Kedge/Stern Rode Sizing

[Muaddib1116]

How should you size your rode for your stern/kedge anchor? Should it be the same strength as your primary anchor rode? or can you size it down?

[thinwater]

That's not going to have one answer.

Generally, yes.

However, many use a Fortress (very light) for their kedge, which doubles as a soft mud bower and back-up bower. In this case, it would be the same strength (but lighter because it's a Fortress).

The secondary rode does not need to be chain, since it won't swing and chain is a bastard to row out. Very little chain, in fact. In fact, I went to a Dyneema leader with a chafe guard on my kedge, because it was easier to handle (I was settling it a lot in the course of anchor testing trials). Really easy to row out and recover.

[tkeithlu]

It doesn't have one answer. If your bow anchor faces you to the wind/current/waves, and the stern is just stabilizing, it can of course be lighter. But, consider a strong and changing tide. When it reverses, your stern takes the load. Your stern (lots of boats) isn't as well designed to take that load as is the sharp entry of your bow, so the drag is greater. You also forgot to tie down your tiller/wheel, and now get an almighty deflecting force from it as the rudder swings to it's stop on one side. Your stern anchor then drags, and soon you are sitting on the bow trying to untangle the two rodes while making really unfortunate comments about your stern anchor's sexual proclivites. That, or the slack bow rode goes into your propeller.

But then, You don't really have a good windlass mounted on the swim platform, so you best follow Thinwater's advice and stick to a rope rode, and use a stern anchor when you do not have reversing situations. We use the smaller of two fortresses, rope, and 25' of chain, which fits in the dinghy OK. We don't do it very often, usually only when anchoring in a narrow river.

[Muaddib1116]

Thanks, I was thinking along similar lines, 3 strand with 30ft-50ft of chain. I had been thinking I could use one of my electric sheet winches to get the line in, but I'd still need to pull the chain and anchor in by hand, so I figured lighter was better.

Using G70 chain and New England Ropes 3 strand nylon, I think 1/4"chain would match with 5/8" line which has 11,650lb average strength. and 5/16" G70 chain paired with 3/4" line would have an average strength of 17,150lbs. To actually match the strength of my primary anchor rode, I'd have to go up to 3/8" chain and 1" 3 strand.

What I'm trying to decide is what type of strength I need.

[tkeithlu]

What's your dead weight? Ours is 65,000 lbs; we use 5/16 G40 and 3/4" three strand nylon rodes. My guess is that your main rode needs to be 3/8" G40 or 70, and that you could do your stern with 5/16" chain and 3/4" rope. How much you put out really does make a difference, because it is a sudden jerk that breaks something (waves, not your brother-in-law). While nylon stretches, and we use a nylon snubber to add some shock aborption with our chain rode, 150' of chain on the bottom also supplies shock absorption.

[Muaddib1116]

My boat is a 50ft pilothouse sailboat that is about 40,000lbs. It came with 100 meters of 10mm (~3/8") duplex stainless chain which has a UBS of 23,380lbs. This is a good deal higher than G40 (16,200)(3xWLL) but lower than G70 (26,400)(4xWLL). I got those numbers from 1stchainsupply, but their G70 isn't galvanized. West Marine has galvanized G70 chain, but they list the breaking strength at 3xWLL. I don't know if they are wrong, or if the galvanizing reduces the strength of the heat treated G70 chain.

I'm still trying to figure out which is the right strength rating, but if 1stchainsupply is right, then G70 5/16" chain will provide at least as much strength as G40 3/8" chain, and I will probably lean towards going with 5/16 chain and 3/4 rope.

[thinwater]

Originally Posted by Muaddib1116

Thanks, I was thinking along similar lines, 3 strand with 30ft-50ft of chain. I had been thinking I could use one of my electric sheet winches to get the line in, but I'd still need to pull the chain and anchor in by hand, so I figured lighter was better.

Using G70 chain and New England Ropes 3 strand nylon, I think 1/4"chain would match with 5/8" line which has 11,650lb average strength. and 5/16" G70 chain paired with 3/4" line would have an average strength of 17,150lbs. To actually match the strength of my primary anchor rode, I'd have to go up to 3/8" chain and 1" 3 strand.

What I'm trying to decide is what type of strength I need.

I'm not sure splicing G70 chain to rope works and I have seen no test data on it. You will have trouble properly fitting the strands of 5/8" through the links, and because of the small link size, the splice may be weaker. You could use a thimble, but finding shackles to match G70 is also problematic. You probably want to use G43.


I would like to see data on splicing G70 to matching rope. All the data I have seen is G43, which is one size larger.

[Muaddib1116]

Since there wouldn't be a windlass involved, I was thinking of using a thimble and shackle. Shouldn't a 3/8" shackle fit 5/16" chain? I know that a 7/16" shackle fits 3/8" chain. A Crosby G209A 3/8" shackle has a breaking strength of ~20,000lbs, which would exceed the 5/16" G70 and 3/4" 3 strand.

[rslifkin]

I wouldn't necessarily try to match line to chain based on breaking strength, but on working load instead. So 3/4" line would work well with 5/16" G43, as the chain has a 3900 lb WLL and the line is around 3400 lbs (assuming 20% of 17k lbs breaking strength for 3/4" 8 plait).


On the shackle topic, yes, a 3/8" G209A fits 5/16" chain. I'm running that same shackle with 5/16" G43 on my setup.

[thinwater]

Originally Posted by rslifkin

I wouldn't necessarily try to match line to chain based on breaking strength, but on working load instead. So 3/4" line would work well with 5/16" G43, as the chain has a 3900 lb WLL and the line is around 3400 lbs (assuming 20% of 17k lbs breaking strength for 3/4" 8 plait).


On the shackle topic, yes, a 3/8" G209A fits 5/16" chain. I'm running that same shackle with 5/16" G43 on my setup.

^^ This too. The working load of nylon is 8-12% of BS, depending on who you ask and the exact use.



However, it is a theoretical rather than practical problem, because a rope rode will have much lower working load than an all-chain rode due to stretch. No shocks.

[Muaddib1116]

Maybe I'm missing something, but I don't really understand matching WLL's rather than breaking strength. It seems to me that every manufacturer/brand of various different components sets their own standard for WLL, whether its 2x, 3x, 4x, or 5x. I understand their desire to say something about WLL, it gives the consumer and manufacturer some protection. But if you are matching component strengths, it really only makes sense, to me, to use ultimate breaking strength.

[Don C L]

As to rode sizing, I guess it depends on where you plan to be anchoring. At least where I am most of us are always anchored bow and stern and occasionally there will be a shift in wind or current that will put the stern under an equivalent amount of stress. In my own case I have equal strength rodes, but I have a smaller boat and I don't need a windlass. And I never row out the stern hook, I always lay out double what I need on bow or stern and then move up or fall back to set the other hook.

[thinwater]

Originally Posted by Muaddib1116

Maybe I'm missing something, but I don't really understand matching WLL's rather than breaking strength. It seems to me that every manufacturer/brand of various different components sets their own standard for WLL, whether its 2x, 3x, 4x, or 5x. I understand their desire to say something about WLL, it gives the consumer and manufacturer some protection. But if you are matching component strengths, it really only makes sense, to me, to use ultimate breaking strength.

Think about when things break. It is NOT when they are brand new. Yes, standard engineering practice is to design based on WLL. BS is mentioned primarily because it is easy to test. WLL is the thing that matters. That, and understanding the range of working load and how it is applied (steady vs. part load cycling vs. slack-to-load cycling).

Ropes and chains do not fail at their BS, they fail at much less after long, long use. The BS is the value when brand new. The WLL is the actual load you can apply, over and over again, without failure. The ratio of WLL to BS ranges from 3.5:1 (non-critical) to >20:1 (suspending people). The ratio of BW to WLL also depends on the material; some materials can withstand repeated cycling better than others (polyester vs. nylon have different ratios). The extent of QC also matters. Some BS are given as average, some are 6 sigma minimums.

Stating a WLL is NOT about giving a safety factor or feeling being conservative. It is where design starts.

[Muaddib1116]

Yeah, I understand the theory, but it's the implementation that I question, which is why I have been thinking breaking strength is a more reliable number to use. Either WLL is just a factor of safety calculation based on the breaking strength data (which you say it is not), or it is determined doing an array of statistically significant testing. Having worked for a large aerospace company, a small family owned medical device company, and trying to start my own company, I can say there is a big range of what a manufacturer might be able to do or think is possible in regards to testing. Without knowing the details of how their WLL number was determined, a manufacturer with even basically good quality control can pretty easily do random destructive sampling and have a meaningful breaking strength number.

When I see that a Crosby shackle has a WLL, a Proof Load of 2x, and a UBS of 5x, and given Crosby's place in the market, I have some faith that they've done the work to make those numbers mean something. Then I look at the specs for my duplex anchor chain, made by Ketten-Walder a well respected German manufacturer of lifting equipment, where they have a WLL of 11,690lbs and a UBS of 23,380lbs for a factor of 2x. This is a very different value, but from everything I've heard, Ketten-Walder also knows what they're doing.

Mantus has duplex shackles, they're the only supplier of them that I've seen. Do they manufacture those themselves? or are they made to contract by somebody else? They use a 4x WLL to UBS. Did they do a full test protocol to determine that? Did their contract manufacturer? or did they do enough tests to failure to determine a reliable breaking strength and then divide that by 4 for the WLL as a matter of course?

1stchainsupply states that G40 chain has a 3x WLL to UBS factor, and all other chain they offer is 4x. New England Ropes has a higher tensile strength on their ropes than the same size offered by Samson, but neither offers a WLL just an Average Strength.

[Muaddib1116]

Another thing that sort of relates to this, Lewmar gypsies for 3/8" or 10mm chain fit a max rope diameter of 5/8", which has an average strength very close to G30 chain's UBS, but much less than 3/8" G40. How was this pairing determined? Was this merely a factor of the chain dimensions, which restricts the maximum rope grabber/slot size they could fit? Or were these gypsy sizes all designed before people started using G40 chain?

If it's the latter, then it seems to me they were basically doing the same as I was, matching the average strength of the rope to the UBS of the chain.