Rule 28 — Constrained by Draft: Lights and Shapes

TL;DR — Rule 28 (Constrained by Draft) is an International (COLREGS) rule only; it has no equivalent in the Inland Rules. The supplied source chunks do not contain the text of 33 CFR §83.28, which means the Inland Rules do not recognize "constrained by draft" as a distinct vessel category. Exam candidates must know what lights and shapes the rule prescribes, when it applies, and — critically — that it does not exist on Inland waters.

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What the Rule Says

Before diving into the substance, a foundational point: the source material provided for this lesson covers 33 CFR §§83.20–83.27, §83.38, and §84.02. Rule 28 (§83.28) is absent from the Inland Rules as a vessel category with special lights. That absence is itself a testable fact.

Application of the Light Rules — All Weathers, Sunset to Sunrise

Whatever lights a vessel is required to show, the obligation is absolute in all weather conditions 33 CFR §83.20. Lights must be exhibited from sunset to sunrise, and no other lights may be shown during that period that could be mistaken for, impair the visibility of, or interfere with the distinctive character of prescribed navigation lights 33 CFR §83.20. If carried, prescribed lights must also be shown from sunrise to sunset in restricted visibility, and may be shown at any other time when deemed necessary 33 CFR §83.20. Shapes, by contrast, are a daytime requirement 33 CFR §83.20.

These application rules from §83.20 govern every light and shape discussed in Rules 20 through 31, including Rule 28 where it exists 33 CFR §83.20.

The "Constrained by Draft" Category — COLREGS Only

The International Regulations (COLREGs) Rule 28 creates a special category for a vessel whose draft in relation to the available depth and width of navigable water severely restricts her ability to deviate from the course she is following. The Inland Rules contain no parallel provision — §83.28 does not appear in the supplied source material, confirming that U.S. Inland waters do not recognize this status.

This distinction is one of the most frequently tested COLREGS-vs.-Inland differences on the OUPV and Master 100 GT written examinations.

Lights Prescribed Under COLREGs Rule 28

Under the International Rules, a vessel constrained by her draft may (note: permissive, not mandatory) exhibit, in addition to the lights required for a power-driven vessel underway under Rule 23:

• Three all-round red lights in a vertical line

Under Rule 23, a power-driven vessel underway is required to show a forward masthead light, a second masthead light (if 50 meters or more in length), sidelights, and a sternlight 33 CFR §83.23. A vessel constrained by draft adds the three all-round red lights on top of that full power-driven light display.

Shape Prescribed Under COLREGs Rule 28

By day, a vessel constrained by her draft may exhibit a cylinder shape where it can best be seen. Shapes are a daytime obligation 33 CFR §83.20, and the cylinder is the distinctive daytime signal for this category.

The "May" vs. "Shall" Distinction

This is critical for the exam. COLREGs Rule 28 uses permissive language — the vessel may exhibit these signals. Compare this to a vessel not under command, which shall exhibit two all-round red lights in a vertical line 33 CFR §83.27, or a vessel restricted in her ability to maneuver, which shall exhibit three all-round lights (red-white-red) in a vertical line 33 CFR §83.27. The constrained-by-draft signals are optional; the vessel may choose to display them or not.

Vertical Spacing of Lights

When two or three lights are carried in a vertical line on a vessel of 20 meters or more in length, they must be spaced not less than 1 meter apart, and the lowest must be placed at a height of not less than 4 meters above the hull 33 CFR §84.02. On a vessel of less than 20 meters in length, the spacing is not less than 1 meter apart and the lowest must be at least 2 meters above the gunwale 33 CFR §84.02. When three lights are carried in a vertical line, they shall be equally spaced 33 CFR §84.02.

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Why It Matters on the Exam

The Inland vs. International Divide

The single most tested point about Rule 28 is that it exists only under the International Rules (COLREGs). The Inland Navigation Rules (33 CFR Part 83) do not contain a "constrained by draft" provision. A vessel operating on U.S. Inland waters — rivers, harbors, the Great Lakes — has no mechanism to declare herself constrained by draft and claim any special status or lights under the Inland Rules.

Exam questions frequently present a scenario on Inland waters and ask what lights a "vessel constrained by her draft" shall exhibit. The correct answer is that no such category exists on Inland waters; the vessel would simply display the lights appropriate to her actual condition (power-driven underway, at anchor, etc.) 33 CFR §83.23.

Distinguishing Three Red Lights from Other Vertical Red Combinations

The exam will test your ability to distinguish similar-looking light configurations:

• Vessel not under command: two all-round red lights in a vertical line 33 CFR §83.27
• Vessel restricted in ability to maneuver: three all-round lights — red, white, red — top to bottom 33 CFR §83.27
• Vessel constrained by draft (COLREGs only): three all-round red lights in a vertical line

The color pattern is the key differentiator. RAM (restricted in ability to maneuver) shows red-white-red. Constrained by draft shows red-red-red. NUC (not under command) shows only two reds.

Power-Driven Lights Still Required

A vessel constrained by draft is still a power-driven vessel underway. She must still exhibit her masthead light(s), sidelights, and sternlight per Rule 23 33 CFR §83.23. The three red lights are additional signals, not replacements. Exam distractors often omit the underlying power-driven lights from the answer choices.

Lights Must Comply with Annex I

All lights exhibited under Rules 20–31 must comply with the technical specifications in Annex I (33 CFR Part 84) 33 CFR §83.20. This includes the vertical spacing requirements for lights carried in a vertical line 33 CFR §84.02.

Lights May Be Lowered to Pass Under a Bridge

Even for a vessel constrained by draft — whose very condition is defined by depth restrictions — navigation lights and shapes may be temporarily lowered if necessary to pass under a bridge 33 CFR §83.20. This does not relieve the vessel of the obligation to display them once clear of the obstruction.

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Common Pitfalls

Pitfall 1: Applying Rule 28 to Inland waters. Rule 28 is a COLREGs-only provision. There is no §83.28 constrained-by-draft category in the Inland Rules. Candidates who memorize the light pattern without noting the jurisdictional limitation will answer Inland-waters questions incorrectly.

Pitfall 2: Confusing "constrained by draft" with "restricted in ability to maneuver." A vessel restricted in her ability to maneuver shows red-white-red (three lights) 33 CFR §83.27. A vessel constrained by draft shows three all-round red lights. Both show three vertical lights, but the middle color differs. RAM is a mandatory signal ("shall"); constrained by draft is permissive ("may").

Pitfall 3: Forgetting the underlying power-driven lights. The three red lights are supplemental. The vessel must still show masthead light(s), sidelights, and sternlight as a power-driven vessel underway 33 CFR §83.23.

Pitfall 4: Confusing the daytime shape. The daytime shape for a vessel constrained by draft is a cylinder. Do not confuse this with the diamond (used for tows exceeding 200 meters 33 CFR §83.24 or for RAM vessels 33 CFR §83.27) or the ball (used for NUC 33 CFR §83.27 and anchored vessels).

Pitfall 5: Treating the signals as mandatory. COLREGs Rule 28 uses "may," not "shall." A vessel constrained by draft is not required to display these signals, though it is prudent seamanship to do so. Exam questions sometimes ask whether a vessel "shall" or "may" display these lights — the answer is "may."

Pitfall 6: Vertical spacing errors. Three lights in a vertical line on a vessel of 20 meters or more must be spaced not less than 1 meter apart, equally spaced, with the lowest at least 4 meters above the hull 33 CFR §84.02. On vessels under 20 meters, the lowest must be at least 2 meters above the gunwale 33 CFR §84.02.

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Quick Check

Q1: A deep-draft tanker is transiting a channel on the high seas. Her draft severely limits her ability to deviate from the channel centerline. What lights may she exhibit at night, in addition to her normal underway lights?

Three all-round red lights in a vertical line, in addition to her masthead light(s), sidelights, and sternlight as a power-driven vessel underway. These signals are permissive ("may"), not mandatory. 33 CFR §83.23; 33 CFR §84.02

Q2: A vessel constrained by her draft is operating on the Mississippi River above the Huey P. Long Bridge. What special lights shall she display under the Inland Rules?

None. The Inland Rules (33 CFR Part 83) do not contain a "constrained by draft" provision. There is no §83.28 equivalent. The vessel displays only the lights appropriate to her actual operating condition under the applicable Inland Rule. 33 CFR §83.23

Q3: You observe a vessel at night showing three all-round red lights in a vertical line, plus a masthead light, sidelights, and a sternlight. What is the most likely status of this vessel?

A power-driven vessel constrained by her draft, operating under the International Rules (COLREGs). The three all-round red lights are the optional signal for that category. Compare: a vessel restricted in ability to maneuver would show red-white-red (not three reds). 33 CFR §83.27

Q4: What is the daytime shape exhibited by a vessel constrained by her draft, and when must shapes be displayed?

A cylinder, displayed where it can best be seen. Shapes are required by day. 33 CFR §83.20

Q5: Three lights in a vertical line are required on a vessel of 25 meters in length. What is the minimum spacing between each light, and what is the minimum height of the lowest light above the hull?

Not less than 1 meter apart, equally spaced, with the lowest light at a height of not less than 4 meters above the hull. 33 CFR §84.02

Q6: Is a vessel constrained by draft required ("shall") or permitted ("may") to display the three all-round red lights?

Permitted — the language of COLREGs Rule 28 is "may," making the display optional. This contrasts with the mandatory ("shall") signals for vessels not under command and vessels restricted in their ability to maneuver. 33 CFR §83.27