Can Dyneema Chafe through Stainless Steel? Yes!

[Capa66]

Good for you mantus, I will now buy your products.

[john manning]

This is not an unusual problem. It is wear not corrosion. Synthetic cord wearing grooves into stainless steel is something I see most often with packed gland seals on stainless steel prop shafts. Wear can be reduced by using a thimble on the Dyneema loop.

[carstenb]

Originally Posted by john manning

This is not an unusual problem. It is wear not corrosion. Synthetic cord wearing grooves into stainless steel is something I see most often with packed gland seals on stainless steel prop shafts. Wear can be reduced by using a thimble on the Dyneema loop.

would you care to elaborate on this? I really wouldn't have thought that Dyneema could wear through stainless, but despite everyone calling this corrosion - I'm also of the opinion that it is wear

Do you know of any tests/scientific articles that explain this?

[pbmaise]

Originally Posted by seandepagnier

Does anyone put zincs on their anchors?

Yes! I did. I had a carbon steel copy made of a common brand anchor in the Philippines. The anchor was painted only and I realized it would corrode pretty dang fast. Therefore, I attached a very large rectangular zinc usually found on the side of a carbon steel hull. I think it weighs about 1 kg.

I have been painting the anchor with Hammerite protective paint to help keep the corrosion level low.

After a few years in the water I repainted the anchor and noted that the zinc was obviously working since it was corroding. This did not prevent the anchor from rusting once the protective paint wore away..however..it is my contention that it reduced the time needed.

..............

I am in greatest agreement with the theory that rough sand particles imbedded into the rope were the cause of the wear. Notice on the right hand side of the thimble the two scratches. These are sharp and defined. A rope nor galvanic corrosion would not make sharp scratches like this. A sharp piece of sand could.

I do see classic pitting corrosion on the thimble on the left hand side. However, this is far away from where the rope attaches.

[john manning]

Originally Posted by carstenb

would you care to elaborate on this? I really wouldn't have thought that Dyneema could wear through stainless, but despite everyone calling this corrosion - I'm also of the opinion that it is wear

Do you know of any tests/scientific articles that explain this?

Before retiring (for the first time) I owned a marine engineering business. We often did work on prop shafts and seals. When the packed gland type seal changed from having jute based packing to teflon packing stainless steel shafts began to wear down under the seal. The teflon was usually between 10mm and 12mm square on the boats I saw. Stainless shafts would wear out a groove roughly the same width. One groove for each ring of packing used. I have seen similar wear from "hi tech" ropes working on cleats and rings. The common feature was that they were synthetic ropes that did not need an outer cover. ie. They were hard wearing materials.

Stainless steel is not a hard steel. It can appear that way when you try to drill it or do other work because it work hardens. ie. Any cutting or bending makes the area worked on harder, but only at the surface.

The notion that steel is harder than plastic disappeared from the engineering world a long time ago. I can buy plastic sanding/scouring pads that work better than wire brushes for smoothing steel surfaces. I can buy plastic bearing materials that last longer than the brass and bronze they replaced.

[Dave_S]

Stainless is stainless because it oxidises and this forms a corrosive barrier. If the dyneema along with some crusty bits of barnacles etc continually rubbing removed that barrier faster than it formed maybe this could be the result ?


Sorry, just being polite. I have to stop that. This is the problem, so its a bit of both.... abrasion allowing corrosion.

[thinwater]

Originally Posted by john manning

Before retiring (for the first time) I owned a marine engineering business. We often did work on prop shafts and seals. When the packed gland type seal changed from having jute based packing to teflon packing stainless steel shafts began to wear down under the seal. The teflon was usually between 10mm and 12mm square on the boats I saw. Stainless shafts would wear out a groove roughly the same width. One groove for each ring of packing used. I have seen similar wear from "hi tech" ropes working on cleats and rings. The common feature was that they were synthetic ropes that did not need an outer cover. ie. They were hard wearing materials.

Stainless steel is not a hard steel. It can appear that way when you try to drill it or do other work because it work hardens. ie. Any cutting or bending makes the area worked on harder, but only at the surface.

The notion that steel is harder than plastic disappeared from the engineering world a long time ago. I can buy plastic sanding/scouring pads that work better than wire brushes for smoothing steel surfaces. I can buy plastic bearing materials that last longer than the brass and bronze they replaced.

\


The error in this argument is movement. The shaft is turning at high RPM for hours and days. Ropes move in chocks. A soft shackle does not move, under load against the thimble. In fact, it is mechanically easier for the chain to flex at any other point than where the rope bears on the thimble.



You can test this. Take a thimble, hang a weight from it using chain, and see if you can get the fiber to move even a millimeter. Instead, the adjacent links will move, because they can pivot without sliding. The rope will flex. Just the weight of the lazy loop of chain is enough to look the sling in place.



Apples to hand grenades.

[qwert]

"The error in this argument is movement. The shaft is turning at high RPM for hours and days. Ropes move in chocks. A soft shackle does not move, under load against the thimble. In fact, it is mechanically easier for the chain to flex at any other point than where the rope bears on the thimble."
I think there may well be movement, every time a wave hits the boat the chain stresses then relaxes. The "grain of sand" held firmly by the plastic fibres of the dyneema can constantly grind in the one spot to which it will return every separate time the anchor is used. The whole load is very concentrated on the one point on the thimble.

[carstenb]

Originally Posted by qwert

"The error in this argument is movement. The shaft is turning at high RPM for hours and days. Ropes move in chocks. A soft shackle does not move, under load against the thimble. In fact, it is mechanically easier for the chain to flex at any other point than where the rope bears on the thimble."
I think there may well be movement, every time a wave hits the boat the chain stresses then relaxes. The "grain of sand" held firmly by the plastic fibres of the dyneema can constantly grind in the one spot to which it will return every separate time the anchor is used. The whole load is very concentrated on the one point on the thimble.

Greg at Mantus has me to send the old bridle back (he replaced it with a new one free of charge). I will let you all know what he says is the cause when I know.

[tunnelvision]

We have heard reports here in French Polynesia that accelerated wear and corrosion can happen especially in the Marquesas. The bottom is made up of finely ground volcanic rock and soil. More than one cruiser has complained of very short chain life if anchored in one area for a good while. I'm wondering if this is from the soft shackle trapping the abrasive bottom material and wearing it down like wet sandpaper. With gusty conditions our bridal goes from resting on the bottom to almost reaching the surface. This would provide the movement to wear it down. Think of a sponge with abrasive grit on its surface. Just my guess.

[kmacdonald]

Has the boat been struck by lightning while at anchor?

[carstenb]

Originally Posted by kmacdonald

Has the boat been struck by lightning while at anchor?

we were a long time in Polynesia and the subsurface is indeed volcanic. But this chafe started a bit before we got there.


No the boat was never hit by lightning while at anchor

[carstenb]

Greg and his folks have now examined the bridle and here is their verdict

Carsten, I really think its anoxic damage.... so far its the only time we heard of this issue we will keep monitoring for customer feedback..
Greg


So - I'm not sujre I'm much smarter now - I will keep a close eye on our new bridle (free warranty replacement from Mantus) to see if any damage occurs