Anchoring in shallow water - chain versus snubber

Anchoring is a very controversial topic, I know! Still, here I would like to put forward a free tutorial tool my son and I have created that allows one to estimate the maximal anchor load and the minimally required chain length after providing input parameters for the vessel as well as sea and weather conditions (i.e. swell and wind / gusts). It works with various physical units, like daN (deka Newton), kp (kilo pond, which is roughly a daN), or imperial units of pound and feet - the button on the top far right. Depending on which country you are coming from, the tool will try to make an intelligent guess to set the default correctly. The tool can be used in Basic Mode and Expert Mode. Advise is to start in Basic Mode and take it from there.

Have a look at the enclosed table, which shows the results for various anchoring scenarios. For the sake of this analysis, wind and (large) swell is always the same and for this analysis the maximally available chain is fixed at 50 metres. I chose three different anchor depths (measured from the bow, of course): 3, 5, and 9 metres. Finally, I varied the quality of the snubber / bridle - I define the quality by the snubber stretch at a given wind strength. First, if you look at 3 metres and no snubber, the tool gives an error, meaning that within my model the chain alone cannot cope with the (large) swell energy. Then, comparing 5 and 9 metres of anchor depth, one finds that, naturally, the pulling angle at the anchor shaft will slightly increase as the chain length is fixed, but at the same time the anchor load will go down from 1322 daN to 480 daN, which is less than half! This massive reduction of the anchor load more than compensates for the slight reduction in maximal holding power caused by the increased pulling angle. Finally, when you use an "excellent snubber", the anchor load gets further reduced to a mere 156 daN. This is less than 1/8 of the anchor load at 5 metres anchor depth and no snubber. What a difference! These results clearly show the importance of using excellent snubbers in shallow water in particular. Using more chain would not make much of a difference, as the chain is not effective in shallow water for absorbing shock loads.

Of course, in this analysis I did assume the swell to be the same at all anchor depths (and rather large), which is not in general true. But when it is, it is definitely safer to anchor in deeper water with that amount of swell present. (In my model, the vessel velocity at anchor is related to the swell energy.) Keeping all but one parameter fixed and only vary a single parameter is a typical scientific approach to study the effect this parameter has. So, please do not misunderstand this as me saying the swell is always the same, regardless of anchor depth. It certainly is not. Also, I am not recommending these shallow anchor depths, not at all. The table is more to illustrate what NOT to do.

So, contrary to popular belief, it is not always better to seek the most shallow place to anchor. Experienced sailors will know this, but at least for me I did not know this when I started sailing - in my sailing classes there was no mentioning of this and the simple rule was: Use a scope of 3:1 in calm weather and more when it gets windy...

Here is the link to the online tool: https://anchorchaincalculator.com

If you do not like to read the tutorial nor watch the short tutorial video, here are some comments on the required inputs: Vessel type refers to whether it is a monohull, a cat, or a tri, and how its built is: slim, medium, or bulky. This information is required to estimate the Windage area of your vessel. Anchor depth is divided into two fields, which you can use as, e.g., freeboard and water depth, but any other split is fine as well. Vessel velocity at anchor is the maximally observed speed over ground in reverse direction away from the anchor, whilst at anchor. It is difficult to get this one precise, as chart plotters are not very precise, but often it is something in the range of 0.1 to 0.7 kn. This entry, together with Vessel weight determines the swell energy. As to Snubber quality - in Basic Mode I define it qualitatively in plain English words. Not scientific, I know, but the names do have a meaning - a good snubber is a good snubber, and a lousy one is a lousy one... If you want to dig deeper here, use the Expert Mode. For instance, an "Excellent snubber" has a stretch of 1.6 metres at 8 Beaufort wind strength, or 40.2 kn (in the absence of swell, which would come on top as a load). If you do happen to have the data sheet of your snubber line, you can use the Rule of Three to work out the input that the tool requires. If you use the qualitative descriptions of the snubber and find that the snubber stretch the tool calculates is very different from what you actually observe, you need to adjust the snubber quality rating you have given your snubber. My observation is that many people overrate their snubber initially - certainly I did when I started this... Obviously, a snubber of only 2 metres length cannot stretch by yet another 1.6 metres!

And when I say swell, I mean all kinds of waves hitting the vessel.

This tool is not meant to be used every time you anchor, but rather as a tutorial tool to play around and see how different scenarios pan out. For instance, it may allow you to make a first quick assessment of the quality of the snubber you are using. I hope it will help you to make better choices for anchoring more safely.

Feedback most welcome!

Cheers, Mathias

PS: If you want to understand the physics and model in more detail, in a nutshell, it is catenary plus a spring (the snubber) bolted onto it with matching loads. For yet more information, please visit: https://trimaran-san.de/die-kettenkurve-oder-wie-ein-mathematiker-ankert/