Anchor types and holding power, scope calculations, the 6-step anchoring procedure, Rule 30 lights, dragging signs, and the seamanship vocabulary tested on the OUPV exam.
5:1
Minimum scope ratio — normal conditions
7:1
Recommended scope in storm or heavy weather
Rule 30
ColRegs rule governing anchor lights and shapes
The OUPV exam tests anchor selection by bottom type and the relative holding power of each design. Know the five primary types.
| Anchor | Also Called | Holding Power | Best Bottom | Notes |
|---|---|---|---|---|
| Danforth | Fluke anchor | Very high for weight | Sand, mud | Excellent for recreational vessels; folds flat for storage; can foul in grass or rock |
| CQR / Plow | Plow anchor | High | Sand, mud, grass | Self-setting; good all-around performance; hinged shank resets with wind shifts |
| Bruce / Claw | Claw anchor | High | Rock, coral, grass | No moving parts; resets easily after dragging; popular on powerboats |
| Mushroom | Mushroom | Very high when set | Soft mud only | Requires time to sink and set; used for permanent moorings, not cruising anchors |
| Kedge | Navy / Admiralty | Moderate | Rock, coral, hard bottom | Good where other anchors foul; traditional design; can be difficult to break out |
Scope is the ratio of rode length to the vertical distance from the bow chock to the bottom. Adequate scope keeps the pull on the anchor as horizontal as possible — increasing holding power dramatically.
Scope Formula
Rode Length = (Depth + Freeboard) × Scope Ratio
Depth = water depth at anchor drop point (feet or meters)
Freeboard = height of bow chock above waterline (typically 3–6 ft for most vessels)
Scope Ratio = 5 (normal) or 7 (storm)
Worked Example
Water depth: 15 ft | Freeboard: 4 ft | Scope: 5:1
Rode = (15 + 4) × 5 = 95 feet of rode
In a storm (7:1): (15 + 4) × 7 = 133 feet of rode required.
Tide — account for maximum water depth
Always calculate scope using maximum expected depth — add the tidal range to your current depth. A vessel anchored at low tide in 10 feet may be in 16 feet at high tide. Using low-tide depth for scope calculation leads to dragging at high tide.
Approach into the wind or current
Identify the strongest force — wind or current — and approach from downwind/downcurrent. This gives you positive control and reduces speed naturally as you slow the vessel. Select a bottom clear of obstructions and marked on the chart.
Check depth and swing radius
Use the depth sounder to confirm actual depth. Calculate your swing radius (rode length) and verify adequate clearance from other anchored vessels, shoals, and obstructions. Account for tidal variation.
Lower the anchor — do not throw
Stop the vessel over the drop point, then lower — do not throw — the anchor to the bottom. Throwing causes the chain to pile on top of the anchor and reduces holding power. Confirm the anchor is on the bottom before paying out rode.
Back down slowly to set the anchor
Pay out rode as the vessel drifts back under light engine power in reverse. This embeds the anchor flukes into the bottom. Increase reverse power gradually to confirm the anchor is set — the rode should come taut and the vessel stop moving.
Pay out full scope and secure the rode
Once the anchor is confirmed set, pay out the calculated amount of rode to achieve the proper scope ratio. Cleat or bitt the rode securely. Chain should have adequate snubber or nylon pennant to absorb shock loads.
Set an anchor watch
Take compass bearings on at least two fixed objects ashore or use GPS waypoint marking. Note your position on the chart. Check bearings every 15–30 minutes or set an anchor drag alarm. Display the required anchor light at dusk.
Both are used in practice — many vessels carry all-chain or combination (chain leader + nylon rode). The exam tests the practical differences.
| Factor | Nylon Rope | Chain |
|---|---|---|
| Weight | Light — easy to handle, less strain on bow | Heavy — lowers center of gravity, good catenary |
| Stretch / Shock absorption | Excellent — nylon stretches 15–25% under load | None — all shock goes to the anchor and cleats |
| Catenary | Minimal unless weighted with a kellet | Natural catenary — improves holding, reduces snubbing |
| Chafe resistance | Poor at chocks and over rocks | Excellent — resists abrasion at the bottom |
| Scope required | More scope needed (lower catenary effect) | Less scope needed (weight creates flat pull angle) |
| Cost | Low — affordable for most vessels | High — significant cost for full all-chain rode |
A vessel at anchor during daylight must display one black ball shape in the forward part of the vessel where it can best be seen. The ball is a solid sphere, approximately 0.6 meters in diameter for vessels over 20 meters.
| Vessel | Signal | Interval |
|---|---|---|
| At anchor — under 100 m | Rapid bell ringing (~5 seconds) | Every 1 minute |
| At anchor — 100 m and over | Bell rapidly forward + gong rapidly aft | Every 1 minute |
| Aground | 3 bell strokes + rapid bell + 3 bell strokes | Every 1 minute |
Bearing drift
Fixed objects ashore change their compass bearing over time. Take a bearing on a prominent landmark and recheck every 15 minutes.
Unsteady rode tension
A properly set anchor maintains steady rode tension. Jerky, slack-then-taut rode suggests the anchor is walking across the bottom.
Depth change
If the depth sounder shows a significant rise or fall in depth while at anchor, you may be dragging into shallower or deeper water.
Pay out more scope
Increasing scope from 5:1 to 7:1 or more flattens the pull angle and dramatically increases holding power. The first corrective action.
Re-anchor in better bottom
If bottom type is poor (rock, grass, or hard sand over mud), move to a better location. The chart often indicates bottom type — mud (M) or sand (S) hold best.
Set a second anchor
Deploy a second anchor off the bow at 30–45 degrees from the first. This doubles holding power and limits swing. Use a Bahamian moor in tight anchorages.
Scope
The ratio of the length of anchor rode deployed to the vertical distance from the bow chock to the bottom (depth + freeboard). A 5:1 scope means 5 feet of rode for every 1 foot of vertical distance. Higher scope = better holding.
Rode
The complete anchor line — including chain, rope, or a combination of both. The rode connects the anchor to the vessel. Chain rode provides catenary; nylon rode provides stretch for shock absorption.
Catenary
The natural sag or curve in a chain rode caused by its own weight. Catenary keeps the pull on the anchor horizontal, dramatically improving holding power and acting as a shock absorber. Light chain or rope rode has little catenary.
Set
The process of embedding the anchor flukes firmly into the bottom. A vessel 'sets' the anchor by backing down on it under power. An anchor that is set has dug in and will resist horizontal pull.
Fetch
The unobstructed distance over which wind can generate wave action. In an anchorage, fetch determines wave height — a short fetch means calmer water. Select anchorages with minimal fetch in the direction of prevailing or forecast wind.
Swing Radius
The circle described by a vessel swinging around its anchor point as wind direction changes. Equals the length of rode deployed plus the length of the vessel. Critical for avoiding collisions with other anchored vessels or obstructions.
Any anchoring question involving rode length requires (depth + freeboard) × scope ratio. The exam frequently omits freeboard as a trap — if given freeboard, add it. If not given, calculate with depth alone.
Vessel at anchor: one all-around white light at night, one black ball forward by day. Vessels 50 m or more add a second lower light aft. The exam tests this exact size threshold — memorize 50 meters.
Fog signal for a vessel at anchor is a rapidly rung bell every 1 minute — not 2 minutes like underway vessels. This is the most common anchor/fog signal trap on the OUPV exam.
A minimum 5:1 scope ratio (five feet of rode for every one foot of depth plus freeboard) is the standard recommendation for normal conditions. In storm or heavy weather conditions, 7:1 or greater is recommended. Scope is calculated on total depth plus the height of the bow chock above the waterline (freeboard). Chain rode requires less scope than nylon because its weight creates catenary — a curve in the rode that acts as a shock absorber — but 5:1 remains the minimum regardless of rode type.
Under Rule 30, a vessel at anchor must display 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 the day, a vessel at anchor displays a black ball shape forward. In restricted visibility, a vessel at anchor less than 100 meters sounds a rapid bell for about 5 seconds every 1 minute. Vessels 100 meters and over add a gong sounded rapidly aft after the bell.
Three primary signs of a dragging anchor: (1) Bearing drift — fixed objects ashore or on radar change their bearing relative to your vessel over time; take a compass bearing on a fixed object and watch it — any consistent change indicates drag. (2) Rode tension — a dragging anchor usually results in unsteady or jerky rode tension rather than a steady catenary curve. (3) Depth change — if your depth sounder readings change significantly while at anchor, you may be dragging into shallower or deeper water. Remedies include paying out more scope, reanchoring in a better bottom, or using a second anchor.
ColRegs Part D — maneuvering signals, fog signals by vessel type, distress signals
Right-of-way hierarchy, light arcs, steering and sailing rules, Inland vs. International
Tides, currents, weather, celestial basics, and chart reading for the OUPV exam
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