Dyneema is an incredible fiber, it's stronger than steel yet so light that it floats! This wonder fiber has some interesting properties that can take some time to wrap your head around, such as: It expands in cold temperatures, and it will creep as it passes through its life cycles.
Dyneema exhibits negative thermal expansion in the direction of the fiber. This roughly translates into the line going slack on very cold days. This is not an issue with sheets because you are always adjusting the sail trim anyway. It is much more apparent when the standing rigging goes slack in the winter. This is not creep, it is simply the dyneema expanding as it cools drastically.
On Wisdom, the rigging was adjusted in temperatures of 60*F to 70*F. This keeps the rigging nice and tight during summer sailing, but when winter sets in, the stays go slack. It is apparent which stays are set tighter than others when this happens. The cap shrouds are still like rods, the headstay is not as tight as it once was, and the lowers are sagging since they have become completely loose. Once temperatures warm back up, they will regain their proper tensions.
Winter provides an opportunity to tension the stays, as they are all going to become even tighter once the warm weather returns. For this reason, be sure not to over tighten the rigging in extreme cold, as this will induce unnecessary stress as they contract in the warmer months.
Now that we know that slackness in the rigging on freezing cold days is not due to creep, lets move on to actual creep.
Dyneema passes through three phases in its life cycle. Phase I, Phase II, and Phase III.
Phase I is characterized by rapid elongation due to creep. During Phase I, you will need to tension your rigging weekly as well as immediately before sailing. The rigging will be tight and well balanced today, less tight tomorrow, and completely sack by the fifth day. This period will last for a few months as the dyneema moves into Phase II.
Phase II is characterized by a slower period of creep. During this phase, the stays will need to be re-tensioned every few months. You will know that you are in this phase when the rigging no longer needs to be tensioned before you go sailing. This is when the synthetic rigging shines! Synthetic rigging weighs only a few pounds (compared to the hundreds of pounds for steel rigging) which reduces the weight aloft as well as the amount of heeling while under sail. Phase II will last for years, providing you with a secure standing rigging that is easy to inspect and dependable. When Phase II finally ends, it will enter Phase III.
Phase III is characterized by rapid elongation again and signifies the end of life of the stay. During this phase, the stay will begin to stretch again, needing frequent tensioning again. This is when the stay is needs to be replaced. If you decide to keep using the stay and simply tension it before each sail, it will snap and could lead to a dis-masting, but there were plenty of warning signs before this would happen.
The points to remember with synthetic rigging are:
It will go slack during the Winter, but will tighten back up once Spring returns.
When it's new, it will creep frequently.
After the break in period of a few months, it will not creep as much any more.
When it starts creeping again (many years later) it is time to replace the stay.