Reversing the engine when digging in the anchor – how much anchor load does this correspond to?

[PilotWolf]

Yep, not one jot of evidence to support his childish argument.

Or anyone's here.

Kind of sad that a leisure sailor has gone to the extent of all this. No longer what would be my idea of leisure.

PW

 

[thinwater]

If the ansatz is reasonable to work out the load on the anchor caused by the engine as f = ? p / v, then I can apply the same principle to work out the load caused by a head-on tidal stream. Only head-on, not from the side!

I do the same procedure as before, but now with engine with full thrust forward. ? and v may be slightly different, but p will be the same.

Then I use the - Bernoulli ? law - of quadratic dependency of the resistance in a laminal flow to scale this force / load to the speed of the tidal current, v_c. This gives me the anchor load due to the tidal current as f_c = ? p / v (v_c/v)^2.

As said, only an approximation for when the tidal stream is coming head on. Normally, wind and tide will mean the vessel is at an angle for both of them, meaning windage area is enlarged, as well as the under-water hull. This will mean the anchor load to the tidal stream will normally be larger than what I have estimated above. It'll make for a factor 2, 3, 4?

Anyway, just toying around with the concept.

I would love to see someone work up the math on wind vs. tide.

Assume the boat pivots around the rode. A safe engineering simplification, because a rode can only function in tension. It cannot push to the side.
Assume there are only 3 forces, each acting in sum on a single point.
Remember that the rode is not aligned with either the current or the wind. It is at an angle, because neither the wind or water force act exactly in opposition to the flow (the water moving over the keel will push to the side).

a. If the wind and water forces act on the same point the boat will not swing to one side (no torque moment around the rode).
b. Each must act on a center of effort. The water COE is typically several feet farther aft of the air COE, which is why boats can yaw.
c. The angle will be determined by the ratio of forces and the ratio of distances from the rode.
d. The actual effect on rode tension will not be that great. If one is much more than the other it will override the other force the boat in line with it. In no case will the rode feel a strong tidal force on its flank, because the ONLY way it can be stable in that position is if the forces are nearly balanced.
e. The resulting tension is probably actually less than either separate force, but you can show us with a vector drawing showing wind and water direction, and resultant force directions and magnitudes.

I'll have to do this when bored some night. While watching an old movie, perhaps.

 

[geem]

Or anyone's here.

Kind of sad that a leisure sailor has gone to the extent of all this. No longer what would be my idea of leisure.

PW

So everybody who responded to you is wrong and your right? There isn't a single contributor to this thread that agrees with you but your right? Wake up

 

[Neeves]

Sticking with the topic raised by the OP.

Reverse thrust on the engine to set the anchor confirms that the anchor is set, not fouled, and that it will withstand whatever tension is developed by the engine were that tension be developed by the wind.

Whether the anchor will foul at a higher tension, if the wind tension is higher than the engine tension, is unknown. There maybe a small piece of wood 1mm beyond the toe of the anchor that will foul the anchor if the tension were increased beyond the set tension. You simply don't know.

When testing anchors you learn what sort of hold to expect. Sometimes the hold is much less than expected and when you dig the anchor out carefully you commonly find a fouling element, piece of wood, seaweed, shell - impailed on the anchor toe.

The other problem that can arise is that the reverse engine tension differs from the wind direction and then the anchor will need to re-align to the new wind direction. If the change is minor the anchor will simply re-oreintate - if the direction is major the anchor may change direction but commonly will list to do so - and until the anchor sets more deeply again then its hold have been compromised.

The biggest issue with a large change in tension direction is that some seabed will have been embedded in the fluke and this embedded seabed alters, completely, the characteristics of the fluke (it becomes convex instead of being concave) and will never develop as high a hold as it should. Think of Morgan's Cloud damning of concave anchors. Though I have also tested for this and agree with Morgan's Cloud there has been no reports of dragging concave anchors, almost, since Morgan's Cloud raised the issue. For those who have invested in a Knox - though it is a concave anchor the slot seems to ensure that the fluke stays clean, my testing would agree with this. The slots of a Viking also help to keep the fluke clean - I'm not sure what the mechanism might be.

Anchoring will always remain a lottery. You don't know what is hidden just below the surface of the seabed. Reverse engine setting increases your chances of success (and safety). The more tension you apply the better will your anchor set. Its not a case of knowing what wind speed your engine tension equates to - go for maximum tension if you want maximum re-assuarnce.

A deep set will also bury some chain and buried chain will increase hold, slightly, but more important will reduce the impact of yawing, up to a point.

Adding a decent snubber will also increase the security of a set anchor as the snatch loads and veers are partially absorbed by the elstciticy of the snubber. Similarly if you have room and the chain then increase the length of your rode will also offer more chances of success (nothing new here).

The increased installation of modern windlass have changed the anchoring environment - if we find our chosen location is more subject to wind and chop than another location it is relatively easy to retreive the anchor and move a few 10s of metres to somewhere offering more shelter.


15 years ago tales of dragging of anchors were common place - now with the high populations of newer anchors the reports of the new generation anchors dragging have reduced, almost completely. What is surprising that despite this refreshing improvement we still see CQRs, Deltas, and Bruce on bow rollers - many on yachts whose owners claim to be liveaboards or long term cruisers.

Jonathan

 

[thinwater]

I would love to see someone work up the math on wind vs. tide.

Assume the boat pivots around the rode. A safe engineering simplification, because a rode can only function in tension. It cannot push to the side.
Assume there are only 3 forces, each acting in sum on a single point.
Remember that the rode is not aligned with either the current or the wind. It is at an angle, because neither the wind or water force act exactly in opposition to the flow (the water moving over the keel will push to the side).

a. If the wind and water forces act on the same point the boat will not swing to one side (no torque moment around the rode).
b. Each must act on a center of effort. The water COE is typically several feet farther aft of the air COE, which is why boats can yaw.
c. The angle will be determined by the ratio of forces and the ratio of distances from the rode.
d. The actual effect on rode tension will not be that great. If one is much more than the other it will override the other force the boat in line with it. In no case will the rode feel a strong tidal force on its flank, because the ONLY way it can be stable in that position is if the forces are nearly balanced.
e. The resulting tension is probably actually less than either separate force, but you can show us with a vector drawing showing wind and water direction, and resultant force directions and magnitudes.

I'll have to do this when bored some night. While watching an old movie, perhaps.

So I ran through several examples, both at a 45 degree angle and at a 90 degree angle to the current. I got that the rode tension is the same or slightly less than if there were no current. The forces offset to a considerable extent, even though the wind force will be perhaps 5x greater beam-on (I have done some measurements of that--I'm sure it varies). The shape of the underbody, specifically the distance between the wind and water COE matters. But it is the balance that put the boat at an angle. But if either force were very high, the boat would swing to face it. Yes, the foce on the keel may be considerable, but the wind from the other side is pushing back.

Not sayin' this is good for anchor stability. Obviously twisting it around is very bad. Very bad. But the forces are not that high.

Unless someone wants to draw the maths.

---

Now, the interesting part is that some of the same things that will steady a boat from yawing--riding sail, lifting the rudder, dinghy off the bow--also balance the wind-tide forces, resulting in less forward sailing and less force imbalance. Chain won't help, other than reducing keel tangles. Reducing windage and reducing underwater profile will. But there are always combinations that will result in a dance; all we can do is make it less violent.

 

[Poey50]

What is surprising that despite this refreshing improvement we still see CQRs, Deltas, and Bruce on bow rollers - many on yachts whose owners claim to be liveaboards or long term cruisers.

It is surprising but even more surprising is how devoted owners can be to those anchors or any specific anchor. The ideas of strengths and weaknesses goes out of the window. On one forum I read a serious plea for Rocna to issue special flags with their anchors so Rocna owners could know that they could safely anchor near each other without fear! (Obviously not an avid watcher of Panope videos.)

I think a cultural anthropologist would say that these items are being given magical properties as a means of managing existential threat - rather like the 'ghost shirts' worn as part of the great Ghost Dance movement when Native Americans were facing the unstoppable spread westwards of white settlers. Sadly the shirts didn't stop bullets but the myth endured regardless. The same is true, for the same reason, as those remarkably durable anchoring myths 'the bigger the better', 'catenary is king' and 'it's the chain that counts'. They offer reassurance and a sound night's sleep but, in the end, reality will always have its say. Science / testing allows more complex object-adequate accounts - boring to some, but better than getting washed-up on the shore.

 

[PilotWolf]

So everybody who responded to you is wrong and your right? There isn't a single contributor to this thread that agrees with you but your right? Wake up

Did I say that? Nope I don't think so.

I chose to lower myself to these discussions when I'm home because it's where I was dragged up from.

Keep playing with the toys and numbers - just don't bother quoting them when a professional mariner or the RNLI, et al saves your butt because they won't care.

W

 

[thinwater]

Fortunately, 97% of the time anchoring is really easy and you can just throw the lump off the bow with some rope attached to it, and go brew some tea. If you live in an area without thunderstorms, avoid foul weather and open roadsteads, and stick to nice mud bottoms, that increases to virtually 100%. No worries.

Really, we are fear mongering, to an extent. If the boat was equipped with half way good gear there is a big idiot--I mean safety factor.

 

[NormanS]

Sticking with the topic raised by the OP.

Reverse thrust on the engine to set the anchor confirms that the anchor is set, not fouled, and that it will withstand whatever tension is developed by the engine were that tension be developed by the wind.

Whether the anchor will foul at a higher tension, if the wind tension is higher than the engine tension, is unknown. There maybe a small piece of wood 1mm beyond the toe of the anchor that will foul the anchor if the tension were increased beyond the set tension. You simply don't know.

When testing anchors you learn what sort of hold to expect. Sometimes the hold is much less than expected and when you dig the anchor out carefully you commonly find a fouling element, piece of wood, seaweed, shell - impailed on the anchor toe.

The other problem that can arise is that the reverse engine tension differs from the wind direction and then the anchor will need to re-align to the new wind direction. If the change is minor the anchor will simply re-oreintate - if the direction is major the anchor may change direction but commonly will list to do so - and until the anchor sets more deeply again then its hold have been compromised.

The biggest issue with a large change in tension direction is that some seabed will have been embedded in the fluke and this embedded seabed alters, completely, the characteristics of the fluke (it becomes convex instead of being concave) and will never develop as high a hold as it should. Think of Morgan's Cloud damning of concave anchors. Though I have also tested for this and agree with Morgan's Cloud there has been no reports of dragging concave anchors, almost, since Morgan's Cloud raised the issue. For those who have invested in a Knox - though it is a concave anchor the slot seems to ensure that the fluke stays clean, my testing would agree with this. The slots of a Viking also help to keep the fluke clean - I'm not sure what the mechanism might be.

Anchoring will always remain a lottery. You don't know what is hidden just below the surface of the seabed. Reverse engine setting increases your chances of success (and safety). The more tension you apply the better will your anchor set. Its not a case of knowing what wind speed your engine tension equates to - go for maximum tension if you want maximum re-assuarnce.

A deep set will also bury some chain and buried chain will increase hold, slightly, but more important will reduce the impact of yawing, up to a point.

Adding a decent snubber will also increase the security of a set anchor as the snatch loads and veers are partially absorbed by the elstciticy of the snubber. Similarly if you have room and the chain then increase the length of your rode will also offer more chances of success (nothing new here).

The increased installation of modern windlass have changed the anchoring environment - if we find our chosen location is more subject to wind and chop than another location it is relatively easy to retreive the anchor and move a few 10s of metres to somewhere offering more shelter.


15 years ago tales of dragging of anchors were common place - now with the high populations of newer anchors the reports of the new generation anchors dragging have reduced, almost completely. What is surprising that despite this refreshing improvement we still see CQRs, Deltas, and Bruce on bow rollers - many on yachts whose owners claim to be liveaboards or long term cruisers.

Jonathan

With reference to your statement that tales of dragging anchors were common place fifteen years ago, and aren't now. It may be that the same people who were dragging then, are now using the increasing number of marina berths and commercial moorings, and now simply don't anchor. You don't see the anchors on boats that are anchored. The anchors that you see on bow rollers, are the unused anchors on the boats tied up in marina berths.
On your other point about a slot in the flukes preventing a build up of mud, someone should tell my Fortress, as when deployed in mud, it frequently comes up absolutely full of the stuff. I've no complaints about its hold.

 

[thinwater]

...
On your other point about a slot in the flukes preventing a build up of mud, someone should tell my Fortress, as when deployed in mud, it frequently comes up absolutely full of the stuff. I've no complaints about its hold.

The ongoing discussion of the problems of clogging relate to reset, not initial hold. Fortress does not reset when clogged. No anchor does, because it horribly upsets the balance. The question and challenge is defining how much is "clogged."

 

[Neeves]

We are a diverse lot and anchor threads bring out some of the differences.

It is a refreshing change that despite a bit of thread drift we have almost, so far one exception, unanimity.

Jonathan

 

[PilotWolf]

We are a diverse lot and anchor threads bring out some of the differences.

It is a refreshing change that despite a bit of thread drift we have almost, so far one exception, unanimity.

Jonathan

Good luck!

Love being the butt of ridicule. Boys and their toys Play on.

PW

 

[Neeves]

Good luck!

Love being the butt of ridicule. Boys and their toys Play on.

PW

PilotWolf, thank you for the best wishes. I'll take it as a compliment to still be called a 'boy'.

I'm puzzled why on earth do you think you are the butt of ridicule?

Contributing to a thread is entirely voluntary. Being a member of YBW is entirely voluntary. If you don't like the comments and think you are being 'picked on' without justification you can use the 'Report' button, bottom left of each post. Or you can simply ignore the comments, do as Thinweater suggests 'don't feed the trolls' and simply move to more interesting activities.

Jonathan

 

[Robih]

Hi PW,

I’m slightly surprised by all of this. I’ve been another random guy kicking about these fora for slightly longer than you have been and I can’t say that I’ve ever noticed you previously getting in to this kind of negativity. in fact the opposite, I’ve always thought your posts worthy of a glance as I might learn something.

I’m not at all interested in anyone being the butt of ridicule (well maybe Johnson our PM, but he deserves it!) but I’m interested to know more of your theory and practical experience, I ’d Just be grateful if you could provide supporting information to your statements so that I could better understand how your opinions are formed.

Rob

 

[Neeves]

Again going back to the OPs thread:

The idea of power setting, whether by wind or engine serves a number of purposes. It holds the yacht in the current position, it gives some indication that the hold is good (upto a point) and if the wind or engine is man enough it may provide more hold than might be needed for the forecast conditions.

I have taken a few liberties with my post to simplify - and keep and script length short to reduce complaints

Anchors when they are dropped to the seabed take up this sort of attitude (this is a mock up but most anchors would lie like this when randomly deployed):


There are exceptions, Fortress being a prime example as it will lie flat and parallel to the seabed



When tension is applied the toe engages and the toe and shackle end of the shank bury together, see above. I've deployed the anchor by hand under water, as in the first image and then applied tension using a sheet winch (our cat being aground), some chain and dyneema rode. I've not tried to set the anchor fully - to show an intermediate stage. I've taken the pictures when the tide has receded. Consequently the tension being applied needs to bury the fluke, shackle end of the shank, shackle and increasingly more and more chain. The shank, shackle and chain don't offer much hold - its the fluke that does all the 'hold' work so maximising fluke size and minimising the size of the shank, shackle, chain and swivel will focus the tension in the rode (whether derived from wind or engine) to the fluke.

The above images are of an Excel and not to show favour this is an image of a Rocna, showing it has buried the shackle and some chain (you cannot see them )



And of a Spade. The green line is attached to the chain, with a buoy so as to easily find the anchor. You can just see the heel of the fluke above the left hand black arrow.



You may think that a swivel and size of chain make little difference but this, below, is a popular swivel and 6mm and 5/16th" (similar to 8mm) chain. If you consider the picture of the Spade - a lot of chain has been buried. By itself chain has an insignificant surface area - but a long length of chain has 'more' surface area - all of which has to be buried and its contribution to hold, in a straight line, is not significant. The featured Boomerang is made from 8mm plate, and sized for 8mm chain and has a smaller cross sectional area than the chain and a much smaller cross sectional area than the swivel (but achieves the same end result as the swivel.



If you want to maximise the depth of burial of the fluke and maximise the hold then you should consider minimising the size of the essential ancillaries, shackle, chain, swivel (smaller stronger shackle, smaller chain G40 or better G70 or use a Boomerang, not swivel - or even no Boomerang)..

Hold is a function of the shear strength of the seabed (in which the anchor is buried) and the size of the fluke. Taking a simple example of a Fortress - if you can bury part of the fluke of a small Fortress FX 16 to 600cm^2 then if you also set a larger Fortress, say FX23 or FX37 to the same tension it will also bury 600cm^2 - no more no less (actually it will be a bit less as the flukes are made fro thicker plate (and increased plate thickness has increased resistance to penetration). Ultimate hold of the bigger anchors is higher - but you are very unlikely (and I would not wish it upon you) to ever develop maximum hold of a FX 16, let alone a FX 23 or 37. So using the recommended size of anchor, recommended by the anchor maker, will allow you to bury the fluke with much potential hold unused, ever.

There are other negativities to using a larger anchor, it will bury less deeply (as the shank becomes an increasing impediment to burial)and the large shank also will be a larger lever arm to capsize the anchor (considering the flukes are offering the same hold but the bigger anchor has the larger lever arm).


Now a bit of controversy and something to think about.

The shackle, swivel and chain (the rode) resist burial. As the anchor is buried the rode is increasingly buried and as it buries it has an increasing resistance imposed by the seabed to its burial. The shackle has the most 'force' against it and that resistance gets larger the more deep the burial (shear strength increases with the square of depth). If you have a rode at say 6:1 that is under tension in the water - the shackle has an increased force of resistance against its burial and it will not 'reflect' that 6:1 angle - its angle will be much higher - maybe 3:1. So though your rode is 6:1 the tension imposed on the anchor is at 3:1 (I pluck some numbers out of the ether to illustrate - I do have 'better' numbers).

Another reason to reduce shackle size (commensurate with maintaining adequate strength and it not locking up), using smaller chain and having second thoughts about your swivel.

Finally - I don't make any of this up. Virtually all of this has been researched by the US Navy, The Royal Navy, Vryhof and Bruce, not forgetting Universities in Houston, Perth and Southhampton (to name only a few).

Take care, stay safe

Jonathan

 

[PilotWolf]

Hi PW,

I’m slightly surprised by all of this. I’ve been another random guy kicking about these fora for slightly longer than you have been and I can’t say that I’ve ever noticed you previously getting in to this kind of negativity. in fact the opposite, I’ve always thought your posts worthy of a glance as I might learn something.

I’m not at all interested in anyone being the butt of ridicule (well maybe Johnson our PM, but he deserves it!) but I’m interested to know more of your theory and practical experience, I ’d Just be grateful if you could provide supporting information to your statements so that I could better understand how your opinions are formed.

Rob

Why would I? Apparently my opinion counts for nothing as I don't have a floaty toy.

My way works for me and 'their' way works for big ships, I'll ask the new captain in the next couple of days assuming I pass the covid test,how he feels about yachties measuring $h1t when anchoring.

PW

 

[NormanS]

Now that anchoring has become so complicated and technically demanding, is it surprising that so many people choose not to, and instead use marina berths and commercial moorings?
Anchoring used to be straightforward and simple. Now it seems that you have to be a techie nerd. ?

 

[geem]

Now that anchoring has become so complicated and technically demanding, is it surprising that so many people choose not to, and instead use marina berths and commercial moorings?
Anchoring used to be straightforward and simple. Now it seems that you have to be a techie nerd. ?

Yeh, and you don't even need an anchor on the end of the chain?