On Wed, 14 Apr 2004 03:44:54 -0500, "QLW" wrote:
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.

============================================

I think this is one of those cases where theory and the real world
break down, probably because of faulty assumptions supplied to the
theory. In the real world of squalls, knock downs, luffing sails and
accidental jibes there are many asymmetric side loads generated which
are trying to force the mast out of column.

That's when the extra support provided by the deck becomes the most
useful.

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

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

On Wed, 14 Apr 2004 18:37:56 -0400, Wayne.B
wrote:

On Wed, 14 Apr 2004 03:44:54 -0500, "QLW" wrote:
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.

============================================

I think this is one of those cases where theory and the real world
break down, probably because of faulty assumptions supplied to the
theory. In the real world of squalls, knock downs, luffing sails and
accidental jibes there are many asymmetric side loads generated which
are trying to force the mast out of column.

That's when the extra support provided by the deck becomes the most
useful.

I'm not so sure about that. If, when there are sideloads in squalls,
knockdowns, etc., the deck stepped mast failed by remining in one piece
but slipping out of it's step, then I'd agree that a keel stepped mast
would solve that problem ... if it didn't snap at the deck. But most
of the mast failures I've see are when it snaps somewhere aloft, like at
the spreaders. How it's stepped doesn't make a difference when it
breaks up there.

I haven't seen many keel stepped masts break at the deck either. If it
did, that would indicate there was enough sideload at that point to
maybe knock it out of the step if it were deck stepped.

Steve

On Wed, 14 Apr 2004 18:37:56 -0400, Wayne.B
wrote:

On Wed, 14 Apr 2004 03:44:54 -0500, "QLW" wrote:
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.

============================================

I think this is one of those cases where theory and the real world
break down, probably because of faulty assumptions supplied to the
theory. In the real world of squalls, knock downs, luffing sails and
accidental jibes there are many asymmetric side loads generated which
are trying to force the mast out of column.

That's when the extra support provided by the deck becomes the most
useful.

I'm not so sure about that. If, when there are sideloads in squalls,
knockdowns, etc., the deck stepped mast failed by remining in one piece
but slipping out of it's step, then I'd agree that a keel stepped mast
would solve that problem ... if it didn't snap at the deck. But most
of the mast failures I've see are when it snaps somewhere aloft, like at
the spreaders. How it's stepped doesn't make a difference when it
breaks up there.

I haven't seen many keel stepped masts break at the deck either. If it
did, that would indicate there was enough sideload at that point to
maybe knock it out of the step if it were deck stepped.

Steve

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

I'm not so sure about that. If, when there are sideloads in squalls,
knockdowns, etc., the deck stepped mast failed by remining in one piece
but slipping out of it's step, then I'd agree that a keel stepped mast
would solve that problem ... if it didn't snap at the deck. But most
of the mast failures I've see are when it snaps somewhere aloft, like at
the spreaders. How it's stepped doesn't make a difference when it
breaks up there.

Mast failure (usually at midsection) is usually due to some rigging
failure that permits the mast to move 'out of column' and permits
catastrophic buckling failure when the compressional loads get off
center. Doesnt matter if its deck stepped of keel stepped, if the
rigging support fails and the mast deflects catastrophically .... the
latent compression load finishes the job.

I'm not so sure about that. If, when there are sideloads in squalls,
knockdowns, etc., the deck stepped mast failed by remining in one piece
but slipping out of it's step, then I'd agree that a keel stepped mast
would solve that problem ... if it didn't snap at the deck. But most
of the mast failures I've see are when it snaps somewhere aloft, like at
the spreaders. How it's stepped doesn't make a difference when it
breaks up there.

Mast failure (usually at midsection) is usually due to some rigging
failure that permits the mast to move 'out of column' and permits
catastrophic buckling failure when the compressional loads get off
center. Doesnt matter if its deck stepped of keel stepped, if the
rigging support fails and the mast deflects catastrophically .... the
latent compression load finishes the job.

On Thu, 15 Apr 2004 12:38:06 GMT, (Steven
Shelikoff) wrote:
I'm not so sure about that. If, when there are sideloads in squalls,
knockdowns, etc., the deck stepped mast failed by remining in one piece
but slipping out of it's step, then I'd agree that a keel stepped mast
would solve that problem ... if it didn't snap at the deck. But most
of the mast failures I've see are when it snaps somewhere aloft, like at
the spreaders. How it's stepped doesn't make a difference when it
breaks up there.
=====================================

The issue of whether or not the mast fails as one piece or multiple
pieces is separate from the structural considerations.

Here's a different way to view the situation: A keel stepped mast is
cantilevered at the deck and thus derives extra support. A deck
stepped mast is essentially pivoted at the deck rather than supported
by it.