Picture this scene: a beautiful day, gently rolling seas, and cool autumn air, the type of day where visible horizon curves infinitely out of sight. Your steadily purring engine draws your course past Owls Head Light, within the channel of the most beautiful cruising grounds in the northeastern US. You have methodically plotted your course through this area reputed for strong currents and rocky coastline. An hour out from your destination the winds back, and in less than ten minutes time that broad, beautiful rocky coast directing you up the Penobscot is swallowed up in a blanket of dense gray fog. Another gray day in Maine. Now the three 100 ft sailing schooners and four lobster boats you had consciously noted moments ago are gone as well, except for the faint distant hum of engines barely audible above your own and some laughter carrying from an otherwise silent schooner. What next? You glance at the chartplotter, it's GPS fix of your own position and heading assuring you that your next nav mark is only 1/2 mile ahead, 1 point off your port bow.
But still, you have no idea where the other traffic is headed, nor of any unseen vessel entering at the channel bifurcations yet to come. Having survived the twinge of doubt and anxiety from navigating blind before, you are grateful for a pair of eyes that sees through all weather. You reach across the helm and push the standby button once. In a short moment the radar sweep registers on the monitor, and you count 7 distinct targets showing you precisely where those boats are in relation to your own. It further confirms via range rings that the shoreline lies 1 mile off your port and 3 miles off your starboard, confirming that you are cruising above nearly 300 ft of charted depth, not the boulder strewn shoals reflected ahead in the distance and off the starboard beam. Sipping hot coffee from a thermos to ward off the chill, you chuckle to yourself, recalling that first nerve racking trip up this bay on a similar foggy day, sans radar. You remember the usual hour run instead taking 3 as you idled ahead under bare steerage to clear all other traffic in your blind, fearful state. You pat the radar confidingly thinking it's the best instrument you ever put on this boat.
Radar is not just the tool of professional mariners anymore. Today radars are available in price and size ranges to fit all types of vessels. When caught out in fog, squall or cover of darkness, there is no better friend to place on lookout. The sweep of a radar displays real time distance and range to any reflective target, meaning it will detect ships, land masses and most navigation aids. Using electromagnetic waves, a radar sees through any condition of visibility.
How It Works
The radar system requires two units, the radar (electronics and monitor) and separate antenna called a radar array or radome. Together, the system is akin to a flashlight scanning the horizon. It illuminates any reflective surface with the sweep of the antenna and displays it on the monitor. Here's how: a specialized antenna, called an array, sends radiowaves out from the boat and receives them back. As it rotates 360 degrees it sends short, rapidly repeated radio wave pulses out across the horizon. Radio waves that strike a hard object scatter. The scattered wave portion that reflects back is heard by a receiver within the antenna. This displays on the monitor. Thus, the wave transmitting and receiving is all accomplished within the same array. Slightly different arrays are used on sailboats, called Radomes. Radomes are encased within a protective pie-dome shaped housing to prevent conflict with sails. As the array hears reflected waves they are amplified and sent along to the radar electronics. The radar keeps time, specifically the elapsed time for each unique pulse to travel out and return. A simple Distance=Speed x Time/2 calculation (Time=speed of sound) figures out the distance to the reflecting object from the boat. The reflecting object ultimately appears as a "target" on the display, at the determined distance and direction. Distance or "range" from the boat is displayed by concentric rings on the monitor. Rings (and signal strength) can be dialed in to 1/4 mile intervals for close range navigation, or expanded out to 10 miles or more for long range navigation or storm cloud detection. The "target" direction is based on where the antenna was pointing at the time, providing a bearing. You can setup the monitor to display "north up" or the 12 o'clock position of the bow as up on the screen (be sure you know which you are looking at.) The process renews constantly: as the array scans across the horizon, it updates results with every sweep of the array.
What are the Benefits?
Radars help those with them and those seen on them. When limited visibility closes in, the use of a radar aids all concerned traffic by preventing collisions. Even if you don't have one onboard, you may benefit from being spotted another radar. How well a radar sees an object depends on the reflectivity of the surface. Less reflective wood and fiberglass boats do not appear as clearly as metal hulls. To make a boat more visible by radar, many boat owners add radar reflectors aloft. With a reflector installed, wood and fiberglass boats are seen on radar as a much larger and clearer target. Whether the boat has radar or not, a reflector greatly increases visibility of the boat on other radars- a good idea anywhere, especially for high traffic seaways.