avoiding collision by eye or radar 147
courts do not excuse ignorance of operation. If there is a radar unit aboard
and it works, you must use it for collision avoidance.
First, energize and optimize your radar (review the section in Chapter 3 on
page 67). Change radar range scales up and down every few minutes. Lower
the range to under 3 miles to pick up small targets close to you. (The maxi-
mum range at which your radar can pick up targets depends in part on radar
antenna height above the water—see Appendix I, Radar Range Table.) Always
combine radar observations with a 360-degree^ visual scan of the horizon. If
you spot a contact, determine its bearing drift right away.
HOW TO DETERMINE VISIBILITY AND SAFE SPEED
Rule 6, Safe Speed, requires you to go slow enough that you can stop your vessel
and avoid collision in the prevailing conditions of visibility, vessel traffi c density,
sea state, shore lights, and depth of water relative to draft. Th e most important of
these factors is visibility, so you must estimate your range of visibility in order to
determine a safe speed.
Radar can help with this. First you need to know how far your radar can “see.”
Base this on antenna height (see Appendix I). Th en use your radar as follows to
estimate range of visibility.
- Scan the horizon with binoculars. Find a vessel or an object just barely
visible in the fog or haze. - Take a bearing to the vessel or object.
- Turn to the radar and set the cursor or electronic bearing line (EBL)
onto that bearing. Move the variable range marker (VRM) out to the
object and read the range. Subtract a half mile for safety, and call the
result your range of visibility.
Having obtained this estimate, what do you do with it? One useful approach
is to ensure that you can stop within half your range of visibility in yards (1 nauti-
cal mile equals approximately 2,000 yards). Th at requires a good estimate of your
stopping distance as well as a good estimate of your range of visibility, however,
which makes it a bit problematic in practice. I prefer to divide my estimated range
of visibility by two, convert the result to a speed, and multiply that speed by 80%.
Follow these examples:
Your visibility is 0.5 mile: 1,000 yards ÷ 2 = 500 yards. To get your speed,
move the decimal to the left two places, which gives 5 knots, and 5 knots ×
80% = 4 knots.
Your visibility is 1.5 miles: 3,000 yards ÷ 2 = 1,500 yards; 15 knots ×
80% = 12 knots.
Your visibility is 0.25 mile or less: bare steerage (just enough speed to steer
the boat); every so often, stop the boat and listen.