One hundred units of rode
With 20 units vertical, taut horizontal projection is about 98 units.
Boat Anchorage Swing Radius Calculator
Created by: Emma Collins
Last updated:
Estimate simplified rode projection, conservative swing radius and diameter, tide scenarios, and clearance to entered neighbours or obstructions.
Boat Anchorage Swing Radius Calculator
BoatingCompare a taut rode projection with a conservative full-rode swing boundary and entered nearby distances.
What is a Boat Anchorage Swing Radius Calculator?
This calculator estimates how far a vessel reference point could move around a fixed anchor under two deliberately simplified geometric models.
The projected circle uses taut rode geometry. The conservative circle uses full rode plus boat length and observation offset, making fewer favourable assumptions.
Neither model represents catenary, yaw, dragging, irregular shorelines, other vessels’ swing circles, or wind/current opposition.
How the Boat Anchorage Swing Radius Calculator Works
Horizontal projection is the square root of rode squared minus vertical distance squared.
Boat length is added to projection; the conservative radius instead adds full rode, boat length, and observation offset.
Entered object distances are compared with the conservative radius for a one-vessel clearance indicator.
Formulas and assumptions
Projection = √(rode² − vertical²)
Conservative radius = rode + boat length + offset
Clearance = object distance − conservative radius
Example Calculations
Outer-bound circle
Adding a 30-unit boat and five-unit offset gives a 135-unit conservative radius.
Common Applications
- Anchor-alarm setup discussions
- Swing-room comparison
- Tide sensitivity
- Nearby-object briefing
Planning Tips
Use consistent distance reference points.
Account separately for other vessels and moving hazards.
Treat anchor movement as an emergency change to the centre.
Frequently Asked Questions
What is an anchorage swing radius?
It is a planning distance around an anchor position or observation point that a vessel may occupy as wind and current change. This calculator shows both taut-line projection and a larger upper-bound circle.
Why is the conservative radius larger than the projected radius?
The conservative case uses full rode plus boat length and observation offset. It intentionally avoids assuming a beneficial catenary or horizontal projection when estimating an outer planning boundary.
Does a positive neighbour clearance mean two boats cannot collide?
No. The entered neighbour distance lacks the other vessel’s rode, length, anchor location, yaw, dragging, and different response to wind or current. Communicate with nearby vessels and use appropriate anchoring practice.
How does tide affect the geometric projection?
With the same rode paid out, greater vertical distance reduces the taut horizontal projection. Real catenary and seabed contact can change differently, and rising water can reduce scope and holding conditions.
What is the observation offset?
It represents the horizontal distance from the bow/anchor reference to the GPS antenna or point used for an anchor alarm. The conservative circle adds it so reference-point placement stays visible.
Does this model anchor dragging?
No. A dragging anchor can move the centre itself, invalidating a fixed swing circle. Use position monitoring, bearings, depth, alarms, weather awareness, and a prepared response.
Sources and References
- Royal Yachting Association. Anchoring with Care, accessed July 16, 2026; https://www.rya.org.uk/environment-and-sustainability/anchoring-with-care/.
- NOAA Center for Operational Oceanographic Products and Services. Tides & Great Lakes Water Levels, accessed July 16, 2026; https://www.tidesandcurrents.noaa.gov/water_level_info.html.
- NOAA Tides & Currents. NOAA Tide Predictions User Guide, accessed July 16, 2026; https://tidesandcurrents.noaa.gov/PageHelp.
- U.S. Coast Guard Office of Bridge Programs. Bridge Guide Clearances and bridge information, accessed July 16, 2026; https://www.dco.uscg.mil/Office-of-Bridge-Programs/.
- Geometry, clearance sign conventions, and interpolation assumptions are documented in each calculator.
Navigation limitation
The circle does not prove holding, collision clearance, seabed suitability, or safe distance from another vessel or obstruction.