The reason most yachtsmen fit a radar set is for help with collision avoidance: the ability to “see” other vessels, navigation marks and hazards in conditions of greatly reduced visibility.
Most yachtsmen are more worried by fog than by high winds. When the viz closes in unexpectedly, it's disorientating, as well as unnerving. It becomes difficult to judge distances. The idea of a mechanical eye that can see through the fog and tell you what is coming towards you - or going safely past you - is very attractive. And although radar is by no means foolproof it is invaluable in these conditions.
Radar instructor Alan Watson points out that the International Regulations for the Prevention of Collisions at Sea (the “rule of the road”) include a number of specific instructions for vessels equipped with radar.
Look up rules 5 (look-out), 6 (safe speed), 7 (risk of collision), and 8 (action to avoid collision).
He also points out that rules 11-18 (conduct of vessels in sight of one another) do not apply in fog. Rule 19 (conduct of vessels in restricted visibility) then supersedes all other notions of rights of way.
Rule 19 requires vessels to proceed at a safe speed adapted to the prevailing conditions, and to slow right down to the minimum speed at which the boat can be kept on her course (do not lose steerage way!) if a vessel is heard ahead of the beam.
The rule also gives specific instructions for boats equipped with radar, and what actions to take “if a close quarters situation is developing and/or risk of collision exists.”
So how do you determine if a risk of collision exists? If another vessel remains on a constant bearing and the range is reducing, then there is a collision situation. In clear visibility, this can be assessed using a hand bearing compass.
With radar, it can be assessed more easily, and more accurately, simply by placing an EBL (electronic bearing line) on the target. If the vessel's echo approaches down the line, there is a risk of collision. If it moves to one side of the line, it is going to pass clear ahead, or clear astern.
Collision risk is a combination of the courses and speeds of two vessels. If either alters course or speed, the relationship is broken. If it seems there is a risk of collision, an early and significant alteration of course (or speed) is required, just as it is in clear visibility, so that other vessel (assuming it is watching its own radar) is not tempted to make a last minute alteration of course the wrong way (the so-called “radar-assisted collision”).
If there is more than one radar target on your screen, there is obviously a need to ensure that giving way to one vessel will not put you at risk from another - something you instinctively check by eye in good visibility, but may find harder to assess on a radar screen.
This is where MARPA (mini automatic radar plotting aid) comes into its own. MARPA helps with collision avoidance by obtaining detailed information for one or more automatically tracked targets, and providing continuous evaluation of the situation.
The set I use allows you to select up to ten targets, which are then automatically tracked, MARPA calculating bearing and range, true speed and course, closest point of approach (CPA) and time to closest point of approach (TCPA). Each target is continually assessed, and a warning alarm sounds if one becomes dangerous or is lost.
Of course MARPA is most effective if your own heading, SOG and COG is steady (ideally, motoring in a flat calm under autopilot!) for every time your heading changes, the relative bearings of any targets change too. This can be significant when hand steering a yacht in a big sea.
Alan Watson points out that because the MARPA display calculates information to several decimal points, it is tempting to regard it as being that precise, but it is not, and it is important to have the alarm set at a distance - half a mile or more - where there is still time to take avoiding action.
Having the alarm set at 0.1 or 0.2 miles would be “within the range of errors”, he told me at Raymarine's Portsmouth headquarters.
“MARPA can occasionally get itself very confused. If there is a ship passing a buoy or another ship you may get what is called target swapping,” he added.
Like everything else to do with radar, it is an extremely useful safety feature, but not to be regarded as in any sense foolproof.
Never forget that there may be things that don't show up on your radar screen, and you can never assume that a big ship has seen your boat on its radar. Even if there is a clear echo, there is no guarantee that anybody is watching it, said Alan.
AIS
You do not need a radar set to overlay AIS (automatic identification system) information on a chart plotter. AIS is another useful tool for collision avoidance, but like radar, it must not be regarded as foolproof. Not all vessels are equipped with AIS. There may still be dangers from for example fishing boats and yachts (and even warships) which do not transmit their positions.
This is why AIS is not a substitute for radar. And thee is a danger that in areas where a lot of vessels are transmitting AIS information, the screen may become cluttered with their symbols, blocking out vital navigation on the chart plotter.
