The simplified illustration shows the interaction
between the cap shrouds and the masthead. The cap
shrouds are in this example prestressed to 150 kg.
The rig is first exposed to a sideways wind force of
200 kg. Without pre-tension the windward shroud
would have taken the whole load and the leeward
shroud would be totally slack. But as the leeward
shroud is prestressed to 150 kg it will be shortened
just as much as the windward one will be elongated
when the masthead is moved sideways by the wind
force.
The windward shroud tension will now increase just
as much as the leeward shroud tension decreases
which is 100 kg. The masthead therefore is only
moved half of the distance to lwrd. that it would have
moved without pre-stressing. The windward shroud
tension will be the pre-tension plus half of the wind
force, namely 150 kg +100kg =250kg. The leeward
shroud tension will then be the pre-tension minus half
of the wind force tension i.e. 150kg-100kg = 50kg. |
If the wind force increases to 300 kg (3), we'll see that the
leeward shroud tension becomes 0 kg. All the pre-tension
has been "used", but so far the masthead has only moved
half the distance to leeward that it would have done without
pretension.
If the wind force increases even more, e.g. to 350 kg (4),
the windward shroud must then take the whole load increase
of 50 kg alone. The windward shroud tension now increases
from 300kg to 350kg, while the leeward shroud has no load
and remains slack.
This causes a movement of the masthead twice as far to
leeward that it would if the shrouds had been pre-tensioned
with 175kg or more: 175 - 350/2=0
It is therefore most important that the cap shrouds are
pre-tensioned so that the leeward cap shroud tension
only becomes zero when the highest wind forces are
experienced. Pre-tension acts as a shock absorber in
variable wind strengths and the masthead is kept under
better control. |
