Anchors - Bigger is Better ?

[Kettlewell]

Isn't it interesting that the USL code seems to worry about windage and length, but nowhere did I see anything about displacement. Did I miss it? I'm in agreement with that--windage is the main factor, with displacement of lesser concern, but obviously it might tilt you one way or the other when in doubt.

[Dockhead]

Originally Posted by barnakiel

I can't help. And it is an example to show things CAN be computed, not an answer to what the actual values are.

If you want to say my computation algorithm is wrong, please point to the apparent mistake. There may be one. Till then considered correct.

20 kg anchor will have roughly 60% larger surface area than 10 kg anchor, so it will also have 60% higher holding power.

b.

Why guess? Some makers publish fluke area as well as weight. For example, Spade:

Range Overview

If you believe these figures, and if we assume that all Spade anchors are the same shape except for size, then fluke area per kilo of anchor weight actually goes down as the anchors get bigger. The smallest Spade is 400cm2 and 6 kilos for 66.7cm2/kg, and the biggest is 2400cm2 and 70 kg, for 34cm2/kg.

Edit: Barnakiel, you are right on the money! The 10kg Spade has 600cm2 of fluke area, compared to the 20kg with 1000cm2 of fluke area, 66% more area for double the weight.

So I wonder whether this actually has anything to do with the widely shared perception that bigger anchors hold disproportionately better?

[estarzinger]

^^

We had a 50kg Bruce, and never dragged in 10 years . . . Then got a 50 kg rocna and dragged several times in 2 years . . . . I just took it off the bow and put a 50 kg Ray there - hoping it will be as good as our old Bruce.

So at least our experience would not support down sizing when shifting to a "new gen" anchor. Of course it's not any sort of "statically valid" test, but it is our real world experience.

[DumnMad]

Originally Posted by estarzinger

^^

We had a 50kg Bruce, and never dragged in 10 years . . . Then got a 50 kg rocna and dragged several times in 2 years . . . . I just took it off the bow and put a 50 kg Ray there - hoping it will be as good as our old Bruce.

When you dragged, can you recall what sort of seabed it was?

[barnakiel]

Originally Posted by Dockhead

(...) So I wonder whether this actually has anything to do with the widely shared perception that bigger anchors hold disproportionately better?

Sort of like to ask if an anchor with 2x the area could have more than 2x the holding power?

I think if we consider a static set up then the holding power relationship will fall close to the area relationship. (Basically, the answer would be NO).

However, if we think about the dynamics, then we must think about potentially involved phenomena like thresholds. Holding power may come from more than one factor related to fluke area (e.g. friction and suction) and the interplay of factors may cause all kinds of bends and kinks in the holding vs. area curve.

A physicist will tell you exactly and you can also run tests to find if they were right.


b.

[Bash]

Originally Posted by barnakiel

(Basically, the answer would be NO).

However, if we think about the dynamics, then we must think about potentially involved phenomena like thresholds. Holding power may come from more than one factor related to fluke area (e.g. friction and suction) and the interplay of factors may cause all kinds of bends and kinks in the holding vs. area curve.

Not to mention the fact that different anchors hold better in different substrates.

It's not just about holding, however. It's also about setting. A Bruce is a great anchor until you try to set it in kelp. A Delta is a great anchor until you try to set it in silt. A Danforth is a great anchor until it needs to reset itself after a 180º tidal shift.

Crazy thing about dynamics is that they're so... dynamic.

[Bash]

Another thought about barnakiel's observation regarding dynamics: I'm wondering whether the "diving factor" (I'll call this the DF) of a given anchor design might have an ideal range. In other words, is it possible that an oversize anchor won't dive as well as an ideally sized anchor.

Case in point: estargazer got rid of his Rocna after the second time it dragged. His Rocna was twice as heavy as mine, even though our boats are the same size. I've never dragged mine, although I admittedly don't spend quite as many nights on the hook per year as he does. Still, I spend a lot of windy nights on the hook every year, and whenever it blows all night I complain about having difficulty breaking my Rocna free in the morning.

Hmmm. Does the DF need to be in an ideal range? Might an oversize anchor not tend to dive as deeply? Or is this just a function that I spend a lot more time anchored in mud than estargazer?

[bobconnie]

Bigger has always been better !! My Colvin only needed a 45 lb Danforth, but we used a 75 lb for years and never had a problem, not sayin we never drug the danforth, but it was very very seldom !! Also had a 100 lb rock hook !!that we used a couple of times !! LOL So even never haveing a Modern anchor, I can't say for sure if they need to be bigger to be better, but it seems to be resonable to me !! I like bigger in boats and anchors !!( like little women tho LOL)

[FSMike]

Originally Posted by Bash

---A Bruce is a great anchor until you try to set it in kelp. A Delta is a great anchor until you try to set it in silt. A Danforth is a great anchor until it needs to reset itself after a 180º tidal shift. ---

+1
Bash, you and I must have anchored in the same places.

[JonJo]

Bash,

If you take anchors of the same design:

For the same load the smaller will dive more deeply than the bigger version. It depends on the difference is size but a small anchor well submerged will have a higher holding capacity (say if tested at 90 degrees) than a bigger one that is still visible on the surface. This is because its easier to break free when the fluke is near or at the surface, there are different forces at work for a fluke that is well buried.

The small one will eventually reach its maximum holding capacity and at this point if you continue to pull it the anchor will squirrel its way at the same depth through the seabed. But as it squirrels its holding capacity will not increase. Basically it will drag, but slowly and remain buried.

If you continue to load the bigger anchor it will continue to dive until it reaches its ultimate depth and holding capacity, which will be deeper and higher than the little anchor.

This is one of those cases where theory and practice actually agree.

So a well set small anchor can be better than an inadequately set bigger anchor.

Load, for a yacht, is developed by engine power, which is finite or windage + momentum. If its a straight line pull then the big anchor will continue to set as the load increases. If the big anchor cannot be set fully and the wind changes?

[Bash]

Originally Posted by JonJo

Bash,

If you take anchors of the same design:

For the same load the smaller will dive more deeply than the bigger version. It depends on the difference is size but a small anchor well submerged will have a higher holding capacity (say if tested at 90 degrees) than a bigger one that is still visible on the surface. This is because its easier to break free when the fluke is near or at the surface, there are different forces at work for a fluke that is well buried.

The small one will eventually reach its maximum holding capacity and at this point if you continue to pull it the anchor will squirrel its way at the same depth through the seabed. But as it squirrels its holding capacity will not increase. Basically it will drag, but slowly and remain buried.

If you continue to load the bigger anchor it will continue to dive until it reaches its ultimate depth and holding capacity, which will be deeper and higher than the little anchor.

This is one of those cases where theory and practice actually agree.

So a well set small anchor can be better than an inadequately set bigger anchor.

Load, for a yacht, is developed by engine power, which is finite or windage + momentum. If its a straight line pull then the big anchor will continue to set as the load increases. If the big anchor cannot be set fully and the wind changes?

In many cases that's true. But there have been many mornings where I know that my anchor dived much deeper than I'd initially set it. For me, this is the big difference between the Rocna and the CQR. In a choppy anchorage, the CQR will plow forward an inch on two every minute, all night long, without ever setting itself deeper. In identical conditions, my Rocna will just continue to dive deeper, and deeper, and....

I'm just wondering whether there's a point when an oversized anchor stops doing that.

[Kettlewell]

I'm just asking whether there's a point when an oversized anchor stops doing that.

Some people have reported that they thought the big roll bar on some anchors impedes burying beyond a certain point. I suppose a huge roll bar anchor must have a proportionally larger roll bar, and so wouldn't it be more likely to resist penetration beyond a certain point? By the way, I have many times seen even old anchors like CQRs completely buried out of sight and I couldn't even reach the anchor shank when trying to find it while diving in both sand and mud in various places. Happens to Danforths and Fortresses more often.

[Bash]

Back in my scuba instructor days I would usually be the first to descend just to check the anchor set, and then I'd wait at the anchor for my students to catch up with me. It was always interesting to watch how different anchors in different dive boats either dived or plowed. That was the point when I decided to put a Bruce on my own boat rather than a CQR, which was what most dive boats used back then.

[JonJo]

All anchors have an ultimate diving depth, depends on seabed. Big ones dive deeper - but they have an ultimate depth at which point they would squirrel through the seabed, holding capacity would not increase.

There is a thought that even when an anchor is set it might worry itslef more deeply. The constant jiggling of the chain on the anchor causing it to settle further - this would explain why CQRs when tested are an abysmal failure but can, sometimes in real life, develop high holding capacity - maybe because they 'settle'. Its not something one can plan to rely on, as it must take time.

[Jim Cate]

JonJo, you have not dealt with my major query about use of the USL codes: what height ("H") do you use to determine what the code says is the correct anchor? And why is there no involvement of the beam of the boat? Or of displacement?

How can you place credence in the application of this set of tables which are not at all formulated on sailing yachts and the anchor loads that they generate?

I'm still confused.

Jim