Anchor Scope Calculator

Why anchor scope matters more than you’d think

When a boat is anchored, the anchor holds against pull from wind and current. The angle at which the anchor rode (chain or rope) meets the anchor determines whether the anchor digs deeper or pulls free.

If the rode pulls upward, the anchor lifts and drags. If the rode pulls horizontally along the seabed, the anchor sets harder and holds better.

Scope is the ratio of rode length to total depth (water depth + bow height). More scope = lower angle = better holding.

The fundamental formula:

Scope = Total rode length ÷ (Water depth + Bow chock height)

A 5:1 scope means 5 feet of rode for every 1 foot of depth. In 20 ft of water with a 5 ft bow height, you’d need 125 feet of rode for 5:1 scope.

The recommended scope ratios

Different conditions require different scope:

Scope below 3:1 has dramatically reduced holding. The angle at the anchor becomes too steep, the pull starts lifting the shank rather than driving the flukes in, and the anchor breaks out. The calculator prints that angle for each ratio so you can see how quickly it worsens: 5:1 gives about 11.5 degrees, 3:1 about 19.5, and 2:1 is 30 degrees, which almost nothing holds against.

Why catenary matters

The chain or rope between boat and anchor forms a curve called a catenary (the same mathematical curve as a hanging chain). The weight of the rode pulls it down toward the seabed, creating a curve that:

1. Keeps the pull on the anchor more horizontal (better holding)
2. Absorbs shock loads as the boat surges in waves
3. Reduces sudden jerks that could lift the anchor

A heavier rode (all-chain) creates a deeper catenary and better shock absorption. An all-rope rode has almost no catenary at all, so it relies purely on scope.

Chain vs rope rode

Two main rode systems:

All-chain rode:

• Best holding (deeper catenary)
• Highest shock absorption
• Resistant to abrasion on rocky/coral bottoms
• Heavy (1.5-2 lbs/ft typical)
• Expensive ($5-10/foot)
• Best for serious cruising

Mostly-rope rode (with chain leader):

• 10-15 ft of chain at anchor end
• Rope (nylon) the rest
• Lighter weight
• Affordable
• Shock absorption from rope elasticity
• Standard for most pleasure boats

All-rope rode:

• Lightest
• Cheapest
• Less shock absorption
• Vulnerable to chafe and abrasion
• NOT RECOMMENDED for overnight or rough conditions

Chain length recommendations

Even with mostly-rope rode, chain is essential:

Anchor types and their characteristics

Different anchors suit different bottoms:

Fluke (Danforth):

• Light, easy to handle
• Excellent in sand, mud
• Poor in weeds, rocks
• Most common type
• Brands: Fortress, Danforth

Plow (CQR, Delta):

• Heavy
• Good all-around holding
• Sets easily in many bottoms
• Standard for many cruisers
• Brands: Lewmar, Rocna

Rocna/Manson (Modern roll-bar):

• Self-righting design
• Sets quickly
• Excellent holding in most bottoms
• Premium price
• Modern cruising favorite

Bruce/Claw:

• Sets quickly
• Decent holding
• Good in rocky bottoms
• Heavy
• Common in commercial use

Mushroom:

• Permanent moorings only
• Heavy
• Settles into seabed
• Not for active anchoring

Anchor sizing

Anchor weight should match boat size:

Storm anchors should be one size larger than working anchors. Bluewater cruisers carry 2-3 anchors total.

Setting an anchor properly

The process:

1. Approach the anchor spot slowly
2. Stop the boat at the chosen drop point
3. Allow wind/current to begin drifting boat backward
4. Lower the anchor steadily (don’t drop it)
5. Let out rode as boat drifts back
6. Reverse the engine gently at idle to set the anchor
7. Maintain reverse until the boat stops, then increase
8. Test by reversing harder. The anchor should hold with the rode bar-taut.
9. Verify with landmarks. Pick a transit ashore; the boat should not be moving against it.

If the anchor drags, retrieve and try again. Don’t anchor in the same spot expecting different results.

Anchoring depth considerations

How deep to anchor:

• Minimum: 1.5x your boat’s draft + tide variation
• Comfortable: 8-15 ft typical
• Maximum: limited by rode length and scope requirements

In deep water, scope requirements become prohibitive:

• 100 ft depth at 5:1 scope = 500 ft of rode required
• Most boats don’t carry that much
• Find shallower anchorage when possible

Tides and scope

Tides change water depth dramatically:

• Atlantic coast US: 3-12 ft tide range
• Gulf coast US: 1-3 ft tide range
• Pacific NW: 8-16 ft tide range
• Bay of Fundy: 30-50 ft range (extreme)
• Mediterranean: under 1 ft (negligible)

Always work the scope from the depth at high water, which is what the tide-rise field above is for. Excess scope at low tide costs you nothing except swinging room. Insufficient scope at high tide is how boats end up on the beach at four in the morning. Watch the low-water figure separately for grounding: your depth sounder reads from the transducer, not the bottom of the keel.

Bottom type effects

Different seabed types affect holding:

Always identify the bottom before anchoring. The chart symbols tell you most of it, a depth sounder’s hardness reading confirms it, and a lead line armed with tallow brings up a sample if you want to be certain.

Anchor watch

After setting anchor:

• Mark position with GPS
• Check landmarks for visual reference
• Set anchor alarm on chartplotter (alerts if you drag)
• Watch for 30+ minutes to confirm holding
• Check during night if conditions change
• Increase scope if wind picks up

A dragging anchor in the middle of the night is how boats end up on rocks. The alarm is worth setting even for a lunch stop.

Multi-anchor systems

For severe weather:

Two anchors in line (Bahamian moor):

• Boat between two anchors set in opposing directions
• Keeps boat from swinging
• Used in tight anchorages
• More complex

Two anchors at 30° (V-pattern):

• Both anchors ahead of boat
• 30-45° angle between them
• Doubles holding power
• Standard for storm preparation

Hurricane preparation:

• 2-3 anchors at multiple angles
• Maximum scope possible
• All chain rodes
• Chafe protection on rope sections
• Boat positioned to face strongest predicted winds

What actually goes wrong

Almost every dragging incident traces back to one of three things, and none of them is the anchor being too small.

The first is scope set at low water. You arrive at half tide, measure eight feet, veer forty, and go to bed at a comfortable 5:1. By 0300 the tide has brought another six feet and you are on 2.9:1 without touching anything. This is why the calculator above asks for the tide rise, and why it is the input people skip.

The second is not testing the set. Backing down at idle proves nothing; the anchor will sit on the bottom looking convincing at idle. Increase the throttle until the rode is bar-taut and hold it there, watching a transit ashore. If the boat creeps, the anchor is not in.

The third is the neighbours. Your swing radius is a circle, and so is theirs, and two boats with different rode lengths, different windage and different keels do not swing together. A yacht on 100 feet of chain and a motorboat on 40 feet of rope will end up in different places every time the wind shifts, no matter how politely they anchored.

The smaller mistakes are worth knowing too: a Danforth on rock, dropping the anchor on top of its own chain, rope rode with no chain leader chafing through on coral, and going to sleep without setting the chartplotter’s anchor alarm.