Anchoring — Ground Tackle, Scope and Holding Ground

What the rule requires

Scope is what makes the anchor hold

An anchor holds best when the pull on it is horizontal, and that is the whole reason scope matters: with more chain out, the catenary — the sag of the heavy chain — keeps the shank pulling parallel to the bottom so the flukes stay buried, while a short scope lifts the shank and can break the anchor out NAVEDTRA 14343 §4-2. The working rule of thumb is a scope of five to seven times the depth of water, measured from the hawsepipe to the bottom, increased in heavy weather.

The same ratio appears with the storm figure attached: 7:1 in good holding ground and calm conditions, 10:1 in storm conditions, where scope is the ratio of rode length to water depth plus bow height USCG Boatswain Manual Ch. 2 §2.2. Less scope reduces holding power, because the rode has to rest along the bottom for the anchor to set.

Chain lying on the bottom does two jobs at once. It adds holding power of its own, and it acts as a shock absorber, cushioning the surges from wind and sea before they reach the flukes .

Chain is counted in shots, and the exam counts in feet

Anchor chain is measured in shots, each shot being 15 fathoms, 90 feet, joined by detachable links — a two-part link secured by a tapered locking pin and lead plug, so a damaged length can be replaced NAVEDTRA 14343 §4-1. Each detachable link and the chain adjacent to it are painted and wire-wrapped to a colour code, which is how the crew reads off how much chain is out as it passes the wildcat.

You have four shots of chain in the water. How much chain is that in feet and in fathoms?

360 feet, 60 fathoms. A shot is 15 fathoms and 90 feet, so four shots is 4 × 90 = 360 feet and 4 × 15 = 60 fathoms. Any conversion question on this topic reduces to those two numbers.

What the law requires of you once the anchor is down

The master or person in charge of an anchored vessel must ensure a proper anchor watch is maintained and that procedures are followed to detect a dragging anchor 33 CFR §164.19. The third requirement is the one exam questions turn on: whenever weather, tide, or current conditions are likely to cause the anchor to drag, action must be taken to ensure the safety of the vessel, structures and other vessels — being ready to veer chain, let go a second anchor, or get underway using the vessel's own propulsion or tug assistance.

The trigger is anticipatory. The duty attaches when conditions make dragging likely, not when the vessel has already started to drag, so an option that has you wait for the anchor to come home before doing anything is wrong on its face.

Dragging is detected by bearings on fixed objects, by GPS anchor-watch alarms, or by a lead line, and a chain that suddenly goes slack then jerks taut — grumbling or vibration felt on the chain — is the same warning coming up through the ground tackle . If the anchor drags, more scope is veered or a second anchor is let go. Where two anchors are used, a running moor or a standing moor spreads the load and reduces the swinging circle.

Reading the bottom before you commit

Good holding bottoms are firm mud, clay, and sand. Poor holding bottoms are rock, coral, loose shingle, and very soft ooze .

The chart tells you which you have. Bottom characteristics are annotated with letter codes — M mud, S sand, R rock, Cl clay, Co coral, Sh shells, G gravel — and they exist precisely so you can assess holding ground before anchoring Light List — Bottom Type Codes.

Where holding power actually comes from

The stockless anchor is standard on most vessels because its crown and pivoting flukes let it stow snugly in the hawsepipe . When it is let go and the vessel drags astern, the flukes trip downward and dig in — holding power comes from the flukes biting the ground, not from the anchor's weight alone.

The chain reaches it by a fixed path: from the chain locker up through the spurling pipe, over the wildcat of the windlass, through the hawsepipe, and out to the anchor. The bending shackle joins chain to anchor, the swivel prevents turns building up in the chain as the vessel swings, and the mooring swivel is used when riding to two anchors. At the far end, the bitter end is secured in the chain locker by a clench or a quick-release pelican hook so it can be slipped in an emergency.

While the vessel rides, the strain is taken by a chain stopper — a pelican hook or riding pawl — not by the windlass; the brake on the wildcat holds the chain when not heaving, but the stopper is what takes the load off the machinery NAVEDTRA 14067 §4-3.

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Telling it apart

The reports when weighing — criterion: where the anchor is relative to the bottom

• "Up and down" — the chain is vertical. The anchor has not yet broken out and the vessel is still anchored NAVEDTRA 14343 §4-4.
• "Anchor's aweigh" — the anchor has broken out of the bottom and the vessel is no longer anchored.
• "Clear anchor" — the anchor is in sight and not fouled. If it is fouled, the report is "foul anchor," and a foul anchor or a fouled hawse — chains crossed from riding to two anchors — must be cleared before proceeding.

Anchor pattern — criterion: the ground it is being asked to bite

• Danforth — lightweight, high holding power in sand and mud, poor in rocks USCG Boatswain Manual Ch. 2 §2.3.
• Plough (CQR, Delta) — good general purpose, harder to stow.
• Bruce (claw) — good in weeds.
• Mushroom — permanent moorings.
• Fluke — small craft.

The Danforth's high holding-power-to-weight ratio comes from its large pivoting flukes NAVEDTRA 14343 §4-1, and that is exactly why it fails over rock: there is nothing for a fluke to bury in.

The chain is up and down and the windlass is still heaving. Is your vessel still at anchor?

Yes. Up and down means the chain is vertical but the anchor has not broken out of the bottom. The vessel is no longer anchored only at "anchor's aweigh," and the anchor is not confirmed clear of obstructions until "clear anchor."

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Working a question

You are to anchor in charted depth 7 fathoms. The chart shows S over the anchorage, the hawsepipe stands 12 feet above the water, and the wind is fresh with more forecast.

1. Convert the depth. A shot is 15 fathoms and 90 feet, so a fathom is 6 feet: 7 fathoms is 42 feet NAVEDTRA 14343 §4-1.
2. Measure to the hawsepipe, not the waterline. Scope runs to water depth plus bow height, so the working figure is 42 + 12 = 54 feet USCG Boatswain Manual Ch. 2 §2.2. Every ratio below multiplies 54, not 42.
3. Read the bottom. S is sand Light List — Bottom Type Codes, which is good holding ground NAVEDTRA 14343 §4-2, so the ordinary band applies rather than the extreme.
4. Pick the ratio for the conditions. Good holding but weather making up puts you at the top of the ordinary band: 7 × 54 = 378 feet. If it comes on to blow hard, the storm figure of 10:1 gives 540 feet.
5. Convert to shots and round up. 378 ÷ 90 = 4.2, so veer 5 shots — 450 feet. Rounding down to 4 shots costs you 90 feet of scope and lifts the shank; there is no penalty in the other direction short of a crowded anchorage. The storm figure works out at exactly 6 shots.
6. Make ready. Chain stopper cleared, brake set, wildcat walked out to place the anchor just clear of the hawsepipe, and the bridge told the anchor is ready to let go NAVEDTRA 14343 §4-4.
7. Take the way off her. She should be nearly stopped or carrying a slight sternway at the chosen spot. Letting go with too much headway can run the chain out fast enough to part it, or simply drag the anchor along the bottom without setting it.
8. Let go and report. On the order, the brake is released; the forecastle reports the number of shots out, the direction the chain is leading relative to the vessel, and its tension.
9. Veer as she falls back, then set the brake and pass the chain stopper to ride the strain.
10. Set the anchor watch. Bearings on fixed objects, and the readiness required by 33 CFR §164.19 — veer chain, let go the second anchor, or get underway — because the forecast has already told you dragging is likely.

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Where candidates lose the point

Working the ratio on charted depth alone. In the example above that yields 294 feet against the correct 378, which is the difference between a distractor and the answer. Scope is rode length against water depth plus bow height USCG Boatswain Manual Ch. 2 §2.2, and the deck source says the same thing in different words: measured from the hawsepipe to the bottom NAVEDTRA 14343 §4-2.

Answering that a heavier anchor holds better. Weight matters, but holding power comes from the flukes biting the ground, not from the anchor's weight alone NAVEDTRA 14343 §4-1. An option that solves a dragging problem by invoking anchor weight rather than scope or bottom type is nearly always the wrong one.

Confusing feet in a fathom with fathoms in a shot. A shot is 15 fathoms; 6 is the number of feet in a fathom. Both appear as options on the same question.

Stopping at "maintain an anchor watch" when the question describes deteriorating weather. The watch and the detection procedures are standing requirements. Once conditions are likely to cause the anchor to drag, §164.19(c) requires action beyond watching — readiness to veer chain, let go a second anchor, or get underway on your own engines or with tug assistance 33 CFR §164.19.

Reporting "anchor's aweigh" at up and down. Up and down is a chain report; aweigh is a bottom report. The vessel is still anchored at the first and not at the second NAVEDTRA 14343 §4-4.

Choosing the Danforth for a rocky bottom on the strength of its holding power. That holding power is in sand and mud; it is poor in rocks USCG Boatswain Manual Ch. 2 §2.3.

Leaving the strain on the windlass brake. The chain stopper — pelican hook or riding pawl — is what takes the strain off the windlass while the vessel rides to her anchor NAVEDTRA 14067 §4-3.

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Check yourself

You are anchored in 60 feet of water with the hawsepipe 10 feet above the surface, in good holding ground, calm. How much rode should be out?

490 feet. The ratio works on depth plus bow height: 60 + 10 = 70 feet, and 7:1 in good holding ground and calm conditions gives 70 × 7 = 490 feet. Working the ratio on 60 feet gives 420 and is the trap answer.

The barometer is falling and the forecast is for a gale over your anchorage. What are you required to do?

Take action to ensure the safety of your vessel, structures and other vessels — being ready to veer chain, let go a second anchor, or get underway using your own propulsion or tug assistance — in addition to maintaining the anchor watch and the procedures for detecting a dragging anchor. The duty is triggered by conditions likely to cause dragging, so it applies before anything moves.

The chart shows Co over the only water shallow enough to anchor in. What does that tell you?

Coral, which is poor holding ground along with rock, loose shingle and very soft ooze. Firm mud, clay and sand are the good holding bottoms. Coral also fouls chain readily, and a fouled anchor holds less and complicates weighing.

You have let go with headway still on the vessel. What is the risk?

The chain runs out too fast and may part, or the anchor is dragged along the bottom instead of setting. She should be nearly stopped or carrying a slight sternway when the order to let go is given.

Your anchor is dragging in a rising wind. Name the two remedies from the ground tackle.

Veer more scope, or let go a second anchor. Riding to two anchors — a running moor or a standing moor — spreads the load and reduces the swinging circle.

You have 5 shots out. How much chain is that, and in 30 feet of water with the hawsepipe 10 feet up, what scope are you riding to?

5 shots is 450 feet. Depth plus bow height is 40 feet, so 450 ÷ 40 is a scope of about 11:1 — beyond the storm figure of 10:1, and more than the situation calls for in a crowded anchorage.

Weighing anchor, the report from forward is "foul anchor." What does that mean and what follows?

The anchor is in sight but fouled — chain wrapped around the flukes, or the anchor snagged on an obstruction or another cable. It reduces holding and complicates weighing, and it must be cleared before proceeding, as must a fouled hawse from riding to two anchors.