Anchor choices

[rslifkin]

Originally Posted by Simi 60

We never back down on our 150lb manson supreme anymore - did it the first time we used it and thought we were going to break something.

Instead we just let wind, current, weight of vessel do the set.
When dived on more often than not it is just the top of the shank and some hoop that is visible.

I usually try to do it gently. Let the boat gently drift back for the initial set, controlling with engines if needed depending on wind. Once it bites, let it settle, then gently bring tension back up on the rode and hold tension on it with the engines for a minute or so. No backing down hard with the rode slack, as with a good anchor the shock loads are nasty, with a bad one you'll just rip it right out. I also don't try to apply the force of a hurricane to it, just enough to let it dig in a bit and make sure it's solid.

[Mike OReilly]

Originally Posted by Dockhead

...John on Morgan's Cloud wrote with tongue slightly in cheek that the size of the anchor on the bow is proportional to experience of the owner, but probably it's not that, but rather the type of cruising one does.

I likes it .

Originally Posted by rslifkin

I usually try to do it gently. Let the boat gently drift back for the initial set, controlling with engines if needed depending on wind. Once it bites, let it settle, then gently bring tension back up on the rode and hold tension on it with the engines for a minute or so. No backing down hard with the rode slack, as with a good anchor the shock loads are nasty, with a bad one you'll just rip it right out. I also don't try to apply the force of a hurricane to it, just enough to let it dig in a bit and make sure it's solid.

I almost always dig our anchor in using a similar technique. I drop, and usually let the wind or current take us back, paying out the rode as we do. Usually around 3:1 scope I'll start putting a little pressure on the anchor by letting the boat tug on the rode. If I feel the anchor set then I might lock it off and start the digging-in process, either with wind or with gentle engine. If it doesn't maintain the set, or won't dig, then I immediately ease off and pay out more rode.

Once I get to around 5:1 (sometimes more) with a good set, and can feel the anchor holding, I slowly dig in the anchor by gradually ramping up reverse thrust on the engine. We do this gently, ensuring we maintain the hold with constant transits and feeling how the rode/anchor is behaving (usually with a foot or hand on the chain).

We continue to ramp up the thrust till we get to our normal cruising level: 2000 RPM. By now the chain is lifted and hard. We hold this for about a minute, and more if there are any doubts. If the anchor drags at any point we stop and try again. We (almost ) never settle for a poor set.

Some people routinely allow natural forces to set and dig in their anchor. This obviously works most of the time, but I prefer the knowledge that my anchor is well dug in and will hold well at high forces, at least along the one axis.

[Dockhead]

Originally Posted by Mike OReilly

. . . I almost always dig our anchor in using a similar technique. I drop, and usually let the wind or current take us back, paying out the rode as we do. Usually around 3:1 scope I'll start putting a little pressure on the anchor by letting the boat tug on the rode. If I feel the anchor set then I might lock it off and start the digging-in process, either with wind or with gentle engine. If it doesn't maintain the set, or won't dig, then I immediately ease off and pay out more rode.

Once I get to around 5:1 (sometimes more) with a good set, and can feel the anchor holding, I slowly dig in the anchor by gradually ramping up reverse thrust on the engine. We do this gently, ensuring we maintain the hold with constant transits and feeling how the rode/anchor is behaving (usually with a foot or hand on the chain).

We continue to ramp up the thrust till we get to our normal cruising level: 2000 RPM. By now the chain is lifted and hard. We hold this for about a minute, and more if there are any doubts. If the anchor drags at any point we stop and try again. We (almost ) never settle for a poor set.

Some people routinely allow natural forces to set and dig in their anchor. This obviously works most of the time, but I prefer the knowledge that my anchor is well dug in and will hold well at high forces, at least along the one axis.

I think this technique is more or less universal among experienced people.


I do it exactly the same way, except only that I back down longer than a minute, and I gradually increase revs right up to redline. Like you, I am very gentle at the beginning and try to feel how it's going in. There is an analogy to similar technique with a different activity, but I'm not going to go there. Suffice it to say that gentleness and sensitivity is really important, and being too abrupt at the beginning will result in failed insertions .



I do it longer than a minute; probably on average 10 minutes actually, even more if there is any question about the bottom. It is surprising how many times the anchor starts to drag over 3000 RPM, although it was holding fine at 2000. Sometimes it takes quite a few minutes to start dragging, so patience is a good thing. If it's really set, it will hold at redline.



Added benefit is that it blows the carbon out of the engine when you run it up to redline.

[thinwater]

Originally Posted by Dockhead

... If it really does work like that, could it be specific to Fortresses?

THAT is a REALLY good question that I am not going to answer. Anchor rotation and resetting in a variety of bottoms is a really, really complicated topic I don't understand well enough. It is easy to watch in coral sand, but other bottoms, not so much. The Panope's 180 reset testing represents a LOT of hard work and we are all very grateful, but like straight pull testing, it still leaves questions.

As for holding of small anchors, I've tested anchors from 2 to 35 pounds, and as nearly as I can tell, in a uniform bottom without weed or excessive trash, holding capacity is proportional to weight throughout the range, with an exponent of 0.85 to 0.95. This is true for both NG and pivoting fluke anchors. Penetrating weed and trash is a different matter.

Much of the information we have is anecdotal (it worked or it didn't work). For example, if I can just lift 150 pounds, a 170-pound weight might as well be infinatly heavy, because I can't move it. It think this is at least part of why people often feel that an anchor that is 10 pounds heavier is twice as good; it works and the old one did not. Fair enough. For them, it is MORE than twice as good, because it works now.


Going big is also a lot simpler then understanding what's going on down there. I'm not teasing. I'm a brown water sailor and we often just don't know. That is what makes this topic so much fun. The more you learn, the more questions you have.

[Dockhead]

Originally Posted by thinwater

. . . As for holding of small anchors, I've tested anchors from 2 to 35 pounds, and as nearly as I can tell, in a uniform bottom without weed or excessive trash, holding capacity is proportional to weight throughout the range, with an exponent of 0.85 to 0.95. This is true for both NG and pivoting fluke anchors. Penetrating weed and trash is a different matter.

OK, here's a data point. I have heard this over and over again. It contradicts a bit the kind of stuff that John on Morgan's Cloud says about bigger anchors holding DISproportionately better, but it sounds right.

Originally Posted by thinwater

. . . Going big is also a lot simpler then understanding what's going on down there. I'm not teasing. I'm a brown water sailor and we often just don't know. That is what makes this topic so much fun. The more you learn, the more questions you have.

Sure. But that's the same old theme again -- we don't size for a perfect bottom, we size for a crappy one we might be forced to anchor in, or to your point, to anchor in one which might well be crappy and we just don't know.

[Don C L]

...well don't forget your lead line with the paraffin or clay in there to test the bottom first!

[evm1024]

With great respect for Drew's work and knowledge I find the (near) linear increase in holding "power" with the increase in anchor weight in the range of anchor weights he has tested to be not surprising.

This is easily shown in strain - stress curves. The material behaves linearly until the yield strength is met. Then it is no longer linear.

I am suggesting that the forces exerted in the testing range is less than the "yield strength" of the substrate for the size anchors tested.

As noted anecdotally when anchors get up around 100# they behave differently than their smaller version. Could this be because the forces induced by the anchors weight move the substrate out of it's linear region?

We all do know that a 50# concrete block behaves differently than a 500# concrete block. And I think we can also see that a ships anchor holds the ship but we would expect that same anchor design if scaled down to our boats size would not perform very well at all. I'm seeing an old Navy anchor in my minds eye.

[Don C L]

...couple other observations.. I don't think it is RPMs that bury the anchor, I think it is the displacement. Think of the force necessary to stop a 10 ton boat coming at you vs. a 2 ton. Sure the RPMs get her moving, but once that weight is stopped by the anchor, the RPMs on the prop are a fraction of the force the displacement offered IMO. Try paying out 5:1 at a good clip before cleating it off. You'll see some real burying then!
...and BTW, as I've said before, as a die-hard Danforth user, I would NOT use a Danforth or Fortress were I not always anchored bow/stern. I trust it completely for one direction (which is what it was designed for!) but not for 360 degrees.
And one more thing, an anchor will be tasked with holding the force of the moving, accelerating, mass, in swells, which is not a linear function I believe as the conditions in an anchorage accelerate. This is very different from applying a steadily increasing force from an engine for testing purposes.

[groundtackle]

Originally Posted by Mike OReilly

My initial reaction on first seeing an Excel anchor was that it looked at lot like the poorly performing Delta. But I've since learned how wrong I was.

When it comes time to replace my current bower (Rocna 25) I'll be looking hard at these Excels. So, thanks Steve (once again).

So nice to hear you say that Mike. The ability to reevaluate previous convictions and assumptions in light of new information is a true sign of a wise and honest man.

[Mike OReilly]

Originally Posted by Don C L

...well don't forget your lead line with the paraffin or clay in there to test the bottom first!

Yup, got one. Nice lead weight with wax tab to pick up bottom. Incredibly useful.

[Dockhead]

Originally Posted by evm1024

With great respect for Drew's work and knowledge I find the (near) linear increase in holding "power" with the increase in anchor weight in the range of anchor weights he has tested to be not surprising.

This is easily shown in strain - stress curves. The material behaves linearly until the yield strength is met. Then it is no longer linear.

I am suggesting that the forces exerted in the testing range is less than the "yield strength" of the substrate for the size anchors tested.

As noted anecdotally when anchors get up around 100# they behave differently than their smaller version. Could this be because the forces induced by the anchors weight move the substrate out of it's linear region?

We all do know that a 50# concrete block behaves differently than a 500# concrete block. And I think we can also see that a ships anchor holds the ship but we would expect that same anchor design if scaled down to our boats size would not perform very well at all. I'm seeing an old Navy anchor in my minds eye.

Someone could write a book on "Scale Effects in Sailboat Anchoring".


We had discussed not that long ago in another thread the fact that bigger anchors of the same size have LESS fluke area per unit of weight.


That's another scale effect. It's probably really complicated.


Less fluke area per unit of weight means more pressure from weight applied to the cutting surface of the fluke, per unit of area. This would be a pretty strong factor making bigger anchors cut in better, something I think I've observed.


You would expect however for ultimate holding power to scale up LESS than proportionally, since that is more a function of fluke area than it is a function of weight, n'est-ce pas?


Maybe we don't even notice that, since we care so little about ultimate holding power. We care about practical holding power in marginal bottoms.

[Don C L]

Originally Posted by Dockhead

You would expect however for ultimate holding power to scale up LESS than proportionally, since that is more a function of fluke area than it is a function of weight, n'est-ce pas?

That's what I am thinking too.
But look at the anchors on aircraft carriers; they really look wimpy!

[thinwater]

Originally Posted by evm1024

With great respect for Drew's work and knowledge I find the (near) linear increase in holding "power" with the increase in anchor weight in the range of anchor weights he has tested to be not surprising.

This is easily shown in strain - stress curves. The material behaves linearly until the yield strength is met. Then it is no longer linear.

I am suggesting that the forces exerted in the testing range is less than the "yield strength" of the substrate for the size anchors tested.

As noted anecdotally when anchors get up around 100# they behave differently than their smaller version. Could this be because the forces induced by the anchors weight move the substrate out of it's linear region?

We all do know that a 50# concrete block behaves differently than a 500# concrete block. And I think we can also see that a ships anchor holds the ship but we would expect that same anchor design if scaled down to our boats size would not perform very well at all. I'm seeing an old Navy anchor in my minds eye.

a. This exponent does not respond to simple analysis, but many investigators have found the same relationship, testing anchors from 2-20,000 pounds. Engineering firms working for oil companies and shipping companies. This is long established fact, not my opinion. Exceptions must be made for certain bottoms, including weeds. I have never found an exception to this simple hold = mass^(0.85 to 0.9) relationship, other than cases where the anchor some how fouled... which happens more often to smaller anchors and is not predictable.



b. A factor that is often ignored is that a larger anchor will reach slightly deeper underground, into firmer substrate. This is a LARGE factor in why larger anchor and anchors that dig deeper hold more. They arn't in the fluff. But at what rate does shear strength increase in submerged sea bed? Complicated. In fact, this is one of the problems with beach testing; the soil characteristic's change with depth is wrong.


It's frigging complicated. It just seems simple.

[thinwater]

Originally Posted by Mike OReilly

Yup, got one. Nice lead weight with wax tab to pick up bottom. Incredibly useful.

I would like to hear of your experience. It seems to me you will only see the top 1/8" and only in that spot. You could feel if the surface is hard or soft, but only the top inch and only that spot.

[Jim Cate]

But look at the anchors on aircraft carriers; they really look wimpy!

Up until you are assigned to chip and paint one!

Do remember that naval vessels ALWAYS have an alert crew on watch when at anchor (in most navies, that is) which is a different situation than us wafis who do not routinely stand anchor watch.

Jim