Anchoring Safely:
Scope, Swing, and Physics
Dragging anchor is one of the most common causes of P&I claims. Learn the math behind the Catenary curve and safe swinging circles.
Dropping the anchor is easy. Staying put is the hard part. Whether you are in the crowded anchorage of Singapore OPL or weathering a gale off the coast, the math remains the same: Scope determines holding power.
Many accidents happen because officers rely on "gut feeling" rather than calculating the forces acting on the chain. This guide breaks down the science.
1. The Physics: It's All About the Catenary
Definition
Catenary: The curve assumed by a heavy flexible cord (chain) hanging freely between two points.
A common misconception is that the anchor holds the ship alone. In reality, the weight of the chain lying on the seabed creates friction and ensures the pull on the anchor shank is horizontal.
If the pull becomes vertical (due to insufficient scope), the anchor flukes will break out of the mud, and the vessel will drag.
- Good Scope: Chain lies flat. Horizontal pull. Max holding power.
- Poor Scope: Chain is tight/bar taut. Vertical pull. Anchor breaks out.
2. Calculating the Scope
How many shackles (shots) should you put in the water? This depends on the Depth of Water and the Weather Conditions.
| Condition | Recommended Scope | Example (Depth 30m) |
|---|---|---|
| Fair Weather / Short Stay | 3 to 4 times depth | 90m - 120m (3-4 shackles) |
| Standard Anchorage | 5 times depth | 150m (5-6 shackles) |
| Heavy Weather / Gale | 7+ times depth | 210m+ (8 shackles) |
*Note: One shackle (shot) is typically 27.5 meters (15 fathoms). Always check your specific ship's particulars.
3. The Swinging Radius (Crowded Waters)
In crowded hubs like Fujairah or Singapore, you don't have infinite space. You need to calculate your Swinging Radius to ensure you don't collide with other vessels or swing into shallow water.
// The Formula
Radius = (Shackles x 27.5m + LOA) / 1852 (for Nautical Miles)
// The "Safe" Calculation
When plotting on ECDIS, add a safety buffer (e.g., +200m) to account for GPS error and anchor dragging before the alarm sounds.
Calculate Before You Drop
Don't guess the shackles. Use our tools to get the exact length required based on depth and the precise swinging area for your ECDIS.
4. The Deep Water Exception
What happens if you need to anchor in 80m of water? If you follow the "5x" rule, you would need 400m of chain (14+ shackles). Most merchant vessels only carry 11-12 shackles port/stbd.
In deep water, the weight of the chain itself is so immense that the catenary curve is deeper. Therefore, you can often safely anchor with a ratio of 3x to 4x depth, provided the weather is calm.
Warning: When anchoring in deep water, do not drop the anchor from the hawse pipe. Walk it out under power until 15-20m off the bottom to prevent damage to the windlass brake.
5. Summary
Anchoring is not a "fire and forget" operation. It requires calculation.
- Check the depth and nature of the seabed.
- Calculate the required scope based on weather (3x, 5x, 7x).
- Calculate your swinging radius (Chain + LOA).
- Plot this circle on the ECDIS.
- Set your swing circle alarm inside this radius to get early warning of dragging.