The 2000 kg holding myth

[noelex]

Quite easily

Independent Anchor Performance Testing

Just check the holds from independent testing for Rocna.

Jonathan

That is Peter Smith’s (from Rocna) own website where he summarises the results of several anchor tests.

It is perhaps not surprisingly a rather biased view.

The tests he quotes are independent, but his summary is not.

Many of the individual results, particularly the poor results relating to Rocnas is omitted. Reading this will not give you a realistic view of the expected performance. For example, the seperate results from the three different substrates measured in the extensive 2006 test is not reported. There are many other omissions.

I would encourage everyone to instead read the oringinal anchor tests. Most are available online if you search.

Every time I mention the 2,000kg hold I mention 'in good holding'. I never say that the anchor holds 2,000kgs in every seabed. You enjoy selective editing .

Modern anchors in a good substrate often do not have holding ability anything like 2000 kg. I think I made this clear in post #1 . Nor do they have a hold of 1000 kg in "difficult" substrates, as you claim in post #23 . The 2000 kg claim (and the 1000 kg claim in "difficult" substrates) are both a myth.

 

[srm]

It might be a bit of water logged wood, an oyster shell - but invariably there is a reason.

Mid 70's I had a 50ft decommissioned fishing vessel. Needed ground tackle so bought a 50kg (newfangled) Bruce and half inch galvanised chain (long before a forum like this could tell me it was too big). First time we anchored in a remote location we dragged across the bay trying to set it. Recovered anchor and found the metal hoop from a herring barrel hooked over the central fluke. Cleared the anchor and tried again, anchor held under power as expected. Next day we recovered the anchor only to find a large slab of rock, around the size of a big grave stone, jammed between fluke and shank with a smaller rock locking it in place.

Taught me that if the Bruces (15kg on my yacht) did not set as expected stop and recover, something is blocking it. More recently we did not like the forecast and decided to move despite having been securely anchored for a couple of days. The Rocna came up wrapped in a sack! Its as well we decided to move as winds were around 50kn with spindrift lifting shortly afterwards.

 

[noelex]

There is an important distinction between dynamic holding capacity (DHC) and static holding capacity (SHC) that should be understood when reading anchor tests.

Nearly all anchor tests measure dynamic holding capacity (DHC). The anchor is pulled with a load cell on the rode and the maximum force experienced is recorded. The anchor test does not try and assess if the anchor is stationary, it simply records the maximum force before the anchor breaks out and the holding force drops. At the limit when the highest load is applied the anchor is not completely stationary, but is creeping slowly through the substrate. This is how anchors typically behave at their limit.

All anchors are tested in the same way so this is a valid comparison of different anchor models. It is also a relatively easy measurement, but boat owners need to know the SHC (static holding capacity). This is the maximium force the anchor can hold without moving at all. This is difficult and time consuming to measure. The pulling power has to be reduced when the anchor starts slowly moving until the anchor is stationary. This is repeated several times until there is no increase (the anchor has set as deeply as it can). The maximum force the anchor can hold without moving is the SHC.

SHC is always a significantly lower number than the DHC. Typically, the value will be around 40-60% lower (although there is some variation in different substrates). Most anchor tests (and all the numbers quoted so far) are only a measure of DHC.

As an example, the graph below is a rare anchor test measuring both DHC and SHC. This is for a 4kg Rocna. The blue dots represent the DHC. This has a maximum value of around 150 kg in this substrate, but as the graph notes, the anchor is creeping at 1.5 cm per second when 150 kg of force is applied. You can see this on the X axis of the graph. An anchor that is continuing to creep at 1.5 cm per second is not acceptable for safe anchoring. The maximum static hold (the important SHC) is shown with the purple dots. Here the force has been reduced until the anchor just stops moving. For this anchor the maximum holding power with the anchor completely stationary (or SHC) is around 80 kg, but the numbers are less important than understanding the principle. The graph below is from this test:
https://static1.squarespace.com/sta...02103d72392/1489319853474/JHK+PBO+article.pdf

Note how both the DHC and the SHC increase in the initial stages as the anchor buries deeper, but at around 2.5 m the anchor has dived as deeply as it can in this substrate and both the DHC and the SHC no longer continue to increase.

Thus in this case the important SHC is around 53% of the DHC.

Keep this distinction between DHC and SHC in mind when considering anchor test results, remembering anchor tests are normally only measuring the DHC value. We need to derate this number by a substantial margin (usually around half) to determine the holding value that is important to boat owners, which is the SHC.

Thus an anchor test that reports a "holding power" of 2000 kg is reporting the maximum hold where the anchor is allowed to creep. The maximum holding power where the anchor remains stationary is likely to be around half of this figure or 1000 kg.

This further shows the 2000 kg claim is a myth.


Sorry for the long post, but these technical matters are hard to summarise briefly.

 

[Bouba]

There is an important distinction between dynamic holding capacity (DHC) and static holding capacity (SHC) that should be understood when reading anchor tests.

Nearly all anchor tests measure dynamic holding capacity (DHC). The anchor is pulled with a load cell on the rode and the maximum force experienced is recorded. The anchor test does not try and assess if the anchor is stationary, it simply records the maximum force before the anchor breaks out and the holding force drops. At the limit when the highest load is applied the anchor is not completely stationary, but is creeping slowly through the substrate. This is how anchors typically behave at their limit.

All anchors are tested in the same way so this is a valid comparison, and it is a relatively easy measurement, but boat owners need to know the SHC (static holding capacity). This is the maximium force the anchor can hold without moving at all. This is difficult and time consuming to measure. The pulling power has to be reduced when the anchor starts slowly moving until the anchor is stationary. This is repeated several times until there is no increase (the anchor has set as deeply as it can). The maximum force the anchor can hold without moving is the SHC.

SHC is always a significantly lower number than the DHC. Typically, the value will be around 40-60% lower (although there is some variation in different substrates). Most anchor tests (and all the numbers quoted so far) are only a measure of DHC.

As an example, the graph below is a rare anchor test measuring both DHC and SHC. This is for a 4kg Rocna. The blue dots represent the DHC. This has a maximum value of around 150 kg in this substrate, but as the graph notes, the anchor is creeping at 1.5 cm per second when 150 kg of force is applied. You can see this on the X axis of the graph. An anchor that is continuing to creep at 1.5 cm per second is not acceptable for safe anchoring. The maximum static hold (the important SHC) is shown with the purple dots. Here the force has been reduced until the anchor just stops moving. For this anchor the maximum holding power with the anchor completely stationary (or SHC) is around 80 kg, but the numbers are less important than understanding the principle. The graph below is from this test:
https://static1.squarespace.com/sta...02103d72392/1489319853474/JHK+PBO+article.pdf

Note how both the DHC and the SHC increase in the initial stages as the anchor buries deeper, but at around 2.5 m the anchor has dived as deeply as it can in this substrate and both the DHC and the SHC no longer continue to increase.

Thus in this case the important SHC is around 50% of the DHC.

Keep this distinction between DHC and SHC in mind when considering anchor test results, remembering anchor tests are normally only measuring the DHC value. We need to derate this number by a substantial margin (usually around half) to determine the holding value that is important to boat owners, which is the SHC.

Thus an anchor test that reports a "holding power" of 2000 kg is reporting the maximum hold where the anchor is allowed to creep. The maximum holding power where the anchor remains stationary is likely to be around half of this figure or 1000 kg.

This further shows the 2000 kg claim is a myth.

View attachment 183046
Sorry for the long post, but these technical matters are hard to summarise briefly.

I did not know this...I just assumed that DHC was a variable wind....and SHC was a steady force...thanks

 

[NormanS]

I'm just so glad that I can anchor successfully without knowing or caring about all the mathematics that some seem to need.

 

[Neeves]

There is an important distinction between dynamic holding capacity (DHC) and static holding capacity (SHC) that should be understood when reading anchor tests.

Nearly all anchor tests measure dynamic holding capacity (DHC). The anchor is pulled with a load cell on the rode and the maximum force experienced is recorded. The anchor test does not try and assess if the anchor is stationary, it simply records the maximum force before the anchor breaks out and the holding force drops. At the limit when the highest load is applied the anchor is not completely stationary, but is creeping slowly through the substrate. This is how anchors typically behave at their limit.

All anchors are tested in the same way so this is a valid comparison of different anchor models. It is also a relatively easy measurement, but boat owners need to know the SHC (static holding capacity). This is the maximium force the anchor can hold without moving at all. This is difficult and time consuming to measure. The pulling power has to be reduced when the anchor starts slowly moving until the anchor is stationary. This is repeated several times until there is no increase (the anchor has set as deeply as it can). The maximum force the anchor can hold without moving is the SHC.

SHC is always a significantly lower number than the DHC. Typically, the value will be around 40-60% lower (although there is some variation in different substrates). Most anchor tests (and all the numbers quoted so far) are only a measure of DHC.

As an example, the graph below is a rare anchor test measuring both DHC and SHC. This is for a 4kg Rocna. The blue dots represent the DHC. This has a maximum value of around 150 kg in this substrate, but as the graph notes, the anchor is creeping at 1.5 cm per second when 150 kg of force is applied. You can see this on the X axis of the graph. An anchor that is continuing to creep at 1.5 cm per second is not acceptable for safe anchoring. The maximum static hold (the important SHC) is shown with the purple dots. Here the force has been reduced until the anchor just stops moving. For this anchor the maximum holding power with the anchor completely stationary (or SHC) is around 80 kg, but the numbers are less important than understanding the principle. The graph below is from this test:
https://static1.squarespace.com/sta...02103d72392/1489319853474/JHK+PBO+article.pdf

Note how both the DHC and the SHC increase in the initial stages as the anchor buries deeper, but at around 2.5 m the anchor has dived as deeply as it can in this substrate and both the DHC and the SHC no longer continue to increase.

Thus in this case the important SHC is around 53% of the DHC.

Keep this distinction between DHC and SHC in mind when considering anchor test results, remembering anchor tests are normally only measuring the DHC value. We need to derate this number by a substantial margin (usually around half) to determine the holding value that is important to boat owners, which is the SHC.

Thus an anchor test that reports a "holding power" of 2000 kg is reporting the maximum hold where the anchor is allowed to creep. The maximum holding power where the anchor remains stationary is likely to be around half of this figure or 1000 kg.

This further shows the 2000 kg claim is a myth.

View attachment 183046
Sorry for the long post, but these technical matters are hard to summarise briefly.

This why having a video monitor or a diver, or test in shallow water allows definition of UHC. Its not rocket science. its all been done.

Jonathan

 

[noelex]

I did not know this...I just assumed that DHC was a variable wind....and SHC was a steady force...thanks

Your welcome.

Thanks for everyone’s participation. I realise the science behind anchor testing is not everyone’s cup of tea.

I have some sympathy with this view. There is much to recommend selecting simply selecting the best anchor model in the largest size you can comfortably manage, dropping it on the seabed and getting a good nights sleep .

But when anchor testing is discussed on the forum this should be accurate information, particularly when the same posts are repeated time and time again. Many of the posts on this forum on anchor threads imply that 2000 kg of holding should be expected when you drop a 15 kg anchor. This is not accurate and hopefully we have put this myth to bed.

 

[srm]

Anchoring used to be such a simple skill, practiced by generations of seafarers.
Then the internet came along and everything became so complicated with anchor tests, plotters and anchor alarm apps seemingly adding to what looks like growing anchoring paranoia amongst leisure sailors.

 

[Bouba]

Anchoring used to be such a simple skill, practiced by generations of seafarers.
Then the internet came along and everything became so complicated with anchor tests, plotters and anchor alarm apps seemingly adding to what looks like growing anchoring paranoia amongst leisure sailors.

Sailing used to be such a lonely pursuit, practiced by generations of seafarers.
Then the internet came along and everything became so social with anchor tests, plotters and anchor alarm apps seemingly adding to what looks like growing social interactions and friendships amongst leisure sailors.

 

[RunAgroundHard]

Anchoring used to be such a simple skill, practiced by generations of seafarers.
Then the internet came along and everything became so complicated with anchor tests, plotters and anchor alarm apps seemingly adding to what looks like growing anchoring paranoia amongst leisure sailors.

It still is simple. Many are anchoring without worrying about it beyond the usual care that a skipper has to take when anchoring.

 

[RunAgroundHard]

Sailing used to be such a lonely pursuit, practiced by generations of seafarers.
Then the internet came along and everything became so social with anchor tests, plotters and anchor alarm apps seemingly adding to what looks like growing social interactions and friendships amongst leisure sailors.

It still is lonely. Wafting your phone about looking for a solution online as you drag towards a leeshore in a F10 with diesel bug, trying to recover a MOB, wondering if you have right of way over a neighboring drager, while complaining about the accuracy of weather reports, and asking if keel bolts on an encapsulated keel will fail, and the names of any good GRP repairer, may not be the best time to look for friendship online interactions.

 

[lustyd]

Anchoring used to be such a simple skill, practiced by generations of seafarers.

Many of whom are still sitting on the seabed due to inadequate technique or poor equipment.

 

[Tranona]

But when anchor testing is discussed on the forum this should be accurate information, particularly when the same posts are repeated time and time again. Many of the posts on this forum on anchor threads imply that 2000 kg of holding should be expected when you drop a 15 kg anchor. This is not accurate and hopefully we have put this myth to bed.

Seems that only you think this - always been pretty clear to me that such hold is not achievable in practice. The key issue has always been determining what load a boat can apply to the anchor and it is very clear that a boat where 15kgs is recommended cannot achieve that load.

The explanation as to how that figure is arrived at and used seems equally clear as is the way that it can be used for comparisons so it seems that implication is yours and not from those providing the (accurate) information on very specific test results..

 

[vyv_cox]

Anchoring used to be such a simple skill, practiced by generations of seafarers.
Then the internet came along and everything became so complicated with anchor tests, plotters and anchor alarm apps seemingly adding to what looks like growing anchoring paranoia amongst leisure sailors.

In those far off blissful days it was assumed that in poor weather a crew member would sit on anchor watch all night. The RN still does. Not so long ago we did. Nowadays everyone would prefer to sleep soundly with nobody on watch, except possibly a mobile phone.

 

[srm]

In those far off blissful days it was assumed that in poor weather a crew member would sit on anchor watch all night. The RN still does. Not so long ago we did. Nowadays everyone would prefer to sleep soundly with nobody on watch, except possibly a mobile phone.

Yes, I have kept an anchor watch, two, perhaps three times, in 50 years as a yacht owner. On each occasion the wind was in excess of 60 knots the whole time and spindrift was being lifted off a sheltered anchorage. Also, another night secured in a marina berth with surprisingly big waves from a 150 metre fetch breaking over the pontoon to windward and sinking another boat. One memorable watch was from breakfast to lunch time, it felt quite calm having a mug of tea in the cockpit when the anemometer eventually dropped to around 50 knots.
I anchored frequently preferring my own ground tackle. Always slept soundly at night with nobody on watch.

 

[geem]

In those far off blissful days it was assumed that in poor weather a crew member would sit on anchor watch all night. The RN still does. Not so long ago we did. Nowadays everyone would prefer to sleep soundly with nobody on watch, except possibly a mobile phone.

We have never set an anchor alarm. When we have squalls above 30kts, I tend to wake up. I will go into the cockpit and do a check on who might be dragging towards us. I rarely worry about us dragging. In particularly bad weather circa 40/50kts, we do anchor watches. Things can deteriate rapidly under such conditions and relying on an anchor alarm is not my idea of good seamanship. Usually at that wind speed it's hard to sleep anyway. Anything under 30kts and we don't drag but we do power set with a big prop and engine combination at 3/4 throttle. If we know there is a storm coming we would possible move anchorage, let out more chain and power set to full revs.
It would be interesting to work out how my power setting correlated to a wind speed load on the hull and rig. Is it equivalent to 30kts?

 

[boomerangben]

The whole anchor holding capacity is moot if you don’t know how much load your boat will put on it in the conditions you might expect as a worst case. So the unit of measurement for anchor holding might as well be bananas or other fruit or vegetable of choice.

Formula for static drag is 1/2 x density of air x wind speed squared x area x drag coefficient.

If I assume my boat is roughly conical in aspect to the wind (Cd about 0.4) with a beam of 2m and air draft of 1m, the area is 2 sqm, so with wind speed of say 20m/s (about 40kts) the drag is 1/2 x 1 x 400 x 2 x 0.4 which is about 160kg. Mast is 8m long by about 100mm in diameter and a Cd of about 0.5, so add another 80kg for that.

Scaling up for a 35’ boat with a beam of maybe 3.5m and an air draft of 2m, the load is going to be roughly 600kg including the mast.

But my estimates of Cd are just guesswork

But even using those figures, I would not bet the boat on my calcs.

But my point remains, unless you have a good idea what load your boat might put on the anchor, 2000kg is irrelevant

 

[noelex]

It would be interesting to work out how my power setting correlated to a wind speed load on the hull and rig. Is it equivalent to 30kts?

I am not sure if this answers your question, but when diving on our anchor I have consistently observed that if the wind is below about 25-30 knots the anchor does not dive further under the substrate than the depth of bury we achieve under motor while setting the anchor.

If the wind is above 25-30 knots the anchor will start to dive deeper than we observed with just the motor setting force.

The above is assuming the wind is light when the anchor was set.

We have a folding propeller (our previous boat had a feathering propellor) and usually set the anchor under motor, gradually increasing the throttle to full cruising revs which we hold for about thirty seconds.

As you have a large motor and prop, your numbers may be a little different.