QLW wrote in message
...
Steve,
As I suspected, my Engineer Friend went on in great detail to
explain why
stepping the mast on the deck or on the keel has no effect on
the strength
of the mast in compression. While some small benefit could
conceivably be
gained by helping to keep the mast in column, he claimed that
would only
occur in the case of a flawed design. If the mast were stepped
on a poorly
supported deck then all of the thinking changes...but that's a
deck problem
not a mast problem. Good reasons for either stepping the mast
on the keel
or on the deck can be argued, but compressive strength is not
one of them.

I think you're talking slightly at cross-purposes here.

Ignoring bendy masts, keel stepping (and its corollary, deck
support) doesn't add to strength in compression (as such), but it
increases the bend stability of a mast under compression. Bend
disturbances will occur due to inertia effects in a seaway, and
the various sail tensions in different sailing conditions. This
is not a design flaw, it's a design case. To keep the mast stable
under compression, these bending moments must be resisted, either
by using a large enough cross section, or by constraining
movement with stays and deck support. With appropriate support,
smaller cross sections can be used.

Most vessels designed to withstand extreme conditions (ignoring
racing) prefer straight masts. Keel stepping either adds to rig
strength, or can be used to reduce weight aloft. An engineer will
correctly say it makes no difference to the (pure) compression
strength of a cross section. But as part of a rigging system, all
other things being equal, it does add strength.

JimB

Nahhhh. an engineer will say that the SHORTEST section (ie. deck
stepped) will have the best resistance to buckling deflection and
harmonic vibrational excitement (pumping).
Especially, If the deck stepped mast is stress connected to the
compression post via a bolted flange it probably gives the best
'stability' of all cases in comparison to a 'pin-set' deck step or keel
stepped.

Side loads are a 'problem' vs. buckling failure and the deck stepped is
better able to support the side loads 'at the deck' (if the deck cross
members are properly engineered); plus - the overal length is shorter
which decreases the vulnerability of buckling due to the shorter
overall unsuported length. A keel stepped is always vulnerable to
movement as it enters the deck, no matter how tight you 'think' the
'wedges' are in place (elasticity of the structure).

Always straight? ... not if the rig is properly tuned! A single
spreader rig (of 'normal' cross section typically needs a 1" forward
'prebend', double spreader 2" of prebend to dampen oscilations / reduce
induced harmonics (pumping)... all of which changes the natural
frequency of the mast, etc. to a much higher frequency, hence better
*dynamic* compressional load handling ability. A dead straight stick
(unless it has a 'bodaceous' cross section and HUGE moment of inertia)
will more easily be vibrationally excited by harmonics induced from the
rigging/sail interaction. Prebend is a 'norm' for a finely tuned rig.




In article q7tfc.17450$4N3.2083@newsfe1-win, JimB
wrote:

QLW wrote in message
...
Steve,
As I suspected, my Engineer Friend went on in great detail to
explain why
stepping the mast on the deck or on the keel has no effect on
the strength
of the mast in compression. While some small benefit could
conceivably be
gained by helping to keep the mast in column, he claimed that
would only
occur in the case of a flawed design. If the mast were stepped
on a poorly
supported deck then all of the thinking changes...but that's a
deck problem
not a mast problem. Good reasons for either stepping the mast
on the keel
or on the deck can be argued, but compressive strength is not
one of them.

I think you're talking slightly at cross-purposes here.

Ignoring bendy masts, keel stepping (and its corollary, deck
support) doesn't add to strength in compression (as such), but it
increases the bend stability of a mast under compression. Bend
disturbances will occur due to inertia effects in a seaway, and
the various sail tensions in different sailing conditions. This
is not a design flaw, it's a design case. To keep the mast stable
under compression, these bending moments must be resisted, either
by using a large enough cross section, or by constraining
movement with stays and deck support. With appropriate support,
smaller cross sections can be used.

Most vessels designed to withstand extreme conditions (ignoring
racing) prefer straight masts. Keel stepping either adds to rig
strength, or can be used to reduce weight aloft. An engineer will
correctly say it makes no difference to the (pure) compression
strength of a cross section. But as part of a rigging system, all
other things being equal, it does add strength.

JimB

Nahhhh. an engineer will say that the SHORTEST section (ie. deck
stepped) will have the best resistance to buckling deflection and
harmonic vibrational excitement (pumping).
Especially, If the deck stepped mast is stress connected to the
compression post via a bolted flange it probably gives the best
'stability' of all cases in comparison to a 'pin-set' deck step or keel
stepped.

Side loads are a 'problem' vs. buckling failure and the deck stepped is
better able to support the side loads 'at the deck' (if the deck cross
members are properly engineered); plus - the overal length is shorter
which decreases the vulnerability of buckling due to the shorter
overall unsuported length. A keel stepped is always vulnerable to
movement as it enters the deck, no matter how tight you 'think' the
'wedges' are in place (elasticity of the structure).

Always straight? ... not if the rig is properly tuned! A single
spreader rig (of 'normal' cross section typically needs a 1" forward
'prebend', double spreader 2" of prebend to dampen oscilations / reduce
induced harmonics (pumping)... all of which changes the natural
frequency of the mast, etc. to a much higher frequency, hence better
*dynamic* compressional load handling ability. A dead straight stick
(unless it has a 'bodaceous' cross section and HUGE moment of inertia)
will more easily be vibrationally excited by harmonics induced from the
rigging/sail interaction. Prebend is a 'norm' for a finely tuned rig.




In article q7tfc.17450$4N3.2083@newsfe1-win, JimB
wrote:

QLW wrote in message
...
Steve,
As I suspected, my Engineer Friend went on in great detail to
explain why
stepping the mast on the deck or on the keel has no effect on
the strength
of the mast in compression. While some small benefit could
conceivably be
gained by helping to keep the mast in column, he claimed that
would only
occur in the case of a flawed design. If the mast were stepped
on a poorly
supported deck then all of the thinking changes...but that's a
deck problem
not a mast problem. Good reasons for either stepping the mast
on the keel
or on the deck can be argued, but compressive strength is not
one of them.

I think you're talking slightly at cross-purposes here.

Ignoring bendy masts, keel stepping (and its corollary, deck
support) doesn't add to strength in compression (as such), but it
increases the bend stability of a mast under compression. Bend
disturbances will occur due to inertia effects in a seaway, and
the various sail tensions in different sailing conditions. This
is not a design flaw, it's a design case. To keep the mast stable
under compression, these bending moments must be resisted, either
by using a large enough cross section, or by constraining
movement with stays and deck support. With appropriate support,
smaller cross sections can be used.

Most vessels designed to withstand extreme conditions (ignoring
racing) prefer straight masts. Keel stepping either adds to rig
strength, or can be used to reduce weight aloft. An engineer will
correctly say it makes no difference to the (pure) compression
strength of a cross section. But as part of a rigging system, all
other things being equal, it does add strength.

JimB

Not that I plan for failure, but I do plan against. I've been told
that, if one is in a position to be towed for a distance in adverse
conditions, leading the tow line to the mast is much stronger than
leading it to some cleat. Unless the mast base on a deck stepped mast
were to go, say, a foot up into the interior of the mast and everything
was bolted to a compression post running to the keel, I'd think that
deck-stepped masts would be more likely to pull loose. A keel-stepped
mast is directly supported horizontally by the deck and would seem to be
better able to resist/sustain the shocks and tugs of the tow line.

john

Not that I plan for failure, but I do plan against. I've been told
that, if one is in a position to be towed for a distance in adverse
conditions, leading the tow line to the mast is much stronger than
leading it to some cleat. Unless the mast base on a deck stepped mast
were to go, say, a foot up into the interior of the mast and everything
was bolted to a compression post running to the keel, I'd think that
deck-stepped masts would be more likely to pull loose. A keel-stepped
mast is directly supported horizontally by the deck and would seem to be
better able to resist/sustain the shocks and tugs of the tow line.

john