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

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

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

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

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

"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. He
is currently building a 90' cat with an unstayed rotating mast but that is
an intirely different problem. No compresson loads there, but at one time
he was considering a stayed mast and must have done the thinking on it.

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.

For offshore work, the ultimate compression strength of the mast is
important for situations like full knockdowns and capsizings. The additional
compression strength also comes into play if you lose a stay or a shroud,
and might just give you the additional reserve strength that would keep the
mast from coming down.

Tom Dacon

"Parallax" wrote in message
om...
I have heard ppl say they would only have a keel stepped mast on an
offshore boat. Why? A well built deck stepped mast is as strong as a
keel stepped one and easier to put up or down. If either loses a
stay, it is coming down in a hurry? I fail to see the reason for this
odd preference.

On 8 Apr 2004 09:52:55 -0700, (Parallax) wrote:

I have heard ppl say they would only have a keel stepped mast on an
offshore boat. Why? A well built deck stepped mast is as strong as a
keel stepped one and easier to put up or down. If either loses a
stay, it is coming down in a hurry? I fail to see the reason for this
odd preference.

I used to be of the school that favored keel-stepped masts as be more
sound.

Some things that have changed my mind.

Pacific Seacraft builds very strong boats with deck-stepped masts, as
do some other builders.

I have heard stories, uncorroborated, of keel-stepped masts carrying
away the coach roof during dismasting.

There are probably well and poorly designed keel and deck stepped
masts. A good solid step and a well engineered compression post are
necessary on a deck-stepped mast.

Jack

__________________________________________________
Jack Dale
Swiftsure Sailing Academy
Director
ISPA and CYA Instructor
http://www.swiftsuresailing.com
Phone: 1 (877) 470-SAIL (toll free)
__________________________________________________

On Thu, 08 Apr 2004 19:14:39 GMT, Jack Dale
wrote:

I have heard stories, uncorroborated, of keel-stepped masts carrying
away the coach roof during dismasting.
===========================================

That can happen, and also the interior can be trashed by the mast butt
whipping around below decks. For that reason, it's very important
(and required by the ORC regs), that the mast butt be mechanically
fastened to the mast step in a secure manner.

I've had some personal experience with this issue since my old Cal-34
started out as deck stepped. As others have pointed out, almost any
kind of rigging failure with a deck stepped mast results in a total
dismasting with the entire rig in the water trying to hole the boat.
Been there, done that, and it's ugly. When I went to re-rig the boat
we did a lot of research. Ben Hall personally told me that keel
stepped masts are about 25% stronger than a comparable deck stepped
mast because of the extra support at the deck. Ben has engineered and
built a lot of masts and should know.

Personally, I would never go offshore again with a deck stepped mast.

On Thu, 08 Apr 2004 19:14:39 GMT, Jack Dale
wrote:

I have heard stories, uncorroborated, of keel-stepped masts carrying
away the coach roof during dismasting.
===========================================

That can happen, and also the interior can be trashed by the mast butt
whipping around below decks. For that reason, it's very important
(and required by the ORC regs), that the mast butt be mechanically
fastened to the mast step in a secure manner.

I've had some personal experience with this issue since my old Cal-34
started out as deck stepped. As others have pointed out, almost any
kind of rigging failure with a deck stepped mast results in a total
dismasting with the entire rig in the water trying to hole the boat.
Been there, done that, and it's ugly. When I went to re-rig the boat
we did a lot of research. Ben Hall personally told me that keel
stepped masts are about 25% stronger than a comparable deck stepped
mast because of the extra support at the deck. Ben has engineered and
built a lot of masts and should know.

Personally, I would never go offshore again with a deck stepped mast.