Why lead angle matters
A mooring line only takes part of a horizontal load as useful holding force. If the line leads steeply up to a high bollard or down from a high fairlead, extra tension is needed for the same horizontal hold. That extra tension can exceed the safe working load even when the ship is not moving much. Short, steep springs are a common cause of parted lines.
How this calculator works
Tension = (Horizontal load ÷ number of lines) ÷ cos(angle). Angle is the vertical angle between the line and the horizontal. At 0° the tension equals the horizontal load. At 30° the tension is about 15% higher. At 45° it is about 41% higher. This is a simplified static estimate. Real mooring loads also include wind, current, surge, yaw and passing ships.
Who should use it
Useful for a quick check during mooring planning. Always follow the ship’s mooring arrangement plan, MEG / OCIMF guidance and the safe working load of ropes, tails and fittings. Never use this result as the only reason to increase tension on a line.