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.

nnnnah, its easy to calculate all the 'joints' as individual 'free
bodies', even when simplifying by omitting elastic strain (of mast
etc.) and then simply balance all the free body connections as sums.
The original poster's question was "calculating stress in the standing
rigging" ... not necessary to calculate wind/sail loads as they can be
equated as reactions via their CE's and resultant distributive loads
as moments versus the 'connections' ... just like you do with any
'beam' calculation (mast).

The above will work as a close approximation ... at least that was the
method of most designers that Ive asked this direct question ... all
have replied: (paraphrased) ... mathematically pull the boat over onto
a 45 degree heel, calculate the restorative loads ... the moment arm
of mass versus center of buoyancy which will result in an close
approximation of the cap shroud tension ... then calculate all the
rest of the shrouds based on trigonometric relationship to the cap
shroud tension, etc. etc.; add/multiply by appropriate factors of
safety, etc. to account for sailing at beyond normal sailing angles
(GT 45 degress), etc. Those scantilings when back calculated seem
to still be appropriate in most cases. :-)

On Jan 31, 5:41 pm, RichH wrote:
... The above will work as a close approximation ... at least that was the
method of most designers that Ive asked this direct question ...

Yeah, there seems to be a lot of rules of thumb and hand waving on
this. I'm certainly no expert. The only rig design I've paid for was
done using some magic and proprietary computer code. But, since the
OP wanted to know how much strain he was looking at and he already has
the rig, wouldn't it be easiest to go look up the wire's specs from
the provider and then make up a strain gage that could be read while
under sail? I think a clever person with a meter stick, some tape and
a sharpie might be able to get pretty good figures. Maybe a good
Loose gage would be adequate if looking for a store bough option.

But if you really want to work it out theres this... Oh, forget
it...

-- Tom.

The 'original' question was:


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?"

Please explain a bit further ... and/or answer the following:
1. Are you setting/tuning the original rigging you have ... based on
some % values of the existing wire?
2. Are you selecting / checking new wire rigging? thinking of
increasing or decreasing the wire diameter, etc.?
3. Do you intend to also 'prebend' the mast? if so how many spreaders
and is the rig mast headed or fractional (what % fractional)
4. Etc. ?

If you are looking for a 'plain vanilla' static tuning; then, set all
wire ar 12-15% tension (by use of a tension gauge) ... then go sailing
at a heel to see if these values of the lowers, intermediates are
correct to keep the mast straight. Otherwise, if you have a plan view
and deck view of the boat, or can post or list the URL of the plan,
etc, views AND if you can give me the relative shape (luff/leech/foot
dimensions) of each sail, I will calculate the approximate % loadings
in all the shrouds for you so that the rigging is 'perfectly tuned/
balanced' on a % of ultimate tensile strength for
you ..... if that is what you are looking
for. :-)

On Thu, 31 Jan 2008 20:33:11 -0800 (PST), "
wrote:

...since the
OP wanted to know how much strain he was looking at and he already has
the rig, wouldn't it be easiest to go look up the wire's specs from
the provider and then make up a strain gage that could be read while
under sail? I think a clever person with a meter stick, some tape and
a sharpie might be able to get pretty good figures. ...
-- Tom.

A meter stick to measure strain is perhaps not the best way, but the
suggestion has merit.
The pro approach to stress testing involves strain gages. They need a
little smooth patch of structure less than a square inch to glue to.
That's a difficulty for rigging, sure enough.
But a thin aluminum strip, 3 or 4 inches long, strapped securely to
the rigging, with the strain gage measuring the stretch of the strip
which is transmitted from the wire would do the job perfectly well.
Low noise amps, data acquisition PC and you're in business.

Brian Whatcott Altus OK

On Fri, 01 Feb 2008 07:08:02 -0600, Brian Whatcott
wrote:

But a thin aluminum strip, 3 or 4 inches long, strapped securely to
the rigging, with the strain gage measuring the stretch of the strip
which is transmitted from the wire would do the job perfectly well.
Low noise amps, data acquisition PC and you're in business.

It might be easy to epoxy the aluminum strip to a rigging wire. Heat
will break the bond without damage to the wire.

Casady

On Feb 1, 8:54*am, (Richard Casady) wrote:
On Fri, 01 Feb 2008 07:08:02 -0600, Brian Whatcott

wrote:
But *a thin aluminum strip, 3 or 4 inches long, strapped securely to
the rigging, with the strain gage measuring the stretch of the strip
which is transmitted from the wire would do the job perfectly well.
Low noise amps, data acquisition PC and you're in business.

It might be easy to epoxy the aluminum strip to a rigging wire. Heat
will break the bond without damage to the wire.

Casady

... and you calibrate the strain gauge against 'what' standard?
Would be a good idea when racing to have a strain gage mounted on the
headstay .... then you can match the headstay sag versus the luff
hollow that the sailmaker cut into the jib/genoas leading edge ....
would insure bombproof genoa leading edge/luff shape.

On Fri, 1 Feb 2008 07:21:59 -0800 (PST), RichH
wrote:

Would be a good idea when racing to have a strain gage mounted on the
headstay

Many serious racing boats do exactly that.