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Good question! We certainly commonly use the phrase, "Hail someone on the radio".
Just to put a fine point on your premise though, more often than not:
the NOR will state that the boat must carry a radio capable of receiving/communicating-on a list of channels.
the SI will state the channel to be use for communication by the RC.
It is rare in my experience that the SI's state that a "boat shall monitor channel ## while racing" (except in some govt-mark races and night races). Without an SI like that, a boat must be capable of monitoring, but is not required to.
So ... in responding to your OP .. forum-members should assume that the SI's state "at all times while racing, boats shall monitor channel ##".
For the protest rules hail means a verbal hail. That leaves informing the other boat "at the first reasonable opportunuty" I think that could be by vhf.
Just to put a fine point on your premise though, more often than not:
It is rare in my experience that the SI's state that a "boat shall monitor channel ## while racing" (except in some govt-mark races and night races). Without an SI like that, a boat must be capable of monitoring, but is not required to.
So ... in responding to your OP .. forum-members should assume that the SI's state "at all times while racing, boats shall monitor channel ##".