Thermal Expansion of Rigging

[jibes138]

I wrote this on another forum but food for thought, comments?

That brings up another subject. How much stress does an aluminum mast put on the rigging due to thermal expansion or contraction. Let's say your mast is 50 feet long. Times 12 = 600 inches. Thermal expansion coefficient = .000013 per inch per degree F. So lets say you tune your rig at 35 degrees for the shroud tension. At 100 degrees the mast will grow 600 x .000013 x 65 = 0.507 inches. Stretching your shrouds by .507 inches is an awful lot of stress, but the shrouds are stainless and will also grow with temperature. So the mast will probably buckle a bit and the boat will probably flex a bit and the shrouds will stretch a bit, hopefully elastically. Point being materials expand and contract and while stainless has a closer expansion rate to aluminum than say carbon steel the rig needs to be tuned for the ambient conditions, meaning several times a year. How many of you tune your rigs in the spring and never again?

 

[Johnb]

Per degree C the thermal expansion of alumnium is about 70 parts per million

Per degree C the thermal expansion of stainless is about 52 parts per million

Therfore for every inch the mast grew the shrouds would grow 52/70 inches

So in you example if the mast grew .5 inches then the shrouds would grow .37 inches. So the differance is about 1/8 of an inch - not too significant.

Note that the above is all in round numbers, does not take into account keel stepping or the effect of temerature on the dimension or compliance of the hull.

I was told by a man who I greatly respect, when you are sailing hard go put your eye right to the mast and look up the track. If it is straight don't mess with it.

 

[Alan]

The tripping point in the equation, though is the initial temp that the rig is tuned at. I generally check and adjust mine some time in mid May. Day time temps at that point are in the 70's. This would of course create a very small expansion in the hottest days of summer but quite the opposite in winter where the rig will automatically decrease the rig loading as the temps drop below 70F. In effect the rig will de-tune itself for winter and run at near specs for most of the sailing season.

 

[kloudie1]

In addition, the long shrouds are generally a couple of feet longer than the mast because of the top triangle and the chainplate being below the mast step or thru-cabintop.. so the difference is really more like 3/32"

 

[PaulK]

Wow. You guys must really be itching for the sailing season to begin. What about the diameter of the wire getting too cold and contracting itself out of the swaged end fittings?

 

[dscribner]

But Paul, won't fittings contract also? And in what direction????? ARRRGGGgg! To much to handle!

 

[rgranger]

How many of you tune your rigs in the spring and never again?

How many tune your rig when you first buy the boat.... and never again....:naughty:

 

[higgs]

I tune her up in the spring and, unless I see something I don't like, don't change anything.