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Old 06-03-2017, 15:43   #1
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Dyneema Steering Cables

After 20k nm our port side 6mm OD steering cable fatigued and parted (at the most inopportune time of course). These cables pass thru a small diameter directional sheave which in my opinion has an insufficient bending radius. Instead of replacing with another set of s/s cables, Im considering a similar diameter dyneema line. It appears these sheave can manage a dyneema line.

Has anyone had this issue and replaced it with a synthetic line? Any concern?

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Old 06-03-2017, 18:50   #2
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Re: Dyneema Steering Cables

In a pinch regular dyneema will work but I would really recommend heat set for its lower stretch. But it works very well.
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Old 06-03-2017, 22:34   #3
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Dyneema Steering Cables

What do you mean by 'heat set'?
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Old 06-03-2017, 23:22   #4
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Re: Dyneema Steering Cables

I made this change.

6mm wire parted after ~ 16k on a mooring. Extremly difficult exercise to get out of anchorage without steering and not hitting other boat. Issue was actually rust .

Replaced with 6mm dyneema using brummel splice, the whole setup has around 80% strength of 6mm dyneema ~ 3.5 T. There is no chafe after 500 nm. Larger size dyneema will require modifications as it will chafe on wheels.

I have stretched it with winch probably around 0.5T. However, after was caught in 45 kn burst, extended for 2 cm. Readjusted and I think now is set for good. Next time will pre stretch more.
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Old 07-03-2017, 01:20   #5
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Re: Dyneema Steering Cables

Heat set Dyneema doesn't tend to suffer from creep (stretch under long term load) like standard Dyneema sometimes does.

As to whether or not it'll work. Yep. Though Vectran has been being used in this application since the early 90's. It doesn't creep, & can tolerate higher working loads than Dyneema. But it's not as UV resistant.


Also, make sure that there aren't any burrs, sharp edges, or scoring anywhere that the cable/fibers will pass or ride on, including the quadrant.
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Old 07-03-2017, 07:05   #6
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Dyneema Steering Cables

Well noted. I have spare 6mm dyneema onboard so can use this for now. I'll pre-stretch as suggested. I'll search for Vectran at the next chandlery as I intend to make up two spares also. Checked already for burrs, etc - all good there. Thanks
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Old 07-03-2017, 09:50   #7
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Re: Dyneema Steering Cables

Both heat set and regular dyneema will creep. The heat set just creeps less. The bigge issue is that heat set dyneema stretches (rubber band like) far less than regular dyneema does (about 75-80% less). For steering obviously you want as low a stretch as possible so heat set is preferred. Vectran is another reasonable option but it has a lower fatigue life versus dyneema. But has a higher strength and lower creep. So it's a trade off, I wouldn't hesitate to use either.

As the loads you are talking about for a steering cable it frankly just won't matter, we are talking about safety margins well above 10:1 which is considered acceptable for life critical applications (10:1 is the safety margin for single point commercial belays, like hanging someone from a building with no backup rope).
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Old 07-03-2017, 10:27   #8
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Re: Dyneema Steering Cables

Dyneema has a minimum bend radius in relation to its size. As well, Dyneema chafes easily especially when slack. The sheeves used would need to have no sharp or hard edges and surfaces should be fairly polished.

I'd be willing to give it a try! But, I'd be very paranoid for the first 100 hours of use, constantly inspecting for chaf.

I have dyneema life lines and I regularly have to tighten them up.
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Old 07-03-2017, 10:55   #9
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Re: Dyneema Steering Cables

This is a podcast by Andy Schell of 59-North. This is the guy that started Colligo Marine, the makers of Colligo Dux synthetic rigging. Aside from being generally entertaining this podcast explains a lot about these synthetic lines and will help give some insight to this topic.

https://www.59-north.com/onthewindpo...st-john-franta

Thanks

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Old 07-03-2017, 11:12   #10
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Re: Dyneema Steering Cables

Any thoughts on using spectra line for this purpose. Either with or without a cover
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Old 07-03-2017, 11:30   #11
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Re: Dyneema Steering Cables

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Originally Posted by MikeFergie View Post
Any thoughts on using spectra line for this purpose. Either with or without a cover
Spectra would stretch even worse then dyneema and the same rules would apply as the dyneema, except sheeve radius could be smaller. But larger is better.
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Old 07-03-2017, 11:37   #12
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Re: Dyneema Steering Cables

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Originally Posted by MikeFergie View Post
Any thoughts on using spectra line for this purpose. Either with or without a cover
Spectra is a trade name of dyneema so all the same warnings above.

I would not cover it at all. If your gear can handle larger line, then just use the largest dyneema possible. If you have a chaff issue then you need to resolve it with a fine metal file.
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Old 07-03-2017, 12:14   #13
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Re: Dyneema Steering Cables

Question: I thought that creep was a function of continuous loading, as in standing rigging where it has static loads of 10-20% of breaking loads. In cable steering, there is little static load most of the time, but high loads when actively steering in big seas.

So, in this application, I'd think that stretch rather than creep would be the limiting factor with Dyneema... what do you all think?

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Old 07-03-2017, 14:00   #14
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Re: Dyneema Steering Cables

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Originally Posted by Jim Cate View Post
Question: I thought that creep was a function of continuous loading, as in standing rigging where it has static loads of 10-20% of breaking loads. In cable steering, there is little static load most of the time, but high loads when actively steering in big seas.

So, in this application, I'd think that stretch rather than creep would be the limiting factor with Dyneema... what do you all think?

Jim
I agree completely. Creep for a steering cable should be close to a non issue. Not zero, but pretty close. The tension wheel I sail going to add a few pounds of static load but not much. Off the top of my head at 20% MLB Dux will creep .001% a year, or something similar, it isn't much.

Elastic stretch is the problem for steering, and there as well heat set dyneema is far preferred over regular. But any very low stretch line will work.

One of the cool thing she about switching to dyneema is you can go up massively in strength compared to wire, which means less stretch total on the line. Not only does it increase sensitivity due to the lower weight, it also increases responsiveness thanks to the lower stretch. Wether you can tell the difference or not, depends on the driver, I can't but know some people who claim to be able to.
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Old 07-03-2017, 14:20   #15
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Re: Dyneema Steering Cables

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I agree completely. Creep for a steering cable should be close to a non issue. Not zero, but pretty close. The tension wheel I sail going to add a few pounds of static load but not much. Off the top of my head at 20% MLB Dux will creep .001% a year, or something similar, it isn't much.

Elastic stretch is the problem for steering, and there as well heat set dyneema is far preferred over regular. But any very low stretch line will work.

One of the cool thing she about switching to dyneema is you can go up massively in strength compared to wire, which means less stretch total on the line. Not only does it increase sensitivity due to the lower weight, it also increases responsiveness thanks to the lower stretch. Wether you can tell the difference or not, depends on the driver, I can't but know some people who claim to be able to.
I'd be too worried about chafe in the longer term but what do I know...I've seen stainless wire last over 40 years on some boats. Maybe I'm too conservative but I can't imagine rope lasting for long periods in this type of use.
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