The main function of mooring lines is to hold a ship fast to her berth against the effects of wind and current or other forces. If the mooring lines are adjusted properly, wind or current loading on the lines is a static load. If you try to use a line to stop a moving ship, that is a dynamic load. The difference between these two types of loads is significant and should be well appreciated by the mariner.
Here’s a simple illustration. If you tie a line to an object and lift it off the ground, the load on the line is equal to the weight of the object. If you drop the same object from some height and try to keep it from hitting the ground by holding onto the line, that is a dynamic load, and it will be much greater than the weight of the object.
Dynamic loads can be calculated using Newton’s second law of motion as expressed in the formula f = ma (force equals mass times acceleration). Deceleration is the reverse of acceleration, and Newton’s Law could just as easily be expressed as f = md. That is, the force required to decelerate a moving object equals the mass of the object multiplied by the rate of deceleration.
The rate of deceleration depends on the initial speed and distance traveled while coming to a stop. The more abrupt the stop (shorter distance, less time), the greater the rate of deceleration and the greater the dynamic force. This is why shock absorbers — such as airbags in cars, crumple barriers on highways and landing cushions for pole vaulters — are designed to lengthen the stopping distance and reduce the dynamic force on the person or object coming to a halt.
In the case of a mooring line used to stop a ship approaching a berth, the velocities are relatively small, but the masses are very large. Consider a ship approaching a berth at 1 knot. If we put a line over to a bollard on the pier and then make it fast to a cleat on deck, as soon as the line starts to become taut, the rate of deceleration will depend on how much the line stretches before we stop. A low-stretch line, such as Spectra or Kevlar, will result in a much quicker stop than a more elastic line like nylon. Skillful checking of the line — letting it take a strain and then easing it out — will reduce the rate of deceleration.
Here are some numbers. If we put 100 feet of line over and hold it, a typical nylon line will stretch 20 percent, to about 120 feet, before reaching its safe working load. At this point we hope the ship is stopped, or we risk damaging or parting the line. A high-strength/low-stretch line like Spectra or Kevlar may stretch only 2 percent, to about 102 feet. If the ship is moving 1 knot, the dynamic load on the nylon line as it stretches to 120 feet would be about 5 pounds per ton of vessel displacement, while the dynamic load on the Spectra line as it stretches to 102 feet (a rate of deceleration that is 10 times greater) would be about 50 pounds per ton. These seem like small numbers until you realize that if you’re dealing with a 10,000-ton ship, you’re loading the nylon line to 50,000 pounds and the Spectra line to 500,000 pounds.
The minimum breaking strength of a typical 1.75-inch-diameter nylon line is going to be about 100,000 pounds, while that for the same size Spectra line will be about 225,000 pounds. In this example, the elasticity of the nylon line gives it a better chance of stopping the ship than the Spectra line, which will probably fail (or rip out the cleat). Of course, one great advantage of the low-stretch lines is that they don’t snap back when they part.
The function of mooring lines is to hold a stopped ship in place. Using mooring lines to bring a moving ship to a stop is inherently unsafe. As you increase the size or speed of the ship, or reduce the stopping distance, you increase the danger. Lines can be effective in removing very small amounts of way, but it is vital that the lines be checked (slipped a little at a time) to increase the stopping distance and slow the rate of deceleration. This is especially important when using low-stretch lines.
Brian Boyce is director of MarineSafety International’s training center in Norfolk, Va.
