1.4 Risk of Collision (Rule 7)

Rule 7 — Determining Risk of Collision

Before you decide who gives way, you must decide whether risk of collision exists. Rule 7 is that determination, and it is one of the most quoted rules on Q100. It also feeds Rules 8, 14, 15, 16, and 17: you cannot apply give-way/stand-on logic until risk is assessed.

Rule 7 requires that every vessel shall use all available means appropriate to the prevailing circumstances and conditions to determine if risk of collision exists. If there is any doubt, such risk shall be deemed to exist. That doubt-resolution rule is pure exam gold: when the stem says "you are unsure," the correct path is to treat risk as present and act accordingly.

The Constant-Bearing Test

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. Repeated compass bearings of an approaching vessel that stay essentially the same while range decreases mean you are on a collision course — regardless of how "comfortable" the geometry feels.

Compass bearing, not relative bearing

Rule 7 specifies compass bearing because a relative bearing ("two points on the starboard bow") swings whenever your own heading yaws. Take bearings with a hand-bearing compass, a pelorus, or by careful sighting referenced to a magnetic/true compass. Field method on a small passenger vessel: hold a steady course and line the other vessel on a fixed structural reference (stanchion, window mullion). If the target stays pinned to that reference and grows larger, the bearing is constant and risk exists.

Record bearings over time (every minute or two). Three successive readings of 112°, 112°, 111° while the target enlarges is CBDR. Readings of 112°, 118°, 125° show the target drawing aft/right and likely passing clear — unless you are already in the large-vessel/tow/close-range exceptions.

Exceptions: Changing Bearing Does Not Prove Safety

Rule 7 expressly warns that 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.

1. Very large vessel. A 900-foot ship is so long that your bearing may open on the bow or stern while the mid-body is still aimed at you. You can "clear the bow on paper" and still hit the side.
2. Tow. The tug's bearing may open while the towline and barge remain on a collision course. Watching only the tug is a classic Master-level error in busy harbors.
3. Close range. At short distance, even a rapidly changing bearing may not open range fast enough to avoid contact.

On Master 100 runs through commercial ports, expect frequent encounters with ship traffic and towing vessels. Train yourself to assess the whole unit (tug + tow + catenary) and the full length of large ships, not just the nearest light or the pilot house.

Radar Duties Under Rule 7

If radar is fitted and operational, Rule 7 requires proper use, including:

• Long-range scanning to obtain early warning of risk of collision.
• Radar plotting or equivalent systematic observation of detected objects — track movement over time; do not glance once and guess CPA.
• No assumptions on scanty information, especially scanty radar information.

Scanty radar information includes a single weak paint, an uncorrelated ARPA vector that has not stabilized, a target lost in sea clutter, or deciding based on one sweep. Scanty visual information is the eyeball equivalent: one hurried glance without a timed series of compass bearings. A Master carrying passengers who alters hard on a single fuzzy radar return can create a worse collision with a third vessel — Rule 7 forbids that kind of guesswork.

Systematic observation in practice

1. Detect early (long range scale, then refine).
2. Acquire and track (manual plot, ARPA, or disciplined visual bearings).
3. Assess CPA/TCPA or bearing trend before close quarters.
4. Decide risk under Rule 7 (including the doubt = risk rule).
5. Only then apply Rule 8 action and the applicable Section II/III steering rules.

Early determination is not optional rhetoric — the rule's structure expects risk to be known while there is still room for bold, obvious action.

Worked examples for Master 100

Example A — Ferry CBDR. Crossing a sound at 12 knots, you sight a ferry fine on the starboard bow. Over three minutes the ferry stays on the same window mullion and enlarges. Conclusion: compass bearing is not appreciably changing → risk of collision exists. (Later chapters: if vessels are in sight and this is a crossing with the ferry on your starboard side, you are the give-way vessel under Rule 15 — but that analysis starts with Rule 7.)

Example B — Tug and barge. The tug's compass bearing is slowly drawing open. A deckhand says "we're fine." You check the barge and towline: the barge's bearing is nearly steady and range is closing. Conclusion: risk may still exist under the tow exception; do not clear based on the tug alone.

Example C — Scanty radar. In drizzle at night a single intermittent paint appears at two miles, no trail, no AIS match. Wrong: hard turn based on that paint alone. Right: continue systematic observation (adjust gain/clutter, change range scale, attempt visual, call on VHF if appropriate), assume risk if doubt remains, and reduce speed under Rule 6 to buy assessment time under Rule 8(e).

Relative vs compass bearings — training point

Beginning Masters often say "she's still broad on the starboard bow" after their own vessel has yawed 15°. Relative language lied; the collision geometry may not have changed. Use compass numbers written down. On a rolling passenger vessel, stabilize the course for each bearing if possible, or average carefully — but never treat a single relative impression as Rule 7 compliance.

Exam Framing for Q100 at 90%

Expect at least one item where "compass bearing does not appreciably change" maps directly to risk of collision exists. Expect a second item where a changing bearing on a tow or large ship still leaves risk. Expect a third on scanty radar information or if in doubt, risk exists. Phrase answers with the rule's words — "does not appreciably change," "scanty information," "deemed to exist" — because distractors often substitute softer language that sounds right but is wrong.

Rule 7 is the gate: no valid Master decision under the Rules skips the risk determination. Master it before Rules 8–19, or every later answer is built on sand.