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Mast expansion on a hot day.


Well-known member
24 Mar 2015
It occurred to me the other day that aluminium being fairly conductive stuff, the mast must expand a few mm on a hot day. More, I’m guessing, than the stainless steel shrouds. So what happens? Does the mast just bend a bit more?


Active member
1 Jun 2001
Conduction has nothing to do with it. The important parameter is Linear Temperature Expansion Coefficient . For Aluminium, that's 24 x 10^-6 /deg C and for 316 Stainless, 16 x 10^-6 /deg C, They are significantly different but they are both TINY so the overall effect is b++++r all.



Well-known member
12 Sep 2001
Home - Southampton, Boat - Gosport
Being a sad git, I fired up Excel.

Since we're only interested in the vertical component, we can ignore both the angle of the shroud and Pythagoras wanting it to be longer.

If I got my formulas right, from a cold winter's day (0C) to a hot summer's day (60C) - that's a mast in the sun, too hot to leave your hand on for more than a second or two, a 10m ally mast will expand 14.4mm and a 10m SS shroud will expand 9.6mm, so the difference is less than 5mm. Actually, on most boats, the shroud chain plates are lower than the mast foot, so the difference will be less.

I reckon my boat bends more than that when I tension the rigging!