Anchors, Guns and Strippers!!!!

[Mike OReilly]

Originally Posted by Lost Horizons

No, the force is the same and is determined only by the wind speed or engine’s RPM (for the sake of an argument, we will neglect water resistance). Smaller anchor will have smaller cross-section, just like a nail or a bullet, and therefore will be exerting higher pressure on the substrate. Consequently, it will go deeper with all other conditions being equal, including anchor design, off course.

No. Two different forces. I'm talking about the force to set the anchor. You're talking about the force to dig it in. I've already said, multiple times, that an anchor must be dug in to work properly. But it must also set first. This is where size matters, or helps.

Citation: Dashews. Or the comments from most of the folks here.

[jtsailjt]

Originally Posted by Lost Horizons

No, the force is the same and is determined only by the wind speed or engine’s RPM (for the sake of an argument, we will neglect water resistance). Smaller anchor will have smaller cross-section, just like a nail or a bullet, and therefore will be exerting higher pressure on the substrate. Consequently, it will go deeper with all other conditions being equal, including anchor design, off course.

But the initial penetration that must first occur in order for the things you cite to come into play depends a lot on weight for some hard bottom surfaces. I think that’s what Mike is trying to tell you. If you are anchoring in soft muck or sand, even a lightweight anchor like a fortress with its large surface area to weight ratio will work great. But on a hard gravel bottom a lightweight anchor won’t penetrate well enough so it’s great amount of surface area can come into play. It’ll just scrape along on top. Anchor weight means little in some anchoring situations but is critically important in conditions where a hard surface must be penetrated.

[Lost Horizons]

Originally Posted by Mike OReilly

No. Two different forces. I'm talking about the force to set the anchor. You're talking about the force to dig it in. I've already said, multiple times, that an anchor must be dug in to work properly. But it must also set first. This is where size matters, or helps.

Citation: Dashews. Or the comments from most of the folks here.

This is anecdotal evidence at best. Since we are talking physics, to prove the point we need a controlled experiment with statistically significant data.

Regarding a set vs being dug. I consider one a definition of the other. It appears what you are calling “setting,“ is for an anchor to be appropriate for the bottom, but in my experience bigger anchor does not cure it. A different design might.

[Lost Horizons]

Originally Posted by jtsailjt

But the initial penetration that must first occur in order for the things you cite to come into play depends a lot on weight for some hard bottom surfaces. I think that’s what Mike is trying to tell you. If you are anchoring in soft muck or sand, even a lightweight anchor like a fortress with its large surface area to weight ratio will work great. But on a hard gravel bottom a lightweight anchor won’t penetrate well enough so it’s great amount of surface area can come into play. It’ll just scrape along on top. Anchor weight means little in some anchoring situations but is critically important in conditions where a hard surface must be penetrated.

Are you saying that if you are anchoring over a granite slab, a heavier anchor will work better than a lighter one? Seriously?

[fxykty]

Originally Posted by Lost Horizons

Are you saying that if you are anchoring over a granite slab, a heavier anchor will work better than a lighter one? Seriously?

Well, the heavier anchor, if same material, will be larger so it will have more surface area and friction on that very hard bottom ;-).

Regarding choosing between steel and aluminium versions of the same anchor, I was speaking to Rex at Anchor Right about choosing between the steel and aluminium Sarca Excels. He said that for the same size (surface area and dimensions) their testing showed that the ultimate holding power is the same, the setting ability of the lighter anchor is marginally less in hard surfaces, and the chisel point on the lighter anchor is of a softer metal and won’t last as long. The latter is a deal breaker for extended cruising, so we’re selecting steel.

[jtsailjt]

Originally Posted by Lost Horizons

Are you saying that if you are anchoring over a granite slab, a heavier anchor will work better than a lighter one? Seriously?

Not saying that at all. I believe I mentioned penetrating hard gravel. I’ll leave it to you to google the difference between that surface and a granite slab if you’re unfamiliar.

[Dockhead]

Originally Posted by Lodesman

I agree with this, but there is a practical limit to this without making major structural changes to the boat.

I'm certain I've seen pic's (possibly in this forum) of ripped-out bitts, and other damage done to anchor chain-securing structures.

I'm sure you're right, but I think one should do one's best.

If I ever manage to put away enough money for a custom build, I will have a proper samson post, tied to the boat's structure. Something strong enough that you could lift the boat by it.


On this boat, I'm using a chain plate strongly tied to a structural bulkhead. It ought to be good enough for loads somewhere near the breaking strength of my 12mm chain (about 8.5 tonnes force). On the previous boat with horizontal windlass, we had a strong chain lock, I believe a Lofrans, rated strength equal to the strength of the 3/8" chain, bolted to a strong part of the deck. Snubber was rigged to the strong point on top of the windlass. Whether it would have ripped out of the deck at loads near the breaking strength of the chain, I don't know, but it never did, and we went through some pretty gnarly nights.

[Dockhead]

Originally Posted by Lost Horizons

This is anecdotal evidence at best. Since we are talking physics, to prove the point we need a controlled experiment with statistically significant data.

Regarding a set vs being dug. I consider one a definition of the other. It appears what you are calling “setting,“ is for an anchor to be appropriate for the bottom, but in my experience bigger anchor does not cure it. A different design might.

You are right -- we are certainly arguing anecdotes here, and an experiment would be really useful.


However, the anecdotes line up quite well for people who have done a whole lot of anchoring, and correspond well to what Dashew and others write.


And I think someone above might have found one possible explanation for it -- with larger anchors being heavier for their fluke area.


Another explanation may be in soil mechanics -- soil has grains of certain sizes, and objects of different sizes stuck into soil do not behave in a way which scales perfectly. A pin stuck into dirt certainly behaves very differently from a pile. So that may also be a big factor. Note that big ships have extremely tiny anchors compared to the mass of the ships -- maybe another hint that the effectiveness of anchors does not scale proportionately with size.


As to the solid granite bottom -- that is a red herring. Obviously nothing will penetrate that. But really hard mud, shingle, kelp.


I have been using Spade anchors of different sizes since the late 90's, starting with I believe 30 pounder and ending up with a 100 pounder, with a couple of sizes in between. The difference in bottom-penetratrating power between the small ones and the big ones is very striking, immediately noticeable. Even in relatively good bottoms the big ones go in noticeably better.



But yes -- that's an anecdote and just a data point, but based on thousands of nights at anchor with different anchors. Make of it what you will.

[Izikalvo]

Originally Posted by Uncle Bob

Have to agree with this and also chuckle at the Viking. It looks so very much like the Mantus, with the press out of the Excell and the holes that Steve of S/V Panope did to his manson (I think) that he modified, all topped off with the Viking knowledge base. Sound familiar?

It took you a lot longer to wake up this time

[Lodesman]

Originally Posted by Dockhead

I have been using Spade anchors of different sizes since the late 90's, starting with I believe 30 pounder and ending up with a 100 pounder, with a couple of sizes in between.

In the same boat?

[Dockhead]

Originally Posted by Lodesman

In the same boat?

A couple of different boats. But including different sizes in the same boat. It's very noticeable.


One data point, but based on a lot of experience.




It's also a very noticeable difference coming from smaller boats to bigger ones -- the ground tackle works better and better. I spent a fair amount of time on a Swan 90 belonging to a friend of mine -- with a Bruce anchor, I think the 170 pound one. Smaller as a proportion to weight of the vessel than any of the above, and not even a new gen anchor, but damn that anchor worked well. It would set in anything and held on any kind of scope down to 2:1. An amazing difference to smaller anchors on smaller boats.


The same friend now has a 30 meter, 175 tonne motor yacht, with pathetic anchors for the size and windage of the vessel. I'm just back from spending Christmas and New Year's with him on board. I think his anchors are 90 kilos, fake CQR's, stainless steel. The pro captain kept the yacht for a whole year at anchor in the Indonesian islands, and said he never dragged, even in storms which had the entire anchorage dragging and, in some cases, being dashed on the rocks. I learned this in extensive conversation because my friend asked me for a list of suggested upgrades, and naturally I started with the ground tackle. Despite this good experience, after consultations with the naval architect, we're going to upgrade to 145kg Mansons, which should be easily handled by the massive twin hydraulic windlasses and 16mm chains. That's a whole heap of stainless steel; glad I'm not paying for it.

[Mike OReilly]

Originally Posted by Lost Horizons

This is anecdotal evidence at best. Since we are talking physics, to prove the point we need a controlled experiment with statistically significant data.

Regarding a set vs being dug. I consider one a definition of the other. It appears what you are calling “setting,“ is for an anchor to be appropriate for the bottom, but in my experience bigger anchor does not cure it. A different design might.

It's not anecdotal. It's purposely built into the design of modern anchors. Why do you think many weight the tip, or use roll bars to orient the anchor?

This is exactly the reason why we don't use Danforth-style anchors for all substrate. These have the biggest holding, by far, so by your understanding we should use them all the time. We don't because they can't penetrate a lot of substrate. They have low initial setting force/capacity.

It's the same problem with the older-style CQR. They are fine anchors once dug in, but are a lot harder to get set properly.

[Mike OReilly]

Digging in an anchor translates a component of the near-horizontal force (the pull on the rode), to an angular vector. This angular force vector can only initiate once the anchor has begun to penetrate. Otherwise the anchor will skip along the bottom, with no angular (digging) force created.

This is where heavier is better. It helps with the initial penetration. Again, it's the reason why we don't use Danforth-style anchors all the time, for everything. Harder substrates, or those littered with debris, weeds, pebbles, etc., are all harder to penetrate. And Danforth-style (along with older designs like CQR) have lower penetrating power.

Once the anchor firmly penetrates, the anchor weight becomes largely irrelevant. It is the surface area, angles and other design features which then produce the holding power. Here size (area) matters. But bigger is always better. Weight is a proxy for size.

This comes to the question I think some of you are zeroing in on; the fact that an anchor must dig into the substrate to create holding power. This is absolutely true. I absolutely agree that if you can't get the anchor dug in, then it's no better than a dead weight on the bottom. This is why I recommend using the largest scoop-style anchor your crew and your boat's systems can reasonably manage.

This would include consideration of engine power, although in my experience (there's that anecdote again), engine HP tends to follow other sizing factors on a boat. Bigger boat, bigger windlass, bigger anchor roller/holder AND bigger engine. But if you have an undersized engine for your boat, then yes, I'd consider that factor when sizing the bower.

[Dockhead]

Originally Posted by Mike OReilly

Digging in an anchor translates a component of the near-horizontal force (the pull on the rode), to an angular vector. This angular force vector can only initiate once the anchor has begun to penetrate. Otherwise the anchor will skip along the bottom, with no angular (digging) force created.

This is where heavier is better. It helps with the initial penetration. Again, it's the reason why we don't use Danforth-style anchors all the time, for everything. Harder substrates, or those littered with debris, weeds, pebbles, etc., are all harder to penetrate. And Danforth-style (along with older designs like CQR) have lower penetrating power.

Once the anchor firmly penetrates, the anchor weight becomes largely irrelevant. It is the surface area, angles and other design features which then produce the holding power. Here size (area) matters. But bigger is always better. Weight is a proxy for size.

This comes to the question I think some of you are zeroing in on; the fact that an anchor must dig into the substrate to create holding power. This is absolutely true. I absolutely agree that if you can't get the anchor dug in, then it's no better than a dead weight on the bottom. This is why I recommend using the largest scoop-style anchor your crew and your boat's systems can reasonably manage.

This would include consideration of engine power, although in my experience (there's that anecdote again), engine HP tends to follow other sizing factors on a boat. Bigger boat, bigger windlass, bigger anchor roller/holder AND bigger engine. But if you have an undersized engine for your boat, then yes, I'd consider that factor when sizing the bower.

I totally agree with all of the above, but I would add that I think weight can be important EVEN after the anchor is well buried. The relative density between the mud and the anchor has a lot to do with the anchors behavior once it's down there, even if fluke geometry is also important.


For the same reason probably I find the Spade anchors to work somewhat better than other "new gen" anchors -- the point of the Spade is full of lead, so the anchor is quite dense, plus with no roll bar there is less resistance to diving down (the thin roll bar on the Mantus is probably a similar advantage). I had a 55kg (121 pound) Rocna for a while on this boat; it was less dense and had a somewhat less sharp point, and was noticeably harder to set than my current 100 pound Spade, despite the extra size.



I anchor a lot in very difficult conditions -- high latitudes, rocky and/or kelpy bottoms, sometimes very deep water, often anchoring in just a thin layer of silt over solid rock.

[Lost Horizons]

Originally Posted by Mike OReilly

Digging in an anchor translates a component of the near-horizontal force (the pull on the rode), to an angular vector. This angular force vector can only initiate once the anchor has begun to penetrate. Otherwise the anchor will skip along the bottom, with no angular (digging) force created.

This is correct. To drive an anchor down with the force that has an upward vertical component, the interaction between the anchor and the bottom is essential. This interaction is facilitated by mainly two factors: anchor geometry, snd anchor density. Geometry must be the reason for the differences between Rocna and Spade that Dockhead has observed. As for the density, it is the same for the different sizes of the anchors of the same design. If the latter was not true, smaller boats equipped with smaller anchors would have hard time setting them, which is not true.
I did have problems setting Danforth style anchor in heavy vegetation: it would consistently collect a “haystack” and lose all its bottom biting capacity. Replaced it with a Claw/Bruce and never had that problem. The Claw was not heavier, just better design.