More maneuverable boats give way to less maneuverable ones.
In wide open, deep water a powerboat gives way to a sailboat, and any moving boat gives way to a stopped boat (for instance, one that has fishing lines out). But in a narrow channel or traffic separation zone, smaller boats (under sail or power) give way to ships, ferry boats, and other large vessels that have little room to maneuver.
The challenge is to recognize the scenario.
Here's another rule with its own sound logic:
Any boat overtaking another boat must give way to the leader.
If you can easily see the other boat's stern, it means that you're overtaking and also that her crew can't easily see you. You may pass, but you must keep your distance. This rule applies even to sailboats overtaking a powerboat.
When similar boats are near each other, the rules assign priorities according to arbitrary rules, including:
When sailboats under sail are near each other, a boat on port tack must give way to one on starboard, and a windward boat must give way to a leeward one.
Those rules sound simple, but in the heat of the action even the best of us can momentarily misread a situation. I've known good racing boats that have the following reminder written in large, red letters on the port side of the boom: "You're now on port tack! STAY CLEAR!!"
Another example of an established rule governing similar boats is the crossing rule for powerboats (which, again, include sailboats under power). We are frequently tempted to cut across another boat's bow in channels and harbors, for example, to reach the slip. The rule that helps makes that action seamanlike can be summarized this way:
The boat on the right IS right.When boats under power are crossing, the stand-on vessel (the one that does not alter course) is the one on the other vessel's starboard side and the give-way vessel is the one on the other boat's port side. The give-way vessel must cross astern of the other, not ahead.
Communications must be clear. You cannot assume that the skipper of the other boat can read your mind. One way to communicate is with clear action. The give-way vessel must make an early and substantial course alteration—say, 20 degrees. An old rule of thumb is "show her your side." A small course change may be easily interpreted as a steering error. "Intent-agreement" horn or whistle signals specified by the Inland Rules (which govern US coastal and inland waters) also communicate intentions: one short blast indicates that the boat intends to turn to starboard; two blasts, a turn to port. (Other signals apply to other situations.) The second vessel indicates agreement by sounding the same signal, at which point the turn is made, or the second vessel warns of a risk of collision by sounding the five-blast danger signal. A third way to communicate between boats is over a VHF radio, which in crowded waters should be kept on, tuned to channel 16, and carefully monitored.
Finally, there are two informal, but helpful rules of thumb (not rules of the road) that have saved many skippers from embarrassment or worse:
The clear visibility rule: If you don't think the crew of the other vessel is able to see you, give way regardless of who is technically correct.
The gross tonnage rule: Give way to vessels much larger than yours, for the same reason.