Diamond (day shape)
A black diamond day shape shown where it can best be seen. On the OUPV / Master rules exam, it usually signals either a tow exceeding 200 meters or the middle shape in a restricted-in-ability-to-maneuver ball-diamond-ball display.
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In depth#
How this term is used in the rules, and where it tends to trip people up on the exam.
What a diamond day shape tells you
A diamond is a daytime shape, not a light. Rule 24 requires a diamond when the length of a tow exceeds 200 meters, measured from the stern of the towing vessel to the after end of the tow. The same diamond can also appear on the towed object itself when that object is displaying its own towing shapes. Rule 27 uses the diamond differently: a vessel restricted in her ability to maneuver shows three shapes in a vertical line, with balls as the upper and lower shapes and the diamond in the middle. A dredge or underwater-operations vessel can also show two diamonds on the side where another vessel may pass.
Exam traps
Do not read every diamond as "restricted in ability to maneuver." A single diamond is the long-tow clue. Ball-diamond-ball is the RAM clue. Two diamonds to one side of a dredge indicate the safe passing side. Watch the wording around tow length: the rule says "exceeds 200 meters," not "equals 200 meters." At night, the question will usually switch from shapes to the matching towing lights or red-white-red RAM light pattern.
Related day-shape clues
Pair this with the cylinder shape for constrained-by-draft vessels and the two-ball display for vessels not under command. The exam often tests day shapes as a group because the answer turns on the combination, not the individual geometric symbol.
Exam questions#
Does a diamond day shape always mean restricted in ability to maneuver?
No. A single diamond usually points to a tow exceeding 200 meters. A RAM vessel uses the diamond only as the middle shape in the ball-diamond-ball combination.
When does a towing vessel display a diamond?
Under Rule 24, a towing vessel displays a diamond where it can best be seen when the tow length exceeds 200 meters.
Sources#
- 33 CFR 83.24 — Towing and pushing (Rule 24) (retrieved 2026-05-13)
- 33 CFR 83.27 — Vessels not under command or restricted in their ability to maneuver (Rule 27) (retrieved 2026-05-13)
Related terms#
See Diamond (day shape) in a real exam question.
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