Anchoring System Sizing vs. Windlass Question

dlochner · May 4, 2021

The gypsy should/must match the chain or it will not grab the chain correctly.

The windlass should never have an anchoring load on it. With a chain/rope rode the rode should be secured to a cleat. With an all chain rode a snubber should be attached and the load secured to a cleat via the snubber and backed up with a chain stopper if the snubber should fail.

Kings Gambit · May 4, 2021

If we’re asking about anchor system “failure”’, that would be that the chain or rode parts, anchor breaks, bends, or pulls out, or a deck cleat breaks out. (The windlass should not be in the force line; use a chain snubber to transfer load to a deck cleat.) The actual minimum tensile strength of 9/16” 3-strand twisted nylon rope is pretty high, ca.7,000 lb. The safe working load is far less, around 600 or 700 lb. I would guess that many, if not most, boats have anchors that are either too small for much adventure or too poorly set generally, that they would pull free long before 9/16” 3-strand would reach its ultimate breaking strength. My system on the Bavaria 38E is similar to your system, a 45# Plowmaster (a bit oversized for my LOA), 5/16” chain (SS), and 9/16” nylon 3-strand. So far, I’ve not had the excitement of holding in more than 30 kt, but I’m “comfortable” with my system if that comes. We have done a lot of anchoring around Southern California.

thinwater · May 5, 2021

The loads described in the table (from ABYC) are for all-chain and no snubber. The rope can be lighter because the worst case snatch load with rope rode is MUCH lower than with all chain. The actual wind load in 48 knots is probably closer to 500 pounds. The rest is surging.

This sort of "undersizing" is normal.

NEVER deploy less than 35 feet of rope. Too much of the stretch work will focus on the rope and it may overload. This has happened, when someone deployed, say, 75' of chain and only 10' of rope. It needs to be either all chain with a snubber, or a considerable portion of rope.

tfox2069 · May 5, 2021

I think I understand what the OP is asking here - and he's not suggesting he put the strain on his windlass. He's wondering if when tables indicate the chain and rope rode capacity for the windlass (e.g. the largest sizes it will take and the load capacity for them) is exceeded by the loads in some use cases (40 knot winds) whether that would dictate switching to a windlass that would allow for larger rode sizes (particularly for rope rode). So, for example, if the windlass can only accept rope rode with an 1,800 lb breaking strength, and the boat exerts 2,500 lbs in a 40 kt thunderstorm, should he upgrade the windlass to accept larger rope rode (I made those numbers up)?

I think you're planning on the rope rode failing at its working load limit, and while conservative, is probably unnecessarily so - assuming you've assembled your rode correctly (e.g. without creating a greater than necessary weakpoint by doing something like tying a knot in it rather than splicing it).

The tensile strength (breaking point) of 9/16" New England Rope 3-strand nylon is 8,800 lbs (when new). The working load is roughly 20% of that, or 1,760 lbs. That leaves a lot of margin for higher than normal loads before reaching breaking point, and stressing it occasionally (a few times a year in high winds in a thunderstorm) should be fine. Inspect it regularly for strain, and replace as necessary.

If you are expecting a particularly bad blow, and have enough warning, consider putting out a second anchor to share the load. I've been in sudden 52kt thunderstorm winds, and expected 35-45kt winds overnite at anchor here in the Chesapeake bay with no issue.

The best thing I did was to add a riding sail to keep the boat pointed into the wind and avoid the huge load increase (and sideways anchor pull) when the boat was sailing (sometimes violently) at anchor. Swinging was decreased by roughly 80%, resulting in a much more consistent load and directional pull on my ground tackle in high winds.

dlochner · May 5, 2021

tfox2069 said:
think I understand what the OP is asking here - and he's not suggesting he put the strain on his windlass. He's wondering if when tables indicate the chain and rope rode capacity for the windlass (e.g. the largest sizes it will take and the load capacity for them) is exceeded by the loads in some use cases (40 knot winds) whether that would dictate switching to a windlass that would allow for larger rode sizes (particularly for rope rode).

Windlass size should be determined by the weight it needs to lift. Some manufacturers, like Maxwell, provide different sized gypsies to accommodate different sized chain.

Language and terminology are important.

shemandr · May 5, 2021

As usual I'm out of my element here but I think the term Safe Working Load Or Working Load are derived numbers based on a practice in Engineering to have an 80% safety margin in a system to account for shock loads or other unpredictable forces. In what little metallurgy I've studied there was another term, Yield Strength, which denoted at what load a component would deform but not break. Think of a bent anchor shaft. So what? So, the Yield Strength is the point at which the component has started to fail. It would be a much better indicator of how much load can be tolerated by chain, 3 strand nylon, Dacron weave, SS wire, etc. but I don't think these numbers are commonly published. Interestingly, when the Yield Strength is close to the Breaking Strength you have a component that will fail suddenly - like a Carbon Fiber spinnaker pole. Like I said, I'm out of my element but I hope this post can stimulate more discussion about what failure actually means.

tfox2069 · May 5, 2021

dlochner said:
Windlass size should be determined by the weight it needs to lift. Some manufacturers, like Maxwell, provide different sized gypsies to accommodate different sized chain.

Language and terminology are important.

Or change your gypsy to allow for larger size chain and rope rode. Excellent point dlochner.

Stu Jackson · May 5, 2021

tfox2069 said:
I think I understand what the OP is asking here - and he's not suggesting he put the strain on his windlass. He's wondering if when tables indicate the chain and rope rode capacity for the windlass (e.g. the largest sizes it will take and the load capacity for them) is exceeded by the loads in some use cases (40 knot winds) whether that would dictate switching to a windlass that would allow for larger rode sizes (particularly for rope rode). So, for example, if the windlass can only accept rope rode with an 1,800 lb breaking strength, and the boat exerts 2,500 lbs in a 40 kt thunderstorm, should he upgrade the windlass to accept larger rope rode (I made those numbers up)?

and

duck21 said:
My question: based on those standards is anyone who regularly anchors out and uses a standard windlass friendly rope + chain as their rode under-sizing their system?

tfox ALMOST nailed duck's question. It's not so much switching the windlass, but rather "Is one component of the system," i.e., the rope part of the rode, not strong enough to meet the load.

duck's OP questioned the tables in the link that he posted. I have posted that link many times. In the text of that link, I wrote:

" 4. Actual forces: I have been suggesting that handy reference sources be used, like Calder's Cruising Handbook. The photos below are the three important tables from that book for sizing anchoring SYSTEMS. You can wander on down to your local WM store or any handy nautical book chandlery and read the rest of the two pages on how to use the tables, but they're pretty self explanatory."

If we put aside the "self explanatory" end of the last sentence, please do what I suggested, and go read the text of Calder's book, or any other text on the issue, including any Anchoring BOOK. It will explain the meanings of WLL and SWL, and how to apply them.

Another answer to go UP to the next size of line.

thinwater · May 5, 2021

We are missing the point:

The load with rope rode is MUCH lower than the worse case possible with chain. It stretches, whereas chain can snatch in shallow water. The ABYC table is NOT an estimate of load, it is a design handbook value intended to be used with all-chain systems. I strongly suggest that before quoting ABYC table values as actual rode tensions estimates that you measure the actual load at a know wind speed and then extrapolate (Load ~ V^2).

The thunderstorm load on the OP's boat is not 2,500 pounds, it is more like 700-900 pounds at 60 knots with a rope rode. I've stood in the rain measuring this. This is why rope traditional recomendations are not as fat as ABYC would seem to recommend. Follow traditional recommendations. They have proven safe.

In general, steel parts have a fatigue limit of about 50% of tensile strength (varies) and nylon does not have a specific limit, but life is dramatically shortened about about 10%.

thinwater · May 5, 2021

duck21 said:
Hi all. Thanks for the good discussion! The main point of my goal has been met--start a conversation.

A couple of clarifications on my original post:

I'm not really questioning my own system/planning on making changes in any significant way. As thinwater noted, traditional recommendations generally seem to work, and I plan on sticking with them. I'm just looking at the individual parts of the system and, for my own edification, looking at weaknesses in the system. Based on the tables, it felt that the rope was a weak point, but also a limit based on what appear to be default components of the system (I.E. the windlass that came with my boat doesn't appear to have additional gypsy options and, yet, was determined to be appropriately sized for the vessel by the manufacturer (knowing that this was the option when the boat was originally outfitted)).

I'm also not questioning the validity of the tables or questioning the definitions of the terminology. Just asking how others apply that knowledge to their choices in anchoring systems given various constraints.

I DO have additional (and slightly more practical) questions: how does one determine when anchor rope has reached its end of life? Example: the rope that came with my boat doesn't have any snags or obvious wear spots, it seems fairly pliable (as pliable as most 3 strand that I've seen), but it is of indeterminate age. PO didn't anchor much, so it's mostly sat in the anchor locker. What are the biggest factors in wear and tear on anchor rope? How often do others replace their anchor rope?

I've tested climbing ropes that were over 20 years old, but lightly used and stored under both ideal and anchror locker conditions. They lost almost no strength. Same with nylon anchor rope, even if stored in damp conditions; 20 years of non-use makes almost no difference.

The effects of fatigue and UV, on the other hand, are nearly impossible to determine with any accuracy. Dock lines that are 10 years old may look perfect, but testing shows they commonly retain only 25-40% of their original strength. Of course, the surface will look and feel sunburned, something that does not happen to most anchor ropes (they are not in the sun that many hours).

This difficulty has long vexed climbers and sailors. Climbing gyms go by an estimated number of climbs, based on attendance, since UV is a non-factor. Outdoor climbers go by physical abration and lumps; falls on rock tend to abuse ropes. Sailors guess, since anchor rode use varies so much. The only sure way is to extract a strand, test that, and multiply by the number of equivalent strands. If you tie a knot on the end for testing, remember that weakened the yarn by about 50%.

Since it has not been in the sun, is not chafed, and is not likely (unlike a climbing rope) to have seen any huge loads, it's probably fine. You should probably re-make the rope-to-chain splice and probably trim a few links of chain. It's not hard.