Chesapeake Bay Anchor Holding Power Test

[Maine Sail]

Originally Posted by estarzinger

I can generate about 1000lbs (in reverse), but I was not sure how it scaled down to smaller hp/props (linear to hp?).

44HP engine, 36 foot boat RPM taken to cruise in reverse.... If I go higher, to 3K, I can break 1000 pounds peak... As one can see the load is not always static probably due to prop slip and the rebound in the rode.....
What Happens When I Back Down on My Anchor - YouTube

[thinwater]

My appologies for being grumpy. Good work characterizing the bottom. The pictures explain the holding power results.

I have always felt that even more than soft mud, bottoms that are layered with problem substrates are the real Chesapeake problem; shell under mud is classic and common case in places that formerly held vast oyster beds that failed and were covered with silt. probably not even obvious if you dive on the anchor, which is not common practice in the Chesapeake anyway.

Though I am a believer in letting the anchor soak when in very soupy coves, I doubt time will change much if you are setting on shells.

Tough problem. Sometimes they come up way too easy in the morning, even after a good set.

[thinwater]

Originally Posted by Maine Sail

44HP engine, 36 foot boat RPM taken to cruise in reverse.... If I go higher, to 3K, I can break 1000 pounds peak... As one can see the load is not always static probably due to prop slip and the rebound in the rode.....
What Happens When I Back Down on My Anchor - YouTube

Think on this, then. Do we...
1. let the anchor soak a while, then
2. backdown with increasing force, then
3. bounce on the anchor, using momentum to create 2-4 times the force for very limitied timed periods. The time periods will be longer with fiber rode and the rebound could be substancial.

I did this with a former boat (outboard and fiber rode) sometimes.

My gut is that that this is a bad idea if taken too far, in part because of the effect of rebound, and in part because the mud is non-newtonian (like silly putty). However, either it is relavant or Mainesail can only count static thrust (about 450 pounds at 2500 RPM).

Down wind sets are old school, we've all done it, and it is based upon this.

Intersting.

[Jim Cate]

Thanks for that, Mainesail. The rapid variation in load is interesting, and reflects what one observes on the foredeck, watching the angle of the rode as one backs down. It is always nice to put some numbers on the eyeball data!

Query: what sort of prop was on that 44 hp engine, and what sort of rode was involved?

Jim

[Maine Sail]

Originally Posted by Jim Cate

Thanks for that, Mainesail. The rapid variation in load is interesting, and reflects what one observes on the foredeck, watching the angle of the rode as one backs down. It is always nice to put some numbers on the eyeball data!

Query: what sort of prop was on that 44 hp engine, and what sort of rode was involved?

Jim

16X9 Campbell Sailor, 100' 5/16" chain and about 65' of 5/8" rode.

[estarzinger]

^^Interesting.

I did my bollard pull using 25' of dyneema to a piling exactly because I wanted to measure the steady state force and not get a bunch of dynamic noise (and it does look like this scales very roughly linearity).

But it is also quite interesting to see how much "bounce load" there is. . . . Makes sense as there is a lot of momentum. I am not sure if that is useful in anchor setting, or whether it might also jerk the anchor out.

But the answer to my initial question seems to be that we all have a hard time generating a steady state reverse bollard pull equal to the 30kt ABYC table, while we can generate that as a transient load with a "bounce".

[Fuss]

Originally Posted by estarzinger

^^Interesting.

I did my bollard pull using 25' of dyneema to a piling exactly because I wanted to measure the steady state force and not get a bunch of dynamic noise (and it does look like this scales very roughly linearity).

But it is also quite interesting to see how much "bounce load" there is. . . . Makes sense as there is a lot of momentum. I am not sure if that is useful in anchor setting, or whether it might also jerk the anchor out.

But the answer to my initial question seems to be that we all have a hard time generating a steady state reverse bollard pull equal to the 30kt ABYC table, while we can generate that as a transient load with a "bounce".

Maybe this topic needs a new thread, but anyway...

It would be interesting to have a table of those figures at different revs.

There are quite a few unanswered questions on this topic that could be laid to rest by some more in depth testing.

For example
I'm not sure that I agree with the load at 30kts of wind especially when it is said that the average boat can only generate a pull equal to around 30kts.

I feel, but have not measured it, that I can generate twice this pull (60 kts)
It is based on the following...
What my chain looks like in 30kts of wind.
What my chain looks like in 60kts of wind.

I mean, think about this without getting too technical about bows blowing off or astern but holding station....
Can you only motor into a maximum wind of 30kts.
Can you motor into 60 kts of wind
Can you motor into 70 kts of wind

Here is a small example that I can remember....I once began anchoring in 45kts of steady wind.

I had to talk to someone on the vhf so I was astern but not much revs to hold position, I am fairly certain that it was not half throttle/hp or I would have remembered.
After about 2 mins I then went into neutral, dumped about 60m of chain and waited while we headed off downwind.
Chain came to the end, bow jerked into the wind, took some transits, let out another 20m for safety and stayed put all night at around 45kts.

So on this occasion I was able to generate the wind force of 45kts easily using my engine.

So to sum up. I'm fairly certain I can motor into 70kts of wind and so I am fairly certain I can generate 70kts of pull.
(Is there something wrong with the theory that if I can motor into a wind strength then I can generate this wind strength?)

That seems to contradict some opinions on here where it is believed that 30kts wind equivalent is all that can be generated by motoring astern.

Of course some proper controlled measuring would be helpful to everyone on this forum I think.

[estarzinger]

^^

There are a couple know points about this:

(1) The ABYC anchor load numbers are known to be 'high' for pure bow in wind load. They say so themselves. They applied either a 2 or 3x factor (can't remember which), which they say is in order to include factors such as: (a) the boat swings beam on to the wind at the end of sailing back and forth at anchor - that by itself will about double the load, (b) wave impact loads in an open large fetch anchorage, (c) shock loads fetching up at the end of the chain, (d) possible current. So the ABYC numbers are intentionally worst case sort of numbers.

(2) motoring forward into wind is rather different than bollard pull for a number of reasons. (a) props are more effecient when moving thru the water than when the boat is held stationary (the water around the prop is turbulant when the boat is held stationary), (b) many props (usually with curved blades) are more efficient in forward than in reverse, (c) when you motor forward usually you are not exactly directly into the wind and may even get 'sailing help' from the keel.

I will say that I know for a fact that I have difficulty making progress under motor alone directly into 40 kts and the resulting waves (from 'medium distance shore' fetch). I know for a fact that I cannot make progress into 60kts under motor alone.

I do completely agree that it would be interesting and useful to have much more data on actual anchoring loads on a variety of boats in a variety of situations, and on bollard pull and forward moving thrust again with a variety of boats in a variety of situations.

[onestepcsy37]

Originally Posted by Fuss

Maybe this topic needs a new thread, but anyway...

It would be interesting to have a table of those figures at different revs.

There are quite a few unanswered questions on this topic that could be laid to rest by some more in depth testing.

For example
I'm not sure that I agree with the load at 30kts of wind especially when it is said that the average boat can only generate a pull equal to around 30kts.

I feel, but have not measured it, that I can generate twice this pull (60 kts)
It is based on the following...
What my chain looks like in 30kts of wind.
What my chain looks like in 60kts of wind.

I mean, think about this without getting too technical about bows blowing off or astern but holding station....
Can you only motor into a maximum wind of 30kts.
Can you motor into 60 kts of wind
Can you motor into 70 kts of wind

Here is a small example that I can remember....I once began anchoring in 45kts of steady wind.

I had to talk to someone on the vhf so I was astern but not much revs to hold position, I am fairly certain that it was not half throttle/hp or I would have remembered.
After about 2 mins I then went into neutral, dumped about 60m of chain and waited while we headed off downwind.
Chain came to the end, bow jerked into the wind, took some transits, let out another 20m for safety and stayed put all night at around 45kts.

So on this occasion I was able to generate the wind force of 45kts easily using my engine.

So to sum up. I'm fairly certain I can motor into 70kts of wind and so I am fairly certain I can generate 70kts of pull.
(Is there something wrong with the theory that if I can motor into a wind strength then I can generate this wind strength?)

That seems to contradict some opinions on here where it is believed that 30kts wind equivalent is all that can be generated by motoring astern.

Of course some proper controlled measuring would be helpful to everyone on this forum I think.

I can't imagine my boat powering in to 70 knots of wind, maybe not even 45.

[estarzinger]

Btw . . . Seaworthy's write up of the test is now "public" on their website.

The Fine Art Of Anchoring - Seaworthy Magazine - BoatUS

[noelex 77]

Now that the dust has settled on this test, it is time to look at results in some detail.

Firstly thanks to Fortress for doing the study. I think each one of these types of tests adds to our understanding of how anchors perform. This was obviously a very difficult test. As Estarzinger has already indicated there was a very high variation in the results. It concerns me that people are drawing very specific conclusions from data with a very high degree of variability.

One aspect that has not been raised is how we translate these these test results into the holding power that the anchors have actually achieved.

The "holding power" power numbers for the anchor have been widely reported and translated into wind speeds that the anchor would "hold" the boat. However, everyone including the magazine write ups seem to miss the the point that the anchors were never stationary.

The force recorded was the force the anchor exhibited while being pulled backwards at about 5cm a second. I don't want to get too technical, but this is often called the Dynamic Holding Capacity (ability, or power) often abbreviated to DHC. The DHC capacity of an anchor is not what we need to know. An anchor that is dragging at 5cm a second (or 100 feet in 10 mins) is no good. Such an anchor would rapidly get us into trouble.

What we need to know if we are going to "anchor" the boat is what force the anchor would hold not while moving at 100 feet in 10 mins, but while stationary. This is often termed the Static Holding Capacity or SHC.

So if we want to determine what holding power the anchors really achieved we need to convert the DHC measured during the test to likely SHC results.

This is actually quite tricky. Professor Knox has done a number of studies in this regard measuring both DHC and SHC and trying to find a relationship between the two. While as boat owners we need to know SHC, DHC is much easier to measure.

The relationship between DHC and SHC depends on a number of factors. The speed of pull is one of the major influences. At 5cm a second (as measured in the Fortress test) the conversion from DHC to SHC is, according to Professor Knox, about 0.24. So if an anchor recorded a pull at 100 feet in 10 mins of 1000lb the force where it would stop moving and hold the boat would be 240 lb. Obviously a very significant difference.

I think yachtsmen understand the implications of windspeed much better than force measurements. If we analyse the graphs and determine the maximum dynamic holding power for the star performer of the test the FX37 set at 45degrees. I get 1300 lbs.

If we use the ABYC guidelines and assume the owner has selected the recommended size anchor for their boat (a mistake in my view) 1300 lbs would start to give way at 27 knots. This is a low number for overnight anchoring especially when we take into account the high standard deviation. In two of the 5 trials the anchor recorded much lower values. However, even this number assumes the anchor was holding the boat at 1300 lbs. It was not. The anchor was dragging at this force to the tune of 100 feet in 10 mins.

If we translate the DHC measured during the test and apply the coefficients recorded by Prof Knox we find to not move the anchors peak holding ability would only be 0.24x1300=310 lbs.

This translates to windspeed of under 14 knots.

I don't think this is the correct number. The conversion for radically different anchor designs are not likely to be the same. The conversion for different substrates is also likely to be very different.

I am left with the the view that the Fortress set at 45 degrees did much better than any of the other anchors in this very soft substrate. However do I want to anchor in this substrate overnight? The actual wind speeds that the anchor would hold and not drag backwards are open for speculation, but they are very low.

[estarzinger]

What was the lat/long of the test - would just be curious if it 'looks like" an attractive anchorage to a visiting cruising sailor.

[Fortress]

Originally Posted by estarzinger

Btw . . . Seaworthy's write up of the test is now "public" on their website.

The Fine Art Of Anchoring - Seaworthy Magazine - BoatUS

Charles Fort did a nice job with his write up of the test. We all had a good hearty laugh over his comment: "During the first test done with the Fortress 37, with its ideal 45-degree mud-bottom fluke angle, it failed to set during the entire 100-foot drag. Chagrinned, the Fortress representatives nervously joked that at least it was obvious the test wasn't rigged in their favor."

I am sure glad that he didn't hear my comment about wondering whether a nearby bridge was high enough....

Originally Posted by noelex 77

Now that the dust has settled on this test, it is time to look at results in some detail.

Firstly thanks to Fortress for doing the study. I think each one of these types of tests adds to our understanding of how anchors perform. This was obviously a very difficult test. As Estarzinger has already indicated there was a very high variation in the results. It concerns me that people are drawing very specific conclusions from data with a very high degree of variability.

One aspect that has not been raised is how we translate these these test results into the holding power that the anchors have actually achieved.

The "holding power" power numbers for the anchor have been widely reported and translated into wind speeds that the anchor would "hold" the boat. However, everyone including the magazine write ups seem to miss the the point that the anchors were never stationary.

The force recorded was the force the anchor exhibited while being pulled backwards at about 5cm a second. I don't want to get too technical, but this is often called the Dynamic Holding Capacity (ability, or power) often abbreviated to DHC. The DHC capacity of an anchor is not what we need to know. An anchor that is dragging at 5cm a second (or 100 feet in 10 mins) is no good. Such an anchor would rapidly get us into trouble.

What we need to know if we are going to "anchor" the boat is what force the anchor would hold not while moving at 100 feet in 10 mins, but while stationary. This is often termed the Static Holding Capacity or SHC.

So if we want to determine what holding power the anchors really achieved we need to convert the DHC measured during the test to likely SHC results.

This is actually quite tricky. Professor Knox has done a number of studies in this regard measuring both DHC and SHC and trying to find a relationship between the two. While as boat owners we need to know SHC, DHC is much easier to measure.

The relationship between DHC and SHC depends on a number of factors. The speed of pull is one of the major influences. At 5cm a second (as measured in the Fortress test) the conversion from DHC to SHC is, according to Professor Knox, about 0.24. So if an anchor recorded a pull at 100 feet in 10 mins of 1000lb the force where it would stop moving and hold the boat would be 240 lb. Obviously a very significant difference.

I think yachtsmen understand the implications of windspeed much better than force measurements. If we analyse the graphs and determine the maximum dynamic holding power for the star performer of the test the FX37 set at 45degrees. I get 1300 lbs.

If we use the ABYC guidelines and assume the owner has selected the recommended size anchor for their boat (a mistake in my view) 1300 lbs would start to give way at 27 knots. This is a low number for overnight anchoring especially when we take into account the high standard deviation. In two of the 5 trials the anchor recorded much lower values. However, even this number assumes the anchor was holding the boat at 1300 lbs. It was not. The anchor was dragging at this force to the tune of 100 feet in 10 mins.

If we translate the DHC measured during the test and apply the coefficients recorded by Prof Knox we find to not move the anchors peak holding ability would only be 0.24x1300=310 lbs.

This translates to windspeed of under 14 knots.

I don't think this is the correct number. The conversion for radically different anchor designs are not likely to be the same. The conversion for different substrates is also likely to be very different.

I am left with the the view that the Fortress set at 45 degrees did much better than any of the other anchors in this very soft substrate. However do I want to anchor in this substrate overnight? The actual wind speeds that the anchor would hold and not drag backwards are open for speculation, but they are very low.

There are certainly many ways of analyzing the data from the testing, which I have found just from the two internal reports that have been written by Bob Taylor, the retired US Navy soil mechanics / anchor design expert who we hired to consult for us on this project, and from Chuck Hawley, formerly the VP of Product Testing at West Marine, who served as the independent reviewer for the testing.

Noelex, as discussed privately, Professor Knox's calculations are based on a hard soil, and I would be glad to send Bob Taylor's comments in response to the Professor's to anyone who has a further interest. Bob has over 45 years in this field and he certainly has impressive credentials to back up his data.

To simplify, and concerning your comments above, in two tests it took a pull load of over 2,000 lbs to continue moving the 21 lb (10 kg) FX-37 @ the 45° angle through the soft mud.....and the tension was climbing in both pulls when the test ended. We also broke the cable at 3,500 lbs after one of the tests when we tried to break the anchor free while we were directly above it and pulling at a 1:1 scope (image of chart below).

During one pull, this anchor reached a tension of nearly 1,900 lbs again before slowly falling back to 1,000 when the test was ended. In another pull, the tension was climbing and reached 800 lbs at the end of test, so in 4 out of 5 tests, the tension was either climbing and/or the anchor remained engaged in the sea bottom when the test was terminated.

By contrast, in 3 out of 5 tests, and to the great surprise of many aboard, the 44 lb (20 kg) Rocna did not engage the bottom, which we were able to determine because we had pulled a 45 lb lead weight and the readings were similar.

There was concern by witnesses aboard the boat whether this anchor would be able to orient itself in soft mud if it landed on its side or upside down...and that it might simply sink and then not be able to orient the fluke in the downward position as it was being pulled forward.

In the two other tests, the Rocna hit peaks of approximately 300 and 700 lbs.

Here are two other stories that have been written about the testing:

All at Sea:
Best Anchor for the Chesapeake Bay? - ALL AT SEA

Boats.com:
Fortress Anchor Testing: When It’s Good To Be a Stick in the Mud | boats.com Blog

Additional stories will be published in future issues of PassageMaker, Soundings, and Sail.

Originally Posted by estarzinger

What was the lat/long of the test - would just be curious if it 'looks like" an attractive anchorage to a visiting cruising sailor.

Evans, if you zoom in on the image below, the lat/long is noted inside a red box.

[jstevens]

"What was the lat/long of the test - would just be curious if it 'looks like" an attractive anchorage to a visiting cruising sailor."

Evans,
Looks like the test were performed in St. John Creek near Solomons, MD. It is a popular anchoring area, but there is a lot of small power boat traffic especially on weekends. Solomons is a nice town with very good marine facilities, but for nice anchorages go further up the Patuxent River to St. Leonard Creek.

John

[estarzinger]

Yes definitely a place a cruiser would consider as an overnight anchorage, especially with a northern sector wind, although definitely with an anchor light up. And the chart does not give any particular indication the holding would be so poor.