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Old 07-04-2019, 11:38   #1
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Thimbles for Dyneema Single Braid

I need to replace a pair of stays which hold up my transom platform, and I want to use shorter pieces of dyneema and rigging screws so I can adjust them.



But the radius of the tiny shackles is too small for the dyneema rope. So I need some kind of a thimble.


Any reason why I can't use a normal wire rope thimble, like this:


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Old 07-04-2019, 11:48   #2
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Re: Thimbles for Dyneema Single Braid

Quote:
Originally Posted by Dockhead View Post
I need to replace a pair of stays which hold up my transom platform, and I want to use shorter pieces of dyneema and rigging screws so I can adjust them.



But the radius of the tiny shackles is too small for the dyneema rope. So I need some kind of a thimble.


Any reason why I can't use a normal wire rope thimble, like this:
I can’t see why not. I would use a slightly oversized thimble so that there is no risk the sides will cut into the dyneema. Just inspect it occasionally for chafe.

Our cockpit locker lids are heavy and would cause tremendous damage if they flopped shut on any body parts. I attached a bit of dyneema to the underside of the handle to enable it to be looped over the rail above it. I eye spliced around a thimble as shown below, as this gave a smooth opening for the catch.

SWL
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Old 07-04-2019, 12:45   #3
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Re: Thimbles for Dyneema Single Braid

Quote:
Originally Posted by Seaworthy Lass View Post
I can’t see why not. I would use a slightly oversized thimble so that there is no risk the sides will cut into the dyneema. Just inspect it occasionally for chafe.

Our cockpit locker lids are heavy and would cause tremendous damage if they flopped shut on any body parts. I attached a bit of dyneema to the underside of the handle to enable it to be looped over the rail above it. I eye spliced around a thimble as shown below, as this gave a smooth opening for the catch.

SWL

So I guess you're happy with that radius -- I doubt you will be testing the strength limits of that cordage.



I suppose I could do 1:1 as well -- should be plenty strong. If I could find the right shackles.


Or I could just have a rigger do up a couple of short pieces of wire rope and forget about it.
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Old 07-04-2019, 13:31   #4
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Re: Thimbles for Dyneema Single Braid

The bend radius thing is not really an issue unless you're putting a terrible lot of stress on the line. SK90 had bend-radius issues, but I'm not sure it's being made anymore. If you use SK75, 78, or 99, you can go as small as 1:1 radius. Think of the bail on a Tylaska T-12 shackle--it's about 5/16," and we commonly splice 1/2" Dyneema halyards onto them.

But that thimble will be fine, or you could get some closed thimbles from Ronstan--really nice, pretty cheap.
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Old 07-04-2019, 14:58   #5
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Re: Thimbles for Dyneema Single Braid

Sail makers one piece thimbles are what you need. The wire rope thimbles can collapse under load that the exotics can sustain and the open ends are a chafe failure waiting to happen.
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Old 08-04-2019, 12:51   #6
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Re: Thimbles for Dyneema Single Braid

Use a low friction ring.
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Old 08-04-2019, 14:26   #7
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Re: Thimbles for Dyneema Single Braid

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Originally Posted by roverhi View Post
Sail makers one piece thimbles are what you need. The wire rope thimbles can collapse under load that the exotics can sustain and the open ends are a chafe failure waiting to happen.
Such thimbles are indeed better, but in this application (holding up a swim platform) it is hard to envision the loads reaching a point that would collapse a conventional thimble, especially when he mentioned using an oversized one.

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Old 08-04-2019, 21:51   #8
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Re: Thimbles for Dyneema Single Braid

To reduce exposure to sharp edges look for closed thimbles. The 2 brands I recall were in a Practical Sailor comparison of thimbles for anchor rodes that resist side loads. They were fully enclosed at the radius, and at the ends.
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Old 08-04-2019, 22:04   #9
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Re: Thimbles for Dyneema Single Braid

Quote:
Originally Posted by Dockhead View Post
I need to replace a pair of stays which hold up my transom platform, and I want to use shorter pieces of dyneema and rigging screws so I can adjust them.



But the radius of the tiny shackles is too small for the dyneema rope. So I need some kind of a thimble.


Any reason why I can't use a normal wire rope thimble, like this:


Attachment 189577

Are they too small?


At d/D= 1 Dyneema looses 50% strength, but since it is an eye, each leg is carrying 50%. It will be full strength without a thimble. This is on their web site. There should be no chafe since the surfaces are smooth. If you need abrasion protection, slide on a length of webbing or rope cover.


So luggage tag it and move on (yes I have pull-tested this sort of rigging).


By the way, the wire rope thimble will collapse at full load.
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Old 09-04-2019, 03:52   #10
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Re: Thimbles for Dyneema Single Braid

Thanks for all the very useful replies!
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Old 09-04-2019, 03:59   #11
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Re: Thimbles for Dyneema Single Braid

What's the breaking load of the tiny shackle? At a guess even with a small bend radius the shackle would go before the dyneema. Getting a long enough bury each end might be tricky if they are only short dyneema strops.
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