Does anybody know how to go about calculating the stresses in standing
rigging. I have an masthead sloop rigged 50 ' FRP boat. 4 lowers, 2
intermediates, 2 uppers, head & back stay. All inboard rigging. I'd
like to know the tensions in the various stays - vs the tensile
strength of the wires. Any ideas on how this is done?
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brought forth on stone tablets:
Does anybody know how to go about calculating the stresses in standing
rigging. I have an masthead sloop rigged 50 ' FRP boat. 4 lowers, 2
intermediates, 2 uppers, head & back stay. All inboard rigging. I'd
like to know the tensions in the various stays - vs the tensile
strength of the wires. Any ideas on how this is done?
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Not simple. Start with Brion Toss' book "The Riggers Apprentice"

bob
s/v Eolian
Seattle

Get a copy of Brion Toss's Rigger's Apprentice. The process takes a number
of steps but it is not really complicated. The main thing is to know your
righting moments which can be hard to find if you don't have the original
design specs.

--
Glenn Ashmore

I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at: http://www.rutuonline.com
Shameless Commercial Division: http://www.spade-anchor-us.com

wrote in message
...
Does anybody know how to go about calculating the stresses in standing
rigging. I have an masthead sloop rigged 50 ' FRP boat. 4 lowers, 2
intermediates, 2 uppers, head & back stay. All inboard rigging. I'd
like to know the tensions in the various stays - vs the tensile
strength of the wires. Any ideas on how this is done?
-----------------
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-----------------

The theoretical calculation is by determining the center of buoyancy,
the center mass of ballast and calculating the righting forces that
would react at the top of the mast ... when the mast is pulled over to
a heel angle of 45 degrees.
You can do the same experimentally by restraining the boat (so that it
can roll unimpeded) and by pulling horizontally (or calculating the
trigonometric difference for other angles) from the top of the mast
until the mast is at a 45 degree angle to the horizontal ... then
MEASURE the stress/strain in the wire (cap shrouds). The induced/
forced 45 degree heel would develope the maximum static loads in the
wire (by the magic of trigonometry).

This would establish the max. 'actual' target load in the wires ....
and (important) then make everything 4 times as strong for 'offshore',
3 times as strong for 'coastal' and twice as string for 'inshore'. I
prefer a higher Factor of Safety (FS=6 ... many of the reknown
'offshore' designers tout FS=4 but when you backcalculate their work I
find FS 'well above' FS=4) for offshore.

Simple answer .... your designer probably knew what he/she was doing
in rigging selection by adding the proper safety factors after
calculating the max. theoretical shroud loads ......... and typically
(on a purely mathematical basis) the cap shrouds would be tensioned
for a 12-15% load (mast/boat not heeled) based on the ultimate tensile
strength of the exisiting wire, ditto forestay/backstay and all the
other shrouds would then be trigonmetrically calculated based on the
12-15% of the caps and backstay. Very simple answer .....12-15% on
all shrouds, then go sailing to see if the mast stays perfectly in
column while on a heel approaching 45 deg.

Intermediates .... probably next to worthless from a stress
standpoint. Do the trig. calcs. and you'll see that they react with
extreme tension to the chainplates because of their very shallow
intercept angle with the mast (hounds) .... use runners instead of
intermediates for better mathematical solution and less stress/strain
as developed on the intermediate chainplates.

;-)

"RichH" wrote in message
...
The theoretical calculation is by determining the center of buoyancy,
the center mass of ballast and calculating the righting forces that
would react at the top of the mast ... when the mast is pulled over to
a heel angle of 45 degrees.
You can do the same experimentally by restraining the boat (so that it
can roll unimpeded) and by pulling horizontally (or calculating the
trigonometric difference for other angles) from the top of the mast
until the mast is at a 45 degree angle to the horizontal ... then
MEASURE the stress/strain in the wire (cap shrouds). The induced/
forced 45 degree heel would develope the maximum static loads in the
wire (by the magic of trigonometry).

snipped

It seems to me that to pull the boat over to 45 degrees using a line from
the top of the mast would put a much higher stress on the cap shrouds than
you would get by heeling the boat to the same amount by sail pressure, which
would spread some of the load onto the intermediates and lowers.

On Jan 31, 6:07*pm, "Edgar" wrote:
"RichH" wrote in message

...

The theoretical calculation is by determining the center of buoyancy,
the center mass of ballast and calculating the righting forces that
would react at the top of the mast ... when the mast is pulled over to
a heel angle of 45 degrees.
You can do the same experimentally by restraining the boat (so that it
can roll unimpeded) *and by pulling horizontally (or calculating the
trigonometric difference for other angles) from the top of the mast
until the mast is at a 45 degree angle to the horizontal ... then
MEASURE the stress/strain in the wire (cap shrouds). *The induced/
forced 45 degree heel would develope the maximum static loads in the
wire (by the magic of trigonometry).

snipped

It seems to me that to pull the boat over to 45 degrees using a line from
the top of the mast would put a much higher stress on the cap shrouds than
you would get by heeling the boat to the same amount by sail pressure, which
would spread some of the load onto the intermediates and lowers.

Doesnt matter where the maximum stress comes from ... the shrouds dont
know the difference as they are in reaction to that stress
application. Doesnt matter if windloading heels the boat to 45
degrees or by a rope tied to the top of the mast heels the boat to the
same 45 degrees .... as if the boat is at 45 degrees over the stress
in the wires will be IDENTICAL. :-)

On Jan 31, 1:12 pm, RichH wrote:
... Doesnt matter where the maximum stress comes from ... the shrouds dont
know the difference as they are in reaction to that stress
application. Doesnt matter if windloading heels the boat to 45
degrees or by a rope tied to the top of the mast heels the boat to the
same 45 degrees .... as if the boat is at 45 degrees over the stress
in the wires will be IDENTICAL. :-)

It isn't really required to do this. Under IOR a method was developed
for estimating a yachts stability from small changes in trim angle
when moving a weight a certain longitudinal distance. They are still
pretty good estimators of stability, particularly if you yacht wasn't
designed to fool them. Again, L&E discuss the methods and work and
example.

-- Tom.

On Thu, 31 Jan 2008 15:12:30 -0800 (PST), RichH
wrote:

On Jan 31, 6:07*pm, "Edgar" wrote:
"RichH" wrote in message

...

The theoretical calculation is by determining the center of buoyancy,
the center mass of ballast and calculating the righting forces that
would react at the top of the mast ... when the mast is pulled over to
a heel angle of 45 degrees.
You can do the same experimentally by restraining the boat (so that it
can roll unimpeded) *and by pulling horizontally (or calculating the
trigonometric difference for other angles) from the top of the mast
until the mast is at a 45 degree angle to the horizontal ... then
MEASURE the stress/strain in the wire (cap shrouds). *The induced/
forced 45 degree heel would develope the maximum static loads in the
wire (by the magic of trigonometry).

snipped
[Edgar]
It seems to me that to pull the boat over to 45 degrees using a line from
the top of the mast would put a much higher stress on the cap shrouds than
you would get by heeling the boat to the same amount by sail pressure, which
would spread some of the load onto the intermediates and lowers.

Doesnt matter where the maximum stress comes from ... the shrouds dont
know the difference as they are in reaction to that stress
application. Doesnt matter if windloading heels the boat to 45
degrees or by a rope tied to the top of the mast heels the boat to the
same 45 degrees .... as if the boat is at 45 degrees over the stress
in the wires will be IDENTICAL. :-)


I don't think so.... I'm with Edgar


Brian Whatcott Altus OK

On 2008-01-31 18:12:30 -0500, RichH said:

Doesn't matter if windloading heels the boat to 45 degrees or by a rope
tied to the top of the mast heels the boat to the same 45 degrees ....
as if the boat is at 45 degrees over the stress in the wires will be
IDENTICAL. :-)

The loads aren't identical, but for the purposes of evaluating the
standing rigging, the differences aren't significant.

As another said elsewhere in the thread: Each boat designer will figure
out those loads and add what they believe is a proper fudge factor to
come up with a wire size. When a boat's been successfully sailed in all
sorts of conditions for 30+ years, you gotta believe that they were at
least in the right ballpark.

--
Jere Lull
Tanzer 28 #4 out of Tolchester, MD
Xan's pages: http://web.mac.com/jerelull/iWeb/Xan/
Our BVI trips & tips: http://homepage.mac.com/jerelull/BVI/

On Jan 31, 5:13 pm, Jere Lull wrote:
... The loads aren't identical, but for the purposes of evaluating the
standing rigging, the differences aren't significant. ...

Well, I don't think that's right. I bet the scantiling rules require
that you consider the reefed condition for a reason rather than to
just make work for designers. Hey, have I mentioned that there's
this book...

-- Tom.