On Tue, 15 Apr 2008 07:20:21 -0500, Brian Whatcott
wrote:

On Tue, 15 Apr 2008 12:55:37 +0700, Bruce in Bangkok
wrote:


Are you stating specifically that a sailing boat that loses its mast
is in more danger of capsizing then when the rig was in place?? I
would appreciate it if you could elaborate on this as it was always my
understanding that once the rig was either cut away or retrieved and
lashed on deck the boat rode no worse then it had with the rig in
place.

It was always my thought that once the rig was gone that stability of
the ballasted hull would become slightly better with no weight above
the deck line.

I emphasize that I have no interest in this discussion other then this
single point which is probably of interest to most cruising sailors.


Bruce-in-Bangkok
(correct email address for reply)

There are conflicting factors (as usual): the pro-stability factor is
the considerable mass far from the roll center (called moment of
iniertia) which slows the rate of roll in adverse seas.
The con-stab factor is the increased windage and weight above the
water line.

Brian W

Certainly the weight of the rig slows the roll speed by some figure
but whether it has an effect on the boat rolling over I seriously
doubt. I have had the spreaders in the water (not on purpose, I might
say) and the boat popped right back up. I don't believe that a
properly designed sail boat will roll over except when it is overcome
with a breaking wave and the boat effectively falls down the face of
the wave.

This assumes some mediocre level of seamanship, i.e., not full sails
in a typhoon....

Bruce-in-Bangkok
(correct email address for reply)

On Tue, 15 Apr 2008 20:41:41 +0700, Bruce in Bangkok
wrote:
/snip/
There are conflicting factors (as usual): the pro-stability factor is
the considerable mass far from the roll center (called moment of
inertia) /snip/

Certainly the weight of the rig slows the roll speed by some figure
but whether it has an effect on the boat rolling over I seriously
doubt.
/snip/
Bruce-in-Bangkok

This is not the first time that you have not quite understood an
engineering input, but felt comfortable about doubting it.
The concept is "second moment of inertia" Bruce, not mass.

Easy to look up though.....

Brian W

On Apr 16, 4:10 pm, Brian Whatcott wrote:
The concept is "second moment of inertia" Bruce, not mass.

Hmmm. I think mass moment of inertia is what you're after (http://
en.wikipedia.org/wiki/Moment_of_inertia). Second moment of inertia is
form stiffness (eg. the "I"'s in a mast section description).

-- Tom.

On Wed, 16 Apr 2008 21:10:11 -0500, Brian Whatcott
wrote:

On Tue, 15 Apr 2008 20:41:41 +0700, Bruce in Bangkok
wrote:
/snip/
There are conflicting factors (as usual): the pro-stability factor is
the considerable mass far from the roll center (called moment of
inertia) /snip/

Certainly the weight of the rig slows the roll speed by some figure
but whether it has an effect on the boat rolling over I seriously
doubt.
/snip/
Bruce-in-Bangkok

This is not the first time that you have not quite understood an
engineering input, but felt comfortable about doubting it.
The concept is "second moment of inertia" Bruce, not mass.

Easy to look up though.....

Brian W

Well, I do understand "moment of inertia" but I do not understand how
a rig that when you put it in the water has a negligible effect on
stability, i.e., the boat rights itself, is going to have a major
effect on a boat rolling over.

Now, for argument's sake we are talking about my boat. the mast can be
picked up by four Asians so lets say, for argument's sake it weighs
500 lbs. It is desk stepped and is forty feet long with the spreaders
about half way up the mast.

I can carry one set of shrouds with no problems so say 100 lbs X 2
sets = shrouds = 200 lbs. Four terminate at the spreaders and two at
the mast head. The fore and aft stays probably weigh a little less
then the stays so say 75 lbs together, both terminating at the mast
head.

The boat displaces 12,000 lbs. It was built in 1971, sailed across the
Pacific Ocean, among other places and hasn't rolled over to date.

It would be a kindness for you to explain it to me the real life
dynamics that will cause my rig to make my boat to roll over.



Bruce-in-Bangkok
(correct email address for reply)

Bruce in Bangkok wrote:

Well, I do understand "moment of inertia" but I do not understand how
a rig that when you put it in the water has a negligible effect on
stability, i.e., the boat rights itself, is going to have a major
effect on a boat rolling over.

Now, for argument's sake we are talking about my boat. the mast can be
picked up by four Asians so lets say, for argument's sake it weighs
500 lbs. It is desk stepped and is forty feet long with the spreaders
about half way up the mast.

I can carry one set of shrouds with no problems so say 100 lbs X 2
sets = shrouds = 200 lbs. Four terminate at the spreaders and two at
the mast head. The fore and aft stays probably weigh a little less
then the stays so say 75 lbs together, both terminating at the mast
head.

The boat displaces 12,000 lbs. It was built in 1971, sailed across the
Pacific Ocean, among other places and hasn't rolled over to date.

It would be a kindness for you to explain it to me the real life
dynamics that will cause my rig to make my boat to roll over.



Bruce-in-Bangkok
(correct email address for reply)





http://en.wikipedia.org/wiki/Metacenter

cavelamb himself wrote:
Bruce in Bangkok wrote:


Well, I do understand "moment of inertia" but I do not understand how
a rig that when you put it in the water has a negligible effect on
stability, i.e., the boat rights itself, is going to have a major
effect on a boat rolling over.
Now, for argument's sake we are talking about my boat. the mast can be
picked up by four Asians so lets say, for argument's sake it weighs
500 lbs. It is desk stepped and is forty feet long with the spreaders
about half way up the mast.
I can carry one set of shrouds with no problems so say 100 lbs X 2
sets = shrouds = 200 lbs. Four terminate at the spreaders and two at
the mast head. The fore and aft stays probably weigh a little less
then the stays so say 75 lbs together, both terminating at the mast
head.

The boat displaces 12,000 lbs. It was built in 1971, sailed across the
Pacific Ocean, among other places and hasn't rolled over to date.

It would be a kindness for you to explain it to me the real life
dynamics that will cause my rig to make my boat to roll over.



Bruce-in-Bangkok
(correct email address for reply)






http://en.wikipedia.org/wiki/Metacenter


Just go study the wiki page.
It's a pretty clear explanation.