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

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 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.

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

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 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

Steven Shelikoff wrote:
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

When my SC22 mast folded into a right angle and spiked into the
water beside the boat, it was because there was no compression box
inside the mast at the point where the spreader was through bolted.
When I built up a new mast from the extrusion, I put a piece of
square aluminium tube inside for the bolt to pass through. I
positioned it with a long stick and duct tape which tore off once I
had secured the box with 2 pop rivets one above, one below, one on
each side, just there to retain it in place when the spraeader bolt
was removed. I had single lowers, as installed by South Coast. I do
not know if the mast was original factory equipment or not.

The side load in a gust caused the spreader and lower shroud mast
tang stresses to crush the mast at that point. I was watching it
when it went. We salvaged the mast lashed it alongside and rescued
the mainsail, there was no other damage except the tabernacle was
partially torn off the deck and bent somewhat.

Lack of a compression box at the spreaders is the most common
failure in design that I know of. Check your spreader mounts.

Mast pumping may have been the root cause, the final straw, so to
speak. Keel stepped mast / deck partners migh have prevented some of
that, while providing a fulcrum to develop gooseneck loads and crush
the mast at the partners in a manner different from those expressed
in a deck stepped system without them.

It seems to me that the main difference between deck and keel steps
in some boats is that the tabernacle bolts passing through the deck
might shear, as most of them seem relatively flimsy. The tabernacle
would never let the mast base get away, as it was all secured
together with bolts. A wad of 1/2 round convex bog faired all around
the base of the tabernacle might help, there.

Terry K

Steven Shelikoff wrote:
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

When my SC22 mast folded into a right angle and spiked into the
water beside the boat, it was because there was no compression box
inside the mast at the point where the spreader was through bolted.
When I built up a new mast from the extrusion, I put a piece of
square aluminium tube inside for the bolt to pass through. I
positioned it with a long stick and duct tape which tore off once I
had secured the box with 2 pop rivets one above, one below, one on
each side, just there to retain it in place when the spraeader bolt
was removed. I had single lowers, as installed by South Coast. I do
not know if the mast was original factory equipment or not.

The side load in a gust caused the spreader and lower shroud mast
tang stresses to crush the mast at that point. I was watching it
when it went. We salvaged the mast lashed it alongside and rescued
the mainsail, there was no other damage except the tabernacle was
partially torn off the deck and bent somewhat.

Lack of a compression box at the spreaders is the most common
failure in design that I know of. Check your spreader mounts.

Mast pumping may have been the root cause, the final straw, so to
speak. Keel stepped mast / deck partners migh have prevented some of
that, while providing a fulcrum to develop gooseneck loads and crush
the mast at the partners in a manner different from those expressed
in a deck stepped system without them.

It seems to me that the main difference between deck and keel steps
in some boats is that the tabernacle bolts passing through the deck
might shear, as most of them seem relatively flimsy. The tabernacle
would never let the mast base get away, as it was all secured
together with bolts. A wad of 1/2 round convex bog faired all around
the base of the tabernacle might help, there.

Terry K

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 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.