Anchoring equations

[MicHughV]

I've often dove on my anchor during a blow and have been surprised to see a goodly length of chain laying flush with the seabed and only about 25' of the 75' motioning up and down when the bow pitches up.

There have been numerous threads on this CF which have addressed, chain/nylon rodes, catenaries, ad infinitum.

At the end of the day, most every sailor has their own personal favorite setup with little in common with other sailors.

For me, it was a trial and error procedure, until I came up with a solution that worked well for me.

Perhaps this is the way for you to determine what works best for you as there are simply too many variables involved here, and trying to figure something out on an online chat forum is not really the way to go.

[Don C L]

Just out of curiosity, since you are happy with option 3, are you asking if it would be better to add more chain or if you can get by with less?

[Uncle Bob]

Originally Posted by Panope

Assuming the angle of pull (at the anchor) remains the same, holding power increases with anchor weight at a rate of (roughly) 1/1 in "normal" seabeds (sands, muds)


The above does not work for special situations where a smaller anchor may not penetrate at all (thick weed, stones, hard sand), and a large version will. In this case the increase in holding power might be many times greater for the anchor that is just large enough to function properly (penetrate).

Originally Posted by Izikalvo

Long story short:
The weight of the anchor does not affect holding, it is only the fluke size, the fluke to-shank angles, and the position of the crown, that does it, as well as the pulling direction (see aluminum fortress for example).
The weight of the chain does.

I know which statement I'm inclined to accept.

[Lodesman]

Originally Posted by Uncle Bob

I know which statement I'm inclined to accept.

They're both true with caveats.
Weight doesn't affect "holding", but can be important for "penetration."
The 1/1 weight/holding stated by Panope essentially assumes a linear relationship between weight and fluke area for particular anchor styles. Spade, who make the same anchors in steel and aluminum, state the holding power is same for anchors of the same size, vice weight.

[carstenb]

Originally Posted by Lodesman

They're both true with caveats.
Weight doesn't affect "holding", but can be important for "penetration."
The 1/1 weight/holding stated by Panope essentially assumes a linear relationship between weight and fluke area for particular anchor styles. Spade, who make the same anchors in steel and aluminum, state the holding power is same for anchors of the same size, vice weight.

Does anyone know if Steve (Panope) has tested this assertion?

[thinwater]

Originally Posted by Lodesman

... The 1/1 weight/holding stated by Panope essentially assumes a linear relationship between weight and fluke area for particular anchor styles. ...

Not quite.


The larger anchor will penetrate just a little deeper, where the bottom is stronger because it is denser and has more soil pressure above it compressing it. The fluke of anchor that is twice as heavy will be about 60% bigger, will also be 30% thicker (because it has to be stronger), and will hold twice as much because it is in firmer soil.


The exponent in consistent (sand or good mud) is usually a little greater than 1 (1.1 to 1.2) for a variety of reasons, but 1:1 is close enough for most reasoning. Difficult bottoms, such as weed or cobbles, are much more difficult to predict.


Aluminum anchors often do very well on a weight basis, particularly in consistent bottoms, but the additional thickness required generally makes them perform a little less well on a size basis than the steel equivalent. Yes, this has been tested with Excel, for example.

[Lodesman]

Originally Posted by thinwater

Not quite.


The larger anchor will penetrate just a little deeper, where the bottom is stronger because it is denser and has more soil pressure above it compressing it. The fluke of anchor that is twice as heavy will be about 60% bigger, will also be 30% thicker (because it has to be stronger), and will hold twice as much because it is in firmer soil.

I disagree. The ability to penetrate to depth is dependent not on weight, but on weight per area. The anchor that is able to put more pressure on its pointy tip, will dig deeper. At a given depth (and therefore compaction) of the soil, its density is sufficient to effectively buoy the anchor. The anchor's ability to dig further is purely a product of its angle of attack and the force applied to it by engine or wind/wave.

[davefromoregon]

Originally Posted by Don C L

Just out of curiosity, since you are happy with option 3, are you asking if it would be better to add more chain or if you can get by with less?

I really just wanted to understand the benefits of what I’d done. I live in the Pacific Northwest and cruise from the San Juan Islands to north of Vancouver island. Last summer, I had multiple occasions where I was anchoring in 20-30+knots. By adding the chain, I’m now comfortable anchoring in 60’ of water rather than being uncomfortable in 40’.

Now I know that in semi reasonable weather, my rig is good for more than 80-90’.

[davefromoregon]

Quick correction that probably isn’t that important. The chain weighs .83 lbs per foot father than 1.0 lbs per foot.

It is 5/16ths g40.

[Don C L]

Originally Posted by davefromoregon

I really just wanted to understand the benefits of what I’d done. I live in the Pacific Northwest and cruise from the San Juan Islands to north of Vancouver island. Last summer, I had multiple occasions where I was anchoring in 20-30+knots. By adding the chain, I’m now comfortable anchoring in 60’ of water rather than being uncomfortable in 40’.

Now I know that in semi reasonable weather, my rig is good for more than 80-90’.

My boat originally came with 50’ on each of 3 rodes. For 90% of my anchorages that’s fine; I have plenty of 3-strand for plenty of scope when needed. My main concern has been chafing against rocks etc so I switched to 100’ of chain plus a few hundred feet of nylon, and I have more chain stowed as ballast should I need it. That’s just my strategy vs. concerns for my area. Ymmv

[davefromoregon]

Só, with 270’ of chain and rope as described at the beginning, how deep would you guys actually anchor in assuming less than 10 knots of wind and 1’ of waves?

20 meters(66’) plus 2x depth would say 100’ with no room to adjust.
3xboat length ( 35*3=105’) + depth would say 160’ with no room to adjust.

Since I’m far from the most experienced person here, I’m really unlikely to ever try more than 100’. But I’m curious where more experienced people would land?

[Lodesman]

Originally Posted by davefromoregon

Só, with 270’ of chain and rope as described at the beginning, how deep would you guys actually anchor in assuming less than 10 knots of wind and 1’ of waves?

20 meters(66’) plus 2x depth would say 100’ with no room to adjust.
3xboat length ( 35*3=105’) + depth would say 160’ with no room to adjust.

Since I’m far from the most experienced person here, I’m really unlikely to ever try more than 100’. But I’m curious where more experienced people would land?

With 10kts of wind you're good to 150' depth, but no back-up. I'd say with it all out, you're reasonably safe up to depths of 75' and winds of 30 kts. Lot of variables though, so might want to download and anchor calculator and play around with it a bit to get a good idea.

[thinwater]

Originally Posted by Lodesman

I disagree. The ability to penetrate to depth is dependent not on weight, but on weight per area. The anchor that is able to put more pressure on its pointy tip, will dig deeper. At a given depth (and therefore compaction) of the soil, its density is sufficient to effectively buoy the anchor. The anchor's ability to dig further is purely a product of its angle of attack and the force applied to it by engine or wind/wave.

Let's take this one step farther.


In the process of holding more, the larger anchor felt more force and set deeper. When the smaller anchor dragged, the larger anchor when a few inches deeper.


Another limiting factor in an anchor's ability to go deeper is the ability to pull the rode down into the soil. A larger anchor can pull the rode deeper, because rode drag is proportionally less.



It feels like circular logic, but it explains the 1.1 exponent, even when the fluke area does NOT follow the same exponent (it can't if the fluke is thicker, obviously).


I've done a lot of anchor testing and dug up a lot of anchors. Google US Navy and oil platform anchor tests and they will support the change in soil density observation, as well as the rode drag problem.


Actually, weight per area is a small factor. Look at Fortress numbers.

[Lodesman]

Originally Posted by thinwater

Actually, weight per area is a small factor. Look at Fortress numbers.

Danforth/Fortress are not particularly renowned for digging into hard bottoms or through weeds. Their strength comes in the last part of my point, about angle of attack and force applied. Seems to me that their capacity for such great hold, comes from the massive surface area of their flukes vice weight.

[thinwater]

Originally Posted by Lodesman

Danforth/Fortress are not particularly renowned for digging into hard bottoms or through weeds. Their strength comes in the last part of my point, about angle of attack and force applied. Seems to me that their capacity for such great hold, comes from the massive surface area of their flukes vice weight.

This is outside of the "exponent" discussion. The exponent rule, and all of the math that goes with it, only applies to consistent mud and sand bottoms, as was explained. The exponent rule does fit Fortress and Danforth performance in those bottoms.


In fact, the most important factor, in hard and weed bottoms, in many cases, is sharp edges.