1.4 Risk of Collision & Compass Bearings (Rule 7)

Rule 7 — Determining Risk of Collision

Before you can decide who gives way, you must first decide whether a collision risk even exists. That is the job of Rule 7, and its central test is one of the most quoted facts on the entire OUPV exam.

"Risk of collision shall be deemed to exist if the compass bearing of an approaching vessel does not appreciably change."

This is the constant-bearing, decreasing-range (CBDR) principle. If you take repeated compass bearings of an approaching vessel and the bearing stays the same while the vessel gets closer, you are on a collision course. It does not matter how the geometry "looks" — a steady bearing is the definitive warning.

How to take the bearing

The rule specifies the compass bearing, not a relative bearing, because a relative bearing changes whenever your own heading yaws. You can take the bearing with a hand-bearing compass, a pelorus, or by sighting across the steering compass. On a small vessel a quick field method is to line the other vessel up against a fixed part of your own boat (a stanchion or window frame) while holding a steady course: if the other vessel stays pinned to that reference point and grows larger, the bearing is constant and risk exists.

The Two Important Exceptions

A steady bearing proves risk, but a changing bearing does not prove safety. Rule 7 explicitly warns:

"Such risk may sometimes exist even when an appreciable bearing change is evident, particularly when approaching a very large vessel or a tow or when approaching a vessel at close range."

Two scenarios defeat the simple bearing test:

1. A very large vessel or a tow. A 900-foot ship or a tug with a long tow is so long that your bearing can be drawn open on the bow or stern while the middle of the vessel — or the tow line and barge trailing behind — is still headed for you. You may clear the bow of a ship and strike its side, or pass the tug and hit the barge on the towline.
2. Close range. When two vessels are already very close, even a rapidly changing bearing may not open the range enough to avoid contact.

The safe habit, reinforced by the rule, is: if there is any doubt as to whether risk of collision exists, it shall be deemed to exist. Doubt always resolves toward caution.

Proper Use of Radar

Rule 7 also governs how radar must be used to assess risk:

• Use it if you have it. "Proper use shall be made of radar equipment if fitted and operational, including long-range scanning to obtain early warning of risk of collision."
• Scan long range for early warning. Detecting a target at 12 miles gives you time to plot and act; waiting until it is at 2 miles does not.
• Plot systematically. The rule requires radar plotting or equivalent systematic observation of detected objects — you must track a contact's movement over time, not glance once and guess.
• Do not trust scanty information. "Assumptions shall not be made on the basis of scanty information, especially scanty radar information." A single fuzzy blip is not a basis for a maneuvering decision.

Worked Example: Ferry on a Steady Bearing

You are crossing a sound at 12 knots and spot a ferry off your starboard bow. You sight it across a fixed window mullion: over three minutes the ferry stays exactly on that mullion and visibly enlarges. Conclusion: the compass bearing is not appreciably changing, so risk of collision exists (CBDR). Because the ferry is on your starboard side, this is a crossing situation in which you are the give-way vessel (previewing Rule 15). Contrast that with a sailboat whose bearing draws steadily aft toward your stern — that bearing change indicates it will pass astern of you, and (absent the large-vessel/tow/close-range exceptions) risk is low.

Relative Bearing vs. Compass Bearing

Beginning operators often confuse a relative bearing (measured from the vessel's own bow, e.g. "two points off the starboard bow") with the compass bearing Rule 7 requires. The problem: if your boat yaws 10 degrees while you hold a rough heading, the relative bearing of a distant vessel swings 10 degrees even though nothing about the collision geometry actually changed. Only the compass bearing, referenced to true or magnetic north, filters out your own yaw. In practice, take a bearing every minute or two and write it down: three readings of 045, 045, 044 degrees over four minutes is a constant bearing and confirms risk; readings of 045, 050, 057 degrees show the target drawing right and destined to pass clear. Do not rely on a single glance, because the rule's warning against scanty information applies to eyeball bearings just as much as to a single radar blip.

Exam Framing

Expect at least one question stating that the compass bearing of an approaching vessel "does not appreciably change" and asking what that means — the answer is always risk of collision exists. Expect another testing the exception: risk can exist despite a changing bearing when the target is a large vessel, a tow, or very close. And remember the tie-breaker built into the rule: when in doubt, assume risk exists.