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.

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.

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