The Anchoring Math: Stop guessing your chain length

Many sailors use the "5-to-1 ratio" rule of thumb. While it's a safe starting point, true seamanship requires understanding the catenary curve—the shape a cable or chain takes under its own weight.

The Basic Physics

The goal is to ensure the pull on the anchor is horizontal. If the angle is too steep, the anchor will break out of the seabed.

The Scope Formula

For a perfectly straight line (maximum safety/wind), the horizontal distance $L$ is:

$$L = \sqrt{s^2 - h^2}$$

• s: Length of chain deployed.
• h: Total depth (depth + bow height).

When to add more scope?

1. Holding Ground: Mud and sand are great; weeds and rock need more scope.
2. Wind Force: As wind increases, the catenary "flattens", requiring more chain to keep the pull horizontal.
3. Current: A strong tide can act much like wind.

Sleep better with NavAI

Calculation on a wet deck at 2 AM is a recipe for disaster. Our Anchor Scope Assistant (part of the Bridge Console) takes the depth, your boat's weight, and the forecasted wind to give you a recommended amount of chain.

Check it out in the Bridge Console.

See also: Mastering GRIB files for better wind forecasting.