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.

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 Thu, 8 Apr 2004 10:19:19 -0700, "Tom Dacon"
wrote:

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.

To that lucid engineering perspective, I would add the following
observations:

1) Dismastings MAY be better with a keel-stepped mast in the sense
that a deck-stepped mast will tend to fail in its entirely, whereas
even if you can salvage ten feet of keel stepped above the deck, you
may be able to rig a jury rig and keep going. Also, if a deck stepped
mast goes, you must IMMEDIATELY cut the shrouds still attached or the
mast will poke a hole in your hull...and this under very likely less
than ideal conditions.

2) Pluses of deck-stepped include no partners to leak water...no mast
boot, no Spartite, and, usually, more room and less obstruction in the
cabin.

3) Keel stepped masts are frequently heavier, but that weight can
translate to the mechanical advantage and lower CG mentioned above.
Also, deck flexing is avoided.

It's a matter of taste and intended use, mostly. I prefer keel stepped
on fiberglass boats, but see no objection to deck-stepped on steel
boats, mainly due to materials used and likely function of the boat. I
suppose the ideal compromise would be an aluminium deck stepped mast
on an aluminum boat! G

For the record, I have a keel stepped mast I am quite happy with.
YMMV.

R.

On Thu, 8 Apr 2004 10:19:19 -0700, "Tom Dacon"
wrote:

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.

To that lucid engineering perspective, I would add the following
observations:

1) Dismastings MAY be better with a keel-stepped mast in the sense
that a deck-stepped mast will tend to fail in its entirely, whereas
even if you can salvage ten feet of keel stepped above the deck, you
may be able to rig a jury rig and keep going. Also, if a deck stepped
mast goes, you must IMMEDIATELY cut the shrouds still attached or the
mast will poke a hole in your hull...and this under very likely less
than ideal conditions.

2) Pluses of deck-stepped include no partners to leak water...no mast
boot, no Spartite, and, usually, more room and less obstruction in the
cabin.

3) Keel stepped masts are frequently heavier, but that weight can
translate to the mechanical advantage and lower CG mentioned above.
Also, deck flexing is avoided.

It's a matter of taste and intended use, mostly. I prefer keel stepped
on fiberglass boats, but see no objection to deck-stepped on steel
boats, mainly due to materials used and likely function of the boat. I
suppose the ideal compromise would be an aluminium deck stepped mast
on an aluminum boat! G

For the record, I have a keel stepped mast I am quite happy with.
YMMV.

R.

"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.

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.

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 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.