Just verifying my standing rigging specs before approving my order and need
someone to confirm my calculations. All the formulas I have found for
standing rigging loads are based on RM@30 but all I have is RM@1. The
formula for RM is RM=Displacement*GM* SIN but I don't have the metacenter
height (GM) either so working backwards;
RM/GM=Displacement*SIN
therefore (I hated that word in 10 grade algebra)
GM= RM/Displacement*SIN
Substituting:
GM=1952/24,400*.01745
GM=4.58
Now, to get RM@30:

RM@30=55,931 (Say 60K lb.)

Am I on the right track?
--
Glenn Ashmore

I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at: http://www.rutuonline.com

On Fri, 8 Aug 2008 21:21:12 -0400, "Glenn Ashmore"
wrote:

Just verifying my standing rigging specs before approving my order and need
someone to confirm my calculations. All the formulas I have found for
standing rigging loads are based on RM@30 but all I have is RM@1. The
formula for RM is RM=Displacement*GM* SIN but I don't have the metacenter
height (GM) either so working backwards;
RM/GM=Displacement*SIN
therefore (I hated that word in 10 grade algebra)
GM= RM/Displacement*SIN
Substituting:
GM=1952/24,400*.01745
GM=4.58
Now, to get RM@30:

RM@30=55,931 (Say 60K lb.)

Am I on the right track?

Probably but I would strongly encourage you to work with professional
rigging specialists such as Hall Spars or Navtec. It is important
that the calculations be backed up with a "reasonability check" based
on what similar boats are successfully using.

Another good resource would be Tom Anderson at Hathaway, Reiser and
Raymond in Stamford, CT. Tom and Ben Hall both gave me a great deal
of help when I re-rigged my old Cal-34.

http://www.navtec.net/home/index.cfm

http://www.hallspars.com/

http://www.hathaways.com/

Glenn Ashmore wrote:

Just verifying my standing rigging specs before approving my order and
need someone to confirm my calculations. All the formulas I have found
for standing rigging loads are based on RM@30 but all I have is RM@1. The
formula for RM is RM=Displacement*GM* SIN but I don't have the metacenter
height (GM) either so working backwards;
....
Am I on the right track?

I would guess so. I have been speculating along the same lines while
dreaming of my little schooner. But I've also done some reading, which may
be of help here.

Brion Toss writes in Riggers Apprentice (p. 135):

Righting moment starts at 0 with the hull upright, then climbs in nearly
straight line to at least 30 degrees, sometimes up to 40 degrees. After
that, RM increases more slowly, to its maximum, and then begins to
decrease. Since the maximum RM indicates the maximum sustained load the rig
will have to bear, you'd think that designers would use this figure as the
basis for mast scantlings. But this is not so; they find the RM for 30 or
40 degrees, depending on the formula they are using, then multiply by a
safety factor to take maximum RM into account. Why? For one reason it is
very easy to find RM at a small angle of heel, then extrapolate along that
nearly straight line to find RM30 or RM40, then work in the extra factor.

In the Nature of Boats, Dave Gerr writes (p.293):
Again, there's a simple rule of thumb. for cruising boats, the total
breaking strength of all the shrouds on one side of the boat should equal
about 1.1 times the displacement of the boat. Racing vessels and day
sailers can get by with the total exactly equalling displacement, while
serious off-shore cruisers should have a total shroud breaking strength of
about 1.2 times the displacement.

(But count only one of the double lower shrouds, he warns!)


This should act as a good sanity check for your calculations.

Regards

Heikki

Glenn,
Did you try to get help from John Fox?
If he is any kind of Sailor/NA, he did a RM curve early on in the design
phase and revised it later when he had better data.

If you can not, you have the RM@1, you have the complete offset table
and so you can calculate the motion of the center of buoyancy. This can
give you elevation of the metacenter at any angle of heel as it is
only a function of hull form.

Your RM@1 is based on an assumed CG. With 8600# of fixed ballast, that
should be close enough to be safe.

Fair Wind and Smooth Sea
Matt Colie

Glenn Ashmore wrote:
Just verifying my standing rigging specs before approving my order and need
someone to confirm my calculations. All the formulas I have found for
standing rigging loads are based on RM@30 but all I have is RM@1. The
formula for RM is RM=Displacement*GM* SIN but I don't have the metacenter
height (GM) either so working backwards;
RM/GM=Displacement*SIN
therefore (I hated that word in 10 grade algebra)
GM= RM/Displacement*SIN
Substituting:
GM=1952/24,400*.01745
GM=4.58
Now, to get RM@30:

RM@30=55,931 (Say 60K lb.)

Am I on the right track?

There in lies the problem. The rigging was specified in rod by my designer,
John Fox but budget constraints require that I drop back to 1x19. What I
got back from the companies bidding on it in 1x19 is considerably beefier.
Almost 50% higher minimum load.

Using an RM@30 of 60K and Larsson & Eliasson I get close to John's numbers.
Gerr's rule of thumb comes out a little lighter. Haven't quite finished
working up Toss's method but what I have so far is a little heavier.

To throw a kink in the mix, I also got some figures from Colingo for Dynex
Dux 75. Their strength figures at 20% of breaking strength come out about
the same as Rod. The up weight is about 1/5 as much and with all the
fittings the total price is about 20% less than 1x19. Even better I can
splice 12 strand braided blindfolded. If they can convince me there will
not be a problem with abrasion at the spreaders I may seriously look at
synthetic rigging.

--
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

"Wayne.B" wrote in message
...
On Fri, 8 Aug 2008 21:21:12 -0400, "Glenn Ashmore"
wrote:

Just verifying my standing rigging specs before approving my order and
need
someone to confirm my calculations. All the formulas I have found for
standing rigging loads are based on RM@30 but all I have is RM@1. The
formula for RM is RM=Displacement*GM* SIN but I don't have the metacenter
height (GM) either so working backwards;
RM/GM=Displacement*SIN
therefore (I hated that word in 10 grade algebra)
GM= RM/Displacement*SIN
Substituting:
GM=1952/24,400*.01745
GM=4.58
Now, to get RM@30:

RM@30=55,931 (Say 60K lb.)

Am I on the right track?

Probably but I would strongly encourage you to work with professional
rigging specialists such as Hall Spars or Navtec. It is important
that the calculations be backed up with a "reasonability check" based
on what similar boats are successfully using.

Another good resource would be Tom Anderson at Hathaway, Reiser and
Raymond in Stamford, CT. Tom and Ben Hall both gave me a great deal
of help when I re-rigged my old Cal-34.

http://www.navtec.net/home/index.cfm

http://www.hallspars.com/

http://www.hathaways.com/

On Sat, 9 Aug 2008 11:52:50 -0400, "Glenn Ashmore"
wrote:

If they can convince me there will
not be a problem with abrasion at the spreaders I may seriously look at
synthetic rigging.

Discontinuous rigging would eliminate the bend/chafe at the shrouds,
albeit with a tradeoff in more fittings and increased tuning
complexity. I went with rod rigging on my boat and never regretted
it. There was no perceptible stretch even when pushing the boat hard
and it is still going strong for the new owner 20 years later.

Is the boat in the water ?

Glenn Ashmore wrote:

If they can convince me there will not be a problem with abrasion at the
spreaders I may seriously look at synthetic rigging.


I have been thinking along similar lines for my project. If all the numbers
I see are right, then it might well make sense to do all the standing
rigging in synthetic rope instead of metal wire. I expect the ropes will
only get better in the next few years before I actually get to start
building my dream boat, not to mention rigging her...

But even today, synthetic seems to be an option to consider! Does anyone
here have a good reason why not?

- Heikki

Glenn,

I was on the verge of trading the remaining 1*19 to rod or aramid on my
race boat when I was made an offer that was just too good. I had
replaced the damaged backstay with spectra several seasons ago and this
also got me better tension control (win-win).

Rutu looks a lot like a blue water cruiser and I guess I have just been
through a bit too much bad weather. I really like knowing that the rig
is likely to still be standing when I can look up at it next.

My problem with the synthetic fiber shrouds is that I'm still afraid of
long term UV damage. I've had two spectra halyards fail.

I've lost rod rigging to fatigue and notch damage (never quite to
disaster - but real close).

I just guess that with the problems with abrasion and UV, I would be
leery of trying to use it to hold up the mast of a boat that I'm going
to ask to keep me well through the storm.

Watermen and their boats, they take care of each other.

Matt Colie
Lifelong Waterman, Licensed Mariner and Congenital Sailor



Glenn Ashmore wrote:
There in lies the problem. The rigging was specified in rod by my designer,
John Fox but budget constraints require that I drop back to 1x19. What I
got back from the companies bidding on it in 1x19 is considerably beefier.
Almost 50% higher minimum load.

Using an RM@30 of 60K and Larsson & Eliasson I get close to John's numbers.
Gerr's rule of thumb comes out a little lighter. Haven't quite finished
working up Toss's method but what I have so far is a little heavier.

To throw a kink in the mix, I also got some figures from Colingo for Dynex
Dux 75. Their strength figures at 20% of breaking strength come out about
the same as Rod. The up weight is about 1/5 as much and with all the
fittings the total price is about 20% less than 1x19. Even better I can
splice 12 strand braided blindfolded. If they can convince me there will
not be a problem with abrasion at the spreaders I may seriously look at
synthetic rigging.

"Heikki" wrote

If they can convince me there will not be a problem with abrasion at the
spreaders I may seriously look at synthetic rigging.


I have been thinking along similar lines for my project. If all the
numbers
I see are right, then it might well make sense to do all the standing
rigging in synthetic rope instead of metal wire. I expect the ropes will
only get better in the next few years before I actually get to start
building my dream boat, not to mention rigging her...

But even today, synthetic seems to be an option to consider! Does anyone
here have a good reason why not?



A ****ed off machete wielding Haitian high on mushrooms was the first thing
that came to my mind. :-)

The Dynex rigging has been out for several years now and the reports seem to
be pretty good. It does creep a little over time so deadeyes are
recommended over turnbuckles to give more adjusting room. Deadeyes look a
little out of place on a modern boat but it is fairly easy to resplice 12
strand with a multiple Brummel so turnbuckles would not be that much
trouble. I have some 5mm Endura 12 for the double lifelines. More than
three times the breaking strength as 1/8" stainless and the wire and
fittings for the gates cost more than all the rest of the lifelines. Could
have used 3mm but I think it would cut you in half if you fell against it.
:-)

--
Glenn Ashmore

I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at: http://www.rutuonline.com

I was on the verge of trading the remaining 1*19 to rod or aramid on my
race boat when I was made an offer that was just too good. I had replaced
the damaged backstay with spectra several seasons ago and this also got me
better tension control (win-win).

Rutu looks a lot like a blue water cruiser and I guess I have just been
through a bit too much bad weather. I really like knowing that the rig is
likely to still be standing when I can look up at it next.

My problem with the synthetic fiber shrouds is that I'm still afraid of
long term UV damage. I've had two spectra halyards fail.

I've lost rod rigging to fatigue and notch damage (never quite to
disaster - but real close).

I just guess that with the problems with abrasion and UV, I would be leery
of trying to use it to hold up the mast of a boat that I'm going to ask to
keep me well through the storm.



Until recently I was of the same mind but the more I study these synthetics
the more I like them. From what I see rod and wire are designed with a
safety factor of about 4:1. The Dynex Dux has to be higher than 5:1 to
minimize creep. Also with steel you can't see what is going on down in that
terminal socket. At least you can see all the rope and keep an eye on UV
and abrasion. The one place I am worried is at the spreaders where it is
hard to check.

Another good point is you can easily stow a length of line and make up a new
stay in a couple of hours.

--
Glenn Ashmore

I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at: http://www.rutuonline.com