Anchors - Bigger is Better ?

[Snowpetrel]

Originally Posted by JonJo

SnowPetrel,

There was a suggestion that its not weight but surface area that engenders holding capacity (or anti drag). I wonder what increased safety margin of wind speed is available for say a 33kg Rocna over a 20kg at say 40 knots. If the 20kg anchor drags at 40knots at what wind speed would the 33kg anchors drag?

Jonathan

Yes I thought surface area was the main factor. And it is very important. But more important is the depth the anchor dives to. And heavier anchors of the same type have more surface area, but not in direct proportion to the weight gain. And also tend to dig in deeper so the holding is roughly proportional to weight in a substrate deep enough for them to reach maximum depth. You should be able to rework my wind speed formula to get the answer.. I otherwise I can do it tonight after work. Cheers

[JonJo]

Originally Posted by Snowpetrel

Yes I thought surface area was the main factor. And it is very important. But more important is the depth the anchor dives to. And heavier anchors of the same type have more surface area, but not in direct proportion to the weight gain. And also tend to dig in deeper so the holding is roughly proportional to weight in a substrate deep enough for them to reach maximum depth. You should be able to rework my wind speed formula to get the answer.. I otherwise I can do it tonight after work. Cheers

SP,

I agree that a bigger anchor can dive more deeply, but (my guess is, backed up by trying) it will only dive more deeply if loaded sufficiently.

The scenario is a yacht of fixed windage, so the load developed is fixed, and 2 anchors, one bigger than the other, Rocna 20kg and 33kg (which have SA of 1415 cm sq and 1945cm sq respectively). The load to 'set' the 2 anchors is the same but I'm suggesting that the 20kg anchor is more deeply set than the 33kg anchor. But they have the same immediate or current holding capacity. So depth is not relevant (or I do not think so), or its not relevant in a straight line pull, being more deeply set might be moreimportant if the wind shifts 90 degrees?

If we were to assume (for the sake of argument) the 20kg anchor now begins its drag at 40kg (as Noelex has described - its slipping though the seabed, measurable by you slippage of the arc of swing on your GPS/Plotter).

At the same 40 knots the 33kg Rocna has some potential still to go before it starts to slip, it can take an increased load from windage, which we can now increase as the wind increases. Its limit is defined by the difference in Surface Areas (everything else is similar - see below). The question is when will the 33kg model start to slip.

The unknown is the fact the 33kg model at 40knots is probably higher in the seabed (less deep) than was the 20kg model when it started to slip (at 40 knots) (drag). But we are increasing the load (same windage/sq of wind speed) and as we increase the load the anchor will dive more deeply., assuming (big assumption?) we can get the big anchor to dive to the same depth as the small one then the seabed consistency will be the same, if it dives deeper the seabed will be firmer (more resistant to dragging and more resistant to diving) if it does not dive as deeply??

But on the basis of a consistent seabed when will the 33kg anchor start to drag - what is the extra margin of safety of increase of wind speed for the difference in SA? My marin for safety is simply the difference in wind speed the 33kg anchors drags vs the 20kg model (and we have assumed 40 knots).

Maybe if we get that figure we can consider whether the bigger anchor will dive less deeply, the same, more deeply - and whether depth matters, its the load that matters and the load at which is slips/drags is dictated by design (and surface area).

I have my own answers on this - but it looks questionable, very. The calculation also turned out to be very, very simply, which is why I think my answer questionable (it cannot be that simple!?) I worked on the basis that Surface Area and ultimate holding capacity is like Windage (or SA of a yacht), its a linear relationship (double SA and you double holding capacity etc), unlike wind speed which is the square? I am also assuming perfect scaling, so a big Rocna has the same proportions as a small one (they do not but its a good start). I also use a Rocna because the SA and weight are both known.

My 40 knots might be challenged, but it depends on the seabed and I wanted a neat figure.

Jonathan

[daddle]

Lighter is better. When some NewGen Anchor genius improves anchors by designing one with more performance per pound I'll consider one. Simply editing the marketing hype to specify a much larger anchor for whatever length or displacement is ... well ... marketing.

I like small because:

(1) Easy to launch and retrieve so one does not need to carry it on the bow.
(2) Reduces displacement.
(3) Drags sooner so the problem can be handled in less-than-typhoon conditions.
(4) Test's anchoring skill beyond just finding the down button after a day of drinking.
(5) Doesn't look dreadful on the bow of a beautiful boat. It's below.
(6) Avoids complacency. Eliminates the arm-chair sailor deception of "never drags."
(7) Smaller chain, smaller rode, all around much lower cost.
(8) Never seen it happen, but if seeing it's size makes some Nancy move away so much the better.
(9) Bragging rights.

So far so good. Cruising out here for many years.

[JonJo]

Originally Posted by daddle

Lighter is better. When some NewGen Anchor genius improves anchors by designing one with more performance per pound I'll consider one. Simply editing the marketing hype to specify a much larger anchor for whatever length or displacement is ... well ... marketing.

I like small because:

(1) Easy to launch and retrieve so one does not need to carry it on the bow.
(2) Reduces displacement.
(3) Drags sooner so the problem can be handled in less-than-typhoon conditions.
(4) Test's anchoring skill beyond just finding the down button after a day of drinking.
(5) Doesn't look dreadful on the bow of a beautiful boat. It's below.
(6) Avoids complacency. Eliminates the arm-chair sailor deception of "never drags."
(7) Smaller chain, smaller rode, all around much lower cost.
(8) Never seen it happen, but if seeing makes some Nancy move away so much the better.
(9) Bragging rights.

So far so good. Cruising out here for many years.

Join the Anchor Pariah Club! The APC is very select and having a thick skin is essential. You need to have a sense of humour - and confidence in your abilities.

Jonathan (APC Member)

[downunder]

Originally Posted by daddle

Lighter is better. When some NewGen Anchor genius improves anchors by designing one with more performance per pound I'll consider one. Simply editing the marketing hype to specify a much larger anchor for whatever length or displacement is ... well ... marketing.

I like small because:

(1) Easy to launch and retrieve so one does not need to carry it on the bow.
(2) Reduces displacement.
(3) Drags sooner so the problem can be handled in less-than-typhoon conditions.
(4) Test's anchoring skill beyond just finding the down button after a day of drinking.
(5) Doesn't look dreadful on the bow of a beautiful boat. It's below.
(6) Avoids complacency. Eliminates the arm-chair sailor deception of "never drags."
(7) Smaller chain, smaller rode, all around much lower cost.
(8) Never seen it happen, but if seeing it's size makes some Nancy move away so much the better.
(9) Bragging rights.

So far so good. Cruising out here for many years.

That changes the 99.99999999999999999999999999999%

[JPA Cate]

Well, fellas,

I think Snowpetrel has kept his cool over this. I notice the chap who had good fortune with proven fisherman anchors and heavy chain went ignored. IMO, we should listen to all these guy, even if we don't want to or are unable to deal with 112 lb. anchors.

You guys are really tolerant, and my hat's off to youse all. You have remained tolerant.

[Snowpetrel]

Originally Posted by JonJo

SP,

I agree that a bigger anchor can dive more deeply, but (my guess is, backed up by trying) it will only dive more deeply if loaded sufficiently.

Very true, but more load will quickly pull it in deeper and any slippage will be small.

The scenario is a yacht of fixed windage, so the load developed is fixed, and 2 anchors, one bigger than the other, Rocna 20kg and 33kg (which have SA of 1415 cm sq and 1945cm sq respectively). The load to 'set' the 2 anchors is the same but I'm suggesting that the 20kg anchor is more deeply set than the 33kg anchor.

I am think you are right, using De Parsons approximate formula, (h/t to Kettlewell for finding the source of this formula here page 83)

Holding power = Constant x Area x depth^2

(Constant is based on soil and anchor type best found through real world tests.)

If the 20Kg anchor is at 1 meter depth then ((1415x1^2)/1945)^0.5=0.853m

So using a crude and approximate formula gives the 33kg anchor at 0.85m depth.

But they have the same immediate or current holding capacity. So depth is not relevant (or I do not think so), or its not relevant in a straight line pull, being more deeply set might be moreimportant if the wind shifts 90 degrees?

Prehaps? but if the soil is 1 meter of soft mud over rock the small anchor cannot develop any more holding power by digging deeper, where the bigger one still can if the wind picks up.

If we were to assume (for the sake of argument) the 20kg anchor now begins its drag at 40kg (as Noelex has described - its slipping though the seabed, measurable by you slippage of the arc of swing on your GPS/Plotter).

At the same 40 knots the 33kg Rocna has some potential still to go before it starts to slip, it can take an increased load from windage, which we can now increase as the wind increases. Its limit is defined by the difference in Surface Areas (everything else is similar - see below). The question is when will the 33kg model start to slip.

no, not just surface areas, because under higher loadings the 33kg anchor will continue to dive deeper, also increasing holding. This is why holding power increases more closely in proportion to weight rather than surface area (in most cases).

The unknown is the fact the 33kg model at 40knots is probably higher in the seabed (less deep) than was the 20kg model when it started to slip (at 40 knots) (drag). But we are increasing the load (same windage/sq of wind speed) and as we increase the load the anchor will dive more deeply., assuming (big assumption?) we can get the big anchor to dive to the same depth as the small one then the seabed consistency will be the same, if it dives deeper the seabed will be firmer (more resistant to dragging and more resistant to diving) if it does not dive as deeply??

Lets do some figures and see...

But on the basis of a consistent seabed when will the 33kg anchor start to drag - what is the extra margin of safety of increase of wind speed for the difference in SA? My margin for safety is simply the difference in wind speed the 33kg anchors drags vs the 20kg model (and we have assumed 40 knots).

51.4 knots, with all the usual reservations, regarding trying to fit idealised and simplified formula to an un-idealised and un-simple world. Note I am using a scaling ratio of ^1 not ^0.9 to simplify my calculations, this would reduce my numbers slightly, to about 50 knots

Maybe if we get that figure we can consider whether the bigger anchor will dive less deeply, the same, more deeply - and whether depth matters, its the load that matters and the load at which is slips/drags is dictated by design (and surface area).

More deeply, The 33kg anchor will dive to about 1.1 meters, with all the usual disclaimers

If for some reason the anchor can not get to that depth, say due to a hard rock substrate at 1 meter the holding will then be in proportion to area, less a small factor for the fact that more of the bigger anchor is higher in the soil. The net result will still be more holding to the bigger anchor, just not by all that much in windspeed terms.

I have my own answers on this - but it looks questionable, very. The calculation also turned out to be very, very simply, which is why I think my answer questionable (it cannot be that simple!?) I worked on the basis that Surface Area and ultimate holding capacity is like Windage (or SA of a yacht), its a linear relationship (double SA and you double holding capacity etc), unlike wind speed which is the square? I am also assuming perfect scaling, so a big Rocna has the same proportions as a small one (they do not but its a good start). I also use a Rocna because the SA and weight are both known.

This is about where I got to in my first attempt to put this all into numbers, see my blog post on this here See my Windage to anchor area ratio = windage (m^2) / (Anchor fluke area (cm^2)/1000.

I have since come to the conclusion that this ratio is not very relevant in most cases due to it neglecting the depth of set. I have now moved to considering anchor weight to windage, and using length^2 as a proxy for windage. This seems to give a good correlation to actual anchor sizes for vessels ranging for tankers to dinghy's.

Many thanks to all contributors and questioners on this and the other threads, for driving me to learn more of what's really happening.

My 40 knots might be challenged, but it depends on the seabed and I wanted a neat figure.

I think this is a good figure to use, It's about where normal sized and good anchors start to have problems. If it is just a gust the stretch in the rode will absorb much of the peak loads, but if it is constant it is quite a lot of wind.

Cheers

[JonJo]

SP,

Good work.

Initially I was not wanting to get too complex, leave that till later. I was simply thinking of a 'simple' seabed. We can get to more complex seabeds afterward.

I'm not sure of the logic that greater weight is more important than greater surface area at deeper diving depth. If this were the case oil rig anchors that are designed to dive much more deeply would have a greater weight vs surface area? Someone in this thread, or the thread started by Factor, was saying the opposite - once an anchor has set (or engaged) then its about Surface Area not weight - and many comments about the Fortress support the idea. if weight engendered diving would Smith have not incorporated weight into the Rocna? If weight is important then the Spade is maybe a better anchor?

But the calculation you have made that the 20kg anchor drags at 40knot and the 33kg anchor drags at 50 knots is not far off my figures (I came up with 47 knots). We have an anchor with 70% more weight and 34% more surface area giving 10 knots of a safety margin. Not very impressive. A considerable opportunity for complacency 70% increase in weight gives 10 knots increase in safety margin?

My assumptions also include the idea that the yacht had the same anchoring practice, uses decent snubber(s), the same for both, maybe hangs an anchor off the bow to act as veering brake etc. So the only difference is the anchor.

The suggestion that big is better has some numerical substance but its not really that impressive.

I'm not denying the problems of twisted rodes but 2 x 20kg anchors set at 45 degrees if the wind is not to veer looks a very interesting proposition? They will take the load together, or one 'drawing ' in the other if the snubbers are doing their job - as the snubber stretches the unloaded anchor should come into play. 2 anchors set in line, one closer than the other look a good idea, as long as you have not succumbed to alcohol.

I wonder if the BIB coterie had realised the differences were so small? I realise of course I am now going to receive a barrage of posts telling me 10 knots is important, if not critical - but I'm guessing most thought the differences would be greater than 10 knots - even if they do not admit it.

My further calculations suggest if you move the 40 knot drag point for the 20kg anchor up or down then the 33kg anchor still looks unimpressive, given its 70% weight advantage.

It will be interesting to read other comments.

Jonathan

[conachair]

More to add to the mix from The AlainFraysse site..

Anchor

Vryhof as well as other anchor manufacturers (e.g. The Bruce Anchor Group ) use the same relation to predict the UHC of their products:

(4.1a)

So the power holding of very big anchors is almost proportional to their weight. The coefficient K depends on the anchor type and on the holding quality of the soil: typical values for the most recent models span from 40 (very soft clay) to 80 (in mud).
What if we apply this formula to steel anchors between 5 and 50 kg?

• For the typical 15 kg range focused on by most holding tests published in boating magazines, we get an UHC around 500 daN in very soft clay and 1000 daN in mud. Actually, modern 15 kg anchors exhibit performances 2.5 times better, even in non optimal conditions (more on that below).
• From 10 down to 5 kg, however, the formula gets closer and closer to the measured holdings.

In addition, an analysis of a few tests than have been carried out on an homogeneous range of anchors of 10, 12, 16 and 20 kg shows their holding powers is best fitted by eq. 4.2:

(4.2)

Looking at it from that formula then with medium holding, a 25% increase in weight leads to a 36% increase in holding capacity.



Or with linear X scale with k=12 (medium holding) from the excellent https://www.desmos.com/calculator page.


https://www.desmos.com/calculator/8tquzxudvi

How does that apply to the real world? Seems the best resource for cruising anchor data around that I have seen anyway, any other data would be very interesting, conformation bias can be as strong as the chain we use

[charliehows]

heres an anchoring story - this happened over easter weekend in my favourite little bay which i have been anchoring in for 3 or so years on and off. i was getting aboard my dinghy to go ashore for a bit of brekky when one of the 50ft motor boats started pulling his anchor - must have had a pretty good winch cos they hauled up a whopping bit of old industrial cast iron - looked about a metre or more in dia. - hooked on the nose of their delta. I putted over because their deck was so high they couldnt get a rope through the bar on the anchor, i put the rope through for them and they tied it off and let the chain out enough to tip the thing off, meanwhile drifting sedately towards the rocks. I did meet a guy there last year who couldnt get his anchor up - i had a go with my winch but i couldnt pull it. My point is this - anchors do some complicated stuff, all this palaver about which is best - it really depends on the situation, not the anchor.

[Cotemar]

Originally Posted by downunder

That changes the 99.99999999999999999999999999999%

I hate it when my math is wrong, sorry
Here is the new % of cruisers that like BIB anchors
99.99999999999999999999999999998%

The smaller anchor is better (SIB) group is now upto two

[noelex 77]

Originally Posted by JonJo

But the calculation you have made that the 20kg anchor drags at 40knot and the 33kg anchor drags at 50 knots is not far off my figures (I came up with 47 knots). We have an anchor with 70% more weight and 34% more surface area giving 10 knots of a safety margin. Not very impressive. A considerable opportunity for complacency 70% increase in weight gives 10 knots increase in safety margin?

A 10k increase makes a very substantial real world difference.
There is big difference in the forces involved and the practical experience.

When its been blowing 50k and it drops back to 40k it almost sounds like the wind has stopped

The most significant difference is frequency of these stronger winds drops off rapidly with a small increase in wind strength.

We get sustained winds of 40knots many times a year, but sustained winds of 50knots and over are only a once a year event (none this year so far ).
So the extra 10k buffer in this hypothetical example would reduce our dragging from "many times" down to 1, or 0. All for the addition of only 13KG.

The wind pattern of most regions follows roughly the bell shaped curve of normal distribution so a small wind increase near the extreme translates to a dramatic decline in frequency.

In addition with the extra weight we gain extra ability to anchor in more questionable holding grounds and with a shorter scope.

I think its a very substantial improvement for only a very small weight gain.

[Snowpetrel]

Originally Posted by JonJo

I'm not sure of the logic that greater weight is more important than greater surface area at deeper diving depth. If this were the case oil rig anchors that are designed to dive much more deeply would have a greater weight vs surface area?

Ha, I was not trying to say much about different anchor designs, only comparing two anchors of the same type with different weights. I agree that for two different anchor designs of the same weight the one with the bigger surface area will in theory hold more at the same depth. And if designed correctly it may also have the potential to dive deeper as well.

It seems to happen that anchor holding scales somewhere between L^2 and L^3 and normally it is nearer L^3 in mud according to Alain Puech. which happens to reasonably closely match weight, which is convenient since most people use weight rather than anchor area. This is why I harp on about weight rather surface area, or though we could just as easily use surface area and add a power say A^1.4 which would do the same thing if you wanted to measure the surface area of every anchor.

By the way I have some doubts about de parsons formula, as it scales to ^4, I am looking into this... Oh and another formula I looked at gave the holding power in wind of around 55 knots for the 33kg anchor, must be time for bed. head hurts!

I realise of course I am now going to receive a barrage of posts telling me 10 knots is important, if not critical

I was just thinking how much difference 10 knots makes... But I won't go there

[Kettlewell]

Does anyone have evidence that there is a negative effect from having too long a snubber? I too can imagine the bungie scenario, but I have not seen it in real world anchoring, and I have been on some long mostly nylon rodes in high winds. In the real world gusts don't turn off and on, they ramp up in intensity, fade off, and then often quite quickly a new gust comes along. So, on mostly nylon you do get some back and forth on the rode, but I would not describe it as being on a bungie cord. The closest I have felt to that effect was when hanging on a parachute sea anchor offshore on 400 feet of 1/2 inch nylon. Breakers would push us back hard once in awhile, and afterwards the boat would spring back, like on a bungie, but not very far and soon the pressure on the boat would pick up and slow the bounce down. Bottom line, I suppose there could be a bad bungie effect given some certain combination of too-small snubber or anchor line, very powerful and abrupt gusts, with pronounced lulls in-between, but it is certainly a rare occurrence.

[conachair]

Originally Posted by JonJo

SP,

But the calculation you have made that the 20kg anchor drags at 40knot and the 33kg anchor drags at 50 knots is not far off my figures (I came up with 47 knots). We have an anchor with 70% more weight and 34% more surface area giving 10 knots of a safety margin. Not very impressive. A considerable opportunity for complacency 70% increase in weight gives 10 knots increase in safety margin?

Assuming those figures and looking at it from a function of force,
as force exerted is proportional the square of the wind speed, 50Kts results in 56% more force per unit area than 40Kts. 20Kg to 33Kg is a 65% increase in mass ( 70% ) , 65% heavier for a 56% increase in holding power puts a very different spin on it from 70% for 10kts.


(are my sums accurate , feel free to correct anyone )