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.

Stress-wise a 'cantilever' connection is very weak and vulnerable ....
and requires much more 'meat' to make it work. It is also subject to
'stress risers' (things that make it 'weaker' by geometry) at the root
and requires some pretty precise shape/form to make it work well.



In article , Wayne.B
wrote:

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.

Stress-wise a 'cantilever' connection is very weak and vulnerable ....
and requires much more 'meat' to make it work. It is also subject to
'stress risers' (things that make it 'weaker' by geometry) at the root
and requires some pretty precise shape/form to make it work well.



In article , Wayne.B
wrote:

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.

Relativisticly the keel stepped is perhaps better if you have rig
failure and NO LOAD on the rigging. But if you have extreme deflection
at the deck interface and create deformation such as a 'teeny notch'
where it goes through the deck ..... step out of the way, its coming
DOWN.


In article , Bill
wrote:

About 15 years ago a friend had an aluminum vessel ~40' designed by Tanton,
a French naval architect who lives in this area (or at least did at that
time).

Tanton insisted that the mast be keel stepped. The issue he was concerned
about was mast pumping. If a deck stepped mast starts to pump it can
theoretically jump out of the step. How the mast gets into this mode, I
don't know.

Now, this boat was for blue water, and this friend has sailed around the
world at least once. As far as I know, he still has the boat.

I also had an Aluminum vessel that was French designed and built. The mast
was deck stepped.

Bill

"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.

"Steve Christensen" wrote in message
...
In article , QLW says...


"Tom Dacon" wrote in message
...
It's a mechanical engineering issue. A mast (called a column by
mechanical
engineers) that's supported only at the ends is less strong in
compression
than a column that's supported at two points at one end. The support
at
the
mast step, for a keel-stepped mast, allows the mast to take more
compression
before failing than a deck-stepped mast can. Because the stays and
shrouds
take sailing loads almost parallel to the mast, the mast column comes
under
significant compression load.

While I like the idea of a keel stepped mast, I'm skeptical about the
reasoning above. I'm not an engineer but I have a good friend that
is...and he has a lot of aircract and boat design experience...so I'll
run
this thread by him this afternoon and get his input before saying more.



I hope your friend agrees with the above post, since this IS the
accepted
wisdom
wrt rigs. Deck stepped masts get less support than keel stepped masts.
Therefore the deck stepped mast must be larger - and heavier - in cross
section
to make up for it. It's always an option, but it adds weight aloft.

Steve Christensen

Relativisticly the keel stepped is perhaps better if you have rig
failure and NO LOAD on the rigging. But if you have extreme deflection
at the deck interface and create deformation such as a 'teeny notch'
where it goes through the deck ..... step out of the way, its coming
DOWN.


In article , Bill
wrote:

About 15 years ago a friend had an aluminum vessel ~40' designed by Tanton,
a French naval architect who lives in this area (or at least did at that
time).

Tanton insisted that the mast be keel stepped. The issue he was concerned
about was mast pumping. If a deck stepped mast starts to pump it can
theoretically jump out of the step. How the mast gets into this mode, I
don't know.

Now, this boat was for blue water, and this friend has sailed around the
world at least once. As far as I know, he still has the boat.

I also had an Aluminum vessel that was French designed and built. The mast
was deck stepped.

Bill

"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.

"Steve Christensen" wrote in message
...
In article , QLW says...


"Tom Dacon" wrote in message
...
It's a mechanical engineering issue. A mast (called a column by
mechanical
engineers) that's supported only at the ends is less strong in
compression
than a column that's supported at two points at one end. The support
at
the
mast step, for a keel-stepped mast, allows the mast to take more
compression
before failing than a deck-stepped mast can. Because the stays and
shrouds
take sailing loads almost parallel to the mast, the mast column comes
under
significant compression load.

While I like the idea of a keel stepped mast, I'm skeptical about the
reasoning above. I'm not an engineer but I have a good friend that
is...and he has a lot of aircract and boat design experience...so I'll
run
this thread by him this afternoon and get his input before saying more.



I hope your friend agrees with the above post, since this IS the
accepted
wisdom
wrt rigs. Deck stepped masts get less support than keel stepped masts.
Therefore the deck stepped mast must be larger - and heavier - in cross
section
to make up for it. It's always an option, but it adds weight aloft.

Steve Christensen

About 15 years ago a friend had an aluminum vessel ~40' designed by Tanton,
a French naval architect who lives in this area (or at least did at that
time).

Tanton insisted that the mast be keel stepped. The issue he was concerned
about was mast pumping. If a deck stepped mast starts to pump it can
theoretically jump out of the step. How the mast gets into this mode, I
don't know.

Now, this boat was for blue water, and this friend has sailed around the
world at least once. As far as I know, he still has the boat.

I also had an Aluminum vessel that was French designed and built. The mast
was deck stepped.

Bill

"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.

"Steve Christensen" wrote in message
...
In article , QLW says...


"Tom Dacon" wrote in message
...
It's a mechanical engineering issue. A mast (called a column by
mechanical
engineers) that's supported only at the ends is less strong in
compression
than a column that's supported at two points at one end. The support
at
the
mast step, for a keel-stepped mast, allows the mast to take more
compression
before failing than a deck-stepped mast can. Because the stays and
shrouds
take sailing loads almost parallel to the mast, the mast column comes
under
significant compression load.

While I like the idea of a keel stepped mast, I'm skeptical about the
reasoning above. I'm not an engineer but I have a good friend that
is...and he has a lot of aircract and boat design experience...so I'll
run
this thread by him this afternoon and get his input before saying more.



I hope your friend agrees with the above post, since this IS the
accepted
wisdom
wrt rigs. Deck stepped masts get less support than keel stepped masts.
Therefore the deck stepped mast must be larger - and heavier - in cross
section
to make up for it. It's always an option, but it adds weight aloft.

Steve Christensen

About 15 years ago a friend had an aluminum vessel ~40' designed by Tanton,
a French naval architect who lives in this area (or at least did at that
time).

Tanton insisted that the mast be keel stepped. The issue he was concerned
about was mast pumping. If a deck stepped mast starts to pump it can
theoretically jump out of the step. How the mast gets into this mode, I
don't know.

Now, this boat was for blue water, and this friend has sailed around the
world at least once. As far as I know, he still has the boat.

I also had an Aluminum vessel that was French designed and built. The mast
was deck stepped.

Bill

"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.

"Steve Christensen" wrote in message
...
In article , QLW says...


"Tom Dacon" wrote in message
...
It's a mechanical engineering issue. A mast (called a column by
mechanical
engineers) that's supported only at the ends is less strong in
compression
than a column that's supported at two points at one end. The support
at
the
mast step, for a keel-stepped mast, allows the mast to take more
compression
before failing than a deck-stepped mast can. Because the stays and
shrouds
take sailing loads almost parallel to the mast, the mast column comes
under
significant compression load.

While I like the idea of a keel stepped mast, I'm skeptical about the
reasoning above. I'm not an engineer but I have a good friend that
is...and he has a lot of aircract and boat design experience...so I'll
run
this thread by him this afternoon and get his input before saying more.



I hope your friend agrees with the above post, since this IS the
accepted
wisdom
wrt rigs. Deck stepped masts get less support than keel stepped masts.
Therefore the deck stepped mast must be larger - and heavier - in cross
section
to make up for it. It's always an option, but it adds weight aloft.

Steve Christensen

On Thu, 15 Apr 2004 15:44:47 GMT, Rich Hampel
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.

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.

Exactly. Which is why I don't think it makes all that much difference.

Steve

On Thu, 15 Apr 2004 15:44:47 GMT, Rich Hampel
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.

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.

Exactly. Which is why I don't think it makes all that much difference.

Steve

On Thu, 15 Apr 2004 13:39:15 -0400, Wayne.B
wrote:

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.

That's exactly how I am viewing it. But the support at the deck of a
keel stepped mast is not going to do a damn thing to keep the mast from
breaking if the rigging fails in rough weather. If anything, I'd think
you'd have a slightly better chance of salvaging a deck stepped mast
after a catastrophy and jury rigging it up again since, if it does come
down with a pivot at the step, you've got a chance it may still be in
one piece. Of course it all depends on what breaks and how it comes
down. The only mast I've ever had come down due to rigging failure was
on a deck stepped boat and everything was salvagable. Lose the rigging
on a keel stepped mast and you're pretty much guaranteed to break it.

Another difference between them is that, when the boat is parked at the
slip, you can take the rigging down and not have the mast fall over if
it's keel stepped. But then it's also a little tougher to get the mast
up and down when you want to if it's keel stepped because you have to
lift it up a bit.

And the last difference I can think of is that you don't have to worry
about a compressed compression post and attendent deck sag with a keel
stepped mast.

Steve