Effect of chain on anchoring

[Dave Chain]

I first bought a sailboat a couple of years ago. Since I like to sleep at night and hike during the day, I replaced the smallish anchor and bought as much chain as I thought I could lift. After some experience, this is still a reasonable criteria for boats with no windlass.

Being an engineer, I later put a little thought into what the effects of chain weight are on the anchor rode and put together a web page that calculates the geometry of anchor rodes under varying conditions. You can adjust various parameters and the page updates. Click the label you want to change and drag the mouse to change the number. The important factor is the angle the mooring force is applied to the anchor.

If you are interested you can try it at tubafour.com/chain. This page is new, so let me know if you have browser issues or have other comments.

These calculations verify many (but not all) of theories previously proposed on this site.

As the load on the anchor increases, the effect of the chain weight are diminishes. However, even when the rode looks very straight standing on deck, there is still some effect from the chain weight.

Under more reasonable conditions, the weight of the chain can have a big effect anchoring in small anchorages. We cruise in the Pacific northwest where the anchorages are often very small and scope is very limited. Adequate chain weight allows you to swing in a much smaller circle without pulling up on the anchor.

Dave

[GordMay]

Greetings and welcome aboard the CF, Dave.

[Zanshin]

I am using Internet Explorer 8 and get a green screen. It seems to work fine when on Google Chrome and I like the visual approach! What I don't understand is that the formula doesn't seem to take wind speed into account, is that correct?

[Seaworthy Lass]

Hi Dave
Welcome to CF.
The weight of chain helps (as do anchor angels) until wind strength is sufficient to lift the chain all the way to the anchor. Then the weight of the chain makes no difference. So in light to moderate air, yes, heavier chain helps. Under gale/storm conditions it has little effect.

[carstenb]

I also get a green screen, using an ipad.

Seaworthy lass is correct, once the wind reaches somewhere around force 7/8 chain weight makes no dofference. The same is true for anchor kettles. After force z7/8, the kettle does not produce any catenary on your rode ( assuming of course you haven't dumped something like a 2 ton kettle overboard)

Peter smith has written about the best dissertation on anchoring and anchors I found.

Petersmith.co.nz or something like that.

[Maine Sail]

Dave,

Mozilla Firefox version 17 on Windows 8 yields no effect on parameters. When I click the yellow boxes nothing happens. Got a blank screen on iPad...

Also used current Firefox on a Macbook Pro and the box turns red but I can't get the parameters to change.

Perhaps clicking the box could allow one to simply change the numbers manually???

[Zanshin]

Maine - click on a box, then press your mouse key and move the mouse left or right to decrease / increas the values of the activate button item.

[Seaworthy Lass]

I don't get a green screen on my iPad, I can open all the options, but "Manual Input" doesn't function for me.

[estarzinger]

Originally Posted by Seaworthy Lass

The weight of chain helps (as do anchor angels) until wind strength is sufficient to lift the chain all the way to the anchor. Then the weight of the chain makes no difference. Under gale/storm conditions it has little effect.

Originally Posted by carstenb

once the wind reaches somewhere around force 7/8 chain weight makes no dofference.

Actually . . . this is quite incorrect. These models only look at the steady state static loads, not at the dynamic loads (wind gusts, waves, and sailing back and forth at anchor) that actually create the peak shock loads. Chain will in fact dampen those peak shock loads.

A secondary problem with these models is that they don't incorporate the important fact that the boat is in water and the bow can move up and down. This reduces the peak static loading - because when the chain comes straight, the bow can still 'give' by being pulling down into the water, but increases the dynamic shock loads because a wave can jerk it up.

No mathematical model is a perfect representation of the real world. Some models are close enough to provide decision making value, but these chain loading models are not. ANyone who has anchored in storm force winds will know that the dynamic component, which these models miss, is more critical than the static loadings.

[Kettlewell]

Or, take a look at table 5-5 on page 113 of Earl Hinz's book The Complete Book of Anchoring and Mooring (first edition 1986).

[carstenb]

Hi evans

I can only discuss the theory behind this. I bow very deeply in respect of your greater experience. So you will get no arguement from me.

How's it going with the sanding/revamping of Hawk?

[cal40john]

In manual mode changing the wind speed doesn't change the load. Setting it to ABYC load, then decreasing wind speed appeared to work as the load started decreasing, the load went through a minimum then went back up to a huge number inconsistent with wind speed.

I'm using Chrome

[estarzinger]

Originally Posted by carstenb

Hi evans

I can only discuss the theory behind this. I bow very deeply in respect of your greater experience. So you will get no arguement from me.

How's it going with the sanding/revamping of Hawk?

I am a mathematician by education, and theory here shows the flaws just as well as experience. That is that the static wind loads on the boat will be (much) smaller than the dynamic snatch loads and these chain models focus entirely on the smaller static wind loads and ignore the higher dynamic snatch loads.

It is certainly possible to model the dynamic loads, and if you really wanted to use the model for decision making it would be necessary to get anywhere near real world results, but it does require a bit more sophisticated math/physics (need to model the deceleration from gusts/waves/sailing back and forth and the effect this has on anchor setting/resetting)

Also, the plain fact is that most of us don't pick chain because of any sort of load calculations. We use chain primarily for chafe resistance and secondarily because it runs thru a windless more trouble free than rope.

Hawk is now back in the water with a fresh bottom (although when they relaunched her the water was too shallow and I had to plow out thru mud which probably wrecked all my good work on the very bottom of the keel ). I am now working on deck and mast paint - that's going to be more of a touch up job.

[conachair]

Best web page on the dynamic unpredictable nature of anchoring I've seen so far is here..
Rode - Dynamic Behavior

Dave's page is fun to play around with though, shows how little force it takes to lift a chain off the sea bed. Well done.


If you get bored a metric/imperial option would be nice for the rest of us

[Kettlewell]

By the way, when you are out there someplace like the Bahamas with crystal clear water it is fascinating to snorkel or dive on your anchor and watch what actually happens down there as the boat pulls on the anchor chain. One thing that is little appreciated is how much extra holding power the chain itself adds to the anchor rig if you have plenty of scope out. Off of the coast of Mexico, near Tulum, I spent a good hour in the water trying to find some depression in the bottom that would give my anchor some grip--it was basically as hard as a runway with a sprinkling of sand. Even though the Caribbean trades were cranking upwards of 20 or so, we ended up hanging on mostly chain and anchor weight, because I just couldn't find a place for the anchor to dig in. I could pick up the anchor, and walk across the bottom carrying it, while the boat just hung on the rest of the chain dragging across the bottom, despite some pretty hefty winds. Similarly, when diving on anchors lots of other places, they are very frequently hardly dug in at all, with the boat hanging mostly on the drag and weight of the chain.