Anchor types and holding power, scope calculation, chain vs rope rode, anchoring procedures, Bahamian moor, Mediterranean mooring, COLREGS Rule 30 lights and shapes, dragging anchor response, and kedging off when aground.
5:1
Minimum scope ratio, calm conditions
7:1
Minimum scope in wind or storm conditions
50 m
Threshold for two anchor lights under Rule 30
6
Anchor types tested on the USCG captain exam
The USCG exam tests anchor selection by vessel size, bottom type, and intended use. Each anchor design uses a different mechanical principle to resist horizontal load. Know all six types, their relative holding power, and their ideal bottom conditions.
| Anchor | Also Called | Holding Power | Best Bottom | Key Notes |
|---|---|---|---|---|
| Danforth | Fluke anchor | Excellent (highest power-to-weight) | Sand, mud | Folds flat for storage. Penetrates well in soft bottoms. Can foul in rock, grass, or hard clay. Most common on recreational vessels. |
| CQR / Plow | Plow anchor | High | Sand, mud, grass | Hinged shank allows self-setting after wind shifts. Good all-around performer. Does not fold flat — requires bow roller or locker. |
| Bruce / Claw | Claw anchor | High | Rock, coral, grass, sand | No moving parts. Resets quickly after dragging through wind shifts. Widely used on powerboats with bow rollers. |
| Stockless / Navy | Patent anchor | Moderate | Mud, clay, sand | Stows in hawsepipe — essential for large commercial vessels. Low holding power per pound; compensated by large size. Rarely used recreationally. |
| Mushroom | Mushroom mooring anchor | Very high after sinking in | Soft mud only | Must sink and become embedded over weeks or months to achieve full holding power. Used for permanent moorings and channel markers — not for cruising anchors. |
| Kedge | Admiralty / Navy kedge | Good in hard bottoms | Rock, coral, hard ground | Traditional design with stock perpendicular to flukes. Good where fluke anchors foul. Standard secondary anchor for kedging off when aground. |
Charts indicate bottom type using standard abbreviations. Selecting the right anchor for the bottom is tested directly on the USCG exam.
| Chart Symbol | Bottom Type | Best Anchor Choice | Notes |
|---|---|---|---|
| S | Sand | Danforth, CQR, Bruce | Flukes penetrate and embed well. Excellent holding in firm sand. |
| M | Mud | Danforth, Mushroom (permanent) | Danforth buries deep. Mud anchors slowly but can drag without warning in soft mud. |
| R or Rk | Rock | Bruce, Kedge, CQR | Fluke anchors foul or slip on hard rock. Bruce and kedge hook crevices. |
| G or Gr | Grass or Weed | CQR (plow), Bruce | Danforth clogs with grass and loses holding. Plow pushes through grass to bottom. |
| Sh | Shell | Danforth, CQR | Similar to firm sand. Shell breaks up and provides good penetration. |
| Cl | Clay | Any — difficult to break out | Excellent holding once set. Can be hard to retrieve — may need to break out under power. |
Scope is the ratio of deployed rode length to the vertical distance from the bow chock to the seabed. The greater the scope, the more horizontal the pull on the anchor, and the higher the holding power. The USCG exam frequently asks you to calculate required rode length using the scope formula.
The Scope Formula
Rode Length = (Depth + Freeboard) × Scope Ratio
Depth
Water depth at the anchor drop point, measured in feet or meters by the depth sounder.
Freeboard
Height of the bow chock above the waterline. Typically 3 to 6 feet on recreational vessels. If not given on the exam, calculate with depth alone.
Scope Ratio
5:1 minimum in calm conditions. 7:1 or greater in strong wind, storm, or overnight stays. Some authorities recommend 10:1 in storm conditions.
Example 1 — Basic calm anchorage
(15 + 4) x 5 = 95 ft of rode
Answer: 95 feet
Example 2 — Same anchorage in a storm
(15 + 4) x 7 = 133 ft of rode
Answer: 133 feet
Example 3 — Tidal anchorage at low water
(16 + 3) x 5 = 95 ft of rode
Answer: 95 feet — using max depth not current depth
Critical Exam Trap: Always Use Maximum Expected Depth
The USCG exam may give a low-tide depth. Add the tidal range to get maximum depth before calculating scope. Anchoring on low-tide depth scope will leave you dragging when the tide rises.
Rode selection affects catenary, shock absorption, chafe resistance, and the scope required for safe holding. The exam tests the practical differences between all-chain and nylon combination rodes.
| Factor | Nylon Rope | Chain |
|---|---|---|
| Weight | Light — easy to handle, less weight on bow | Heavy — lowers bow, improves catenary, good for offshore use |
| Stretch and shock absorption | Excellent — nylon stretches 15 to 25 percent under load, absorbing snatch loads | None — all shock loads transfer directly to the anchor, cleats, and windlass |
| Catenary effect | Minimal catenary — requires a kellet (weight) to improve pull angle | Natural catenary from chain weight keeps the pull on the anchor nearly horizontal |
| Chafe resistance | Poor at chocks, over rocks, and in surge. Requires chafe protection at the bow | Excellent — chain resists abrasion at the bottom and through chocks |
| Scope required | More scope needed to compensate for lack of catenary | Less scope needed because chain weight creates the flat pull angle |
| Practical use | Best for day sailing and light use. Use snubber if anchoring overnight | Best for cruising vessels, exposed anchorages, and overnight stays |
| Cost | Low — affordable for any vessel | High — all-chain rode adds significant cost and weight |
Catenary is the natural curve or sag formed in a heavy chain rode due to gravity. A straight rode transmits every surge and snatch load directly to the anchor, risking breakout. A sagging chain rode acts as a shock absorber: as the vessel surges forward, it first lifts the chain before tension reaches the anchor. This dramatically improves holding and reduces the risk of breaking out in chop or swell.
With a nylon rope rode, catenary is achieved by hanging a kellet (a lead or chain weight) at the midpoint of the rode. This adds weight and restores the beneficial sag.
The USCG exam tests the correct sequence for anchoring. The seven steps below represent the authoritative procedure. Know the order and the reasoning behind each step.
Select the anchorage and check the chart
Identify the anchorage on the chart. Confirm bottom type, depth range, and swinging room. Check for shoals, rocks, submarine cables, or restricted areas. Identify the prevailing wind and current direction.
Approach into the wind or current (stronger force first)
Approach from downwind or downcurrent of the chosen spot. This slows the vessel naturally and gives positive directional control. If wind and current oppose, approach into whichever is stronger.
Check depth and calculate scope
Use the depth sounder over the intended drop point. Calculate scope based on maximum expected depth (adding tidal range). Confirm adequate swinging room for the rode length you will deploy.
Stop the vessel and lower the anchor
Stop the vessel directly over the drop point. Lower the anchor to the bottom slowly and under control. Do not throw the anchor overboard. Throwing the anchor causes the chain to pile on top and prevents the flukes from orienting to set.
Veer scope as the vessel falls back
As the vessel falls back on wind or current, veer (pay out) rode steadily. Veering scope allows the chain to lay out on the bottom and lets the flukes orient in the direction of pull before setting. Do not veer all rode at once.
Set the anchor by backing down under power
Apply steady reverse throttle to load the anchor and embed the flukes. Increase power gradually until the vessel stops moving astern and the rode comes taut. The rode should lead forward from the bow at a low angle when the anchor is set. Confirm the anchor is holding before increasing power.
Set anchor watch and display required signals
Take compass bearings on at least two fixed objects. Mark your GPS position. Display the anchor ball (day) or all-around white anchor light (night) as required by Rule 30. Check bearings every 15 to 30 minutes or set a GPS drag alarm.
A vessel at anchor swings in a circle around its anchor point as wind and current direction change. The radius of that circle determines safe clearance from other anchored vessels, shoals, and fixed objects. Underestimating swinging room is a leading cause of anchor collisions.
Swing Radius Formula
Swing Radius = Rode Length Deployed + Vessel Length
If you deploy 95 feet of rode and your vessel is 35 feet long, your swing radius is 130 feet. No other anchored vessel or obstruction should be within 130 feet of your anchor point in any direction.
Different Vessels Swing Differently
Sailboats and powerboats swing at different rates and in different arcs depending on windage and hull shape. A deeply keeled sailboat tends to lie to the wind; a high-windage powerboat may lie to the current. When anchored near other vessels of different types, ensure that your swing circles do not intersect even under worst-case wind shift.
The Bahamian moor limits swing to a small arc, making it ideal for tight anchorages and areas with strong tidal reversals. Two anchors are set 180 degrees apart off the bow so the vessel rides on a bridle between them.
Mediterranean mooring combines an anchor off the stern with dock lines to a quay, seawall, or pier. The bow or stern lies against the dock depending on local convention. It is commonly used in Mediterranean ports where dock space is limited and vessels cannot lie alongside.
Tandem anchoring connects two anchors on the same rode in series, with the second anchor shackled to the crown of the first. Both anchors work in the same direction. This effectively doubles holding power without requiring a second rode and is useful in storm conditions or poor holding ground where a single large anchor is unavailable.
Rule 30 of the International Regulations for Preventing Collisions at Sea governs the lights and shapes required on vessels at anchor. The USCG exam regularly tests Rule 30 requirements and the 50-meter threshold.
| Vessel | Required Light(s) | Arc |
|---|---|---|
| Any vessel at anchor — under 50 m | One all-around white light, placed where best seen | 360 degrees |
| Vessel at anchor — 50 m or more | All-around white light forward + lower all-around white light aft | 360 degrees each |
| Vessel at anchor — under 7 m | May omit if not in or near a channel, fairway, or anchorage used by other vessels | Exception applies |
| Situation | Signal | Interval |
|---|---|---|
| Vessel at anchor, under 100 m | Rapid bell ringing for approximately 5 seconds | Every 1 minute |
| Vessel at anchor, 100 m and over | Rapid bell forward + rapid gong aft | Every 1 minute |
| Vessel aground | 3 bell strokes + rapid bell + 3 bell strokes | Every 1 minute |
Bearing drift
Take compass bearings on two or more fixed objects ashore. If those bearings change consistently in the same direction over time, the anchor is dragging. This is the most reliable indicator.
GPS position change
A properly set anchor holds the vessel within a small circle. If the GPS track shows the vessel moving steadily in one direction beyond its normal swing circle, the anchor is dragging.
Unsteady rode tension
A set anchor produces steady, even rode tension. A dragging anchor produces jerky, uneven tension as it skips across the bottom.
Depth change
Rising or falling depth readings on the sounder while at anchor indicate movement into shallower or deeper water.
Vessel not lying to wind or current
A vessel with a properly set anchor aligns with the dominant force. A vessel beam-on to wind or current while at anchor may indicate the rode has fouled or the anchor is skipping.
Pay out more scope
The first and fastest response. Increasing scope from 5:1 to 7:1 or more flattens the pull angle and can recover a dragging anchor. Veer additional rode immediately.
Back down to reset
If more scope restores holding, apply engine astern power to re-set the flukes. Increase power gradually and confirm the anchor has re-set before leaving the helm.
Re-anchor in better holding ground
If the bottom type is unsuitable (rock with a Danforth, grass with no plow anchor) or if the anchor continues to drag, weigh anchor and move to a better location.
Set a second anchor
Deploy a second anchor at 30 to 45 degrees from the first to double holding power and restrict swing. This is the preferred solution in crowded anchorages where re-anchoring is not practical.
Emergency maneuver
If the vessel is dragging toward shoals or another vessel and anchoring solutions are failing, start the engine, weigh anchor, and motor clear. Never anchor closer to a hazard than the swing radius.
An anchor watch is a scheduled lookout maintained while a vessel is at anchor to detect dragging and respond to changing conditions. The USCG exam treats a proper anchor watch as part of safe anchoring practice.
Rotating Watches
On overnight passages or extended anchorage in deteriorating conditions, a rotating watch schedule keeps a crew member on deck at all times. Typical watch rotation is 2 to 4 hours. The watch officer is responsible for taking bearings at the start of each watch and before waking the off-watch crew for any reason.
Emergency anchoring uses the anchor to arrest the vessel when engine or steering failure threatens collision with a shoal, pier, or other hazard. Speed and correct technique are critical.
Do Not Let Go All at Once in a Fairway
Dropping anchor in a channel or fairway with no scope control can create a hazard for other vessels. If possible, veer scope steadily and notify VTS or port authority on VHF. An anchored vessel aground is required to show distress signals if in immediate danger.
Kedging off is the technique of pulling a grounded vessel into deeper water using a kedge anchor deployed from a dinghy. It is one of the most important practical seamanship skills tested on the USCG exam and in real-world coastal operations.
Wait for the Tide When:
Kedge Off Immediately When:
Any rode-length question requires (depth + freeboard) x scope ratio. The exam often omits freeboard as a trap. If freeboard is given, always add it. If not stated, use depth alone.
Vessels under 50 m show one all-around white anchor light. Vessels 50 m or more show one forward and one lower aft. This threshold appears on nearly every USCG exam anchoring question.
Fog signal for a vessel at anchor is a rapid bell ring every 1 minute. Underway power vessels in restricted visibility sound one prolonged blast every 2 minutes. These two intervals are the most common fog signal confusion points.
The mushroom anchor requires weeks or months to sink and embed. It is never the correct answer for a cruising anchor or a vessel at anchor. Choose Danforth for sand and mud, Bruce or plow for grass and rock.
The USCG exam tests the correct anchoring procedure. The anchor must be lowered, not thrown. Throwing piles chain on top of the anchor and prevents the flukes from orienting to set.
Chain rode creates catenary (the natural sag) that keeps the pull on the anchor horizontal. More horizontal pull equals more holding power. Nylon rope rode has minimal catenary and requires more scope.
Ground Tackle
The complete anchoring system including the anchor, rode (chain and rope), shackles, swivel, windlass, bits, cleats, and chocks. Ground tackle refers to the entire system, not just the anchor.
Scope
The ratio of deployed rode length to the vertical distance from the bow chock to the seabed. Scope = rode length divided by (depth + freeboard). Higher scope produces more horizontal pull and greater holding power.
Rode
The complete anchor line connecting the anchor to the vessel. May be all-chain, all-rope (nylon), or a combination rode with a chain leader and nylon body.
Catenary
The natural sag or curve in a chain rode caused by the weight of the chain. Catenary keeps the pull on the anchor horizontal and absorbs shock loads. A taut chain rode has lost catenary and transfers all shock directly to the anchor.
Veering Scope
Paying out or easing additional rode after the anchor is on the bottom. Scope is veered as the vessel falls back to allow the flukes to orient and set. Veering too quickly piles the rode and prevents setting.
Set
The process of embedding anchor flukes firmly into the seabed. The anchor is set by backing down under controlled astern power until the rode comes taut and the vessel stops moving.
Kellet
A lead or chain weight attached to the midpoint of a rope rode to increase catenary. The kellet adds weight that sags the rode and improves the angle of pull on the anchor. Also called a sentinel.
Swinging Room
The circular area swept by a vessel as it swings around its anchor point. Swinging room radius equals the rode length deployed plus the length of the vessel.
Snubber
A nylon line attached from the bow cleat to the anchor chain to absorb shock loads and take the load off the windlass. Essential when anchoring on all-chain rode in surge or chop.
Windlass
A mechanical device on the foredeck used to haul in heavy anchor chain under power. May be electric or hydraulic. The windlass must never be used as a cleat — the rode must be transferred to bitts or cleats after anchoring.
Kedge
A small secondary anchor, traditionally of the Admiralty or stockless pattern, used for kedging off when aground, mooring in tight quarters, or as a storm anchor. Also the action of using such an anchor to move a grounded vessel.
Freeboard
The vertical distance from the waterline to the bow chock (the anchor exit point). Added to depth when calculating scope. Typically 3 to 6 feet for most recreational vessels.
Scope ratio equals rode length divided by the sum of depth plus freeboard. To find required rode length: multiply (depth + freeboard) by the scope ratio. Minimum scope is 5:1 in calm conditions and 7:1 or greater in wind or storm conditions. Freeboard is the height of the bow chock above the waterline, typically 3 to 6 feet for most recreational vessels. Always calculate using maximum expected depth, adding the tidal range to your current depth.
The USCG exam tests six primary anchor types: (1) Danforth or fluke anchor, excellent in sand and mud with very high holding power for its weight; (2) plow or CQR, a self-setting all-around anchor good in sand, mud, and grass; (3) Bruce or claw, with no moving parts, good in rock and coral, resets easily after dragging; (4) stockless or Navy anchor, used on large commercial vessels, stows in hawsepipes; (5) mushroom anchor, provides very high holding in soft mud after sinking in but is for permanent moorings only; (6) kedge anchor, traditional Admiralty design good in rock where fluke anchors foul.
Under COLREGS Rule 30, a vessel at anchor must show an all-around white light where it can best be seen. Vessels under 50 meters display one all-around white light. Vessels 50 meters or more display an all-around white light forward and a lower all-around white light aft. During daylight, any anchored vessel shows one black ball shape in the forward part of the vessel. Vessels under 7 meters may omit anchor lights if not in or near a channel, fairway, or anchorage used by other vessels.
Three primary signs: (1) Bearing drift, where compass bearings on fixed objects ashore change consistently over time; (2) Unsteady or jerky rode tension instead of a steady catenary curve; (3) Depth change on the sounder as the vessel moves into shallower or deeper water. Additional indicators include GPS position drift and the vessel pointing upwind or up-current while the anchor is forward. Remedies include paying out more scope, re-anchoring in better holding ground, or deploying a second anchor.
A Bahamian moor uses two anchors set off the bow in opposite directions, typically 180 degrees apart. The vessel rides on a bridle between the two anchors and can only swing in a limited arc, making it ideal for tight anchorages with strong tidal reversals. To set: drop the first anchor, motor upwind to the second drop point, lower the second anchor, then fall back to the midpoint between them and equalize scope on both rodes. The Bahamian moor limits swing radius to approximately the rode length rather than twice the rode length.
Kedging off is the technique of using an anchor deployed from a dinghy or by hand to pull a grounded vessel into deeper water. The kedge anchor is carried out by dinghy in the direction of deepest water, dropped, and the rode is then hauled in by hand, winch, or power from the grounded vessel. Simultaneously, crew weight should be shifted to change trim, and the engine used in short bursts. The technique is named after the kedge anchor, a small secondary anchor traditionally carried for this purpose.
OUPV, Master, and endorsement licensing — requirements, sea time, exams, and the NMC application process.
Comprehensive seamanship coverage: knots, deck work, towing, line handling, mooring techniques, and practical anchoring.
Propeller effects, backing and filling, docking, undocking, wind and current compensation, and close-quarters maneuvering.
Over 1,600 USCG exam questions with full Navigation General coverage — scope calculations, anchor types, Rule 30 lights and shapes, fog signals, dragging anchor response, and kedging. Spaced repetition flashcards and instant explanations. Free to start.
Start Free Practice