Ropework Question

[Dockhead]

I've finally settled on something for my dinghy lift problem, and it involves ropework, a skill which I don't have much of.

I am going to solve the clearance problem by suspending a block from one side of the davit end, with the end of the rope fixed on the other side of the davit end. That way, the load will be centered. In order not to have to drill into 3mm thick stainless more than necessary, I will do this with ropes.

On the bottom surface of the davit at the davit end is a sturdy eyebolt. I am planning to use that to prevent the rope from slipping. I will take two pieces of 6mm Dyneema, and tie them both to the eyebolt. First question -- what knot? I would be inclined to use a round turn and two half-hitches -- is that the right knot?

Then, I will take each piece of rope and wrap it around one side of the davit, over the top, and down the other side. I will attach a block to one of these, and tie the end of the lifting rope (10mm polyester braid on braid) to the other.

Here's the hard part -- I want to tie the two ropes together at the top, so that the load is balanced, rather than going to the eyebolt. What's the best knot for that? I struggle to come up with the right knot for that, or indeed any knot at all.

I want the whole assembly to be pretty tight around the davit to minimize slipping forward. There will obviously be a forward component to the forces involved, but if the ropes are tight, this shouldn't be a problem, since the davit is tapered. So it will jam and hold fast after slipping a bit forward, which is ok. Because of the purchase, the forward component of the load will be half the vertical component.

Keeping in mind that this is racing dyneema rope and so very slippery.

Any tips?

The lifting rope will run through a clutch at the other end of the davit. That will require drilling two holes in the massive 3mm thick stainless, which will be a royal PITA, but I don't really see any other way to do it. Cobalt bit, slow speed, lots of lube . . .

There will be plenty of vertical clearance this way for the lifting rope to pull the dinghy up tight against the davits. I'm planning to use a Wichard MXL soft block on the top of the original lifting slings. The ingenious Wichard soft block is extremely short, so I will have more clearance than the original lifting mechanism had.

For a safety, I will attach two pieces of Dyneema to the lifting eyes in the hull of the dinghy, using snap shackles, and pull them through the eyebolt in the davit end, and tie them to a shackle close to the eyebolt. There will be a block on the shackles, and I will use a ratchet strap slung around the forward end of the davit to pull that tight and hold it with effectively double-purchase.

That should work excellently, IF (a) I can figure out the right ropework, subject of this thread; and (b) IF I can manage to get the holes drilled in the davits to fit the Spinlock clutch for the lifting rope.

All of the above describes just one end of the dinghy. If it all works well, I will repeat the whole thing for the other end.

I will continue to try to repair or replace the original powered lifting mechanism, but it will be great to have this in place even if I succeed -- as a built-in backup and safety system.

[Randyonr3]

There are a couple way of attaching Dyneema that work well.. one is (and this is if its perminant) to do a loop and feed the line back into itself, a couple inches and sew the two together and then whip it.. works like the old chineese finger cuff.. the harder you pull it the tighter it gets..
Second, if its a tempfix, after you loop the dyneema, after it is looped, its run through, across the origional strand, down an inch or so and run it bach through, down an inch and again through the origional.. kinda like you are sewing with the end in a zig-zag pattern throu the origional strand.. using a fid makes it work very well..
You treat it like you would a poly tow rope..

[Hydra]

For a piece of rigging intended to stay permanently outside, I would not use bare Dyneema because it is vulnerable tu UV light.

I would rather use polyester braid with Dyneema core. It is more difficult to splice but keeps knots better.

Alain

[Dockhead]

Originally Posted by Hydra

For a piece of rigging intended to stay permanently outside, I would not use bare Dyneema because it is vulnerable tu UV light.

I would rather use polyester braid with Dyneema core. It is more difficult to splice but keeps knots better.

Alain

Good point. I will do that. Thanks.

[Dockhead]

Originally Posted by Randyonr3

There are a couple way of attaching Dyneema that work well.. one is (and this is if its perminant) to do a loop and feed the line back into itself, a couple inches and sew the two together and then whip it.. works like the old chineese finger cuff.. the harder you pull it the tighter it gets..
Second, if its a tempfix, after you loop the dyneema, after it is looped, its run through, across the origional strand, down an inch or so and run it bach through, down an inch and again through the origional.. kinda like you are sewing with the end in a zig-zag pattern throu the origional strand.. using a fid makes it work very well..
You treat it like you would a poly tow rope..

I don't want it to be permanent. I'm having trouble visualizing this zig-zag-- is it a knot?

[conachair]

Originally Posted by Dockhead

I will take two pieces of 6mm Dyneema, and tie them both to the eyebolt.

Could you just make up a single dyneema strop with an eye each end and wrap it once or twice round the davit with an eye ending up hanging each side? With a single wrap around the eyebolt to stop any slipping?
Then some seizing with whipping twine to tidy it up.

Maybe.

Or even a climbing tape. Though getting a tidy wrap round the eyebolt would be easier with dyneema.

You might be overthinking the safeties, with a snap shackle it's not too hard to overhaul the lifting lines a little to get the shackle in then back off a tiny bit.

And I can't see UV being too much of a worry, dyneema in general is so outrageously strong you're going to rip something else apart long before some doubled up 6mm goes I'm on a project at the moment where there must be heading towards 10Km of marlow d12 max up in the air - absolutely incredible stuff! All those offcuts along the way have been assured a happy retirement home

[Dockhead]

Originally Posted by conachair

Could you just make up a single dyneema strop with an eye each end and wrap it once or twice round the davit with an eye ending up hanging each side? With a single wrap around the eyebolt to stop any slipping?
Then some seizing with whipping twine to tidy it up.

That's a great idea, but then -- what knot or other fixing method to use, to fix the block, on the one side, and the end of the lifting rope, on the other?

Is there a knot I can make in the rope, to create an eye, which will be strong enough?

[Stumble]

Dockhead,

A few things.

1) dyneema is the most UV resistant line made. Full stop. After 5 years of Carribean sun dyneema has been tested to 75% of its original MBL though most people assume a loss of 35% to be safe. Long term exposure isn't an issue.

2) as you have found dyneema is pretty resistant to knots. It's just too slippery to really hold them well, so wherever possible it is better to splice it in place. I know a lot of people have an aversion to splicing, but because of its construction dyneema is honestly the easiest line I know of to splice. A simple eye splice takes longer to find my tools than to actually do. It is litterly just sliding one part of the line into itself, takes about a minute, then a lock stitch that takes me maybe 2-3.

I recommend http://www.samsonrope.com/Documents/...UL2012_WEB.pdf but there are plenty of resources that will teach you how.

3) there is a splice called an end-to-end splice that can join two ropes together, or make a continuious loup. It takes a few minutes longer, but just that. Your first one will take 10 minutes, after that just a few seconds.

http://www.bethandevans.com/pdf/Clas..._Splice_08.pdf

4) I am having a hard time visualizing the issue, but to join two lines together eye splice the dyneema then tie the other line to the eye. It is possible to do an end-to-end splice dyneema and double braid, but it gets tricky. It's a more advanced trick. Can you draw a diagram of what you are thinking? I may be able to help more that way.

If all you are doing is trying to end the dyneema into a larger line to make it easier to handle then the easier way is to add a cover to that portion of the dyneema, or use a line like endurabraid and strip the cover away from the area you don't need it on. Again this is a pretty easy process but takes a little more time than the above.

[Dockhead]

Originally Posted by Stumble

Dockhead,

A few things.

1) dyneema is the most UV resistant line made. Full stop. After 5 years of Carribean sun dyneema has been tested to 75% of its original MBL though most people assume a loss of 35% to be safe. Long term exposure isn't an issue.

2) as you have found dyneema is pretty resistant to knots. It's just too slippery to really hold them well, so wherever possible it is better to splice it in place. I know a lot of people have an aversion to splicing, but because of its construction dyneema is honestly the easiest line I know of to splice. A simple eye splice takes longer to find my tools than to actually do. It is litterly just sliding one part of the line into itself, takes about a minute, then a lock stitch that takes me maybe 2-3.

I recommend http://www.samsonrope.com/Documents/...UL2012_WEB.pdf but there are plenty of resources that will teach you how.

3) there is a splice called an end-to-end splice that can join two ropes together, or make a continuious loup. It takes a few minutes longer, but just that. Your first one will take 10 minutes, after that just a few seconds.

http://www.bethandevans.com/pdf/Clas..._Splice_08.pdf

4) I am having a hard time visualizing the issue, but to join two lines together eye splice the dyneema then tie the other line to the eye. It is possible to do an end-to-end splice dyneema and double braid, but it gets tricky. It's a more advanced trick. Can you draw a diagram of what you are thinking? I may be able to help more that way.

If all you are doing is trying to end the dyneema into a larger line to make it easier to handle then the easier way is to add a cover to that portion of the dyneema, or use a line like endurabraid and strip the cover away from the area you don't need it on. Again this is a pretty easy process but takes a little more time than the above.

Wow, extremely useful -- thanks!

If it's really that easy to splice, then that should be the answer to all of my problems.

This rope does not need to be easy to handle -- its sole function is to hold up the block on one side of the davit, and the end of the lifting rope on the other. It will not be handled. So something like 6mm Dyneema should be fine.

If it can splice it, then that answers a ton of questions. Maybe I would just make two pieces with an eye on both ends of each. Cow hitch them both to the eyebolt at the bottom of the davit, and sling both pieces across the top in opposite directions. Then that leaves as the only puzzle how to knot them together or otherwise join them at the top. Is it possible to put an eye in the middle of a piece of line?

[Stumble]

This is a slightly sped up example of a locked brummel eye splice in dyneema (run time 3:20). This is technically a better splice since it doesn't rely on lock stitching, but I don't like them since it can only be done on one side of the line and I prefer to always do the same one. That way I can try to build repeatability.

https://m.youtube.com/watch?v=d0B6tYTVHVM

It is possible to splice an eye in the middle of a line, and then add an eye splice in each end. But the eye in the middle needs to be a locked brummel like in the video. However instead of doing the taper as shown you pull the inner portion out, then throw in another brummel. Finally you toss an eye splice in the ends.

I probably wouldn't do this though since it would dramatically reduce the line strength. Probably by 50% or more, though admittedly 6mm dyneema is likely overly strong any way. I would make two short legs with an eye splice in each end, then attach them to the working line (also with an eye splice in the end) with a soft shackle attaching all three eye splices. Or splice all the eyes to a stainless ring.

Edit: I should mention that while a set of fids is nice almost anything the same size or smaller than the line can be used to pull the line thru. For really small line (1mm) I use repurposed fish leader. But I have also used Bic pen barrels as fids.

[Seaworthy Lass]

Originally Posted by Dockhead

First question -- what knot? I would be inclined to use a round turn and two half-hitches -- is that the right knot?

I second the suggestions that splicing is the way to go.
A round turn and two half hitches is near useless in unsheathed UHMWPE (such as Dyneema or Amsteel Blue).

The best knots in UHMWPE don't give much more than 50% of line strength. If you need to quickly attach a line to a bolt, the EStar Hitch (or EStar-XX ) is by far the best option. Evans Starzinger found the strength of this to be around 54%.

Links to the EStar (the only thing that makes the first two links a little confusing is that the EStar is not actually based on a correct Buntline):
http://www.bethandevans.com/pdf/estar.pdf
Buntline Hitch | How to tie a Buntline Hitch | Boating Knots
(the EStar is presented lower down the page of the above)
http://www.cruisersforum.com/forums/...ch-127822.html

Originally Posted by Hydra

For a piece of rigging intended to stay permanently outside, I would not use bare Dyneema because it is vulnerable tu UV light.

Sorry, this is incorrect. Unsheathed UHMWPE such as Dyneema has been treated to give it very good UV resistance.

Originally Posted by Stumble

....
If all you are doing is trying to end the dyneema into a larger line to make it easier to handle then the easier way is to add a cover to that portion of the dyneema, or use a line like endurabraid and strip the cover away from the area you don't need it on. Again this is a pretty easy process but takes a little more time than the above.

The only problem with this is that the core of sheathed line is natural UHMWPE, and not UV resistant. If you strip the sheath and join the core to unsheathed line the join need some kind of covering on the join for UV protection.
Excellent suggestions otherwise .

SWL

[Stumble]

Seaworthy,

The coatings used for Amsteel is called Samthane type S, and is primarily used to add a little stiffness because the raw fibers are so flexable. In addition Type S adds a little intra fiber wear resistance. It does nothing for UV stability.

There are coatings that can help with UV stability, but they are more commonly used on Polypropaline and other UV sensitive lines.

There are coated Dynex lines that are used for standing rigging, but really they don't apply here. Just figure a 5-10 year service life and don't worry too much about it. This is getting a little esoteric, but for standing rigging heat treated and coated dyneema (a version of Dynex Dux) is getting service lives in excess of 10 years.

[Seaworthy Lass]

Originally Posted by Stumble

Seaworthy,

The coatings used for Amsteel is called Samthane type S, and is primarily used to add a little stiffness because the raw fibers are so flexable. In addition Type S adds a little intra fiber wear resistance. It does nothing for UV stability.

There are coatings that can help with UV stability, but they are more commonly used on Polypropaline and other UV sensitive lines.

There are coated Dynex lines that are used for standing rigging, but really they don't apply here. Just figure a 5-10 year service life and don't worry too much about it. This is getting a little esoteric, but for standing rigging heat treated and coated dyneema (a version of Dynex Dux) is getting service lives in excess of 10 years.

I will have to do a bit more reading when I have a chance. As I understand, unsheathed UHMWPE line is treated to increase its UV resistance (certainly sheets of it are). Untreated, it deteriorates fairly rapidly.

To save me a bit of hunting, do you have any references to indicate no treatment is applied to Dyneema and Amsteel Blue?

SWL

[Seaworthy Lass]

This article discusses UV stabilised UHMWPE:
EngArc - M - Ultra High Molecular Weight High Density Polyethylene

"Ultraviolet-stabilized UHMWPE. UHMWPE can be attacked by ultraviolet (UV) radiation. The attack is known as UV degradation, and can be a problem in products highly exposed to sunlight or in some cases artificial sources. Continuous exposure for a long period of time is a more serious problem than intermittent exposure, since the attack is dependent on the extent and degree of exposure. Examples of UV degradation on natural UHMWPE include chalking, a yellow discoloration and cracking. In severe cases, complete product disintegration can occur. The attack can be detected before serious cracks are seen in a product using infrared spectroscopy. UV attack by sunlight can be slowed or prevented by dispersing UV stabilizers in the polymer prior to extruding. The mos effective methods for protecting plastic polymers for UV attack is the addition of specific carbon blacks which when added in sufficient quantities absorb the UV radiation and give it off as heat; protecting the polymer chain. Properly loaded carbon black can give 10+ years of protection from normal exposure. Simply coloring the part black doesn't mean the product willbe optimally protected from UV attack."

Stumble, do you think Dyneema and Amsteel has not received any treatment such as this?

[Seaworthy Lass]

Hmmmm. I am having a lot of trouble finding out much about any UV resistant coatings (or lack of them).

A company called DSM has been manufacturing Dyneema since 1990. I have searched their website, but can find zero reference to UV .
As far as I understand, they produce three types of Dyneema - SK75 (first produced in 1996 and used in Amsteel Blue and probably in Dyneema®), SK78 (2003) and DM20 aka Dyneema Max (2012).

Sampson ropes have a section in their brochure on slings, indicating 8mm Dyneema (probably SK 75) loses 60% of its strength after 10 years of open air UV conditions (less as rope diameter increases). This was a study conducted by the manufacturers:
http://www.samsonrope.com/Documents/...CT2013_WEB.pdf
(See below)

The Marlow Ropes website simply states:
"UV Resistant: Dyneema® has very good resistance to photo degradation, maintaining its performance when exposed to UV light"

Evans Starzinger comments on his website:
Load testing
"UV exposure damages all lines and fibers, but it particularly degrades the strength of single braids, as there is no cover to shield the load bearing core fibers. How much the line strength will be degraded depends on what sort of coatings are applied to the fibers and exactly what sort of braid is used."

I have previously taken this to mean the fibres are coated to increase UV resistance, but I may have got hold of the wrong end of the stick .
Amsteel Blue, for example uses a Samthane coating, which provides "abrasion and tension fatigue resistance for superior wear". It may or may not help with UV.

Anyone have any more info on coatings or lack of?
I have written and asked the manufacturers of Dyneema and will report back .

SWL