vyv_cox
Well-known member
Interesting that 3:1 is now regarded as a short scope. Cruising pioneers the Hiscocks apparently always anchored on 3:1 in the 1950-60s, using anchors of the period.
Catenary?
- Thread starter vyv_cox
- Start date
noelex
Well-known member
Yes, I have noticed that as well. Scopes used for routine conditions are becoming longer despite the improvement in anchor technology. 3:1 has become 4:1 etc.
There does not seem to be same trend for scopes used in strong conditions. These seem to have become shorter. Typically in “the good old days" we would use as much rode as possible in winds that I would now regard as only moderately strong. In conditions where 6 : 1 or 7 : 1 may be deemed appropriate by modern boats, it was not unusual for all the rode to be deployed when I first started cruising.
I think the reason (although this is just speculation) why many people routinely use longer scopes routinely is because modern anchor windlasses make recovering long lengths of chain and heavy anchors easy even for a small crew.
My first yacht had no anchor windlass and the thought of having to pull in long lengths of anchor chain was not appealling. Every meter let out had to heaved back on board.
On the other hand, when stronger wind eventuated dragging was very common even if you used the best available equipment at the time . Not keeping any chain in the locker seemed sensible and with the less crowded anchorages these long scopes did not pose the inconvenience they would in modern times. A very long scope and the deployment of multiple anchors was the only defence and these measures were frequently adopted (whereas these days using multiple anchors to provide adequate holding is very rare). Perhaps the ”good old days” is not the appropriate term
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thinwater
Well-known member
The topic this brings up is Yawing. The wind is oscilating 50-80 degrees and the boat is swinging 50-70 degrees. Unless the poster corrects me, I think this is yawing, not wind oscilation, based on the exacness of the timing. The boat is really dancing.Not a regular here any more so there will be no replies to this post from here.
Breaking from forum tradition here by actually looking & measuring the real world ..
And having a play around with the data in grafana to see if there might be some patterns hiding away, whech there is! 5m average boat speed * 100 & 5m average wind speed correlate pretty well.
Heavy steel 10m long keel cruising boat. Little tidal flow in the anchorages.
Which also matches the physics very nicely, 20Kts exerts 64 X more force & same goes for the increase in kinetic energy when the boat accelerates.
Oscillations as the boat yawed through the wind seemed fairly regular against changes in aws but there might be something of interest in there somewhere.
If anyone fancies a chat pop over to this discord server > Join the Raspberry Pi boat monitoring Discord Server!
Some csv data will get uploaded for a play.
Masses of data is easy to save & peruse for patterns using signalk & influxdb. Runs Linux & windows, not sure about Mac.
- The anchor is seeing tugs from ~ 30 degrees from the average every cycle. That will cost you a little hold, but not much (15-20%) if well set, because the chain is in the muck. (yes, I have tested this--the results vary with the angle of yawing, bottom type, anchor type, and how deeply it is set--that figure is an average--some are a little better and some are worse).
- The rode tension is increased about 20-40% becase of the yawing and because the windage is much greater the bow is 50 degrees off the wind.
There are two things that worry me when anchoring. A bad bottom and yawing. I've never had anchor move unless both of these two were very bad.
thinwater
Well-known member
... I think the reason (although this is just speculation) why many people routinely use longer scopes routinely is because modern anchor windlasses make recovering long lengths of chain and heavy anchors easy even for a small crew ....
That is the biggest reason for the popularity of all-chain as well. If it was not for the electric windlass we'd have a lot more conversation about rope. Chain is now the easier choice instead of the more difficult.
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I will add again that speaking of scope without mentioning depth is only half the story. 3:1 in 25 feet of water places nearly 100 feet of chain in the water. 3:1 in 4 feet of water (yes, 4' is common with my trimaran) is only 20 feet, and I can practically shackle the anchor to the bridle. Very jerky and not sensible. Scope is a valuable metric, but when thinking of catenary, the mass of chain in the water (which includes scope and depth) is nearly more important than the scope, because it is the mass you are lifting. The same with stretch on nylon rodes. I've found that as a rule, I never put out less than 50 feet. It just does not ride right if any chop comes up.
noelex
Well-known member
Very true.
The chain mass does start to have more benefits in deep water. In the deep anchorages of Norway (often around 30m) anchoring at around 2:1 was quite secure for most conditions. You are right depth should be included when discussing scope, but I try (often unsuccessfully) to keep posts simple.
GHA
Well-known member
The quite hasty post did say yawing, seemed pretty obvious from the data fields what was happening, though maybe not to anyone who hadn't spent a while playing with the data. Data is up in discord f anyone wants it. Join the Raspberry Pi boat monitoring Discord Server! Guessing few here would actually even know what Grafana even is....
"The boat is really dancing." - nope, not really. I'd always noticed the yawing with the shadows moving but never paid too much attention. it's very gentle. Plotting data changes everything, stuff you wouldn't expect hidden in the data, I never knew it yawed so much. At a guess this would apply to many. Don't get the data & plot it, so you just don't know.
"The anchor is seeing tugs from ~ 30 degrees from the average every cycle. That will cost you a little hold, but not much (15-20%)"
"The rode tension is increased about 20-40% because of the yawing and because the windage is much greater the bow is 50 degrees off the wind."
For a guess that seems reasonable, but neither in this case. From lots of time watching the snubber , 12m of 10mm nylon 3 strand though some pulleys on deck so it touches nothing apart from some pulleys and a length of dyneema though the bow roller. The chain slowly would straighten just a little bit round about when the bow turned but not enough to overcome the friction in the bow roller. You could gave a bit of a pull manually to overcome the friction & the nylon would stretch maybe 20mm sometimes. But very much next to nothing. The boat felt like it wanted a change before it turned, hard to describe but it wasn't in the least bit jerky. Thinner chain I suspect things would be very different.
"The end result is that holding is probably reduced 40-50%". Sorry, not the case. From lots of observation the tension on the chain varies very little up to 25Kts in this boat in that anchorage. Hard to see how the anchor would even know it was yawing. Maybe slight change in pull, though change in plan view the angle would be small. below zoomed in view of AWA, HDG (minus 200 to get it next to the rest) & GPS speed * 100 Max speed. Max about 0.3Kts though only 1 second sampling time so might be some peaks hidden.
Zooming out, how come the various anchor tests don't record every variable they can possible think of? Though in fairness most were a while ago, simple nowadays with signalk & influxdb/grafana. Seems odd that no effort was put into saving everything they could think of.
And never any mention if the load cells were calibrated (or the vertical load on the chain from gravity which seems to get ignored on the data) , I`ve been using commercial load cells on site for decades, the calibration sheet will always be different & sometimes the factor substantial but never linear.
Next will be a load cell from bust kitchen scale, be hard to get any kind of calibration done in the anchorage but they do seem to have a pretty linear output so hopefully will save some interesting, & maybe even useful, data. What would be just lovely would be a little knee in the plot just as the chain lifted completely off the sea floor, little bit of code should be able to calc that from anchor & boat position/ depth as well, imagine the joy if it tallied up
GHA
Well-known member
Have play > Equal force scope
Think the maths is correct, or close enough to be useful.
So with 200Kg HORIZONTAL load in 2m water, 10mm chain a scope of 10:1 will just lift the last link of chain at the anchor, 10m water more like 4.5:1
Red line is chain, blue line is (20 + 2 * depth of water), matches the 200Kg catenary curve pretty well & quite a handy day to day rule of thumb. Though only looking at catenary, there are so often many other factors going on.
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thinwater
Well-known member
The quite hasty post did say yawing, seemed pretty obvious from the data fields what was happening, though maybe not to anyone who hadn't spent a while playing with the data. Data is up in discord f anyone wants it. Join the Raspberry Pi boat monitoring Discord Server! Guessing few here would actually even know what Grafana even is....
"The boat is really dancing." - nope, not really. I'd always noticed the yawing with the shadows moving but never paid too much attention. it's very gentle. Plotting data changes everything, stuff you wouldn't expect hidden in the data, I never knew it yawed so much. At a guess this would apply to many. Don't get the data & plot it, so you just don't know.
"The anchor is seeing tugs from ~ 30 degrees from the average every cycle. That will cost you a little hold, but not much (15-20%)"
"The rode tension is increased about 20-40% because of the yawing and because the windage is much greater the bow is 50 degrees off the wind."
For a guess that seems reasonable, but neither in this case. From lots of time watching the snubber , 12m of 10mm nylon 3 strand though some pulleys on deck so it touches nothing apart from some pulleys and a length of dyneema though the bow roller. The chain slowly would straighten just a little bit round about when the bow turned but not enough to overcome the friction in the bow roller. You could gave a bit of a pull manually to overcome the friction & the nylon would stretch maybe 20mm sometimes. But very much next to nothing. The boat felt like it wanted a change before it turned, hard to describe but it wasn't in the least bit jerky. Thinner chain I suspect things would be very different.
"The end result is that holding is probably reduced 40-50%". Sorry, not the case. From lots of observation the tension on the chain varies very little up to 25Kts in this boat in that anchorage. Hard to see how the anchor would even know it was yawing. Maybe slight change in pull, though change in plan view the angle would be small. below zoomed in view of AWA, HDG (minus 200 to get it next to the rest) & GPS speed * 100 Max speed. Max about 0.3Kts though only 1 second sampling time so might be some peaks hidden.
Zooming out, how come the various anchor tests don't record every variable they can possible think of? Though in fairness most were a while ago, simple nowadays with signalk & influxdb/grafana. Seems odd that no effort was put into saving everything they could think of.
And never any mention if the load cells were calibrated (or the vertical load on the chain from gravity which seems to get ignored on the data) , I`ve been using commercial load cells on site for decades, the calibration sheet will always be different & sometimes the factor substantial but never linear.
Next will be a load cell from bust kitchen scale, be hard to get any kind of calibration done in the anchorage but they do seem to have a pretty linear output so hopefully will save some interesting, & maybe even useful, data. What would be just lovely would be a little knee in the plot just as the chain lifted completely off the sea floor, little bit of code should be able to calc that from anchor & boat position/ depth as well, imagine the joy if it tallied up
You need to measure rode tension during yawing before you make statements based on looking at the snubber. Really. You siad the changes were minor, and then go on to describe conditions so light the rode isn't pulling across the roller, but then the snubber stretches a little. It sounds to me like my 20-40% figure is not at all wrong. Put a strain gauge on it. The peak strain varies, but can occur anywhere from mid-swing to near the end of the swing. Much depends on whether a gust coninsides with the beam-to-wind. I have done this. When I said it was "dancing" I was referring to range, which is quite broad, not the speed. For compaison, my cat an tri yaw about 1/3 that amount in a steady wind, because they have broad bridles. The speed of yawing (slow) is obvious from the data and did not need to be commented on. For typically yawing, the speed was in the range of normal for a boat that size.
I was not guessing about anything that I said. I have taken measurements of rode tension on multiple boats and I have measure holding capacity of multiple anchors in multiple bottoms while yawing. Hundreds of data points. It's been published, just not here. There is a range of behaviors, but you can Google ships and yawing and find rode tension and holding ranges very similar to those I quoted. It's a big deal and has been extensivly studied.
You say you have not lost 50% of your holding capacity. More likely you just don't know that you have. This is normal and is true off most boats. Google ships.
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Why do anchor tests skip over topics like yawing? Because you get really deep in the woods and anchor testing is difficult, generally un-funded work. You really need to either test anchors, or study the dynamics. Mixing the two gets very complicated, very fast, with too many variables. And then the weather changes. Lets see...
* Anchor brand. Some anchors are far more vulnerable than others.
* Size
* Chain vs rope
* Scope
* Depth (with chain this is nearly as important as scope)
* Depth of set. With a deep set the anchor will not feel it in light conditions because the chain is buried. But it isn't light conditions that matter.
* Fetch
* Bottom type. Liquefaction and consolidation.
* Snubber.
* Bridle. Can practically eliminate yawing on some boats. So can twin anchors.
* Yawing. Both range and frequency matter, and the nature of the bottom (prone to liquefaction?)
* Boat type. This influences speed of yawing and the range of rode tension.
* Stuff on deck. A furled reacher or a dinghy on the bow make a big difference.
* Wind. gusting, yawing, or steady?
Small wonder that testers simplify things. The effect of yawing on holding is very complex and the best you will ever do is general ranges. It could be one thing to day and another tomorrow. Your results will likely be only for your boat, but that's OK.
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The last time I was testing I rigged the boat for variable yawing, from 20 degree range to 120 degree range. The anchor sailed out on the last test, at just 15-20 knots, and the boat headed off on a slow reach (bare poles) and the anchor never reset.
The last time
GHA
Well-known member
Nope, I was there, in 25Kts sustained there was very little increase in chain angle at any time during the yawing. Not enough to have the snubber do anything. In effect, the snubber was superfluous because the dynamic load was so small. Every time, watched it enough. Your overview is just plain wrong compared to the real world on that boat on that day. No idea of the mechanism though. I fit a snubber every time the hook goes down, just to keep in the habit as much as anything. It just doesn`t do anything in those wind speeds.
"Why do anchor tests skip over topics like yawing? " - That wasn't the question asked. It was about the failure to record everything and anything available. Also questioned was no mention of load cell calibration. Vital!! It could over read at 200Kg & under read at 750Kg or the other way round, without recent calibration you won't know anything for sure and won't know how wrong it is. Doesn't ever seem to get mentioned on any tests. Do they just not bother? Pretty major oversight if so.
"I was not guessing about anything that I said. "
Might as well have been, you did not describe reality in the slightest
"You say you have not lost 50% of your holding capacity. More likely you just don't know that you have."
Go on then, how? Chain angle barely changed at any time, how does the anchor know that anything is going on when the rode tension varies so little?
After a lifetime of industrial rigging I did pick up a fair bit of instinct about dynamic loads & getting a feel for loads just by hand. Very little dynamic going on in these winds, whatever the mechanism. If you were actually there you would have no choice but to agree.
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noelex
Well-known member
Thinwater, I agree with much of what you post, but I cannot understand your very significant concern about yawing.
When a boat yaws it exposes the side profile to the prevailing wind, which increases the force compared to the more aerodynamic condition where the boat is aligned with the wind. This is a legitimate concern, but the rise in force is modest. The concept that the yawing exposes the anchor to a significant side load is not accurate, at least at reasonable anchoring depths. I know because I have (perhaps foolishly) dived on anchors in very strong wind.
If the boat yaws say 20° there is an impression that the anchor must also be exposed to side force of 20°. This is not accurate. The side force is related to the lateral displacement of the boat rather than the angle of the bow. In many cases when the bow is significantly blown away the angle of the chain at the anchor end is dead straight or only at very slight angle.
Yawing is someting that should be minimised where possible, but you anchor is not going to unduly concerned if this is not practical, at least if you are in reasonable anchor depths with a reasonable scope (as you are likely to be in bad conditions).
It is true that an anchor will often drag when the boat yaws, but this is because boats yaw in gusts and this higher wind strength is when the force is high and invariably when a drag starts. It has nothing to do with a side force on the anchor. If the anchor does drag the bow will fall away which further creates the impression that the yaw caused the drag when in reality the drag has caused the yaw In many cases.
Owners of small anchors have described their anchor as twitching or shaking in strong wind. This is indeed concerning. This movement is only seen when an anchor is close to its holding limit and something should be done to increase the holding capacity. The anchor will only twitch or move in gusts if it is close to letting go. The answer is more scope a better anchor, a larger anchor etc.
When a boat yaws it exposes the side profile to the prevailing wind, which increases the force compared to the more aerodynamic condition where the boat is aligned with the wind. This is a legitimate concern, but the rise in force is modest. The concept that the yawing exposes the anchor to a significant side load is not accurate, at least at reasonable anchoring depths. I know because I have (perhaps foolishly) dived on anchors in very strong wind.
If the boat yaws say 20° there is an impression that the anchor must also be exposed to side force of 20°. This is not accurate. The side force is related to the lateral displacement of the boat rather than the angle of the bow. In many cases when the bow is significantly blown away the angle of the chain at the anchor end is dead straight or only at very slight angle.
Yawing is someting that should be minimised where possible, but you anchor is not going to unduly concerned if this is not practical, at least if you are in reasonable anchor depths with a reasonable scope (as you are likely to be in bad conditions).
It is true that an anchor will often drag when the boat yaws, but this is because boats yaw in gusts and this higher wind strength is when the force is high and invariably when a drag starts. It has nothing to do with a side force on the anchor. If the anchor does drag the bow will fall away which further creates the impression that the yaw caused the drag when in reality the drag has caused the yaw In many cases.
Owners of small anchors have described their anchor as twitching or shaking in strong wind. This is indeed concerning. This movement is only seen when an anchor is close to its holding limit and something should be done to increase the holding capacity. The anchor will only twitch or move in gusts if it is close to letting go. The answer is more scope a better anchor, a larger anchor etc.
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thinwater
Well-known member
True about the angle the anchor was exposed to. It can be very different or the same. You need to measure both.
Guys, get out the gauges and do the measurements. There is no progress in discussing qualitative observations. Go on google and search ship yawing and anchors. Remember, I collected data. I read the studies of others, including maritime studies re. ships. The results on your boat may be different, but this is well accepted in maritime circles.
Obviously anchors only move near the holding limit. GHA reported yawing in steady winds (no significant gusts). Yes, an answer is always more scope, more chain bigger anchor ... and part of the answer is often reducing yawing. It's all part of smart anchoring. Rigging methods and technique can be nearly as important as the ground tackle.
Guys, get out the gauges and do the measurements. There is no progress in discussing qualitative observations. Go on google and search ship yawing and anchors. Remember, I collected data. I read the studies of others, including maritime studies re. ships. The results on your boat may be different, but this is well accepted in maritime circles.
Obviously anchors only move near the holding limit. GHA reported yawing in steady winds (no significant gusts). Yes, an answer is always more scope, more chain bigger anchor ... and part of the answer is often reducing yawing. It's all part of smart anchoring. Rigging methods and technique can be nearly as important as the ground tackle.
KLBH
Member
Vyv, you say you reverse at 2500 rpm to give you the same pull on the anchor as wind force 6. Presumably the engine revs required to produce this force is dependent on the size / power of the engine, am I correct thinking this.
For example a boat with a 40hp engine would have less pull on the anchor than a boat with 110hp if both were running at 2500 rpm?
westernman
Well-known member
How many revs do you need with your engine to make headway against a given wind?
I know I have been in situations where I struggle to make any headway at all at full throttle.
This would imply that I would need to back down at full throttle to ensure that anchor would hold in those conditions.
(Ideally you measure this reversing while into the wind, but that is likely to be very unpleasant and wet when trying to do it into a gale).
Tranona
Well-known member
The load required is in the order of 250-300kgs which is roughly what you can expect with a 30hp engine on a 34' yacht. It is also roughly the load generated by 25-30 knots of wind on a similar size boat. So you would expect the anchor to dig in similar amount after power setting. Of course a more powerful engine would achieve the same at much lower rpm. The point of posting is to demonstrate that catenary is largely lost at relatively low wind speeds, confirmed by other examples such as the Canope video exploring the nature of catenary
Pretty sure I explained that in an earlier post on this thread but probably lost in all the other issues that are raised.
KLBH
Member
Tranona, yes I get the catenary point, it's just that I hear so many people saying that you need to reverse at 2500 rpm, when it seems to me it is relative to engine power.
geem
Well-known member
I was having the discussion with regard to yawning with a friends who has been at anchor for a couple of days with a stiff breeze. His boat yaws. We talked about riding sails. He lifted his dinghy up and strapped it to the transom and the yawing stopped. Just that extra bit of windage in the stern of the boat solved the problem.
geem
Well-known member
It's different for every boat. We don't even rev to 2500rpm!
We do have 86hp available and a large prop so we can exert a lot of force
Tranona
Well-known member
That is because for most small to mid range yachts with 20 say 40 to 40hp engines that is the sort of revs you need to generate that pull. People like rules of thumb which mostly like this have some underlying theoretical basis. In reality you know you have hit the point when the boat stops with a jerk (particularly if you whack the throttle open) and the boat does not move. There are individual variations boat by boat. For example my boat has a different pitch forward and reverse - deliberately - to get a more progressive increase in thrust so needs more revs. On the other hand my previous boat had the same pitch forward and reverse but a lower reduction (2:1 rather than 2.65:1) in reverse so came in with a bang and would only rev to 1500, but still provide good pull.
Life can get too complicated if you dig too deep, but on the other hand sometimes the knowledge enables you to refine your approach.
thinwater
Well-known member
Exactly. Little adjustments can make a big difference.
The reason many boats begin to yaw in the gusts is not because of the gust per se, but because that iw when the chain lifts, and chain on the bottom damps yawing in light winds. Logically, the imbalance is not actually wind speed dependant.
Many things can help:
- Bridle. This is the main thing for multis, not as much for monos.
- Move the dinghy off the bow.
- Drop sails on bowsprits.
- Anything that adds windage aft. Anything that reduces windage forward.
- Anything that increases lateral plane forward. Anything that reduces plane aft.
- Kellet (for rope rode in light-moderate conditions--makes the boat swing more like boats on chain). A loop of chain is easier to handle than a lump of iron (comes over the rollers).
- Hammer lock mooring (a second anchor on very short scope, like 1.5 :1).
- Riding sails. The best are twin luff and V-shaped. Paratech designed the best one, but they don't sell it.
- Drogue on the rode ~ 15-20 feet forward. Also reduces up-down pull on the anchor during hobby horsing.
- Lift rudder if practical.
geem
Well-known member
We don't yaw. Ketch rig sees to that.
Friends with a Jeaneau 45 added a riding sail this season as their boat yawed alarmingly. Totally cured. Not a twin luff variety. You just set it bar tight with a slight offset. Works perfectly
