Choosing an Onboard Computer

By Daniel Piltch, Marine Computer Systems
(as originally published in Ocean Navigator Issue #96) 
  See Also:

Tips for Maintaining Your Computer

The NMEA Standard

Choosing an Onboard Printer
 

 

So you've decided to buy a computer for your boat. Or perhaps you've decided to upgrade the computer that you already have onboard. The obvious question is, "What kind of computer should I buy?" Unfortunately there isn't any universally correct answer to this question. Each boat and each boat owner will have different requirements and therefore will wind up with different computers.

The computer is at the center of many modern navigation stations. It has the capability to integrate information from various sources and present it in a way that adds more value to that information than if it was presented individually. In order to be able to assimilate and present this information, your computer will need to have sufficient processing power, adequate display capabilities, the capacity to collect information and the ability to resist the harsh environment of a boat's navigation station.

Processing Power

Let's tackle the first of these four requirements. The processing power of a computer is basically the speed at which it performs its tasks. A faster computer can work on more concurrent tasks without noticeably slowing down or crashing. You've probably noticed this when you've had multiple programs open at the same time, and everything seems to move along very slowly.

The vast majority of onboard computers are based on one of Intel's microprocessor chips. The performance of a computer is related to the model of microprocessor, the amount of memory and the clock speed which can be thought of as the speed of the computer's heartbeat. There are other factors involved, but for our needs they're relatively minor. The most recent lineage of available microprocessors from Intel can be seen in figure 1.

Note that clock speed alone is not the best indicator of performance. Faster models of the same chip will perform better (i.e., a Pentium 200 will perform better than a Pentium 166). However, a Pentium Pro running at 200 MHz is a much better performer than a regular Pentium running at the same clock speed. The biggest break comes with the introduction of the Pentium Pro. Due to a number of architectural improvements inside the chip, the Pentium Pro and Pentium II are much higher performers than the Pentium and Pentium MMX series. These newer chips actually try to predict the future by "guessing" what your program is going to do next, and already have the answer waiting at hand. This style of processing, known as Speculative Execution, allows the Pentium Pro and Pentium II to run programs much faster than their predecessors.

One of the most significant challenges in making processors even faster is the dissipation of heat. As the chips get loaded with more and more circuits, they generate more and more heat. There are about 7,500,000 transistors on a Pentium II chip! Even if each one of these transistors emits only a tiny bit of heat, you quickly wind up with a very hot chip when you add them all together. To further complicate the problem, there is a lot of pressure to produce processors that draw less power so that they can be used for longer periods of time while running on a laptop computer's battery. One way this is accomplished is to make the circuits inside the chip narrower. The electrical pathways inside the Pentium II are only 0.25 thousandths of a millimeter wide! However, as these circuits get smaller, the resistance inside them builds up and they generate more heat.

One way to dissipate this heat in a regular desktop or deskside model computer is to supplement the main cooling fan in the back of the computer's case by attaching a small fan directly to the processor using a thermally conductive glue. This works fine in a desktop computer, however, in a laptop computer, this gets more difficult due to lack of space and power. In a marinized computer with a completely watertight case, the problem is significantly more complex.

Because the problems get increasingly complicated as you go from desktop to laptop to marinized computer, you'll find the fastest processors being introduced first in desktop systems, then later in laptop systems, and sometimes later in marinized laptop systems. The price of these systems increases along with the complexity.

Another key performance factor is the amount of memory you have in your computer. A common mistake is to confuse the amount of random access memory (or RAM) with the amount of hard disk space. Most computers have between 8 megabytes (MB) and 96 megabytes of RAM. Each megabyte is roughly equivalent to one million individual characters or bytes. Hard disk sizes vary much more than memory, with smaller computers having less than one gigabyte (around one billion bytes), and more powerful computers having between four and eight gigabytes.

Adding more hard disk space to a computer allows it to store more items. This would be equivalent to adding more chart stowage space to your nav station. A computer with more hard disk space will be able to store a larger number of electronic charts without resorting to the relatively slow method of reading each chart as it's needed from a CD.

Adding more memory, on the other hand, allows the computer to work with more information at the same time. This is equivalent to enlarging your chart table. If your computer only has a small memory bank, it will process your electronic chart by breaking it up into chunks and working with one chunk at a time, reading the next chunk from the disk when it's needed. You can usually notice this by watching the small LED on the computer's case that indicates disk activity. If this light persistently stays on as you move around the chart on screen, chances are that your computer is busy swapping the relevant chunks of the chart to and from the disk.

Now that all this has been explained, you're probably still asking, "So which one should I buy?" Again, there's no simple answer. I'll go through each class of system and point out some of the advantages and challenges typically found in each one.

Marinized v. 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 onboard 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 not unfeasible 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 dekstop system, a conventional laptop system, or a marinized system. I'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 the tropical temperatures cruisers are fond of.

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 source. 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" CRT color monitor draws 1.6 amps at 110 volts AC not including the computer, while my entire laptop system including computer, 13" 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 the adjacent 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, knot meter, or depth finder. This is the same port you would use to interface with your SSB for receiving weather fax on your computer, or to interface your Inmarsat unit to your computer to send and receive e-mail. Once a computer is onboard, it won't take long before you have more things to plug into it than you have available ports. I'll cover more on connectivity later in this article.

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 anywhere from 3600 to 10,000 revolutions per minute inside a sealed case. As the disks spin, 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 2 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 5 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 resulting in a "head crash" and a loss of data.

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, 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 which is 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 isn't a problem for the typical mariner. These protective rubber or plastic membranes are often found on computer keyboards in restaurants or plants.

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 might 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 it's being transported from place to place, but as long as it's 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 in down on the chart table and go sailing. Use a well-built hinged arm like the one shown in the accompanying photograph, 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: CRT and LCD. Cathode ray tube (CRT) based 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 very 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: 640x480, 800x600, and 1024x768.

Usually, the larger resolutions are found on the larger screen sizes. It wouldn't be all that useful to try to cram a 1024x768 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, I find it convenient to be able to see a larger section of the chart at once.

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 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 onboard 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 onboard instruments, it needs to offer a method of connectivity.Fortunately, the original PC designers thought of this and provided for a communications (or COM) port. This is commonly known as a serial port, also known by it's 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's port with also has 25 conductors, but this will be a female port on the computer 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 ten serial ports is a nice feature that should save some extra expense down the road.

Another option is a NMEA multiplexor. Most modern navigation instruments, such as GPS receivers, wind speed indicators, knotmeters, depth finders, 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 multiplexor. This nifty little device accepts up to ten different NMEA signals and sends them out one by one to a single port which you would then connect to your computer. However, NMEA multiplexors don't work with non-NMEA signals such as those coming from an Inmarsat or SSB unit.

 

In figures 3 and 4, you'll see schematic drawings for two common systems. Figure 3 shows a system that might be found on a modest coastal cruiser, while figure 4 is more typical of a serious ocean-going vessel. Notice the demodulators in figure 4. 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. It may be the amplitude or wave height that's modified as in amplitude modulation or AM radio, or it may be the frequency of the wave that's modified as in frequency modulation or FM radio. There are other types of modulation that are commonly used as well. 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. This is also where the term modem originates. A modem is a device that connects your computer to a telephone line, by modulating the computer's digital signal into an analog one that can be transmitted over the phone lines, and then demodulating the return signal from. By combining the words modulate and demodulate, the term modem was born.

Conclusion

With things changing so quickly in the computer marketplace, buying an onboard 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.

See Also:

Tips for Maintaining Your Computer

The NMEA Standard

Choosing an Onboard Printer

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