Expected load on anchor rode

[David Mathis]

We've never anchored in weather with some of the severe wind gusts described in a few of the posts as we've always been fortunate to be able to head to a somewhat protected area to spend a night or a couple days. As you know, even the lee side of a small island will offer a significant reduction in the wind and wave force. With that stated, we have a 43' powerboat using a 20k Bruce as a primary anchor with 30' of 5/16 chain then 200' of 9/16 nylon. The Bruce was slightly modified by sharpening the leading edge with an angle grinder after experiencing a couple failures when trying to penetrate hard sand. After the sharpening, we've never had a problem with setting or dragging. For soft mud, we have an appropriate size Danforth.

[Shrew]

I find the Rocna Sizing guide handy for chain:

https://rocna.com/fitment-guide/

At 40ft and 32K, this has you at 3/8" G40 chain. They do suggest upsizing the anchor for multi-hulls. That weight recommends wither 5/16" (8mm) or 3/8" (10mm). I'd go with the 3/8" (10mm) chain.

[MathiasW]

So, I have started to work on the mix of chain and rope for my anchorchaincalculator tool...

The number crunching part is almost done. Some corner cases remain to be worked on, and stability of numerical convergence is also not yet where it should be, but I will get there in the end...

The daunting part will be the update to the interface to keep it a simple to use tool!

Cheers, Mathias

[MicHughV]

There are about a dozen anchoring... scope....rope..chain...anchor selection, etc, threads on this forum. Just snoop around and you'll find them.
Pretty much anything that can be discussed, has been discussed, ad nauseam.

[MathiasW]

Originally Posted by MicHughV

There are about a dozen anchoring... scope....rope..chain...anchor selection, etc, threads on this forum. Just snoop around and you'll find them.
Pretty much anything that can be discussed, has been discussed, ad nauseam.

Yes, I know, I am to blame for a few of them...

And despite of this all having been discussed at great lengths, I keep watching folks making serious mistakes at anchorage, sometimes resulting in the loss of their vessel. I had to watch three such incidents in as many years.

So, there is still some work to do to get the message out...

[MicHughV]

Yes, I recall your name.

I don't think there is a one size fits all situation here, as I've posted many times, there is an art to laying an anchor properly, in addition to all the other variables, which has as much to do with successful anchoring as anything else.

At the end of the day, it's as much to do with one's personal experience as anything else. When I first started sailing I also drug my anchor and had other mishaps. It took some time before I got it right. I don't think reading about it really adds much, one has to try different things on one's own boat under different circumstances. It takes time to develop these skills.

I'm at a point now, where I've got my system down pat. It works for me, but may not work for anyone else. I'm unlikely to change what I do, or be persuaded to try different anchors, etc.

Anyway, good luck with your thread !! I'll lurk along for funsies

[fxykty]

Originally Posted by MathiasW

My son and I have recently developed a tool to calculate the anchor load stemming from wind and swell. It is based on energy calculations very much like a pendulum, where chain and snubber act as energy storage elements. As Thinwater said, it is not only the chain, but the snubber is a very important part of the system as well. In particular in very shallow water, when the chain does not absorb energy burst from gusts / swell well.



Here is my tool:



Anchor Chain Calculator



One or two parameters need further explaining - there is a document linked to.



You can also try Bjarne's tool, which includes even more effects than mine, but may be a bit more complicated to use:



http://svamanda.dk/anchor/intro



He uses the same model as me, but also includes effects like the actual elasticity of each metal link of the chain.



Cheers, Mathias



PS: If you have a lot of time to waste on this topic My long essay on the topic:

https://trimaran-san.de/die-kettenku...atiker-ankert/ (In English)

With your calculator the result for non-trivial wind speeds always seems to be to use all of the available chain - that’s patently ridiculous in the real world. So I find the calculator not useful for anchoring, but useful for deriving loads for sizing the components.

[MathiasW]

Originally Posted by fxykty

With your calculator the result for non-trivial wind speeds always seems to be to use all of the available chain - that’s patently ridiculous in the real world. So I find the calculator not useful for anchoring, but useful for deriving loads for sizing the components.

Well, that is so by design. If you do not restrict the chain length in the MAX field, the calculator will spit out the length of the chain needed to make the angle at the anchor shank vanish. A perfect catenary will get arbitrarily long if only enough wind and swell is present, even if the additional gain is marginal.

So, what you should do is to use a restriction of the chain length to what you have available, or is feasible to pay out given neighbours etc. The thing to watch out for then is the angle at the anchor. If that remains small, all is well.

Cheers, Mathias

[MathiasW]

Originally Posted by MicHughV

Yes, I recall your name.

I don't think there is a one size fits all situation here, as I've posted many times, there is an art to laying an anchor properly, in addition to all the other variables, which has as much to do with successful anchoring as anything else.

At the end of the day, it's as much to do with one's personal experience as anything else. When I first started sailing I also drug my anchor and had other mishaps. It took some time before I got it right. I don't think reading about it really adds much, one has to try different things on one's own boat under different circumstances. It takes time to develop these skills.

I'm at a point now, where I've got my system down pat. It works for me, but may not work for anyone else. I'm unlikely to change what I do, or be persuaded to try different anchors, etc.

Anyway, good luck with your thread !! I'll lurk along for funsies

Absolutely! If you do have a lot of experience in anchoring - and have been successful at it - there is no reason to change anything.

But I disagree that this experience has to be gained by trial and error as you suggest. The risks of damage or worse are just too high. The importance is not to use a calculator, but to understand the physics of anchoring. When does a chain work well (in deep water), and when does it not (in shallow water with strong gusts or swell), where it needs to be helped by a good long snubber / bridle. This is what I am trying to teach with my long essay on anchor chains. This general knowledge will already help a novice to avoid some mistakes and that makes it worthwhile to pursue. There is no reason why everybody should start at absolutely zero when they start with sailing.

After all, you would not approve of an airplane pilot to learn his art by trial and error, would you?

If there is knowledge out there, why not use it when starting as a novice. Details will differ, but the general physics governing anchoring will be the same on out vessel and mine.

Cheers, Mathias

[wsmurdoch]

Originally Posted by Benz

There is a new company now making waterproof inline load cells that can sync to a phone or B&G display. Designed for tack line loads on racers, but it'd be interesting for someone to put one on their anchor tackle and experiment with different rode/snubber/boat size configurations for some real-time load experiments. It can graph the results, so one could see the load cycles during a blow.
Sadly, I don't have $3k to spare for one, and embarrassingly, I can't remember the name of the company, though I talked with the CEO just three days ago...
But it would be a neat thing to have for the anchoring nerds out there.

A few years ago I bought a 0-2000 lb Dillon dynamometer on ebay. Among other things I have measured the loads on my anchor. It has a tell-tale needle that shows the maximum load. It has hit 450 lb in 40 kt wind with 1-2 ft waves.



I have been unwilling to use it in the rain because it is in no way waterproof, but it has been useful in getting some idea of what my anchor loads are.

Later, I bought a 600 lb electronic load cell based hanging scale on Amazon. I took it apart, removed the load cell, mounted the load cell inside a piece of accordion hose, and ran enough wire between the load cell and the display to reach the nav desk in my boat.





Covid has slowed my work with it, but I have it in use right now in the Bahamas, and it shows about 75 lb at 20 kt in flat water.

[Rumrace]

I don’t see the advantage of a lot of chain in the waters I anchor. I did run a SeaRay 40’ Sedan and now a 40’ Jeanneau. The. Jeanneau came with a double roller but just one anchor. I’ve anchored with two anchor and two bands to take the load off the hardware. The chain really is just to lay an anchor properly. I’ve scuba’d Around anchors and found some boats with a big pile of chain on top of the anchor. Those guys need 250’ of chain.

[Adelie]

Originally Posted by B4A

Hi all,



I'm in the process of changing my anchor chain. I've looked everywhere for the formula used to calculate the expected load on anchor chain for given wind and boat weight, but I only found an old ABYC table where the load is based not on the weight of the vessel but on its length. And there is no mention of windage or Cat Vs Monos.



Attachment 253330



I have a 40 feet catamaran (light ship 10 tons, max 15 tons). I'm trying to determine what would be the Safe Working Load (SWL) for the new chain.



A G40 galvanized 10mm chain is only rated about 1.2 tons. That would be enough for my boat, according to the ABYC table. Is this chart accurate or am I missing something?

Counterintuitive but broadly correct that anchor loads are dependent on vessel dimensions and not vessel mass.

If you have 2 vessels the same except one masses twice the other what will happen is that when a wave hits the boats both will experience the same force but the heavier one will not accelerate as much before the wave passes. Since force divided by mass equals acceleration the heavier boat has half the acceleration. Once the wave passes the heavier boat has half the backwards speed thru the water as the lighter boat. Probably it is some other fraction but close enough.

Once the rode starts tighten up the force on the heavier boat is less than on the lighter boat because it hasn’t moved as far but it acts for a longer period because it is moving more slowly.

The weird thing is that the energy in the lighter boat is a lot greater because energy = 1/2 x mass x velocity^2. So even though mass is half the heavier boat the effect of speed being twice that of heavier boat nets to twice the energy that the rode has to absorb.

But if 2 boats of equal weight are in the same waves the bigger one will incur more load on the rode.

The boat with the bigger area presented to the wave will absorb a greater force the same way that a boat with a larger sail will absorb more force from the wind and go faster.

Since a boat does not exactly face the wind at anchor but yaws about area presented to a wave is some combination of length and beam.

The ABYC tables I have seen have both length and beam. The ABYC tables are also very conservative so even if your rode is getting a little old, if it was chosen to meet the storm loads from the table it will probably do just fine in a real storm that actually inflicts lighter loads.

Alas the tables are for powerboats and monohull sailboats.

I would suggest getting and reading Earl Heinz’s book on anchoring which walks you thru calculating all the loads including for a catamaran.

Attached is somebody’s reproduction of the ABYC table.
The following is a link to Practical Sailor’s article on the subject.

https://www.practical-sailor.com/sai...our-rode?amp=1

[MathiasW]

I've always found the real-life measurements of the late Robert Smith most helpful:

http://www.plaisance-pratique.com/IM...0-17-Smith.pdf

They are for standard mono-hulls, but of different lengths. For cat and tri one needs to use a fudge factor.

Cheers, Mathias

[Benz]

Originally Posted by wsmurdoch

A few years ago I bought a 0-2000 lb Dillon dynamometer on ebay. Among other things I have measured the loads on my anchor. It has a tell-tale needle that shows the maximum load. It has hit 450 lb in 40 kt wind with 1-2 ft waves.



I have been unwilling to use it in the rain because it is in no way waterproof, but it has been useful in getting some idea of what my anchor loads are.

Later, I bought a 600 lb electronic load cell based hanging scale on Amazon. I took it apart, removed the load cell, mounted the load cell inside a piece of accordion hose, and ran enough wire between the load cell and the display to reach the nav desk in my boat.





Covid has slowed my work with it, but I have it in use right now in the Bahamas, and it shows about 75 lb at 20 kt in flat water.

This is awesome. Thanks for posting.
Also, I'm astonished at how low the loads are. Maybe I need thinner anchor rope.

[wsmurdoch]

Originally Posted by Benz

I'm astonished at how low the loads are. Maybe I need thinner anchor rope.

Considering that I was anchored in a well sheltered spot, that there was no wave action, that the boat was sitting calmly bow into the wind with a riding sail up, and that my Pacific Seacraft 34 has a low free board, 75 lbf is not much less than the loads shown in Smith's tables mentioned above.

In my experience waves add a lot to the anchor load as does my boat's orientation to the wind.