Anchoring Protocol revised

[thinwater]

Originally Posted by Don C L

I've actually been curious about the math on that and how to calculate the force required to pull a given chain at a given length and angle up to taught. Anybody have a link to the formula?

https://www.spaceagecontrol.com/calccabl.htm.

"Last link on the bottom" is easily found by doubling the length of the cable. Obvious.

You can Google Wiki and find the actual equation.
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There are a bunch of variables that effect the minimum scope. Banging on specific numbers is just noise.
* Depth of water. The mass of chain is actually more important than the actual scope in determining shock absorption. The math will show this.
* The grade of chain. Higher grade chain is typically speced lighter, so you need just a little more. Not much.
* Wind speed. Some never see high winds. Some think 60 knot squalls are normal. It's best not to presume. At high wind speeds some of these people will see peak loads up to a ton. That will get ANY chain pretty straight.
* Bottom. Some bottoms resist soil liquification better than others. Start hobby horsing at short scope in shallow water with a soft mud bottom and you are out of here.
* Anchor type. Some withstand up-lift better than others. At 3:1 true angle (if you used rope) most anchors are down to about 20-25% holding capacity. Most of what you read about short scope ability ASSUMES the chain is holding the shank down. re-test with rope and the answer changes.
* Does the boat sit still? Yawing can be very bad for holding. Will there be swell? A nice swell can really pick up the chain.

Just too many variables to generalize. The correct answer can vary from 15:1 to 2:1.

[Simi 60]

Originally Posted by rabbi

I measure chain length between anchor and the point where the bridle is attached to the chain. Under heavy load this point is around or just under the surface.

My snubber is just under the surface as well.

. So 1:3 in 3 meters the bridle is attached at the 10m mark.

And if measuring from sea floor to deck I would be closer to 18m
I have rope markers in the chain at 25m, 50, 75m intervals so 25m would be us

[StuM]

Originally Posted by Don C L

I've actually been curious about the math on that and how to calculate the force required to pull a given chain at a given length and angle up to taught. Anybody have a link to the formula?

Fixed it for ya


You may find this interesting


Catenary Freeware


If you google "catenary calculations", you will find lots of explanations of the math behind it.

[Mike OReilly]

Ok, so now that we’ve sorted out that rabbi is laying out about the same amount of rode, here’s a question, or query:

When you set and then dig in your anchor, how taut is your rode? When I dig in the anchor we always aim for a sustained pull such that out rode (all 3/8” chain) is fully taut, rock hard. We’re simulating a blow that fully stretches out the rode and keeps it there. This is why I say anchoring is a tactile experience; I can feel how the anchor and rode are behaving at this point. Is this normal practice for others?

I would not expect any reasonably sized anchor to hold at 3:1 (from anchor roller to bottom) under this pull.

In my experience, my rode rarely stretches out fully. Even with full gale conditions, our rode at ~7:1 rarely straightens out for long. It will tighten up, then catenary and snubber bounce will act to lighten the load. I’ve sustained 60 knots with this effect. I have yet to feel a full hurricane. I assume as some point it would become constantly ‘bone hard’, but that is storm conditions, and I’d pursue different approaches to anchoring if I could.

[Jim Cate]

FWIW: Folks keep saying their chain is "bone hard" or completely straight all the way to the anchor and the likes. I have to ask, how do you know that? All you can see is the very last bit of the chain, even in pretty clear water. The only real data your eye gets is the angle of the rode as it enters the water, and that changes VERY little as the last part of the catenary is diminished and the end of the chain lifts from the bottom. If using a snubber that attaches below the water's surface, it is even worse, for the snubber stretches as the load varies and its angle of entry shifts. It is all very subjective and IMO there is no way you can tell from the boat that the chain has lifted all the way to the anchor.
.
My experience of diving and watching our chain showed me that even in sustained 20 knots with gusts to ~25 there is quite a lot of chain lying on the bottom (10 mm chain in 20-30 feet depths, ~ 5:1 scope. Yet, standing on the bow the chain looks quite taught, and the angle of entry to the water isn't changing visibly in the puffs. Easy to draw the wrong conclusions...

Jim

[Don C L]

Originally Posted by StuM

Fixed it for ya


You may find this interesting


Catenary Freeware


If you google "catenary calculations", you will find lots of explanations of the math behind it.

I wish I could have blamed that on autocorrect!

What a great link, thanks!
I have looked up catenary calculations but didn't find any specific to what we are talking about yet.

[thinwater]

Originally Posted by Jim Cate

FWIW: Folks keep saying their chain is "bone hard" or completely straight all the way to the anchor and the likes. I have to ask, how do you know that? All you can see is the very last bit of the chain, even in pretty clear water. The only real data your eye gets is the angle of the rode as it enters the water, and that changes VERY little as the last part of the catenary is diminished and the end of the chain lifts from the bottom. If using a snubber that attaches below the water's surface, it is even worse, for the snubber stretches as the load varies and its angle of entry shifts. It is all very subjective and IMO there is no way you can tell from the boat that the chain has lifted all the way to the anchor.
.
My experience of diving and watching our chain showed me that even in sustained 20 knots with gusts to ~25 there is quite a lot of chain lying on the bottom (10 mm chain in 20-30 feet depths, ~ 5:1 scope. Yet, standing on the bow the chain looks quite taught, and the angle of entry to the water isn't changing visibly in the puffs. Easy to draw the wrong conclusions...

Jim

This is actually not complicated.
1. Determine the rode tension from wind load. Either a load cell or assume about 1/4 of the ABYC working load for your boat.
2. Plug the numbers into a catanary calculator.


It is also not difficult to estimate reverse engine thrust. There are rules of thumb or you can measure it at the dock. Although this varies a LOT with the boat, WOT is typically equivalent to 30-45 knots, depending on the engine and prop.



I've dove on anchors that were being monitored (load cell) and confirmed that theory and observation are a close match.


20-some knots is just breezy. 60 knots has 10 times the force.


This also depends on the depth. I know that my boat will lift the chain off the bottom at about 20 knots at 7:1 scope in 5 feet of water, for example. I've watched it. By 60 knots it will be nearly indistinguishable from a straight line. Check the math (1500-2000 pounds tension, 70' rode, 1/4-inch chain). 20 knots is more like 275 pounds. I see that all the time.


And see, this is why answers vary. I've anchored thousands of times, but never in over 12 feet of water. Never needed to. I consider 6 feet normal.

[Don C L]

Originally Posted by Mike OReilly

This is why I say anchoring is a tactile experience; I can feel how the anchor and rode are behaving at this point. Is this normal practice for others?

Definitely tactile for me too, but I have a much smaller boat. However since I am setting bow and stern anchor it is much easier to test both by pulling them against each other, it is fairly easy to feel which one is not setting.

[Jim Cate]

And see, this is why answers vary. I've anchored thousands of times, but never in over 12 feet of water. Never needed to. I consider 6 feet normal.

Your experience is, compared to most cruising yachts, unusual. In contrast, I've anchored thousands of times too, possibly anchored in anger more times than you, and I can count the times in less than 12 feet of water on, well, maybe two or three hands. There's Canoe bay in Fortescue harbour in Tasmania where such depths are all there are, but the protection is great. There have been a few times on sand areas behind the reef near Tenia Island in New Cal in such depths, but only for day anchorage. And maybe a few I've forgotten, but for us, and for most world cruising folks in monohulls, >15 feet is minimal and 20-30 our targeted depth when we can choose, and it isn't unusual to be in 40-60 feet.

I don't have access to load cells and other test equipment that might be interesting, and so I have not real life data on rode loads. That does not change the observations that I have made, watching the chain move about while our own boat is tugging on the end, and that is what I was commenting on. I do realize the superlinear rise in loads with increase in wind speed, and have never been brave (foolish?) enough to be in the water in 60 knot conditions. I agree that the chain might well have lifted and the catenary flattened substantially in those conditions...no argument there!

What I was commenting on is the assertion(s) of folks that their chain was "bar taught" whilst only observing the upper end of it, and in fairly low wind speeds.

Jim

[Don C L]

Originally Posted by thinwater

I've anchored thousands of times, but never in over 12 feet of water. Never needed to. I consider 6 feet normal.

In 6' of water I'd be saying "yikes!" and if it is high tide, double "yikes!"

[conachair]

Originally Posted by Don C L

I've actually been curious about the math on that and how to calculate the force required to pull a given chain at a given length and angle up to taught. Anybody have a link to the formula?

Some here - An Analysis of Anchor Rode Length versus Depth

And scope against water depth i had a play around with here - https://www.desmos.com/calculator/4a3u2b8jjo


For 10mm chain a pretty close aproximation is 2 x depth +22m of chain to lift the last link with 250kg horizontal force.

Just one aspect of a very complex system of course.

[rabbi]

Originally Posted by StuM

But angle of pull only changes when the last link lifts.

Until the last link lifts, the angle of pull is horizontal. Once it lifts, the angle of pull starts to change.

Yep. But that would still be an almost exactly horizontal pull. Only as the chain catenary is straightened out the angle of pull starts to change.
This requires way more force than just lifting the last link.



Experiment: take out 5m of chain and pull hard enough by hand to lift the last link. Entirely possible. But I doubt a normal humand can straighten out the chain to the point wherethere is no noticable catenary.

[rabbi]

Originally Posted by Mike OReilly

Ok, so now that we’ve sorted out that rabbi is laying out about the same amount of rode, here’s a question, or query:

jaja. sorry for confusion

Originally Posted by Mike OReilly

When you set and then dig in your anchor, how taut is your rode? When I dig in the anchor we always aim for a sustained pull such that out rode (all 3/8” chain) is fully taut, rock hard. We’re simulating a blow that fully stretches out the rode and keeps it there. This is why I say anchoring is a tactile experience; I can feel how the anchor and rode are behaving at this point. Is this normal practice for others?

We drop the anchor at (our) 1:3 scope. That is bridle attached at 10m in 3m of water.
Let the wind push us back, then go in reverse for a few moments at 1000rpm, increasing throttle by some 500rpm every 30sec or so up to 2500RPM.
Wife sits at the bow and "feels" the chain, if there is vibration when the anchor sets this sometimes indicates we have just hooked a rock.
I watch the surrounding to see if we are moving. At the same time I feel if the anchor slips or just digs deeper. The boat moves slightly differently.

At 2000-2500rpm I set the anchor app (which uses direction & distance to anchor), watch it a little while and then do a 30sec pull at WOT.
Of course with the rudder midships.
If the anchor holds well, we may add some more scope depending on distances to rocks or neighbors. If we expect bad weather we use more scope to be on the safe side.


The chain looks straight during the maneuvre but I know its not. Just get into the water and let the wife do the backing and you see what I mean.

[conachair]

Originally Posted by rabbi

Interesting plot.
Even more interesting would be a plot showing the angle of pull depending on chain length, water depth, and force

Because its not really important if the last ink lifts half a milimeter. Only the angle of pull is important.

Beyond my meager maths and negligible programming skills I'm afraid

And a very complex thing to look at, with energy being proportional to the square of speed concentrating on keeping the boat slow with riding sails/drogue or bucket off the chain / very good snubber all might well help more than some more scope. I've found usually the peak chain loads come from the boat getting yanked to a stop after sheering about in the wind or a huge gust from a slightly different direction rather than the sustained wind speed. The the sawing action from waves as well which a good snubber should probably take care of . Hopefully...

ISTR on a manufacturers site above 8:1 or so there's little to be gained regards chain angle to the hook.

[rabbi]

Originally Posted by StuM

But angle of pull only changes when the last link lifts.

Until the last link lifts, the angle of pull is horizontal. Once it lifts, the angle of pull starts to change.

Agreed. But the angle of pull changes very little with increasing force.

If one needs say 1to to lift the last link of 60m chain in 20m water, my gut feeling says its more like 3to or 4to to make straighten out the chain to produce a change in pull angle of 15 degree.

No clue about the detailed maths but the plots seem to support this gut feeling as the last few meters of chain are always horizontal in the plots. Maybe not exactly horizontal but good enough.