⚓ Anchor Scope Calculator
Calculate the optimal anchor rode length for secure anchoring based on depth, conditions, and equipment.
Understanding Anchor Scope
What is Scope?
Scope is the ratio of anchor rode deployed to the total vertical distance from the bow roller to the seabed. It's the most critical factor in anchor holding power. A scope of 5:1 means you deploy 5 meters of rode for every meter of depth.
Why Scope Matters
Proper scope ensures:
- Horizontal pull: Anchors hold best when pulled horizontally along the seabed
- Shock absorption: Longer rode absorbs wave and wind loads through catenary
- Reduced vertical forces: Prevents the anchor from breaking out
- Better holding: Allows the anchor to dig deeper into the seabed
Scope Guidelines by Conditions
| Conditions | All Chain | Chain/Rope | Mostly Rope |
|---|---|---|---|
| Lunch stop, calm | 3:1 | 5:1 | 7:1 |
| Overnight, moderate | 5:1 | 7:1 | 10:1 |
| Storm conditions | 7:1+ | 10:1+ | 15:1+ |
Calculating Total Depth
Always calculate from the bow roller to seabed at high tide:
- Current water depth
- Plus: Expected tide rise
- Plus: Height from water to bow roller
- Plus: Any storm surge (if expecting bad weather)
- Equals: Total depth for scope calculation
Rode Types and Characteristics
All Chain Rode
- Advantages: Excellent catenary, weight helps anchor set, abrasion resistant, shorter scope needed
- Disadvantages: Heavy, expensive, harder to handle, can damage topsides
- Best for: Rocky bottoms, coral, strong winds, limited swinging room
Chain and Rope Combination
- Typical setup: 10-20m of chain, then rope
- Advantages: Good compromise, chain protects against abrasion, lighter than all chain
- Disadvantages: Requires more scope than all chain, connection point can be weak
- Best for: Most cruising situations, mixed bottoms
Mostly Rope Rode
- Advantages: Light, easy to handle, stretches to absorb shock, inexpensive
- Disadvantages: Requires maximum scope, vulnerable to chafe, less holding power
- Best for: Deep anchorages, sandy bottoms, emergency rode
Anchoring Best Practices
Setting the Anchor
- Choose appropriate bottom (sand or mud preferred)
- Account for swing room (2x rode length circle)
- Drop anchor while boat moves slowly astern
- Pay out rode gradually while backing down
- Set anchor with reverse at 1500-2000 RPM
- Check holding with transit bearings
- Consider setting anchor alarm on GPS
Safety Considerations
- Always know your swing circle and nearby hazards
- Check weather forecast and adjust scope accordingly
- Use snubber or bridle to reduce shock loads
- In strong winds, deploy second anchor at 45° angle
- Keep anchor light on at night
- Monitor position regularly, especially with wind shifts
Frequently Asked Questions
How much chain should I use with a rope rode?
A minimum of 5-10 meters of chain is recommended, but 15-20 meters is ideal. The chain serves multiple purposes: it helps the anchor set properly by providing horizontal pull, protects the rope from abrasion on rocks or coral, and adds weight for better catenary. The chain should be at least as long as your boat for adequate protection. In areas with coral or rocks, consider using all chain or at least 30 meters of chain leader.
What if I don't have room to swing?
In tight anchorages, you have several options: Use a stern anchor to limit swing (Bahamian moor), or deploy two anchors at 45° from the bow. You can also use shore lines to control position, or anchor bow and stern in line with prevailing wind. Consider using less scope with all-chain rode (minimum 3:1), but monitor carefully. Mediterranean mooring (anchor off bow, stern to quay) is another option. Always communicate with nearby boats about your anchoring arrangement.
How do I know if my anchor is dragging?
Watch for these signs: GPS position changes more than normal swing would account for, bearing to shore references changes steadily, depth sounder shows changing depths, boat's heading doesn't match wind direction, unusual vibration or rumbling from dragging anchor. Set up anchor alarms on your GPS/chartplotter (typically 50-75m radius), take transit bearings on two shore objects 90° apart, and monitor regularly during the first hour after anchoring and during weather changes.