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If you failed to see something your radar, if on, would've seen, then you probably haven't lived up to what Rule 5 says about "all means appropriate to the circumstances".
But if you already saw what radar would've told you was out there (the "bright sunny day"), then you didn't need radar to "help", and likely wouldn't be faulted for not having it on, or using if it was on--the collision was caused by factors other than your lookout.
Different situation, though, if it's restricted viz or obstructed view behind your big jenny while you declined to use radar, and the radar would've seen what you didn't.
Note also that Rules 6 (safe speed rule) and 7 (determining risk of collision) have additional requirements for vessels "fitted with radar". So if you have it, you have to use it, or at least take its capabilities into account.
The article Jackdale linked give much more explanation about all this.
Betcha the courts will find the same principle applies, rules or no, about some of the newer stuff, like AIS, to find out who's out there and communicate with them if in doubt.
The lookout rule is vague on specifics for a reason--they don't want you to have your head stuck in a radar scope (at least in the absence of pea-soup fog) to the detriment of a good visual lookout, no vice-versa either. "Lookout" mean listening too, for signals or foghorns. Or for other vessels talking on the radio (if you have radio, which is whole 'nother topic in the same vein as radar). If you have all this stuff and can't effectively use it all by yourself, then you may be judged to have been short-handed if things went south due to incomplete lookout.
Last edited by nolatom; 07-07-2011 at 05:53 PM.