Shackle SWL- am I misunderstanding something?

[Gin]

Having identified that my 10mm anchor warp is no longer fit for the job, I decided to replace it with a larger 12mm diameter for added safety- Breaking strain quoted as being 3300kgs.

The 3/8" anchor chain, which is sound, is said to have a SWL/breaking strain of at least 2449kgs according to commercial tables.

However, having increased the rope size and its thimble I need a bow shackle to take the thimble and since the largest size of pin which will pass through the chain links is the same as the chain link diameter it seemed sensible to go for a matching 10mm body and pin galvanised job which would be a little larger than the 'D' shackle previously attached.

This is where I became puzzled as it appears the shackle is literally the weakest link and the beefed up rope size will not count for anything.

Commercial i.e. unrated, although cheap have no indication of their load capacity- on the other hand much dearer rated shackles but possibly of similar materials/construction have very much lower ratings than I expected.

For example, some state a SWL of 0.75tonnes, others 1.0 T and it is not until one gets up to about 16mm diameter that a SWL of 3T is reached- such a shackle cannot be used in my situations.

It would seem therefore that the best I can expect is about 1T which even for my little boat of about 1.3T displacement that seems too modest. Then again given that the existing shackle has a 9mm body and a 10mm pin and a rotten 50metres of warp (no idea of how much abuse its had of course) perhaps I am worrying too much.

Have I missed something here? Comments please

 

[macd]

The 3/8" anchor chain, which is sound, is said to have a SWL/breaking strain of at least 2449kgs according to commercial tables.

Depends on grade but the SWL of the most commonly available 3/8" chain is around the number you mention, but in pounds, according to Vyv Cox's website: https://coxengineering.sharepoint.com/Pages/Chaindefinitions.aspx

SWL and breaking strain are very much not the same thing.

10mm Forged Wichard HR shackles are rated at 2640kg (SWL)/6000kg (breaking load). As here: http://www.seateach.com/wichard-hr-shackles.html

 

[jakeroyd]

SWL is what it suggests. Safe working load.
This means the part can take this load probably indefinitely.
There may also be an allowance for wear on a component like a shackle.
SWL will be well inside the elastic limit ( this is the limit where the part will cease to be elastic and will deform permanently. This is known as the yield point.
Further up the load scale is the breaking load where , well , it breaks.

There is also a fatigue limit which again is significant with a shackle.
below the fatigue limit life in terms of the number of applications of the load is infinite for a ferrous material.
Above the fatigue limit the life of the part is not infinite and the part will eventually fail.


Any component subject to many applications of a particular load will need a SWL well below the yield point to be safely usable in its intended application.

 

[Ruffles]

Did your 10 mm anchor rope actually break or wear out? I've always thought rope wears out long before it snaps. In fact I've always assumed that a deck cleat would fail before a rope snapped.

Thinking about it, in all my years pottering about on boats I don't think I've ever had a rope, halyard or sheet snap. I've replaced loads because they're showing wear from chaffing.

I tow a heavy old dinghy often part filled with water putting the painter under serious strain. The painter is a retired laser main sheet from the mid 80s. I replaced it then because I thought it was worn out!

 

[dovekie]

Others will suggest too. and indeed you may already have discounted the idea for good reason, but I think most people splice the chain to the rope, for this very reason.. Apologies if I have misunderstood.

 

[sailorman]

Others will suggest too. and indeed you may already have discounted the idea for good reason, but I think most people splice the chain to the rope, for this very reason.. Apologies if I have misunderstood.

And 18mm anchorplait would be more suited to 10mm chain

 

[Gin]

Did your 10 mm anchor rope actually break or wear out? I've always thought rope wears out long before it snaps. In fact I've always assumed that a deck cleat would fail before a rope snapped.

Thinking about it, in all my years pottering about on boats I don't think I've ever had a rope, halyard or sheet snap. I've replaced loads because they're showing wear from chaffing.

I tow a heavy old dinghy often part filled with water putting the painter under serious strain. The painter is a retired laser main sheet from the mid 80s. I replaced it then because I thought it was worn out!

The synthetic section 20+ metres was grey and very chafed- the natural fibre section (sisal like), another 20metres, typically hairy but looks OK otherwise . However, there were two connections along its length and all in all I wouldn't want to trust it- hence the replacement.

Sheets snapping? Yes, been there and only recently on another boat a sheet snapped without warning apart from the fact that the braid on braid looked very tired at the outset

 

[vyv_cox]

It would seem therefore that the best I can expect is about 1T which even for my little boat of about 1.3T displacement that seems too modest. Then again given that the existing shackle has a 9mm body and a 10mm pin and a rotten 50metres of warp (no idea of how much abuse its had of course) perhaps I am worrying too much.

A SWL of I ton is way over what you need for such a small boat. The figure is either 1/4 or 1/6 of the MBL minimum break load, dependent upon the duty for which it is sold. Even at 1 ton I doubt very much that your deck cleats would still be attached to the boat, even assuming that your presumably quite small anchor had not given up the struggle a long time previously.

Similarly, 3/8" chain is very large for the boat, I use 8 mm with a boat weighing 7 tons. With 8 mm chain I can fit a 10 mm shackle, which made in stronger material than the typical chandlery product, matches the strength of the chain.

 

[Gin]

A SWL of I ton is way over what you need for such a small boat. The figure is either 1/4 or 1/6 of the MBL minimum break load, dependent upon the duty for which it is sold. Even at 1 ton I doubt very much that your deck cleats would still be attached to the boat, even assuming that your presumably quite small anchor had not given up the struggle a long time previously.

Similarly, 3/8" chain is very large for the boat, I use 8 mm with a boat weighing 7 tons. With 8 mm chain I can fit a 10 mm shackle, which made in stronger material than the typical chandlery product, matches the strength of the chain.

Thanks Vyv. that's just what I needed- the existing anchor is a 6kg CQR type- a design I don't like at all as they nip fingers so easily but clearly has served the boat for years (as has the rode by the look of it). So it would seem I would be best to buy a rated shackle at least for the comfort of knowing what it was designed to cope with (and get myself a new anchor which, happily, a good friend has already given me as a birthday gift)

A quick, final update- rated shackles now purchased for thimble to chain and chain to new anchor connections- I can now rest easy Thanks to everyone who contributed