Rope for various lines

Moderator: GreenLake

Re: Rope for various lines

Postby GreenLake » Tue Aug 22, 2017 12:59 pm

Here's my variant of a sheet bend with extra loop.
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P1030598-c1_900.jpg
Sheet bend with extra loop
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Re: Rope for various lines

Postby talbot » Tue Aug 22, 2017 6:16 pm

The blue line in the blocks might be 6mm Marlow 8-Plait-Matt.

I have started buying rope in scrap ends from our boatyard. Inexpensive, but sometimes you have to do some detective work to know what you are getting if the scrap isn't something the yard ordered. It's particularly tricky when you have a jacketed line that doesn't weigh much. It might be polypro; it might be Dyneema. In any case, the cover may be different from the core. You have to look for distinctive jacket colors or get the boatyard to let you dissect the end of the scrap. Sometimes it's impossible. Once it's off the spool, I have no idea how to tell the difference between first-generation gray Dyneema and the latest SK-99. (It doesn't make a difference on control lines, but I'm considering replacing all the wire on our cabin sloop (stays, shrouds, lifelines) with fiber.)
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Re: Rope for various lines

Postby GreenLake » Wed Aug 23, 2017 2:43 am

I totally get the pleasure of "making do".

The rope I use for my jib sheets was a "cutoff" I couldn't resist. The rope ended up having just the right length for the purpose, and it handles reasonably well, but it's unnecessarily bulky and heavy. The DS is not not a lightweight, so a few extra ounces doesn't affect the boat weight so much, as it's hanging off the jib in light airs, distorting it. Would do something different today, but don't feel like swapping out what I got.

Most ropes are ridiculously oversized because the minimal size for the purpose is too small to be handled. Halyards and similar lines can be sized to the load, and, with modern rope, slimmed down without loss of usability. That's where scrounging becomes a challenge.

However, for mission-critical applications where it's not possible to oversize (like standing rigging) I would rather purchase by spec.
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Re: Rope for various lines

Postby GreenLake » Wed Aug 23, 2017 2:48 am

talbot wrote:The blue line in the blocks might be 6mm Marlow 8-Plait-Matt.


2328

Very plausible. The store I purchased the original has change their assortment of ropes over time, so I couldn't fit it again in their online catalog. But if not the exact match, it looks like it's at least a close match for what I got. 4mm would seem to be the size.
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Re: Rope for various lines

Postby talbot » Wed Aug 23, 2017 12:08 pm

4mm? As in 5/32"? You must have hands like steel.
If it's the Marlow polyester line, the breaking strength is only 240kg/529lbs. Even assuming I could grip the line, I would want a bit more strength to account for safe working load and the effects of knots and wear.
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Re: Rope for various lines

Postby GreenLake » Thu Aug 24, 2017 2:25 am

We are talking about the vang? The blue line is in a 6:1 purchase. That is, the force on the line is 1/6th of the force applied to the orange line. The orange line is a 2:1 cascade, so the force on the boom is 2 times the force on the orange line, or 12 times the force on the blue line.

For a short pull, I can manage 40 pounds even on a 4mm line. Usually, I think I pull at most 10-20lbs when I adjust the vang. Say, 50 lbs is the absolute ceiling. Then, the force I apply to the line is 50, the orange line sees 300 (it's 3/16th) and the boom sees 600.

In my estimate, the weakest link is the SS strap on the boom. All the ropes are within spec (for reference, the orange line is Samson ultra-lite, and per their specs it rates 1200 lbs, or 600 after things like knots. They don't say whether they list breaking strength or SWL, so I assume the former. I estimate it gets loaded to 300 at most, which would leave a factor 2).

At 14knots wind pressure is about 1lb/sqft. With a typical DS main, the total wind force on the main should be 100lbs, (factor 4 or) 400lbs if it gusts to 28knots (factor 2). Somewhere along this curve, you end up exceeding the righting force from the crew hiking - that force is the maximum load on the sail because after that, you capsize.

With a beam of 6', and crew weight (fully equipped) of 400lbs, you can get 4' x 400lbs or 1600 lbft. Assume center of effort of the sail at 10', and you can hold a sail force of 160 lbs! I think the actual number is a bit higher, because the boat, when heeling, doesn't do that around its centerline, but I don't think the distance between center of buoyancy and center of mass ever gets to be much larger than 5'. In that case you can hold 200lbs of sail force.

772

Because of vang location and angle, the vang takes higher forces than the main sheet. You loose about 50% from the angle, and another 50% from the distance, so your vang needs to pull 4 times as hard as your mainsheet, assuming center boom sheeting. With a 4:1 mainsheet, pulling 35bs on the free end would get you 150lbs of pull on the center of the boom. With that estimate, you get the same pull on the boom, as with 600 lbs of vang tension (accounting for the fact that the main sheet and vang don't pull in the same direction (it pulls in and down), you need to pull a bit harder on the main to get the boom equally far down, so now you get to 40~45 lbs on the main sheet.

Unlike a vang, you'd hold that continuously, and that's where ratchet blocks come in :)

I don't get to sail my DS at the upper end of the wind range often enough, but I know I can manage to hold the mainsheet in hand when it blows into the 20's. But I use 3/8" and a ratchet block.

So running all these numbers, the vang strength should be just about adequate for the extreme case where the entire sail force counterbalancing the maximally hiking crew is held by the vang only. (In reality, the jib takes a good share).

Let me know if you spot any flaws in the reasoning so far.
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Re: Rope for various lines

Postby talbot » Thu Aug 24, 2017 2:20 pm

Nice analysis. Along the way, It helps explain why a lot of cruisers like the larger O'Days use stern sheeting, because it allows them to handle larger sails with minimal rigging by pulling from the end of the boom. (Or lets you handle the DS main with less than 4:1 purchase.)

As for the size of the vang line, I accept that it's unlikely to fail. I just like my 6mm or 7mm line for its grip.
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Re: Rope for various lines

Postby GreenLake » Thu Aug 24, 2017 4:43 pm

That Marlow rope is easy to get a grip on (not slippery).

While I understand that for jib and mainsheet there is a good reason to size them for their feel in the hand, I think that same is not true for most of the other lines on the boat, and there's an unexamined tendency to oversize them needlessly.

Maybe I'm doing it wrong, but I don't adjust my vang at every gust. I'm also not immune to this oversizing tendency, case it point are the lines I use for the jib-sheet inhaul. They pull at right angles to the sheet, so their max load is, what, 1/10th of the load on the sheet; I think used 1/4", 1/8th or 3mm would have been sufficient with no loss in terms of sufficient working load or ability to adjust them.

I bought a 15' dinghy off someone who had rigged it with 3mm for the halyards. Dyneema/spectra core, so not stretchy, and a grippy cover, so no need for tails. Totally no problem, although it looks like a gossamer thread that should fail at any moment :)
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Re: Rope for various lines

Postby GreenLake » Thu Apr 18, 2019 4:36 pm

looking through this thread, I can't find whether I made mention of replacing my jib and main halyards by Dyneema (with spliced on braided line tails for handling). The instructions for that are on L36.com. I thought I had discussed that in more detail somewhere, if I can find it (or find the earlier post that I thought should have been in this thread, I'll post a link here).
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