Dyneema Backstay

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Tim Daniel

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Dec 30, 2020, 3:53:07 PM12/30/20
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Hi!

I'm replacing the backstay on hull #71 and was thinking 3/16" Amsteel SK78 with a breaking strength of 4900 lbs.  

Is that adequate for an SC 27, or should I go up one more size to 1/4" which has a 7500 lb breaking strength?  Anybody successfully using 3/16 dyneema for their backstay?

I can't imagine a 4900 lb load on the rig appearing on the backstay without something else breaking, but I'm not a mechanical engineer.  

thanks in advance for any advice!

Tim 

Ian Sprenger

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Dec 30, 2020, 4:03:19 PM12/30/20
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Tim,

I replaced mine with 1/4" (6mm) Heat Set Dyneema (New England STS-HSR) for the main section and the 3/16 SK-78 you mentioned in the cascade.  The 1/4" in combination with being heat-set is sized for stretch. For the main section use a quality sailmaker's thimble (ronstan, etc.) for radius and strength.  The regular thimbles can collapse or chew into your line with their sharp edges.

Cheers,

Ian
WILDER #122

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David Garman

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Dec 30, 2020, 6:41:28 PM12/30/20
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i have used 3/16 dynema, cheap sk72  and expensive for 5 PacCups.  i have used thimbles and low friction rings. and blocks.    I currently have low friction rings and a 48:1 cascade with the backstay lead to both sides. 20 and 35 mm sizes.  Good setup. 

if you use thimbles, use this one for the reasons that Ian states.  avoid thimbles that are intended for wire on rope.  
Save More on Your Suncor Extra Heavy Thimble at Fisheries Supply. Excellent Customer Service, Ready to Ship. Marine Supplies Since 1928!

Thimbles

Thimbles, 3/16"

SKU: 92963 Item ID: RON RF2182


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Sent: Wednesday, December 30, 2020 1:03 PM
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Subject: Re: Dyneema Backstay
 

Tim Daniel

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Dec 30, 2020, 7:38:28 PM12/30/20
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Thanks!

I built this this afternoon, using 3/16.  Based on the advice, I will change the thimble to a less nasty one....



thanks for the help,

Tim

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Sent: December 30, 2020 3:41 PM

Ian Sprenger

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Dec 30, 2020, 8:52:51 PM12/30/20
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Tim,

Looks good.  Chances are that thimble would be fine, but with it at the masthead and now that we mentioned it, well you know..... Murphy....

Another nice touch, and an added layer of security against a potentially unloaded brummel splice, is to heat shrink the bury and taper.  If you go this route use the good stuff from Ancor and a heat gun.  Be careful to not apply too much heat to the line to cause it to singe.  I set the splice under load before I did this.  Or you can lock stitch.  Or not.  I'm just paranoid.  If you lower the rig often while trailering and coil up the backstay this would certainly be a good reason for this step.

Ian

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Tim Daniel

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Dec 30, 2020, 9:03:43 PM12/30/20
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Thanks Ian, et al

Those are locked brummels on the splices.  Even though they apparently reduce the strength a little, I don't like the possibility of the unloaded splice unravelling.  We would likely not notice it happening, especially at the top of the mast.

I might still upgrade the line to 1/4" depending on what we end up using the boat for.  Most of what we sail in is <15 kts of wind.  I know one owner wants to do Oregon Offshore, whenever that happens again.  If he decides to so that we'll upgrade.  

It is comforting to hear that 3/16" backstays made it to Hawaii 5 times.

cheers

Tim

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Sent: December 30, 2020 5:52 PM

m d

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Dec 30, 2020, 11:16:50 PM12/30/20
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I'm a bit late to the draw here. I started typing this earlier and just came back to finish it. Looks like a nice job on the line you made up today.  A few points below; bottom line I am concerned the 3/16 amsteel used is not as strong as the original wire in this application.

--------------------------------

Size the backstay to be least as strong as the original, after derating the new one to account for the differences between dyneema and steel. Hayn shows the breaking strength of 3/16" 1x19 316 Stainless wire as 4000lbs - reference here: https://hayn.com/tech/wire_breaking_strength.html . If the original was 304 stainless wire, breaking strength is even higher at 4800lbs. If 5/32, strength is 2800lbs (316 stainless) or 3335lbs (304 stainless) 

Derating factors for Dyneema might include (but are not necessarily limited to, 'cause there's a lawyer in the house...):
1. Strength loss due to UV exposure
2. Strength loss due to wear and abrasion (internal to the line, or external) over time (e.g. for dyneema in a cascade rubbing on rings, or around pulleys, or wear between fibers as load is cycled on a backstay)
3. Strength loss due to stress concentration in splices or as the line bends around pulleys, low-friction rings, or other terminations and, more generally, small radii (knots, pulleys, low friction rings, etc...)
4. Fatigue. At cursory inspection, Dyneema appears to have similar  fatigue characteristics to steel. For the sake of this example, I'm going to assume dyneema is comparable to steel.

#1: 50% strength loss after 5 years due to UV exposure. Source: Eurofibers PDF showing % strength remaining in an 8mm dyneema line after some months of UV exposure. See page 12 here: https://issuu.com/eurofibers/docs/name8f0d44.
#2: Assume 10% due to wear and abrasion; I haven't been able to find good data for this. It's more than zero, probably less than 50%. How sporty do you want to be?  The Netherlands tugboat line test data here is a good source for info, but doesn't directly address internal abrasion.
#3: Assume 15%; though it can be up to 50% when wrapped around a bend of diameter equal to the line diameter. If using low friction rings instead of pulleys in a cascade, this factor may be greater than 15%.  If the splice bury is not tapered very carefully, this factor is about 15% according to test data from L-36.com Let's assume 15% for the standing portion of the backstay.
#4: Assume 0%

Total strength loss is then (1-.50)*(1-.10)*(1-.15) = .5*.9*.85 = 38% rated breaking strength after 5 years.  If sizing a dyneema backstay, make sure it is at least 2.5x the breaking strength of the steel wire. That way after a year or two in the sun & salt your backstay is still as strong as the original steel wire was.

Original          SS Strength      Dyneema BS
5/32 316SS      2,800 lb                7,000 lb
5/32 304SS      3,335 lb                8,338 lb
3/16 316SS      4,000 lb              10,000 lb
3/16 304SS      4,800 lb              12,000 lb

If the original wire is 3/16" 316SS, I recommend using dyneema with no less than 10,000 lb breaking strength.  New England Ropes lists breaking strength for their 1/4" (6mm) SK-78 at 9,700lb (close, and maybe close enough). 5/16" (8mm) SK-78 is 14,500lb.  Using NE SK-78 instead of amsteel might be the difference between 6mm and 8mm in this application. If the original wire is 5/32, breaking strength per Hayn is 2800lb, and 2.5x that is 7000lb which also requires 6mm SK-78 from New England. Per your email, the 1/4" Amsteel is also above 7000lb breaking strength.
 
For reference, 3/16" Amsteel SK-78 with initial breaking strength of 4,900 lbs is about equivalent to 1/8" 316SS wire after a year or two in the sun.  I would not sail with a 3/16 dyneema backstay.

Sanity check:
The dyneema tugboat tow lines tested after 1 or 2 years of use fail at 50-60% their original strength.
Colligo marine says to size Dyneema to be 2-5x the strength of the original wire. Their sizing is driven by stiffness and creep characteristics, but it's another data point that confirms the "at least" ~2.5x strength recommendation above.

For what it's worth, my boat has a nitronic stainless rod for the backstay, and 7x19 wire going through pulleys in a 24:1 cascade.  The nitronic rod is smaller diameter than equivalent strength 7x19 wire, so presents a smaller aero cross-section for lower drag. It should last 10+ years, and I don't have to worry about mainsail battens chafing dyneema over time. The backstay purchase is set at 24:1 for less line in the cockpit, and to make it harder to inadvertently break the forestay. If you're using rings instead of pulleys in your cascade, the math says there's enough friction to make going beyond a certain ratio basically pointless beyond some ratio which, if I remember correctly, is around 20:1 (i.e. 48:1 isn't easier to pull than 24:1). Use pulleys in your cascade to make it more efficient. Best bang for your buck is to use pulleys on the 'control line' side of the cascade, as they're smaller, lighter and cheaper and give similar benefit. 

It's possible (difficult, but possible) to break the forestay by pulling a 48:1 backstay cascade with one hand. You don't want your crew accidentally breaking your forestay on a windy day. Without a mechanical 'safety' that limits the load, the only indications that loads are too high on the forestay are:
a) It's hard to pull the backstay control line (which isn't the case with a 48:1 cascade, unless it has a ton of friction...in which case, might as well reduce friction and got to a 24:1)
b) Cracking sounds from the hull and deck
c) The 'twang' of a separating forestay

Consider a 24:1 backstay purchase with pulleys at each location. It'll require pulling only half the line (less spaghetti factory in your cockpit), with similar force. You can still nearly break the forestay with an enthusiastic, double-handed pull on the backstay line.  I can share a parts list if interested. Selden makes some nice, reasonably lightweight, and cost effective pulleys. Check these out:

First:

Second:

Third (or the Harken equivalent wire block):

Lower pulley for the third cascade (or the Harken equivalent)


Regards,
Manolis


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Tim Daniel

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Dec 31, 2020, 2:17:31 AM12/31/20
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Wow.  Much appreciation for the thought, time and effort that went into your analysis.  While some factors work in favour of the 3/16 line, such as having relatively large radius bends around the thimble and friction ring, quite a few of the derating factors do apply.   We can overcome some, but not all, of them by replacing the line often.

Given the catastrophic effect of a backstay failure on an SC 27, and your analysis, I will at least upsize it to 6mm (1/4").  The line isn't very expensive and splices are easy to make.  I don't know what size wire the original backstays on the boat were, and no backstay came with our boat when we bought it, so we were guessing at the size.  I used the next size down from the running backstays I made for my Dash 34, which were 1/4".  We put serious tension on those, I would guess 2-3K lb and ran them for 4-5 years with no failures.  However, losing a running backstay on a Dash 34 does not mean a rig over the front of the boat as there is also a permanent backstay.

The most interesting part of your analysis is the possibility of a 48:1 purchase breaking the forestay.  This will need further analysis on my part - what size wire is the forestay?  I will have to go and measure it.

We have had a lot of trouble getting enough forestay tension on our boat.  When we sail upwind we want to get the forestay sag down to 3-4", but even pulling very hard on the backstay purchase we were looking at 5-6".  Not having a lot of experience sailing a SC 27 - are people normally able to get enough tension on the forestay or are my expectations too high?

Thanks again for your analysis, this is exactly the kind of information I was looking for.  I jumped the gun a bit because it was a rainy day and not much else going on!

cheers

Tim 

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