Dyneema and its Coefficient of Thermal Expansion

Dyneema, while being incredibly strong, light, unaffected by water, and UV resistant, it is still a material of this mortal world. As such, it has various physical properties that can not be ignored.

One of these physical properties that can not be overlooked is the way Dyneema will change in length as it heats or cools. Sure, all normal materials experience this phenomenon, so what makes Dyneema special?

If you look at a bridge, you will see those nifty expansion joints in the roadway. These allow the bridge to expand and contract without breaking the bridge. If you look at these joints during the hot days of the summer, you will notice how the finger joints are fully interdigitated. Yet, in the winter on a very cold day, the finger joints will be pulled apart.

Concrete behaves as most materials do, expanding as they heat up and contracting as they cool down.

This expansion and contraction actually happens at a fixed rate that is known (by experimentation) for each material. The rate is called the Coefficient of Thermal Expansion and for all things that expand when they heat, this coefficient is a Positive Number.

Dyneema is a special material because it actually behaves backwards to this common convention. It will contract as it heats and expand as it cools, resulting in a Negative Coefficient of Thermal Expansion. This rare trait means that it will change length on you throughout the year and probably in an opposite direction to everything else on your boat.

In the summer, as your mast grows slightly longer, your stays will become slightly shorter. In the winter as your mast grows ever shorter your stays will become even longer!

Dyneema has a Coefficient of Thermal Expansion of -12 x 10^-6 m / K. This means that for every change temperature equivalent to 1 Kelvin (also equivalent to 1 degree Celsius) a meter of Dyneema will change its length by 0.000012 m, or 12 μm.

A little more clearly, for every degree change in Celsius, Dyneema will expand or contract by 12μm for every meter of length of the line.

This might not sound like much, but the temperature fluctuations throughout the year on a yacht can easily be 40 Kelvin (or 40*C) and stays on a yacht are very long. Each meter of Dyneema is now fluctuating by 480μm. That’s almost half a millimeter per meter!

This all adds up and in the dead of winter, your rigging can be significantly longer than you expected and thus very slack, or by inverse it could be very tight as it contracts on hot days.

Being how Dyneema expands as it cools and contracts as it warms, it is imperative to always tune your rigging on a warm day that way, worst case, your rigging is a little loose. If you tune your rigging to perfection on a frigid day, by Spring your rigging will have contracted so much that it will break something else on your yacht. Let’s face it, the synthetic standing rigging is going to be the strongest part of your rigging so something else is going to break when the stays all contract!

By being mindful of this physical property, you can safely enjoy the ease of inspection and reduced weight aloft that come with synthetic standing rigging.

No Winter Sailing in the Mediterranean Sea

In the Mediterranean, it is common knowledge that you can’t sail in the winter. The winds are too strong and the seas are too punishing!

This is what everyone who sails there says, but most people who sail there have “only” sailed there. Bad conditions exist everywhere so it’s a bit of a stretch to say that “this is the worst”. When I read accounts of circumnavigators who sailed the Mediterranean in the winter, they did so on “good days” and said it was rough but still manageable.

So the two types of stories are: “Can’t be done” by people who haven’t sailed elsewhere and this is their first large body of water and “It’s really rough” by people who have crossed an ocean before.

Being how we have weathered a storm off Cape Hatteras and faced Force 11 winds in the middle of the North Atlantic, we felt that we could confidently manage the conditions of the Mediterranean in this winter season.

We waited for what seemed to be a calm period between punishing storms. We were in the middle of the Alboran Sea (the first sea in the Mediterranean) and the conditions became very punishing.

While the winds reached Force 10 with gusts of Force 11 and 12, the waves were only 12 feet high! The problem was that the waves were very close together and very steep, not giving the yacht time to rise and fall over the waves; instead the waves crashed over the boat with punishing blows every few seconds.

Modern European production boats with low build quality would begin to break apart from such relentless pounding, as the deck/hull joint would begin to break open and wreck the whole yacht. Most of the people who only sail the Mediterranean are also cruising on these European production boats and are wise to not venture out into the sea if they wish to return alive and with their yacht still floating! We are cruising on a very old and very heavily built yacht that can take a pounding and keep going. It’s low freeboard means that it offers less of a face to a crashing wave and its heavy construction allows it to resist such punishment.

One thing is being able to do something and the other thing is being nonsensical about such decisions. Just because your car has bullet proof windows doesn’t mean you shoot at it every time you get the chance!

We decided that enough was enough and this punishment was not necessary or worth it!

Crossing the Mediterranean in the Winter

The Mediterranean Sea is known to have very fickle winds. Think about all the old tales of shipwrecks and storms at sea; think of The Odyssey! Well, those tales are true.

In the summer, the Mediterranean is plagued by no wind, while in the winter it is plagued by gales! These gales are not called storms because a storm is strong and unusual wind that is blowing; these are just “the winds”.

We leave our anchorage in Gibraltar and make our way through the Strait of Gibraltar and officially into the Mediterranean Sea. The winds were forecasted to be following us and a bit strong, but nothing we haven’t seen out in the Middle of the Atlantic (or off Cape Hatteras). The winds started out as expected, but then the winds continued to build further and further until they were way beyond our comfort zone!

Sailing the Straits of Gibraltar

The Straits of Gibraltar are famous. They are the gateway from the Mediterranean Sea and the Atlantic Ocean. This very narrow body of water separates two MASSIVE bodies of water and two continents which see very high shipping traffic. Winds and currents in the Strait are perilous and live up to their reputation for being unforgiving!

The Blue City

Chefchaouen, also known as The Blue City to tourists, is an ancient city perched high up in the Atlas Mountains of Northern Africa. The city gets its name from its history. Back in the 15th century, when the Spanish Inquisition was killing everyone, all the Jewish people in Spain fled to these mountains and established this city. Jewish people back then always painted their homes blue and white and since the city was predominantly Jewish, it was also predominantly blue. Fast forward a few hundred years and the city is still blue and is a sight to behold!