Chesapeake Bay Anchor Holding Power Test

[estarzinger]

A got a peek at the test result graphs . . . . and tentatively (it was just a quick peek) drew three conclusions:

1. There was a lot of variability in the results and there was not a statistical difference (95% confidence interval) in the results between the 'plow types'. There was also not a statistical difference between the danforth and the plow types. There might or might not be a statistical difference (better) between the fortress (set at 45 degrees) and the danforth.

2. If you have a fortress - set it at 45 degrees in soft mud - much better (3x) than 32 degrees. . . . . but how do you know it is in fact 'soft mud' if you are just passing thru and not a local?

3. Soft mud is a difficult/tricky bottom and the minimum set for all the anchors was very very low (the mantus had the highest minimum at 300lbs, the others were all near the 100-140lb range) . So, with all anchors in soft mud, be sure to check your set . . . . and bigger/heavier anchors will be useful/safer as they will probably both dive more quickly to harder sub-strait and have more holding power.

[transmitterdan]

Originally Posted by estarzinger

A got a peek at the test result graphs . . . . and tentatively (it was just a quick peek) drew three conclusions:
:
:
and bigger/heavier anchors will be useful/safer as they will probably both dive more quickly to harder sub-strait and have more holding power.

Dear Lord no, please don't tell the guys from down under that bigger is better, for pity's sake...

[DumnMad]

I missed the chance to say earlier but;
I'll bet that with your 250+ft length of rode, in soft mud a 3/8" all chain rode with no anchor would compare quite well. At the low test loads used the drag of the rode could have skewed the results.

[estarzinger]

Originally Posted by transmitterdan

Dear Lord no, please don't tell the guys from down under that bigger is better, for pity's sake...

These were 45lb-ish anchors, which are generally specified for 40'ish boats. The ABYC table says you should be prepared for a 1200 lb anchor load in 30kts of breeze on a 40' boat (we can argue with the ABYC table, and I would not take it as gospel, but let's just use it as a very general bench mark for a 'strong wind' loading).

Of the 60 anchor sets (12 anchors each set 5 times) only two held +1200 lbs (just looking at the final load on each set), and remember these include most of the best 'next gen' designs. That suggests to me that if you want to be more confident of being secure . . . one possible conclusion is . . . . go bigger.

Another possible conclusion is 'don't anchor here', but that's not a super attractive answer for a tired visiting cruiser as the sun is setting and it is getting dark.

A side question for the 'experts' here - whats is the typical 'bollard pull' for a 40' cruising sail boat (lets say with a 45hp engine with lets say a folding or feathering prop)? eg - how hard can the set be tested by going into reverse hard?

[donradcliffe]

Originally Posted by estarzinger

A side question for the 'experts' here - whats is the typical 'bollard pull' for a 40' cruising sail boat (lets say with a 45hp engine with lets say a folding or feathering prop)? eg - how hard can the set be tested by going into reverse hard?

Depends on how much wind you are backing down in, but at zero wind and with an efficient prop, about 1000 lbs. With a folder, maybe 200. Don't you have a load cell lying around??

[estarzinger]

Originally Posted by donradcliffe

. Don't you have a load cell lying around??

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

[Fortress]

I tried to post the test results here as well, but the CF forum cannot accept a large (12) page PDF file, so I broke it down below into two separate PDF files.

I have also included an image of the ABYC "Horizontal Loads Table" to which Estarzinger is referring, which I think is important in order to keep the pull results in perspective as to how they relate to the holding power requirements of various sizes of boats in rising wind conditions.

Bob Taylor, a retired US Navy soil mechanics and anchor design expert who consulted for us on this project, had this comment about the ABYC table:

"I checked on the basis of the ABYC table. It includes a factor of safety of 3 to account for waves and for current so the numbers are probably realistic. What this means is that the loads specified are high if all I have to deal with is wind but as soon as current and wind enter into the equation a boat may assume an orientation that results in a larger projected wind and current drag areas.

Wind and current can come from different directions so the boat will assume an equilibrium position that resolves all the forces. If I presume that this load is realistic then I need to select an anchor that will safely hold this load so I need to provide a factor of safety to the anchor selection. If I do a full up dynamic analysis to determine loads then most regulatory agencies suggest sizing an anchor with a factor of safety of 1.5.

So, let's assume that the loads in the table reflect that of a dynamic analysis because they presume that wind, waves and current are considered. At a minimum an anchor should hold 50% more than that in the table (my suggestion). If I were being really safe I would suggest a factory of safety of 2 because of the uncertainties in bottom condition.

Regarding the guys from Down Under, when considering that the performance of most anchors will drop off a cliff in soft mud and fall far below the holding power requirements needed for securing a boat in anything but minimal wind conditions, bigger is definitely better.

That said, we saw one "new generation" anchor with a very large surface area only seriously engage the soft mud bottom one time, so having an "effective fluke angle" capability in the anchor's design is a key component of performance as well.

Brian

[dugout]

Originally Posted by estarzinger

Another possible conclusion is 'don't anchor here', but...

... or any place like it, overnight.
This is the only conclusion which makes sense to me.

Brian,
Thanks for the test results. I sincerely hope you got more out of it than I do reading the results. I'm a simple person and have no problem accepting the reality that there are some bottoms which just don't hold well with any anchor. You make a great product.

[thinwater]

Something that is very common in that portion of the Bay is soft mud over very hard mud (can't poke a screwdriver into the lower layer, though it is not quite rock. Lots of hard underlying compressed sediment layers extending from the cliffs (this area is very close to Calvert Cliffs). The anchor will grab, but it simply NEVER will dig into a firm layer. This is common on the western shore from Deale to 10 miles south of Solomons. I've anchored along these shores many times, typically to go beach combing (lots of fossils), and the anchoring is quite variable.

Was any sort of investigation of the bottom conditions in this regard made? Some coring of the material ~ 4 feet deep? It seems to me that coring the bottom is the first step, before the first anchor is dropped. Otherwise, how do you describe the experiment, since one variable is not only changeable but wholly uncharacterized?

[Pete7]

Originally Posted by estarzinger

TA side question for the 'experts' here - whats is the typical 'bollard pull' for a 40' cruising sail boat (lets say with a 45hp engine with lets say a folding or feathering prop)? eg - how hard can the set be tested by going into reverse hard?

Can't answer for a 40ft yacht, but a Beneteau Oceanis 323 can generate the following bollard pull depend on prop:

http://www.flexofold.com/upload_dir/...onthly_low.pdf

Pete

[Hydra]

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?).

With the same propeller (conserving the same thrust coefficient), bollard pull scales with HP^(2/3).

With different propellers, the question is much more difficult because bollard pull in reverse is not generally the main design point.

Alain

[Fortress]

Originally Posted by thinwater

Was any sort of investigation of the bottom conditions in this regard made? Some coring of the material ~ 4 feet deep? It seems to me that coring the bottom is the first step, before the first anchor is dropped. Otherwise, how do you describe the experiment, since one variable is not only changeable but wholly uncharacterized?

We have exhibited at the US Sailboat Show in Annapolis for the past 15 years or so, and during that time we have heard many stories from the local sailors about the anchoring difficulties in the soft mud throughout the Bay.

Earlier this year, we found and contacted the University of Maryland Center for Environmental Science in Solomons, MD and the crew of their research vessel, the Rachel Carson, and we were able to confirm that the nearby bottoms were the soft mud that we had heard so much about at the shows.

We then conducted preliminary testing, and based on what was coming up with the anchors, particularly those that were able to penetrate the deepest, we found that the bottom there consisted of a soft mud that was several feet deep, which was then followed by a layer of oyster shells, and further down a harder granular type of mud.

I have soil samples that I am going to have analyzed. From Bob Taylor's calculations of anchor penetration depth and holding power, he determined that the bottom has a soil shear strength that is typical of soft mud.

The top two images show the harder granular material found at the deepest part, and the middle images show the very slick surface mud, which by the way, smelled horrible.

In the bottom two images the shells were evident after the soft mud was washed away with the aft power hose.

Brian

[estarzinger]

Based on the results I have an owner/user question about the fortress angle setting.

If I am a visiting cruising sailor, and may suspect the bottom is some sort of mud but don't know if it is soft or hard, what angle do you recommend I use?

The fortress was the only variable geometry anchor in the test, and the variable geometry appears to be useful as set at 32 degrees it had very average performance while at 45 it had better performance. But it raises the question about what angle to use when we usually don't know what the bottom actually is?

How much downside is there in using 45 degrees on a harder bottom?

As an aside . . . . I suspect the test results would have been different (better) if the anchors had been left to "soak in" on the bottom for a hour or two. I know that is a difficult use if time in a test, but is common in actual anchoring practice in these sort of bottoms.

[Fortress]

Originally Posted by estarzinger

Based on the results I have an owner/user question about the fortress angle setting.

If I am a visiting cruising sailor, and may suspect the bottom is some sort of mud but don't know if it is soft or hard, what angle do you recommend I use?

The fortress was the only variable geometry anchor in the test, and the variable geometry appears to be useful as set at 32 degrees it had very average performance while at 45 it had better performance. But it raises the question about what angle to use when we usually don't know what the bottom actually is?

How much downside is there in using 45 degrees on a harder bottom?

As an aside . . . . I suspect the test results would have been different (better) if the anchors had been left to "soak in" on the bottom for a hour or two. I know that is a difficult use if time in a test, but is common in actual anchoring practice in these sort of bottoms.

Evans, if cruising in an area where you are not familiar with the bottom conditions, then I would definitely assemble the Fortress at the 32° angle, as you are likely to have difficulty setting it at the 45° angle if you encounter a hard soil.

I think that the 21 lb FX-37 @ 32° did better than average, and it was relatively comparable to the 35 lb Danforth HT.

Our late founder used to say, "When anchoring, time is your friend" and letting the anchors "soak" would have likely resulted in improved performance.

We did pull them slowly, at a rate of 10 feet (3 m) per minute, as advised by Bob Taylor for soft mud bottom testing.

Brian

[estarzinger]

Originally Posted by Fortress

Evans, if cruising in an area where you are not familiar with the bottom conditions, then I would definitely assemble the Fortress at the 32° angle, as you are likely to have difficulty setting it at the 45° angle if you encounter a hard soil.

Ok, thanks much. That's what I have done in the past, but I wanted to double check because I was both surprised and impressed by the difference in this test between the 45 and 32 degree performance. The variable geometry is obviously useful if you know when to use it.