| My boat has one of these. It is physically connected from a spot
about 18" below deck level on the front of the mast, to the mast
collar. It appears to pull the deck in a downward direction. I
would guess it either compensates some for the forces from the standing
rig load which would pull the hull sides together and squeeze the
bulkhead, or it is intended to keep the mast collar from pulling
up when one attaches halyard turning blocks to the collar for those
who use halyards fed to the cockpit.
Walt
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| I am certainly no expert on this but I think that tie rods (which
are mounted as described) are intended to keep the mast parteners
(i.e. the heavy deck beams which surround the opening for the mast
in the deck or coachroof) from bowing upwards from the LATERAL forces
of the mast loads. needless to say, if these beams deform, major
stresses are added to the shrouds, hull-deck joint and the mast
itself while allowing the mast to attack the step at an angle.
This, in turn, would potentially cause more lateral loading of the
keel, another not so great thing as this would be a force opposite
that of the external, water/weight lateral forces which the keel
is designed to take.
The structure which is designed to distribute the compression loads
(downward forces) from the mast step to the keel and hull is the
step itself. In properly designed wooden boats there is often a
'web' wherein the step transfers its loads not to a point on the
keel, but to six or more floors which are attached to the keel
as well as the frames of the boat and often to the ends of the
chainplates. A tie rod would only transfer these compressions loads
to the deck beams or coachroof; structures which are inherently far
less strong than the keel.
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| Allow me to step into the mud, too:
If you think of a cross-section of mast/hull/deck/standing rigging/tierod
as a system, perhaps you can see how the shrouds pull up against the
mast's compression -- down at the base on the keel. If you removed the
deck & tierod from the system, the hull would merely fold a little and the
rigging would slacken.
To counter that, one could add a straight beam athwartship between
hull/shroud connections P&S. (Look at many small dinghies which do just
that -- a Thistle or Highlander are excellent examples.) However, in
bigger boats, since people like to live in them, as well as for several
other very good reasons, decks are crowned and cabin tops often are, too.
Now the system has somewhat more resistance to the rigging loads, but
still can tend to bow the deck/cabintop upwards in the direction of and
beyond the designed curvature, thereby slackening the rigging.
Now add that tierod, and you see what is to be gained: The deck is kept
from lifting, thus keeping the rigging tighter and marketable headroom maxed.
J.
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