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wrote:
I fly glassfibre aircraft, sailplanes actually - structure lifespan is
a topic of interest when you are at 20 000' above ground level flying
at 200 kph plus and the outside air temperature is below -15 degrees
Celcius.

It's of great interest if you're sailing farther away from
shore than you can swim... or in some cases, wade...

The technical body that co-ordinates the efforts of a lot of research
and development around sailplanes ( OSTIV ) recently published a rather
reassuring article giving an estimated service lifespan of a glassfibre
sailplane of around half a million flying hours. and that was with a
safety factor of at least 1 order of magnitude.

Although yacht hulls are stressed by rigging and water loads I suspect
that the stresses involved ( in a normal cruising boat ) are somewhat
less than experienced by an aircraft structure.

I disagree. In fact I'd suggest that since yachts experience
structural failure more often, they are more highly loaded.
Also they experience greater load transients . (AFAIK... it
may be that aircraft experience more structural failures
that I don't hear about).

Think about how your airplane would react if you picked it
up & dropped it 10 or 20 feet (3 ~ 6 meters) three times a
minute, at varying angles, for several days.

..... The wings on my 25m
span Nimbus regularly deflect by a metre or so in normal flight. Even
using the same figure of half a million hours one gets 47 odd years of
'use', not just sitting in a berth.

I'd be willing to bet that this is well within the range of
unrelieved strain (ie does not accumulate fatigue) and the
wing is designed for this much deflection. Sailboats have to
be a good bit more rigid... I'm guessing that parts of the
plane are too, like for example how much does the fuselage
flex & deflect when you put on some rudder?

I would say that for all intents and purposes the useable lifespan of a
'well built' glassfibre hull is probably in the order of several
hundred years with only minimal attention to care.

'minimal attention to care' is the real killer in cases
where lack of maintenance allows water penetration into
coring, or increases UV exposure.

Of course if you insist on building a hull only millimetres thick out
of Nomex, Kevlar, Carbon and other exotics, sail it in across Biscay in
a gale with only one hull of three in the water and driving through 5m
high swells then 'all bets are off' and you may well run into
mechanical limits of the structure.

Could happen in planes too. And parachutes are cheaper than
ocean rescues

Fresh Breezes- Doug King