"Dag Stenberg" wrote in message
...
Dave wrote:
On 18 Jan 2005 09:00:27 GMT, "Dag Stenberg"
said:

Another thing is, if you leave the mast up, you have to loosen the
stays and shrouds anyway to prevent the stress on the hull from the
shortening of wire in cold, so the additional nuisance of taking the
whole thing out is not so big.

How does the coefficient of expansion of aluminum compare to the
coefficient
of expansion of stainless wire?

Good point! I found the following values (in m/m.K) from different
sources:

Al 23.5 E-6
Al 23.7 E-6 Steel 12 E-6
Al 22.2 E-6 Steel 13.0 E-6
that last source Stainless steel (304) 17.3
Stainless steel (310) 14.4
Stainless steel (316) 16.0

"The thermal expansion coefficient of aluminum near room temperature is
about twice that of steel (the exact values depend on the type of
aluminum and the type of steel, but for most types it's around 24 ppm
per deg C for aluminum and 13 ppm per deg C for steel)"

OK, so we'll believe that pure aluminium has nearly double heat expansion
compared to stainless steel.

Aluminium alloy 6063 T6: 22 E-6, Stainless steel (306) 17 E-6

Aluminium alloys
(LM 25) 22 E-6
(6061) 24 E-6
(6082) 23 E-6

So we can still believe that aluminium alloys have much greater thermal
expansion than even stainless steel.

Where does that put us? I suppose one can then argue, that when it gets
colder, the aluminium mast shortens more than the stainless steel
shrouds, so those should actually loosen. Apparently I have been misled
earlier by incomplete information....

This jives with what I have observed. Not so much that shrouds appeared
to be loose, but I have noticed occationally that forestays and backstays
appear a bit slack. I chauked this up to the fact that to be hauled with
a Travelift one often has to release either the backstay or forestay to fit
into the Travelift without hitting the crossbar. Once blocked, I assumed
that
the fore or backstay was not tightened as it would be for normal operation.

Dag Stenberg


Glad to learn something new and useful.

Doug
s/v Callista

Doug Dotson dougdotson@nospamcablespeednospamcom wrote:
"Dag Stenberg" wrote in message
...
... pure aluminium has nearly double heat expansion
compared to stainless steel.
Where does that put us? I suppose one can then argue, that when it gets
colder, the aluminium mast shortens more than the stainless steel
shrouds, so those should actually loosen. Apparently I have been misled
earlier by incomplete information....

This jives with what I have observed. Not so much that shrouds appeared
to be loose, but I have noticed occationally that forestays and backstays
appear a bit slack.

Yep, now I heard something from our club: somebody left the mast up, and
it started to wobble in the wind, putting more stress on the hull.

About the risk of toppling over - there seems to have been an occasion
in Cornwall about five years ago, when 50 boats toppled over, because
the gusts had rocked the supports, making them loose. They used logs for
supports, no cradle.

With the mast up, the rocking against the supports of the cradle will be
stronger than without the mast. I hear that one yacht in a marina close
by our place got one of the supports through the hull because of this.

So I remain happy with unstepping the mast for the winter.

Dag Stenberg