Upsizing anchor - suggestions?

[Doughdish]

Vulcan 12 or Vulcan 15 with 20 - 30 feet of 5/16 in chain and 3-strand nylon line. Best all round anchor system for this boat in the Bahamas. SWAN. All chain rode causes very high force on the anchor system in high winds. Snubbers do not really offer enough flex in high winds (>45 kts). IMHO.

[rslifkin]

Originally Posted by Doughdish

Snubbers do not really offer enough flex in high winds (>45 kts). IMHO.

If a snubber isn't giving enough stretch, the solution is to make the snubber longer. Depending on what size you need for adequate strength and the conditions, you may need 30 - 50 feet of snubber in heavy weather to get enough stretch.

[noelex 77]

Originally Posted by Lodesman

Is this really the case? The fluke areas for 15 and 10Kg models are 1030cm^2 and 795cm^2 respectively. The 15 is only 30% larger than the 10.

The best studies on anchor size vs ultimate holding ability were conducted by Professor Knox. The studies have been published in several magazines such as Practical Boat Owner around 2011.

He looked at wide range of anchors measuring the ultimate holding ability over several sizes. The results showed that when you compared anchors of the same design and construction material, the ultimate holding ability was at least proportional to the weight. In other words, doubling the weight at least doubled the ultimate holding ability. In some cases the gains were significantly greater.

As you correctly point out, fluke area does not rise linearly with weight for anchors of the same design and construction material, but heavier anchors have the ability to penetrate deeper into the substrate at their ultimate holding capacity.

In difficult substrates, especially weed, I suspect the difference is greater again. Small, light anchors such as the 10kg model used by the starter of this thread have great difficulty cutting through thick heavy weed even when the design of the anchor is fundamentally sound. Larger and heavier models have much less difficulty.

It is important to select a good anchor design (and Rocna is a good anchor design in my view), but once this is done then increasing the anchor size is the best way to increase anchor security. As well as more security, there are gains in versatility enabling anchoring in poorer substrates and at shorter scopes. These are all valuable improvements for a cruising boat. However, there are limitations, especially for boats without an electric anchor windlass. It is important the anchor is not too large to manage comfortably.

[soopad00pa]

Originally Posted by OldManMirage

Soupa, I think your setup is fine. I had the same Rocna with my Cape Dory 28, and it worked great. Knowing your area a little bit I think what might have happened was with all the shifting around the anchor came across a patch of hard sand, the kind that gets scoured off by the current, and just didn't have enough time or scope to get dug back in before your alarm went off.

I'm sure with practice and experience you'll know and do better. But I would not change out what you have, it should be more than enough. I would probably have gone with more scope. Make sure your adding in the height of your bow from the water to your depth, probably an extra 5 feet for the C30. I'm old school and I try to get 7 times when I have the room, and enough anchor line. It doesn't hurt a thing and it will make motion at anchor easier.

Right on. I do think it's likely I fouled or bounced across something unsticky, but who knows. Would've liked more scope, but there were other considerations that made me feel there was only so much rode I could let out. But that's also a function of inexperience - eyeballing a site and figuring out how much room you've got.

[rslifkin]

As Noelex pointed out, fluke area isn't the only factor in scaling. As you make the anchor bigger, individual grains of sand, bits of junk on the bottom, weeds, etc. get smaller relative to the anchor. That has an effect on behavior.

[Lodesman]

Originally Posted by noelex 77

The best studies on anchor size vs ultimate holding ability were conducted by Professor Knox. The studies have been published in several magazines such as Practical Boat Owner around 2011.

He looked at wide range of anchors measuring the ultimate holding ability over several sizes. The results showed that when you compared anchors of the same design and construction material, the ultimate holding ability was at least proportional to the weight. In other words, doubling the weight at least doubled the ultimate holding ability. In some cases the gains were significantly greater.

This is what I found:

The
corresponding efficiencies of the
5kg and 13kg Spades are
therefore 24 and 32 respectively.
The heavier anchor is 30% more
efficient than the lighter one.
25 APRIL 2011 - 7.3 k

https://static1.squarespace.com/stat...BO+article.pdf

This didn't seem to necessarily apply with all anchor types. Curiously in this same test the larger Bruce was less efficient than the smaller one. Maybe there's another test somewhere?

[noelex 77]

I think you are misunderstanding the results.

The efficiency in this study is the Ultimate holding capacity/ weight. If the efficiency of the smaller anchor was identical to the larger anchor the holding power would be propotional to the weight. In other words doubling the weight of the anchor would double the holding capacity.

The test is not indicting that the larger Spade is 30% better than the smaller Spade, but rather that the larger Spade is 30% better than would be expected from assuming the holding power is proportional to the weight.

The fact that the heavier Spade has 30% higher effeciency than the smaller Spade means that doubling the weight would result in about 2.3 x the holding capacity. This is a great result and shows the value of even a small increase in anchor size.

All the other anchors on test (Spade, Rocna, Manson Supreme, Delta and CQR) had a higher efficiency factor for larger than smaller anchors (in other words doubling the weight of the anchor will more than double the hold). Many models did substantially better. The only exception was the Bruce anchor. For the Bruce doubling the weight increased the holding capacity by only slightly less than double (1.7x).

Professor Knox did not test the 10kg Rocna owned by the original poster, but did test a 5kg Rocna and a 15kg Rocna. Increasing the weight of the anchor by 3x resulted in a remarkable 5.7x increase in the holding ability for this test.

The results from this study suggests that my prediction of increase in holding ability of 50% for a size increase of only 5kg (from 10-15kg) is conservative.

[seandepagnier]

Originally Posted by Panope

Others have suggested increasing the size of the anchor from 10 to 15 kgs., but this will only increase holding by about 50%.

anchor holding power is not always directly proportional to anchor weight. For example, a 15kg rocna will have twice the holding power of a 10kg one. They designed each one differently it is not a scaled up version. The 20kg has less of a relative increase so it would not even fit a straight line for all of the anchors it is a bit random from what I recall.

These results may show a different shape curve in different bottom types as larger surface area will give more holding power if the bottom is easy to penetrate but perhaps a heavier anchor can penetrate harder bottom types.. Also sharpening the anchor will help it cut into the bottom just as sharpening fish hooks will make them more likely to catch fish.

[seandepagnier]

Originally Posted by noelex 77

However, there are limitations, especially for boats without an electric anchor windlass. It is important the anchor is not too large to manage comfortably.

There are half a dozen ways to raise even a very heavy anchor that don't involve an electric windlass. The difficulty is in breaking it free when it is well set. Lifting the weight of the anchor is the easy part, so I am not sure an electric windlass is really required on boats under 40ft at least, and I did manually raised a 25kg anchor on a 45ft boats once so its very possible.

[Panope]

Here is some data from some of my testing a "Sandy Mud" seabed in Washington State (note: The original poster of this discussion described the seabed of his dragging event as "Sandy Mud").

In these tests, I chose to measure the maximum resistance during a pull with an electric winch. In all cases the anchors were moving when developing peak values. The fact that winching "speed" slows as load increases means that the larger anchors are (generally) moving slower than the smaller anchors. It is not clear to me what effect this has, but, it may account (in part) for the larger anchors having higher efficiencies than the smaller anchors.

All tests were conducted with the same rode: 12' of 5/16" chain + Dyneema rope. The fact that smaller anchors will have MORE difficulty burying this chain than the larger anchors may account (in part) for the larger anchors having higher efficiencies than the smaller anchors.

I believe that this amount of chain (12') is small enough, and the loads high enough, that measurable catenary will be eliminated.

The data was derived by dividing the Peak resistance by the weight of the anchor.

---------------10kg (range)------------20kg (range)

Spade (steel)------75----------------------107
Vulcan--------------67---------------------109
Mantus M2---------67-----------------------95
Quickset------------55-----------------------88
Viking---------------52---------------------104
Excel----------------51----------------------87
CQR-----------------50----------------------72
Mantus M1----------43----------------------78
Delta----------------41----------------------41
Rocna---------------27----------------------15
Bruce---------------15-----------------------16

Average------------49.4--------------------73.8

-------------------------------------------------------------------------------------

The next set of data is from my testing in a "Cobblestone" seabed in Washington State. In these tests, a true maximum HOLDING was measured (anchor not moving) using the propeller thrust of a test vessel.

All tests were conducted using all chain rodes with enough scope to ensure that the chains did NOT lift at the anchor. In other words, the angle of pull on all anchors = zero.

Because all anchors used similar chains (that were laying on the seafloor), the small anchors likely have an advantage due having a higher percent of their holding power coming form the friction of the chain on the seafloor.

The data was derived by dividing the holding power by the weight of the anchor:

---------------10 kg (range)----- 20 kg (range)

Excel----------------8.9--------------8.6
Rocna (rollbar)-----8.2--------------7.2
Mantus M2---------8.2-------------11.4
Mantus M1---------8.2--------------8.6
Vulcan--------------7.9--------------8.4
Quickset------------7.5--------------6.3
Spade (steel)-------7.1--------------8.2
Delta----------------6.9--------------4.3
Viking---------------6.7-------------16.5
Bruce----------------6.4-------------3.4
CQR-----------------5.2---------------1

Average------------7.4--------------7.6


--------------------------------------------------------------------------------------


This next set of data should be taken with a HUGE GRAIN OF SALT because a rode that included 80 FEET OF CHAIN was used on all of these test anchors. This means that the smaller anchors received a LARGE benefit due to the effect of greater chain catenary (lower angle of pull). Yes, the small anchors were hindered by having to drag a proportionally larger chain through the seabed, but I believe this factor is far overshadowed by the increase in chain catenary.

The test was conducted in a "Soft Mud" in Washington state (creamy organic material, no sand).

The rode was 80' of 5/16" chain + nylon at 5:1 scope.

The data was derived by dividing the peak resistance by the weight of the anchor.

------------------------10kg (range)--------------20kg (range)

Mantus M1----------------37--------------------------20
Viking---------------------32--------------------------21.5
Rocna---------------------27--------------------------18
Bruce---------------------25.5------------------------18
Vulcan--------------------22--------------------------16.8
Mantus M2---------------21--------------------------12.2
Excel---------------------20---------------------------13.7
CQR----------------------20---------------------------10.8
Spade (steel)------------19---------------------------10.3
Quickset------------------14---------------------------8.2
Delta----------------------5.6--------------------------3.6

Average-------------------22.1------------------------13.9

--------------------------------------------------------------------------------------


The next set of data is from my testing of 5 Genuine Bruce anchors of different sizes in a "Soft Mud" seabed in Washington State.

A rode consisting of 3/8" Dyneema (only) was used for each anchor.

The data was derived by dividing the peak resistance by the weight of the anchor.

-------------5kg----------15kg----------20kg-----------30kg-----------50kg

Bruce-------25-----------13.5----------12.7------------10.6------------15.2


-------------------------------------------------------------------------------------

Steve

[Panope]

Originally Posted by seandepagnier

anchor holding power is not always directly proportional to anchor weight. For example, a 15kg rocna will have twice the holding power of a 10kg one. They designed each one differently it is not a scaled up version. The 20kg has less of a relative increase so it would not even fit a straight line for all of the anchors it is a bit random from what I recall.

These results may show a different shape curve in different bottom types as larger surface area will give more holding power if the bottom is easy to penetrate but perhaps a heavier anchor can penetrate harder bottom types.. Also sharpening the anchor will help it cut into the bottom just as sharpening fish hooks will make them more likely to catch fish.

I have examined two different sizes of Rocna (rollbar) anchors and found no differences in design. They were scaled versions of each other with the exception of plate thicknesses being scaled slightly differently due to (presumably) material being readily available in a limited number of thicknesses.

Also, I have tested two different Rocna anchors (10 and 20kg) and have found that performance increases by less than a factor of 1. In other words, doubling the weight of the anchor results in less than double the Performance.

That said, these "scale factors" appear to be strongly affected by seabed type. I'll guess that there are perhaps 10 or more seabed types with significant variation and maybe 100's of seabed types with slight variations.

I believe it is impossible to assign a "single number" to an anchor's scale factor as it will likely vary significantly by seabed type.

Steve

[Dockhead]

Originally Posted by Panope

I have examined two different sizes of Rocna (rollbar) anchors and found no differences in design. They were scaled versions of each other with the exception of plate thicknesses being scaled slightly differently due to (presumably) material being readily available in a limited number of thicknesses.

Also, I have tested two different Rocna anchors (10 and 20kg) and have found that performance increases by less than a factor of 1. In other words, doubling the weight of the anchor results in less than double the Performance.

That said, these "scale factors" appear to be strongly affected by seabed type. I'll guess that there are perhaps 10 or more seabed types with significant variation and maybe 100's of seabed types with slight variations.

I believe it is impossible to assign a "single number" to an anchor's scale factor as it will likely vary significantly by seabed type.

Steve

Thanks again for all this great work, Steve. You've done a huge service to the cruising community.


Another anchor quality which in my subjective experience does not scale 1:1 is setting behavior. And I think setting behavior is far more important than ultimate holding power, since it is not that hard to choose an anchor which, when well set, has more than enough ultimate holding power even in weak seabeds. Not being able to get the anchor well set is in my experience a far more common problem than lack of ultimate holding power.


My experience, and others have written about similar experiences, is that setting behavior improves disproportionatley with size, and seems to take some kind of leap when the anchor is over 100 pounds. I think Dashew and someone else have written about this, and I think I've experienced it myself. Have you done any work on this?

[rslifkin]

Originally Posted by Dockhead

Thanks again for all this great work, Steve. You've done a huge service to the cruising community.


Another anchor quality which in my subjective experience does not scale 1:1 is setting behavior. And I think setting behavior is far more important than ultimate holding power, since it is not that hard to choose an anchor which, when well set, has more than enough ultimate holding power even in weak seabeds. Not being able to get the anchor well set is in my experience a far more common problem than lack of ultimate holding power.


My experience, and others have written about similar experiences, is that setting behavior improves disproportionatley with size, and seems to take some kind of leap when the anchor is over 100 pounds. I think Dashew and someone else have written about this, and I think I've experienced it myself. Have you done any work on this?

I'll second Steve's work being an excellent contribution to the community.


As far as setting, I can vouch for the fact that my 73 lb Vulcan "just sets". There's no fussing or anything. Even when dropped into fairly soft river mud or a pile of weed salad, it's just drop and gently bring up tension. As the tension comes up, it's already set and pulling harder just digs it in deeper. The only movement I've ever noticed has been from stretch in the rode, save for the time I dropped it into a pile of cobblestone without knowing it, where with only 3:1 scope out (only staying for a couple hours in good weather) it would hold a gentle pull, but not even idle reverse on both of my engines. Considering it started skipping along the rocks only when the chain got stretched enough, more scope likely would have helped.

[soopad00pa]

To chime back in on setting my 10kg roc with 20ft of chain, I've never in the 10ish times I've set the anchor had an issue with it setting quickly and securely.

I drop the anchor and play out a 5 to 7 scope (7 if I have the space and if it feels to tight to get 5,I move on) and wait a couple of min doing other boat stuff. Then I back down slowly increasing rpms as I sight markers. Never any slippage.

Of the half dozen overnights, there've been no problems except for the one prompting this post. I was beginning to wonder what all the fuss was about...

[thomm225]

Originally Posted by soopad00pa

To chime back in on setting my 10kg roc with 20ft of chain, I've never in the 10ish times I've set the anchor had an issue with it setting quickly and securely.

I drop the anchor and play out a 5 to 7 scope (7 if I have the space and if it feels to tight to get 5,I move on) and wait a couple of min doing other boat stuff. Then I back down slowly increasing rpms as I sight markers. Never any slippage.

Of the half dozen overnights, there've been no problems except for the one prompting this post. I was beginning to wonder what all the fuss was about...

You'll learn more as you anchor in winds to 25 knots or more and other times when the tide swings you sideways or beam too to the waves in the middle of the night

Or when a squall comes through with gusts in the 40 knot range.......my CQR is 20 lbs but my boat is narrower than yours and about 4,000 lbs lighter.

Luckily in the video below I had offshore winds so the biggest worry was the lightning since I had 30 miles of Bay behind me.

In the video I still had the old chain and rode with chain being around 35' now I have hooked the CQR to the new chain about 40' and heavier and new rode which had been attached to my spare anchor a one-off Bruce.

Also replaced the old shackle at the Chain / rope rode connection. This after 9 years of use. It was on the bow when I bought the boat.

https://www.youtube.com/watch?v=okRZs6os5Eo