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Seamanship & Navigation Forum devoted to seamanship and navigation topics, including paper and electronic charting tools.


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  #11  
Old 12-29-2006
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Wow what great responses. Thank you all for taking the time.

It sounds like the consensus is that the standard rigging is enough on a boat that is sailing where it was designed to sail. I'm encouraged by the words of so many of you that have a lot of experience, and even direct experience of having had stays break and being able to repair them without losing the mast.

I still can't say I have the warmest feeling about it, but I will continue to read about it and maybe I will come around. It doesn't seem intuitively right to me to not have some kind of a backup for any particular cable that holds the mast up because then everything comes down to diligent maintenance on that one chain of hardware, a crack you couldn't see or bad metal could lead to it snapping and you wouldn't have any way to know ahead of time. But I'm hearing what you are all saying, and I will try to keep an open mind about it.

I have to admit though I still don't see a lot of downside to having backups, but I can't support it with a good argument so I won't try to. I am, afterall, inexperienced, and all I have working for me (or against me!) is intuition. I'm very suspicious of the standing rigging for some reason, but I can't put my finger on why. And it's not just because it's light and airy and looks fragile, it's more than that.

Most of the time I really pay attention to the voice of experience, and will listen to the consensus view. But this might be one of those times that I will just have to agree that I'm the odd one out and go my own way ...

Last edited by wind_magic; 12-29-2006 at 01:03 PM.
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  #12  
Old 12-30-2006
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Shrouds & Mast Attachment

Hi Wind Magic,

My boat has one set of lower shrouds forward of the upper shrouds, and one aft. They attach to the mast at the speaders by way of a couple of plates which are through bolted to the mast and each other: http://www.swainsons.com/gallery/dis...p&cat=0&pos=22
The upper shrouds have attachments on the upper end which accept a bolt, and again they're both through-bolted to the mast, about eight inches below the mast cap.

The fore and backstays attach to the cast aluminium mast cap by way of cotter pins through the casting, securing them in vertical channels cast into the cap.

At the deck level, the shrouds and stays (with the exception of the forestays) are all attached to stainless steel chainplates 3/8" (7mm) thick by way of stainless swivels and bottle screws, and these pass through the deck and are bolted to substantial knees (roughly triangular in section and about 12" high, 5" deep and 1 3/4" thick) which were glassed into the hull when she was originally laid up. At the stern I have a supplementary knee in the centre of the boat (although no chainplate is attached to it) as she was also offered with a single backstay option.

The forestays are attached to the stem head fitting via bottle screws, swivels and a tri-angular tension equalising plate.

In terms of tensioning the rig, I've only had to mess about with the fore-and backstays so far, but I put them back on by reconnecting all of them and gradually tensioning them enough to take the sag out of the wires with the mast vertical (as squinted up from lying flat on the deck). I then put enough tension in the backstays to introduce the four or so inches of backwards bend that I like in the mast, and then dial up the tension on each stay evenly so as to be able to deflect the wire by not more than 6 inches when holding onto it 5 feet above the deck plate and leaning back on it (i weigh approx 200 lbs and I'm just over 6 feet tall, so those who can be bothered with the trigonometry can work out the deflective load I'm putting on the wire - It just seems about right to me)

The way to tension the shrouds, so I'm told, is to tension them equally so that the stick is vertical, then go out sailing, and while the boat is heeled check the tension on the leeward shrouds. If they're slack, do them all up a few turns, tack, do the shrouds up on the new leeward side up the same number of turns, and then check the tension again (just with your hand). The tension is right when the leeward shrouds are just taut - not hanging like washing lines, and not as stiff as rods.

Brion Toss's website www.briontoss.com has some good info / links for rigging and the Good Old Boat website has some more stuff.

Rather than re-run the standing rigging on your boat, if you feel uncertain about it, why not have a rigger check it for you and replace any wires / connections that he feels aren't up to scratch. You might then also ask him about installing running backstays to give you some additional security in heavy weather.

What is your boat?

Cheers,

Blue Eagle
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  #13  
Old 12-30-2006
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Wind Magic, by all means DO go with your own intuition (viz. to use redundant back-up stays or shrouds) rather than to just make everything stronger, as advised by some posters! The relevant failure statistics are pretty straightforward. In short, if you are dealing with a low probability failure event (such as rigging failure) redundancy wins hands down, whereas for high probability failure events upgrading/reinforcing may make more sense.
Here is why: Imagine you have a standard shroud or stay with a .01 (1 in 100) failure probability under a particular shortlasting set of circumstances. By installing gold-plated wire/rod/fittings etc. you would be very lucky indeed to bring that failure probability down by 10-fold (i.e. to .001), simply because even the most carefully manufactured metal component has a finite threshold probability of possessing hidden flaws.... Incidentally, similar threshold phenomena will plague the efficacy of your installation and/or maintenance procedures as well. In other words, it will never become perfect.
On the other hand, by backing that stay or shroud up with an independently attached stay or shroud of equal strength the probability of near-simultaneous failure will basically be reduced by a factor 100 (i.e. to .0001).
Nonetheless, for high failure event probabilities, let's say .5 (1 in 2) , redundancy will only buy you .25 (1 in 4). Thus, careful upgrading (plus similarly careful installing and maintaining) could then be the preferred approach.

In its most common configuration, redundant rigging only requires a suitable inner stay plus running backstays. About 80% of the mid-sized cruising sloops here in the S Pacific appears to have an inner stay of some kind and perhaps 60% do have running backs. Nearly all 50+ft vessels have both.

Fair winds,

Flying Dutchman
"Rivendel II"
(currently on the hard in Port Vila, Vanuatu)
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  #14  
Old 12-30-2006
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The problem with most "rigging components" is that they do not include "material" or "mill certifications" ... chemical and physical analysis certification to 'prove' that the base metal is exactly what its supposed to be. Such 'certifications' would probably double/triple the cost of such materials but would give greater credibility and assuredness of the stucture.

Stainless Steel especially is prone to 'fatigue failure' or embrittlement which starts to 'accumulate' from the very first high load it encounters. For all metals there is an "endurance limit" for fatigue considerations (about 1/3 of the ultimate tensile strength) that should NEVER be exceeded to affect good resistance to fatigue. This "1/3 rule" which essentially designs 90kpsi stainless at a maximum 30kpsi leads to a 'safety factor' of 3:1 ... which is essentially equal to the 'structure safety factor' typical of most 'coastal' boat designs. .... but without those 'mill certifications' one never knows what the 'exact performance' will actually be. To my way of thinking, a blue water boat should probably have a 6:1 saftey factor to allow contingency for extreme unpredicted loading, excessive cyclical loading (to remain UNDER the all important 'endurance limit') , etc. .... all resuilting in a "bodaceous structure".
Simply stated, stainless is a BAD material for cyclically applied loads (rigging, etc.) because of its non-uniform molecular structure, its poor ability to resist fatigue .... and probably only used on boats because it's 'shiney' and relatively cheap.
So, anytime you 'oversize' rigging and its structural attachments, you will decrease the (psi) loading to the structure, increase the useful 'service life', etc. .... until the 'rig' is so heavy that all that added weight aloft promotes such a boat to be a very poor performer. Always a 'compromise'.
Best advice I can give is: get your butt UP the mast every few months and do a complete visual inspection to SEE whats happening - especially for the beginnings of fatigue failure because fatigue is a non-elastic failure mode that is 'additive'. Metals get 'tired' - they 'fatigue' when subject to repetetive stresses that exceed the 'endurance limit'.
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Old 12-30-2006
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Quote:
Originally Posted by RichH
The problem with most "rigging components" is that they do not include "material" or "mill certifications" ... chemical and physical analysis certification to 'prove' that the base metal is exactly what its supposed to be. Such 'certifications' would probably double/triple the cost of such materials but would give greater credibility and assuredness of the stucture.
.
In fact upon request some do, and still fail. And it is not that much expensive.

Sparcraft performance and custom, does that upon request.

Search for a post I made a few months back called "Here is a story for you".

And see how even certified paper work full material fails, and a photo of what it did.
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Old 12-30-2006
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All of those certifications would either come with a reel of wire rope or be available from the manufacturer of the wire rope, at least in the USA. The end user would probably not see them with the finished product. Merchant ships, for instance, are required to carry and keep up to date, a cargo gear record book and each length of wire rope, shackle, and even mooring line is required to be entered in it along with the manufacturers certificate of compliance. The cargo gear record book also keeps track of when blocks were disassembled and inspected as well.
Your point on the inherent structural weaknesses of stainless steel and the reasons for it's continued use are spot on.
The Coast Guard standard for rigging dictates a safety factor of 5 to 1 and sets standards for intervals of inspection as well as periodic proof testing.

Having said all of that, it is probably not a bad idea for the consciencous boat owner to have their rig inspected periodically by a professional if they themselves do not have the knowledge. Failing that, periodic replacement would be desirable. And I'm not at all sure why galvanized improved plow steel wire rope, IWRC, is not used or even commonly available. Perhaps style has subsumed substance?
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Old 12-30-2006
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Gentlemen; the math is pretty simple:

Whether standardized or not, certified or not, regularly inspected and maintained or not, the fact remains that as long as it is a low-probability failure event the use of redundant stays/shrouds and fittings is the only effective approach from a statistical point of view.

By contrast; going for stronger/better/frequently inspected/constantly maintained neither keeps you completely safe nor gives you all that much bang for your bucks.

Have fun

Flying Dutchman
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Old 12-30-2006
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I hate to beat a dead horse but here goes. What percentage of rig failure is due to lack of maintainence as opposed to improper design for the loads, I suspect the vast majority of failure is from lack of maintenance. I have a rod rigged boat. One her sisterships owner called me a few years back and warned me of his lost rig due to uninspectable corrosion. I have since had my rigger replace with an open tang that can be inspected. While a second system will help if the first one fails, your boat rig was designed with only so much weight aloft and will perform differently otherwise. I believe inspection, maintainance and replacement at proper intervals is the key.
pigslo
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  #19  
Old 12-30-2006
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I disagree somewhat with you Hank.
Redundant forestays make it nigh-on impossible to affect good rig tension & sail shape. Here's an example: A typical foresail has its luff shaped so that it matches the normal sag of a SINGLE stay when the sail is fully wind-loaded to its max. design (about 15 knots) ... which a good sailmaker usually 'assumes' to be fit to a forestay with a 'basic' tension of 12-15% (ultimate breaking strength). If you use 'double' forestays then for proper rig tension balance (fore/aft), the shared tension of each forestay will be half of 'normal' (6-7%) and the 'sag' of the sail's luff will be tremendous and will induce a LOT of bad shape to the leading edge of the sail. So, if you use double forestays (solent rig, etc.) you have to account for the 'differential loading' on each forestay AND/OR cut the luff curves of foresail to match the greater 'sag' at the lower tensions.

I have a cutter rig and am a 'fanatic' about sail shape, etc. and I have to clearly state that those 'two' stays up front are not very easy to adjust .... with respect to sail shape as if you want a tight forestay you usually have to loosen the jibstay OR apply massive backstay, etc. tension (and you sometimes get near that magic "endurance limit" of 30% applied stress) .... always a damn compromise. Even on a 'cutter' each headsail being of different area will induce quite differnt loads to its own stay (depending on sheeting loads, angle of attack, etc.) and NEVER will you get close to the proper luff 'sag'/curve with out lots of tweaking, etc. Its just plain EASIER to have a single forestay and a single foresail and dont have to continually go through an additional/continuous 'balancing act' between the forestay and jibstay. Dont just think of the wire-stays as a single 'process' all by them selves, as when you add sails the whole combination becomes very 'complex'; adding more stays than absolutely necessary adds exponential and needless complexity.

Double forestays are a GREAT pain in the ass, especially with respect to 'sail shape'.

For what its worth, on my racing scow that has a *running* forestay (for on-the-fly mast rake with jibs that have 'wire' luffs), I do have a short "safety-jumper" that is 'sistered' to the forestay and runs to a pin on the stem .... so if the forestay that goes over a large sheeve on the stem ever 'goes', the loose redundant 'jumper' will - hope save - my bacon. That type of set-up would be quite extreme on a kroozing boat.
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  #20  
Old 12-31-2006
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If you are looking for efficiency and speed, Rich, the arguments I used in favor of redundant forestays (including a forestay plus large inner stay) could very well not be relevant.

On the other hand, if you want to have maximum rig security (i.e. the topic of this thread) and ease of foresail area reduction a second stay is all but unavoidable on modern offshore cruising vessels.

As always, every choice is a compromise. However, I remember seeing double forestays (or forestay/inner stay combinations) on around-the- world racers. If the performance penalty due to less efficient tuning was really that large you would not expect to see that.

Season's Greetings

Flying Dutchman
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