Dyneema, used in synthetic standing rigging, is remarkably strong and light weight. This makes it an excellent choice for standing rigging, as it supports the mast without adding unwanted weight aloft.
The dark secret about synthetic standing rigging is that the dyneema has a negative coefficient of thermal expansion, meaning that it actually elongates as it cools. Aluminum, a common spar material, has a positive coefficient of thermal expansion, meaning that it will shrink while cooling. Stainless steel rigging also has a positive coefficient of thermal expansion, so it will shrink along with the aluminum at roughly the same rate.
As winter approaches, the mast will shrink ever so slightly, as will steel rigging, meaning that it will all stay roughly the same tension. This is why steel rigging seems to stay the same tightness year round, as it expands and contracts with the mast.
Synthetic standing rigging, on the other hand, becomes rediculously loose as winter approaches. The mast contracts ever so slightly, while the dyneema expands a bit. The combination is a longer than normal stay on a shorter than normal mast! You might be tempted to simply tighten the deadeyes during winter, that way the standing rigging is tight again, but the problem is that once summer returns, the mast will expand slightly and the dyneema will contract drastically.
It is very fair to say that synthetic standing rigging is temperature sensitive. I prefer to tune synthetic rigging at 80*F. I find that this gives you wonderful sailing in summer, when it is much hotter, and rigging that is still functional into the cooler times of spring and fall. In the dead of winter, it is not uncommon to find that your headstay has expanded a full 1/2 inch (12mm).
While it might look as if the rigging is lost during the winter, you must admit that no one wants to go sailing when it is 20*F outside! As the weather warms up, the rigging will all go back into its place without any need for adjustments or tuning.
This works well for fair weather sailors who only sail when the weather is inviting. What happens if you need to sail in the winter? Should you tighten your rigging for the sail and be sure to loosen it as the weather warms? What if you forget to loosen it and the contracting stay becomes too tight and breaks a fitting or rips out a chainplate?
While I have yet to go sailing with synthetic rigging in the winter, we are probably going to be encountering this scenario soon. Maddie and I have set out cruising and we will be crossing the North Atlantic in the winter. We hope to arrive in the Azores by December and may be sailing over to Portugal in February! The winds will certainly be cold and the rigging will become slack!
I have the standing rigging tuned to perfection at 80*F, and I know that if I tighten it during the winter, I will have to go through all the trouble of re-tuning the rigging as spring rolls around. If only there was a way to mark the 80*F position on the lashing!
There is! Our plan is to setup a secondary lashing that will run over the current lashings to tension the rigging without disturbing the current setting. The current lashings will remain, but the new lashings will go over them and pull the stay tight once again. With the new lashings pulling everything tight, we will be able to safely sail during the winter to reach our next destination, and simply untie the new lashings as spring approaches. As soon as we untie the new lashings, the old lashings will take over and allow the stays to contract to their old settings!
This idea has not been tested, but it is my solution to winter sailing on my own boat as we cruise the North Atlantic in the winter with synthetic standing rigging.