294 seamanship secrets
Maximum Target Radar Range Based
on Height of Radar Antenna [H(ra)](See Chapter 7, Avoiding Collisions the E.A.S.A. Way and Radar Scope Plotting)
Before deciding on radar height installation, check the manual or talk with the manu-
facturer or installing electrician. Make a final decision based on these factors:
Vertical and horizontal offset. A gain in target range by increasing height also
means a loss in acquiring close-quarters targets. Radar vertical beam width is quite
narrow. For example, install an antenna 30 feet off the water and expect to lose targets
inside a 50-yard radius.
Sailing vessel heel. Without an antenna leveling device, expect loss of target acqui-
sition on the side of tack. On starboard tack, the beam will shoot over starboard-side
targets and vice versa. This problem increases with radar height.
Power vessel squat. Squatting in the stern causes the antenna to aim skyward, missing
targets close to the boat. Use trim tabs to keep the bow down or slow the boat to acquire
targets in reduced visibility.
How much height do you need?
Mount your radar antenna high enough to acquire targets at a range that allows time
to maneuver. Th e following recommendations are conservative, in that they only take
into account your own radar’s horizon distance. In most circumstances, you will pick
up large vessels at somewhat greater distances. Th is depends on many factors, such as
hull refl ectivity, radar signal output, sea height, and atmosphere.
Small power vessels: 11 feet (minimum height above water) = 4-mile radar horizon.
Small sailing vessels: 24 feet (minimum height above water) = 6-mile radar
horizon.
If sailing near shipping lanes, fall off to a deep reach every so oft en to level the
radar antenna. Check for targets on the radar; if you are clear, return to your course.
Otherwise, perform collision avoidance maneuvers described in the chapter sections
noted above.