Anchoring — Types, Scope, and Holding Ground
TL;DR — Scope should be 7:1 in calm conditions and 10:1 in storm conditions (ratio of rode length to water depth plus bow height); the master is legally required to maintain an anchor watch and take action if the anchor drags. USCG Boatswain Manual Ch. 2 §2.2 33 CFR §164.19
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What the Rule Says
Equipment Requirements
Every commercial vessel must be fitted with an anchor or anchors, along with chain, cable, or rope appropriate for the vessel and the waters of the intended voyage. 46 CFR §28.235 The regulation does not specify a single anchor type or a minimum size — the standard is fitness for purpose given the vessel and intended waters. 46 CFR §28.235
Nonmetallic hull vessels have an additional requirement: unless the vessel is rigged with gear that provides a radar signature detectable from 6 miles, a radar reflector must be carried. 46 CFR §28.235 This matters for fiberglass and composite-hull vessels common in the OUPV world.
Anchor Watch and Dragging Anchor Procedures
Once anchored, the master or person in charge bears a specific legal duty under federal regulation. Three obligations attach immediately:
1. A proper anchor watch must be maintained. 33 CFR §164.19 2. Procedures must be followed to detect a dragging anchor. 33 CFR §164.19 3. When weather, tide, or current conditions are likely to cause the anchor to drag, the master must take action to ensure the safety of the vessel, nearby structures, and other vessels — examples include veering chain, letting go a second anchor, or getting underway under the vessel's own propulsion or with tug assistance. 33 CFR §164.19
Note that the regulation says "likely to cause" dragging — the duty to act is triggered before dragging actually occurs, not after. 33 CFR §164.19
Scope
Scope is defined as the ratio of rode length to the sum of water depth plus bow height. USCG Boatswain Manual Ch. 2 §2.2 Two specific values are tested:
- 7:1 — appropriate in good holding ground and calm conditions. USCG Boatswain Manual Ch. 2 §2.2
- 10:1 — required in storm conditions. USCG Boatswain Manual Ch. 2 §2.2
The mechanical reason scope matters: the rode must rest along the bottom for the anchor to set properly. Insufficient scope causes the rode to pull upward on the anchor shank, breaking it out of the seabed and reducing holding power. USCG Boatswain Manual Ch. 2 §2.2
Anchor Types and Their Characteristics
The exam tests recognition of anchor types and their appropriate applications. USCG Boatswain Manual Ch. 2 §2.3
| Anchor Type | Characteristics | Best Bottom | Limitations | |---|---|---|---| | Danforth (lightweight) | High holding power for its weight | Sand, mud | Poor in rocks | | Plough (CQR, Delta) | Good general-purpose anchor | General use | Harder to stow | | Bruce (claw) | Good performance in weeds | Weedy bottoms | — | | Mushroom | Designed for permanent moorings | — | Not for temporary anchoring | | Fluke | Suited for small craft | — | — |
USCG Boatswain Manual Ch. 2 §2.3
Holding Ground — Reading the Chart
US charts annotate bottom characteristics with standardized letter codes. These codes are used to assess holding ground quality before selecting an anchorage. Light List — Bottom Type Codes The codes you must recognize:
- M — mud
- S — sand
- R — rock
- Cl — clay
- Co — coral
- Sh — shells
- G — gravel
Light List — Bottom Type Codes
Sand and mud are generally favorable holding grounds for fluke-type anchors such as the Danforth. USCG Boatswain Manual Ch. 2 §2.3 Rock bottoms present poor holding for that anchor type. USCG Boatswain Manual Ch. 2 §2.3
Designated Anchorage Areas on Charts
Designated anchorages are outlined with a dashed magenta line on US charts and labeled with the anchorage type: general, prohibited, restricted, or quarantine. Light List — Anchorage Area Knowing the chart symbology is directly tested. A vessel less than 20 meters in length anchored in a special anchorage area designated by the Coast Guard is not required to exhibit anchor lights or shapes. 33 CFR §83.30
Anchor Lights and Shapes at Anchor
While lights and shapes are covered more fully in the Rules of the Road module, the anchor light requirements interact directly with anchoring operations and appear on the exam in this context.
A vessel at anchor shall exhibit, where it can best be seen: an all-round white light in the fore part, and at or near the stern, at a lower level, a second all-round white light. 33 CFR §83.30 A vessel less than 50 meters in length may substitute a single all-round white light where it can best be seen. 33 CFR §83.30 A vessel 100 meters or more in length shall use available working or equivalent lights to illuminate her decks. 33 CFR §83.30
A vessel less than 7 meters in length, when at anchor not in or near a narrow channel, fairway, anchorage, or where other vessels normally navigate, is not required to exhibit anchor lights or shapes. 33 CFR §83.30
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Why It Matters on the Exam
The OUPV and Master 100 GT written exams draw heavily from three areas within this topic:
1. Scope calculation. Expect a scenario question: "You are anchoring in 18 feet of water. Your bow height is 4 feet. What is the minimum rode you should veer in storm conditions?" The calculation is: (18 + 4) × 10 = 220 feet. The formula is rode length ÷ (depth + bow height) = scope ratio. USCG Boatswain Manual Ch. 2 §2.2 Candidates who forget to add bow height to depth will select a wrong answer.
2. Anchor type identification. Questions present a bottom type or a scenario (e.g., anchoring in a weedy bottom, or needing high holding power in sand) and ask which anchor is most appropriate. Match the anchor to its described characteristics. USCG Boatswain Manual Ch. 2 §2.3
3. Master's legal duties at anchor. The exam tests whether candidates know that the obligation to act on a dragging anchor is triggered by conditions "likely to cause" dragging — not by confirmed dragging. 33 CFR §164.19 The specific remedial actions listed in the regulation (veer chain, let go second anchor, get underway) are also tested as correct responses.
4. Chart symbology for anchorages and bottom type. Candidates must recognize the dashed magenta line as the chart symbol for a designated anchorage area Light List — Anchorage Area and must be able to decode bottom type codes. Light List — Bottom Type Codes
5. Anchor light exemptions. The size thresholds — less than 7 meters in certain locations, less than 20 meters in special anchorage areas — are classic exam distractors. 33 CFR §83.30
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Common Pitfalls
Forgetting bow height in scope calculations. Scope is rode length divided by depth plus bow height, not depth alone. USCG Boatswain Manual Ch. 2 §2.2 Omitting bow height produces a shorter calculated rode and a wrong answer.
Confusing 7:1 and 10:1 conditions. 7:1 is for good holding ground and calm conditions; 10:1 is for storm conditions. USCG Boatswain Manual Ch. 2 §2.2 Candidates sometimes reverse these.
Misidentifying the Danforth's weakness. The Danforth has high holding power in sand and mud but performs poorly in rock. USCG Boatswain Manual Ch. 2 §2.3 A question describing a rocky bottom and asking for the best anchor is testing this specific limitation.
Assuming the anchor watch duty is discretionary. It is not. The master or person in charge is legally required to maintain a proper anchor watch and to follow procedures to detect dragging. 33 CFR §164.19
Misreading the 7-meter exemption. A vessel under 7 meters is only exempt from anchor lights when it is not in or near a narrow channel, fairway, anchorage, or area where other vessels normally navigate. 33 CFR §83.30 The exemption is location-dependent, not automatic.
Confusing mushroom anchors with temporary anchoring. Mushroom anchors are designed for permanent moorings, not temporary anchoring. USCG Boatswain Manual Ch. 2 §2.3
Radar reflector requirement for nonmetallic hulls. A fiberglass vessel without gear providing a 6-mile radar signature must carry a radar reflector. 46 CFR §28.235 This is frequently overlooked by candidates focused on anchoring mechanics.
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Quick Check
Q1 — You are anchoring in 20 feet of water. Your bow height is 5 feet. Conditions are calm with good holding ground. What is the minimum rode you should veer?
(20 + 5) × 7 = 175 feet. Scope of 7:1 applies in good holding ground and calm conditions. The denominator is depth plus bow height, not depth alone. USCG Boatswain Manual Ch. 2 §2.2
Q2 — A storm is forecast. You are anchored in 15 feet of water with a bow height of 3 feet. What scope ratio is required, and how many feet of rode does that represent?
10:1 in storm conditions. (15 + 3) × 10 = 180 feet. USCG Boatswain Manual Ch. 2 §2.2
Q3 — The chart shows the bottom type code "R" at your intended anchorage. Which common anchor type performs poorly in this bottom, and which anchor would be a better choice?
"R" indicates rock. Light List — Bottom Type Codes The Danforth performs poorly in rock. USCG Boatswain Manual Ch. 2 §2.3 A plough-type anchor (CQR or Delta) is a better general-purpose choice. USCG Boatswain Manual Ch. 2 §2.3
Q4 — At what point does the master's legal duty to take action regarding a dragging anchor arise — when dragging is confirmed, or earlier?
Earlier. The duty arises whenever weather, tide, or current conditions are likely to cause the anchor to drag. The master must act before dragging is confirmed. 33 CFR §164.19
Q5 — A 45-meter vessel is at anchor at night. What anchor light display is required?
A vessel less than 50 meters may exhibit a single all-round white light where it can best be seen, in lieu of the two-light (fore and stern) arrangement. 33 CFR §83.30
Q6 — A fiberglass-hulled vessel has no special radar-reflective gear installed. What equipment requirement applies under federal regulation?
A nonmetallic hull vessel must carry a radar reflector, unless it is rigged with gear that provides a radar signature from a distance of 6 miles. 46 CFR §28.235
Q7 — How are designated anchorage areas depicted on US charts?
Designated anchorages are outlined with a dashed magenta line and labeled with the anchorage type (general, prohibited, restricted, or quarantine). Light List — Anchorage Area
Q8 — A vessel is 6.5 meters in length and anchored in open water well away from any channel, fairway, or anchorage. Is it required to display anchor lights?
No. A vessel less than 7 meters in length, when at anchor not in or near a narrow channel, fairway, anchorage, or where other vessels normally navigate, is not required to exhibit anchor lights or shapes. 33 CFR §83.30