Anchoring Perfectionism vs Realities of Soft Mud

[noelex 77]

Originally Posted by ranger42c

That's not our experience in the soft mud round here. With the smaller fluke angle, the anchor often won't set at all, the boat won't stop moving.

Chris, I think we are perhaps saying the same thing in a different way. .

When you say that with a small fluke angle the anchor "often won't set at all, the boat won't stop moving" , I would say that if I looked at the anchor underwater it is likely the anchor was setting ie burying under the substrate but that your engine's power was greater than the holding power of the anchor. So the anchor sets, but drags while set, or sets and then breaks out.

It sounds a bit pedantic to differentiate between an anchor that sets but has insufficient holding ability, verses an anchor that does not set at all, but in this case it is an important distinction.

If the fluke angle is shallower than optimum, the anchor will set but have reduced holding ability. The engine force may be enough to break the anchor out, but an anchor in this circumstance that can set will develop at least some holding ability. On a sailboat full reverse for 30 seconds is equivalent to around 25- 30 knots of wind (I realise with your powerful engines the force is potentially higher). If we leave the anchor to soak, as Boatman suggests it likely to gain some further holding ability, especially in soft substrates. A larger anchor of the same design and fluke angle may well develop sufficient holding ability to hold the engine force.

Contrast this with an anchor that has been adjusted for too large a fluke angle for the substrate. An anchor like this will never truly set ie never dig into the substrate in a meaningful way. The holding will be very low so the anchor has no hope of holding the boat except in extremely light wind. In addition, if the anchor cannot set because the fluke angle is too large, no amount of upsizing to a larger model or soaking the anchor longer will help significantly.

The distinction between an anchor that cannot set in a substrate and an anchor that has insufficient holding ability in a substrate is perhaps a semantic difference, but it does have practical consequences.

As an aside, in the Fortress Chesapeake Bay tests the anchors were always moving at a reasonable speed. This was part of the test protocol. I think it would be wrong to say the anchors did not "set" because none of the anchors were ever stationary. Rather, I think it more accurate to say that the powerful thrust of the large tow vessel was always enough to overcome the anchor's holding ability. The published anchor force was measured "dragging" ie with the boat moving backwards at a constant 10 feet per minute.

[Fortress]

Originally Posted by noelex 77

As an aside, in the Fortress Chesapeake Bay tests the anchors were always moving at a reasonable speed. This was part of the test protocol. I think it would be wrong to say the anchors did not "set" because none of the anchors were ever stationary. Rather, I think it more accurate to say that the powerful thrust of the large tow vessel was always enough to overcome the anchor's holding ability. The published anchor force was measured "dragging" ie with the boat moving backwards at a constant 10 feet per minute.

This was not how the testing was conducted. During preliminary testing, we quickly realized that the 81-ft research vessel was too large, the bottom was too soft, and the anchors too small (ranging from 21 to 46 lbs / 10 to 21 kg) for controlled and repeatable pull tests.

The protocol was changed to use the aft drum winch to slowly pull the anchors towards the boat while the state-of-the-art Dynamic Positioning System (DPS) aboard kept the vessel stationary.

The initial starting scope was 5:1 + 100-ft (30 m), which equaled about 8.3 to 1, and then we pulled back the 100-ft at a carefully monitored and calibrated slow rate of 10-ft (3 m) per minute.

Each of the 60 tests were concluded at the end of 10 minutes, and so every anchor was given the exact same opportunity to engage and perform in this soft mud bottom.

There were several surprises and unexpected results, some of which occurred during the preliminary testing and so they were not reported in the media. The very first anchor that we splashed was the 32 lb / 15 kg FX-55 (which we did not use during the public tests) at the 45° angle and it kept shutting down the aft winch at approximately 4,000 lbs of tension.

The crew initially thought that they had an electrical problem, but maybe we all should have noticed the S.W.L 4200 LBS in the top left of this photo.

upload an image


Meanwhile, it was rare during the public testing for any of the 44-46 lb (20-21 kg) new or old generation anchors to exceed 1,000 lbs (450 kg) of tension.

Once again, this was came as a surprise to some viewers, but as our US Navy consultant Bob Taylor stated afterwards, "Anchors which are designed and optimized for harder soils will only have a holding ratio (holding capacity divided by anchor weight) of 10-15x in a softer soil."

This was almost exactly what we found during this testing.

[UNCIVILIZED]

Originally Posted by thinwater

I've taken to eliminating the chain leader and substituting Dyneema. There are caveats:
* only in a V rig (no yawing, 2 anchors set)
* only in very mud (less cutting risk, it's the only time I feel I need to anchors)

So far I've seen no chafe. A significant benefit is that the anchor sets deeper, making it stronger. Also easier to lay out and recover. For the complete discussion:

Sail Delmarva: A Better Secondary Anchor Rode?

It's easy t o fit a chafe guard over the Dyneema, if you so choose.

Anyone have much experience in using stainless or galvanized wire rope in lieu of the Spectra in this app? As in theory it's supposed to allow for better anchor burying, due to reduced cross sectional area. But I'm wondering how well it works in the real world. And also what gear folks use to form the eyes on the ends?

Thanks

[noelex 77]

Originally Posted by Fortress

This was not how the testing was conducted. During preliminary testing, we quickly realized that the 81-ft research vessel was too large, the bottom was too soft, and the anchors too small (ranging from 21 to 46 lbs / 10 to 21 kg) for controlled and repeatable pull tests.

The protocol was changed to use the aft drum winch to slowly pull the anchors towards the boat while the state-of-the-art Dynamic Positioning System (DPS) aboard kept the vessel stationary.

Brian my point was the anchors, in fact all the anchors in your Fortress Chesapeake Bay test, were always moving. This was part of the test protocol, as your post confirms.

An anchor moving at 10 feet per minute is slowly dragging and not holding. The force measured is not the anchors holding power, but rather the force generated while the anchor is dragging.

These slow drags are a problem, and more common than people realise.

[ranger58sb]

Originally Posted by noelex 77

Chris, I think we are perhaps saying the same thing in a different way. .

When you say that with a small fluke angle the anchor "often won't set at all, the boat won't stop moving" , I would say that if I looked at the anchor underwater it is likely the anchor was setting ie burying under the substrate but that your engine's power was greater than the holding power of the anchor. So the anchor sets, but drags while set, or sets and then breaks out.

If the fluke angle is shallower than optimum, the anchor will set but have reduced holding ability.

Etc. Etc. Etc.

Fair enough, perhaps we're agreeing (at least slightly) but using different words.

That said, if our anchor is buried and the boat is moving, in my language the anchor is not set. Certainly I can overpower an anchor, but we don't usually do that; we just use normal "setting" power -- not a specific RPM or horsepower, but rather a reading of the situation (including rode tension, etc.) from experience in this area. Mostly I'm simply trying to replicate expected wind on our superstructure, so we're not engaging our full 900 hp or anything like that.

That "buried but not set" situation has most often happened here in our soft mud with the smaller fluke angle... and the "buried" part is usually judged after the fact, if I have to bring it back up to try again. At that point it's semi-easy to gauge (guess) how deep it was from the mud build-up... and from how slimy the soup was when I got it all over my hands....

It's murky here, though, so there's a significant amount of guess-work involved on that last part.

And part of my point earlier was an expansion of your wording. Another would be:

If the fluke angle is not optimum, the anchor may set but have reduced holding ability.

Back to what I meant about operator selection. Good that the manufacturers have provided an adjustment feature, but it's still up to the operator to deal with all the related follow-thru, including an attempt to ascertain info about the target substrate to the extent possible.

Have to admit, we're (usually) pretty cavalier about it. I know it's mud. Our current main anchor (SuperMAX) is pre-adjusted for normal conditions here. I (usually) just lower the anchor and set it. Not much struggle involved.


-Chris

[Fortress]

Originally Posted by noelex 77

Brian my point was the anchors, in fact all the anchors in your Fortress Chesapeake Bay test, were always moving. This was part of the test protocol, as your post confirms.

An anchor moving at 10 feet per minute is slowly dragging and not holding. The force measured is not the anchors holding power, but rather the force generated while the anchor is dragging.

These slow drags are a problem, and more common than people realise.

As per Bob Taylor: "The load measured was indeed the holding capacity of the anchor if it reached a peak. Remember that the anchor is penetrating the bottom during drag and that causes the higher capacity with drag in a soil like that at Chesapeake."

During the testing, many anchors rarely peaked nor did their tension continue to increase dramatically (if at all) as they were being slowly pulled through the soft mud.


Additionally from Bob, "If you stopped pulling at any time and waited a bit, my experience is that it takes about 20% more to start pulling the anchor again in a mud bottom. This is due to local consolidation of the soil around the anchor.

All consolidation means is that the pore pressure in the soil developed when it is under load dissipates (water drains out of the loaded area) and the soil becomes stronger. Once the anchor starts to move again after the 20 or so percent increase in load it will move back to its normal drag load."


I think that if we had pulled the anchors longer than 10 minutes / 100 ft (30 m) then the better performing models would have eventually peaked and reached their holding capacities.

[ranger58sb]

Originally Posted by ranger42c

That said, if our anchor is buried and the boat is moving, in my language the anchor is not set. Certainly I can overpower an anchor, but we don't usually do that; we just use normal "setting" power -- not a specific RPM or horsepower, but rather a reading of the situation (including rode tension, etc.) from experience in this area. Mostly I'm simply trying to replicate expected wind on our superstructure, so we're not engaging our full 900 hp or anything like that.

Thought about this some more...

Given that big boats can overpower small anchors... I think what I should really be saying is that if the anchor is buried and the boat is still moving... we're not anchored.

Without regard to whether the anchor was "set" or not.

Which might also be getting closer to what you were saying about setting, Noelex.

-Chris

[thinwater]

Originally Posted by noelex 77

Brian my point was the anchors, in fact all the anchors in your Fortress Chesapeake Bay test, were always moving. This was part of the test protocol, as your post confirms.

An anchor moving at 10 feet per minute is slowly dragging and not holding. The force measured is not the anchors holding power, but rather the force generated while the anchor is dragging.

These slow drags are a problem, and more common than people realize.

Yes... and no.

If the load and distance data had been taken at finer intervals (this has been reported in land-based testing, where the anchor point is fixed) you would have observed that a this low rate, the pattern was hold/move/hold/move, with sharp spikes in the force. This is most true at high load. The dragging was not actually steady, that is an artifact of the data collection and that the the anchor point was a boat.

If they had taken the additional time to stop near max load, in all but the worst cases (those might have been oozing along), the holding power would be slightly greater than recorded.

I understand the point you are trying to make, but as one who has performed these tests, I can tell you it is a matter of semantics and not substance. The result is the same either way so long as the strain rate is low, which is was.

Yes, slow dragging is common with poor anchors, as waves and gusts cause spikes in load. Obviously. However, it is something that only happens when a given anchor is near it's limit, and to anchors that do not bury well. In other words, it applies to the failures, not the successes.

[Fortress]

I know that Thinwater is well familiar with the soft mud bottom conditions in the area where the test was conducted, and he has conducted his own tests as well, and so I consider him to be a local authority.

Additionally, Chris /ranger42c is also well familiar with the area and I certainly consider his opinion to hold a high value.

Maybe to simplify a bit (or spark more debate?), there is obviously a huge difference between an anchor dragging along at 500 lbs of tension (with maybe a few minor spikes / drop offs along the way), during which time the anchor is simply not burying into the soft mud (possibly due to its poor design for that bottom condition).....

And then comparing this to tension recordings of 1,500+ lbs when the anchor continues to "drag," but it is actually burying deeper and deeper into the sea bottom while it is being loaded as the tension increases.