There is so much to consider with the various forms of elongation in synthetic ropes - elastic elongation, hysteresis, permanent elongation while working, permanent elongation after relaxed, and cold flow, or creep - but what does the sailor need to know about it? You need consistency from tack to tack, so what happens when a synthetic rope is placed under a load?
The first time a new rope is loaded, the braid compacts slightly, and depending on the construction, small helical changes take place. The result is a permanent extension that will not change over the life of the rope. By cycling the rope under a load a number of times before you use it in competition, this constructional elongation can be eliminated.
Elastic elongation (EE) is a characteristic of the fiber or fibers used in the construction. It is that portion of the extension that is immediately recoverable when the load is released. Elastic elongation is expressed as a percentage of the length at a standard load based on a percentage (10%, 20%, or 30%) of the breaking strength. Elastic elongation can only be reduced by moving up to a larger diameter line, a line with higher breaking strength, or a line with a lower measured elasticity.
Hysteresis, another form of elongation, is not recovered immediately when a load is released, but over hours or even days. This is generally negligible in the scheme of the racing sailor.
Creep: Is it a problem for most sailors?
Creep is the most misunderstood concept of rope elongation. It is defined as fiber deformation (elongation) due to molecular slippage under a constant, static loading situation.
Some fibers such as polypropylene and HMPE exhibit characteristics of creep, but if minor fluctuations occur in the rate and frequency of load levels, these characteristics are negligible.
Creep becomes problematic over time at constant loads. On a sailboat—racing or cruising—a constant load is extremely rare. The effect of tacking, and changes in wind and sail trim all create nearly continuous changes in the loads under which your lines are subjected. Creep is often measured in state-of-the-art fibers by maintaining constant, very high static loads over periods of weeks, not hours or minutes.
Not all HMPE fibers have the same creep characteristics. Dyneema® SK90, used in Samson’s AS-90, has been specially developed for enhanced creep performance. Blending an exceptionally low creep fiber like Vectran® with Dyneema® fiber with negligible creep, results in a line with the exceptionally low creep of Vectran® fiber, and lighter weight than a 100% Vectran® fiber rope.