively, you can buy a similar membrane that will fit over the top of your keyboard. This usually means a less responsive feel for touch-typing, but that isn’t a problem for the typical mariner. These protective rubber or plastic membranes are often found on computer keyboards in restaurants or industrial sites. In the unfortunate event of a direct hit by a large amount of seawater, be prepared to live without your computer for a little while. Even the best of marinized computers usually can’t tolerate submersion in water. The best thing to do if this does happen is to immediately disconnect the power supply. If you find that salt water made its way inside of your computer, you’ll need to flush it out. Sometimes the best way to do this is to flush the case with distilled water and then let it dry. A fan or hair dryer can help. Be careful with heat guns as they may produce enough heat to damage certain components. If in doubt, check with the manufacturer.Some marinized computers will be constructed with a toughened case either of rubber or a durable metal alloy. This helps to protect the computer when its being transported from place to place, but as long as its installed securely in your nav station the type of case isn’t likely to make much of a difference.No matter what kind of case your computer comes in, it’s important to mount it properly in your boat. The two priorities here are security and ventilation. Whether you have a laptop or desktop system, make sure it’s mounted in such a way that it doesn’t get jostled around unnecessarily. For a desktop system, use heavy-duty Velcro, nylon webbing, or rubber straps to hold the case securely to a shelf. Leave enough slack in the cables connected to the case so that they won’t unduly strain their connectors. For a laptop system, don’t just plop it down on the chart table and go sailing. Use a well-built hinged arm like the one shown in the photograph on page 70, or design your own mounting. If you have a permanent spot for your laptop, you can often get by with two straps holding down the computer, one above the top row on the keyboard, and one below the spacebar. Make sure the screen has adequate support. Broken screens are a common failure on laptop systems. Try not to allow the screen to flop back and forth with each wave. Many hatch-cover arms can be fitted with hardware to support a laptop screen.Displays Display units primarily come in two flavors. Cathode ray tube (CRT) displays are bulky devices shaped like a small television set. CRT displays are typically used with desktop computers. Liquid crystal display (LCD) units are shaped like a flat panel and are typically found in notebook computers. CRT displays tend to be much brighter than LCD displays but draw significantly more power. The sizes of both types of screens are given in inches along the diagonal axis of the screen. Make sure you get the measurement of the actual viewable size and not the measurement of the casing of the screen. Another significant measurement is the resolution of the screen. This is measured by dividing the screen into the smallest individually addressable units, called pixels. Essentially every screen is a grid of pixels that can individually be set to a particular color. The resolution is measured with two numbers representing the number of pixels on the horizontal axis and the number of pixels on the vertical axis. Typical combinations are 640 x 480, 800 x 600, and 1,024 x 768. Usually, the larger resolutions are found on the larger screen sizes. It wouldn’t be all that useful to try to cram a 1024 x 768-resolution grid onto a small 10.4 screen. This would be analogous to printing an entire coastal chart on the back of a business card. The larger the screen, and the larger the resolution, the more chart you’ll be able to display without having to scroll to the right or to the left. While most electronic charting systems will automatically scroll the chart as your boat moves across it, it’s more convenient to see a larger section of the chart and reduce scrolling time. Another important specification to compare is the brightness of the screen. On a bright, sunny day it can be incredibly difficult to read a computer screen, especially a laptop’s LCD display. The brightness of a screen is measured in a unit called nits, with most LCD screens putting out between 80 and 200 nits. A daylight-readable screen usually puts out more nits, but again there are no industry standards for what makes a screen daylight readable. An LCD screen with 250 nits might be billed as being daylight readable, but for truly bright, sunny days in the cockpit, look for a screen with at least 1,000 nits.Connectivity As computers have become more affordable and available to the average boater, there has been a lot of demand to use the power of the computer to process information from other on-board instruments. A computer can decipher SSB signals and convert them into readable charts. It can plot a GPS fix directly onto an electronic chart. It can send and receive electronic mail from the middle of the ocean by connecting to a satellite transceiver. In order for the computer to be able to communicate with these on-board instruments, it needs to offer a method of connectivity. This is done via a communications (or COM) port. This is commonly known as a serial port, also known by its standards designation as an RS-232 port. You can recognize your serial port(s) by looking at the back of the computer and noticing a connector with nine pins in two rows (five in one row, four in the other) or a connector with 25 pins in two rows (13 in one row, 12 in the other). Don’t confuse this with your printer connection port, which also has 25 conductors; serial ports are “female,” with 25 sockets, not 25 pins. Unfortunately, most computers only have one or two serial ports available on the back panel of the unit. Laptop computers will typically only have one of these ports, while desktops will often offer two serial ports. This poses a problem to the mariner who wants to plug in a GPS, wind-speed indicator, SSB radio, and Inmarsat unit. One solution to this common problem is to create more serial ports by installing a card designed for the purpose. These cards can offer you an additional one to four serial ports by plugging the card into the side of your laptop or inside your desktop. Some marinized computers will have more than the standard number of serial ports. Having two, four, or even 10 serial ports is a nice feature that should save some extra expense down the road. Another option is a NMEA multiplexer. Most modern navigation instruments, such as GPS receivers, wind speed indicators, knotmeters, depthfinders, and autopilots offer the capability to communicate via the National Marine Electronics Association (NMEA) 0183 standard. This standard defines a type of language that can be used by these devices even though they’re manufactured by different companies. If you want to plug in a number of different NMEA-0183-compliant devices into your computer, then you have the option of using a NMEA multiplexer. This nifty little device accepts up to 10 different NMEA signals and sends them out one by one to a single port that you would then connect to your computer. However, NMEA multiplexers don’t work with non-NMEA signals such as those coming from an Inmarsat or SSB unit. In the accompanying figures you’ll see schematic drawings for two common systems. One shows a system that might be found on a modest coastal cruiser, while the other is more typical of a serious ocean-going vessel. Notice the demodulators in the second figure. Most electromagnetic signals are transported through the air by starting with a sine wave at a specific frequency called a carrier frequency. This wave is then modified by applying the signal to be sent. This application is called modulation. In order for a computer to read this signal, it needs to be separated from the carrier frequency and converted from an analog signal into a digital one. This is the job of the demodulator. The NMEA standard The NMEA-0183 standard has details for electrical specifications as well as protocol conventions. The electrical specifications actually recommend using the RS-422 standard, which is slightly different than the RS-232 standard mentioned above. The RS-422 standard is a newer standard that specifies two wires each for transmitting and receiving, for a total of four signal wires, while the older RS-232 standard specifies a common signal ground and one wire each for transmitting and receiving for a total of three signal wires. In most cases, an RS-422 signal coming from an electronic device will be able to be read by an RS-232 port on a computer. The NMEA protocol is based on ASCII (American Standard Code for Information Interchange) characters being sent along the wire at 4,800 bits per second or about 600 characters a second. Since any personal computer can read ASCII characters, its easy to snoop on the NMEA transmission coming into your computer and see what it looks like. With things changing so quickly in the computer marketplace, buying an on-board system is not an easy decision. However, by evaluating your anticipated needs it shouldn’t be too difficult to find a solution that fits your boat and your budget.