Sizing a Soft Shackle

[Slipperman]

Can anyone advise me of the size (in terms of breaking strain) of a soft shackle for use on genoa sheets for a 21 sq metre genoa?

 

[Poignard]

Good question. I hope someone has an answer because I want to do the same.

 

[tudorsailor]

Can anyone advise me of the size (in terms of breaking strain) of a soft shackle for use on genoa sheets for a 21 sq metre genoa?

Surely the strength of the soft shackle does not need to be much more that the breaking strain of your sheets? Do your sheets have a soft eye through which you'd put the shackle.

TS

 

[Slipperman]

Hi yes agree the sheets are relevant too - actually I have just taken delivery of a new genoa, smaller than the previous one, and am thinking about ordering new sheets with eyes in one end and a suitable soft shackle, having never tried anything but bowlines before. I should probably just ring Jimmy Green.

 

[gregcope]

It will be dependant on the dia of Dyneema used (ie its breaking strain) as well as type of shackle (either standard or one of the newer improved designs) and assumptions on how well the Crown knot is tied/tightened. Let assume well made/tightened.

I use 6mm dyneema as it gives a usable size. I also make them in 4mm but these are tiny. 6mm D12 from Marlow has a Breaking strain of 3,140kg. Soft Shackles breaking strain is roughly the same as the line or higher. Marlow Double braid 12mm (my sheet size) is 4000kg (approx). The eye splice will reduce strength by 20% or so, so the breaking strains become similar or the sheet becomes the limiting factor.

So depending on line size of the shackle I would not worry.

See the following for reference;
Soft Shackles
Soft Shackle

 

[vyv_cox]

When Graham and I tested soft shackles, having previously tested steel ones, we found that 5 mm Dyneema was about as strong as 10 mm stainless ones. Dyneema is remarkably tough stuff.

 

[Slipperman]

Thanks all. So going by what gregcope says above, it seems that if I go for sheets of 12mm Marlow Double Braid, I should go a soft shackle with a breaking strain of around 4000kg. Force 4 sell exactly that (in Dyneema) with their next size up being 6000kg, which would seem like overkill.

 

[Laminar Flow]

I think what you need to do is determine the load. Generally speaking, 1lbs per sqft is what most rigs are calculated at or 4.88kg/m2. You can add a safety factor of 2.
Ergo: 21 m2 x 4.88 x2 = 295kg or 452 lbs
Now you can pick your shackle.
Sheets are chosen more for ease of handling than just required strength.

 

[gregcope]

@Slipperman you could go smaller on the shackle as @Laminar Flow points out from a breaking strain POV. I was just trying to show that the shackle will not be the weak point.

However a smaller one might be ickle and hence get a slightly bigger one!

Another consideration on the smaller ones is the size/circumference of the loop and what you need to fit in it; Basically two sheets and the eye of the sail.

 

[Stemar]

The strength of the sheets should be irrelevant. Sheets are usually sized for comfort in handling, so tend to be several times the size needed for strength, at least on a cruising boat.

 

[Slipperman]

Yes thanks - I have plenty of experience with sheets just not soft shackles. I am grateful for all the various bits of advice above, though some do seem a bit contradictory! Anyway I will have a ponder and maybe try out a couple of options.

 

[Poignard]

I don't doubt that Dyneema soft shackles are a very strong alternative to traditional shackles but isn't chafe a problem?

 

[gregcope]

I don't doubt that Dyneema soft shackles are a very strong alternative to traditional shackles but isn't chafe a problem?

Any more than sheets being chafed? Dyneema is very strong so less of an issue for that material.

 

[GHA]

I don't doubt that Dyneema soft shackles are a very strong alternative to traditional shackles but isn't chafe a problem?

Not really for decent dyneema like Marlow d12 max, it has an anti chafe coating. And it's so crazy strong that even half worn through something else onboard will go bang first. Back to the op, as well as a soft shackle for connections to the clew I spliced in a few metres of dyneema into the sheets so at the sail end there's very little weight and it tacks past the inner forestay on a cutter rig much easier. Not that hard to splice, how to on youtube. Also well worth getting some dyneema off ebay and making your own soft shackles. 5mm not a bad dia. Such handy things on board.

 

[vyv_cox]

I don't doubt that Dyneema soft shackles are a very strong alternative to traditional shackles but isn't chafe a problem?

I have been using the same soft Dyneema shackle on my genoa for something like 7 years. No chafe at all.