Safer Anchoring Using an Anchor Chain Calculator!
Diese Seite ist auf Deutsch hier zu finden: https://trimaran-san.de/ankerketten-rechner/
Cette page se trouve en français ici: https://trimaran-san.de/calculateur-de-chaine-dancre/
- Calculates required minimal anchor chain length based on vessel characteristics and weather / sea conditions.
- Calculates anchor load and pulling angle at anchor shaft.
- Works with snubbers & bridles.
- Chain not long enough? The Anchor Chain Calculator will tell you what effect this has.
- Video to App.
A sure way to create a long and heated debate is to walk into a bar full of sailors and start a discussion about how to anchor correctly. It seems no topic is discussed more controversially than anchoring! And yet, it would appear so simple – your throw out your hook, make sure the chain and/or rode is long enough, make sure the hook bites, and that’s it. Yet, everybody seems to have their own recipe how to do things correctly when it comes to anchoring.
And certainly, if you are alone in a large bay at anchor, you can pay out all the chain you have. Then you may think, oh well, I could not possibly have done more than that.
Or could you?
It starts already here – will it be enough, what you did? Wouldn’t it be great to get a better sense of how your anchor gear will stand up to the weather forecasted for the next night? Do you have any safety margins built in? Is a scope of 5:1 really enough? Or should you go up to 7:1? Or even more? Or should you relocate the vessel to another place? What about snubbers and bridles? Do you need them? Or, perhaps you need to use as little chain as you possibly can, as you are rather close to a friendly neighbour at anchor, or a not-so-friendly corral reef. How much can you reduce the chain length and still be safe?
In short, what is the metrics of anchoring that gives you a good assurance that you have done everything reasonably possible to be safe at anchor and not dragged away into other vessels, onto a reef, or onshore! At night…, in a storm…
This Anchor Chain Calculator App is designed to help you get this assurance. It will help you determine the minimal length of chain that you need to pay out, based on a number of weather conditions and vessel parameters. With this minimal length of chain the anchor only needs to bear the smallest possible load, which maximises the chances that the anchor will actually hold. And if you cannot pay out that amount of chain, for one reason or another, the App will tell you how much the anchor load increases, and at which angle the chain will pull at the anchor. The larger this angle is, the less the anchor will hold, and so you will want to keep this angle small.
An important part of your anchor gear is the snubber / bridle, which is essential to absorb swell and often is not recognised for this. I encourage you to play around with this parameter to see what huge benefits good snubbers / bridles have.
This App supports Dark Mode for iOS and is backwards compatible down to iOS 10.0 and Android 4.1! So, it should still work on this very old Android / iPhone / iPad of yours… 🙂
Video to App.
Below you see an example for a 12 metres monohull vessel in strong swell. Wind strength is 27 kn, anchor depth is 7 metres. We assume the maximal reverse vessel velocity at anchor to be 0.7 kn, and no snubber present at all. The App calculates that 72.5 metres of chain are needed for a perfect catenary – which corresponds to a scope (so the ratio of chain length to water depth) of more than 10:1!
Now, suppose you do not have that much chain in the locker. All you have is 55 metres, which is not enough. So, we punch that value into the Max field. As a consequence, the chain pulls at almost 5º at the anchor shaft, and the anchor load has increased from 732.9 daN to as much as 1126 daN. This is a substantial increase, and if the seabed is not good, it may be too much for the anchor to bear.
So far we have not deployed a snubber or a bridle yet. So let’s add that – and whilst we are at it, let it be a ‚very good‘ snubber / bridle. Now much less chain is needed, only 48.7 metres. Even better the anchor load has come down to 326.8 daN, which is a little more than a quarter of the load we had in the absence of this good snubber. Just FYI, the App also tells you how much of the swell energy is absorbed by the snubber, and how much by the chain. Below we see the snubber takes on as much as 70.7% of the swell energy, which explains why the anchor load and chain length have been so much reduced. The snubber is simply very effective!
General advice: Better safe than sorry! In order to add some safety margin to the results of this app, please do the following: Rather than just adding a few metres of chain to the result calculated, which provides only for an unknown margin, it is much better to add an explicit safety margin by increasing the wind strength and swell / vessel velocity a little bit beyond the values actually forecasted. This way you know you have a margin of, say, 5 kn, in the wind strength for particularly nasty gusts, or an additional 0.1 kn in the vessel velocity for calculating the swell energy.
I encourage you to play around with all the parameters to see what effect they have. This way you will be in a better position to make your own judgement call in a particularly tricky situation. For instance, seabed angle, albeit often unknown, has a strong effect on the minimal chain length needed. Interestingly enough, it is not always better to have a positive seabed angle, where your vessel is in shallower water than the anchor. Yes, it will be beneficial for the angle at which the chain is pulling at the anchor shank, but it will be bad in terms of how much swell the chain can absorb.
Talking about swell: When you play around with the snubber / bridle you will soon realise how important this gear is for minimising the anchor load when swell is strong. In particular in shallow water swell is difficult to deal with if no snubber is present. In such a case it may actually be beneficial to ‚escape‘ to deeper water to reduce the anchor load. Possibly, you even do not need to spend more chain then! Sadly, the vital importance of good snubbers / bridles is only too often overlooked.
When the App starts, it comes up with a main screen that allows you to enter and review all information needed. Two modes are available here: Basic Mode and Expert Mode. You can always toggle between the two modes, using the switch at the top of the screen, to see what a parameter in one mode corresponds to in the other mode. Also, right next to this is a switch to select the physical units used for input and output: daN for metric SI units and forces given in daN, kp again for metric SI units, but using kp for forces, and finally lbs when you want to use imperial units such as feet and pounds.
To provide input in a text field simply select it with your finger and use the keyboard popping up to edit old values or punch in a completely new value. For iPhone / iPad, there is a small knob at the right side inside every text field that allows you to clear the entire text field in one go. A few inputs – like the chain size – will be presented using a pull down menu. All input will be stored and thus will be already pre-set the next time you start up the App. You only need to update those inputs that have changed.
Once all input has been provided, you select the red Calculate! button in the lower part of the screen to calculate all outputs. If this button is black, it means the output parameters shown are consistent with the input parameters. If it is red, you need to press this button again to update the output.
In Basic Mode, the following inputs need to be made:
Vessel weight: The vessel’s displacement – in kg, not tonnes. It is only needed to work out the swell energy.
Vessel Length: In Basic Mode we use the vessel’s length rather than working out its windage area. For this we use the late Robert Smith’s Ground Tackle Loads Table for monohulls. These values then get augmented by heuristic factors for catamaran, trimaran and slim, medium or bulky built, which you need to select. You can always switch to Expert Mode and see what your input means in terms of Windage area.
Anchor depth: Measured at the position of the anchor, not the vessel. Zero reference level is the bow roller, not the water surface! For convenience, this value is split into two input fields, which you can assign as you please. For instance, you can store in the first field the fixed vertical difference between the bow roller and the zero reference point of your depth sounder – which most often is either the water level or the bottom of the keel. Then you can always use the second field for the depth sounder reading without having to make any further adjustments. In tidal waters use the water depth at high tide, but do check also at low tide when the swell is excessive and the snubber / bridle poor or not existent. Remember that swell is more aggressive in shallow water.
Seabed angle: If not known, set it to 0. To be used when the seabed has a substantial slope between vessel and anchor. A positive angle means the anchor is in deeper water than the vessel. Play around with this parameter to see what effect it has. Do mind changes in wind direction as they affect the seabed angle! (For Android, it may be that you need to tap the ‚-‚ soft key twice to get the minus sign…)
Chain size: Use a pull-down menu to select from a large range of chains in metric and imperial units.
Max [chain length]: If this parameter is empty or 0, this app will calculate the length of a perfect catenary, where the chain pulls horizontally with respect to the anchor shank, which is optimal in terms of holding power. However, this may result in more chain than you have – or you are willing to spend, e.g., when the swinging circle must not exceed a certain value in order to stay clear of obstacles or neighbours anchoring around you. In such a case use Max to restrict the chain length. Please note that constraining the chain length does come at the expense of a reduced maximal holding power of the anchor.
Swell energy, Vessel velocity and Vessel weight are all related via the vessel’s kinetic energy, so only either swell energy or vessel velocity needs to be entered. Use your chart plotter and determine the maximal speed over ground (SOG) component pointing away from the anchor to get a rough estimate for the velocity. Typical values are 0.2 to 0.7 kn. Larger values generally mean you are anchoring in the wrong spot! Often, a snubber / bridle is needed to keep this contribution in check.
Snubber [quality]: In Basic Mode, you only need to select one of a few preset choices for your snubber / bridle, ranging from ’none‘ to ‚excellent‘. You can always switch to Expert Mode to see what these preset choices correspond to in terms of Snubber stretch @ 8 Beaufort / 40 kn. If you select ‚custom‚, you can set any value even in Basic mode. This value will be maintained as long as you do not overwrite it again in Basic or in Expert mode.
There is an Info button to the right of the input parameters, where most of this information is also made available within the App.
Outputs of the Anchor Chain Calculator
The App calculates a number of results for your anchor gear, such as:
Chain length: Either the minimally required length of a perfect catenary with zero pulling Angle at anchor shank, or the Max chain length of a constrained catenary as given at input, if this one is shorter than the perfect catenary. In the latter case the pulling Angle will be larger than zero – leading to a reduced maximal holding power of the anchor! Angles less than 6º may still be ok. Please note that the chain length calculated by this app is anchor to bow roller.
Anchor load / Angle: The load the anchor needs to be able to bear / the angle at which the chain pulls at the anchor shaft (measured relative to the sea bed). However, whether the anchor can sustain this load is outside the scope of this app! It will depend on the quality of the anchor and the nature of the seabed. Please note that swell has a massive effect on the anchor load – in particular in shallow water. When this angle is zero, the chain is a perfect catenary. Angles larger than zero can only happen when the chain length gets constrained. If the chain pulls at the anchor shaft at an angle, the holding power of the anchor will get reduced. Commonly accepted rough values are 85% @ 8.2°, 70% @ 11.5°, 40% @ 19.5°, 10% @ 30°. Up to 6° the result is coloured in orange, beyond that in red.
Bow load / Angle: The load at the bow roller, and the angle of the chain there with respect to the water surface. This load is slightly larger than the anchor load because of the weight of the chain.
Bow swinging circle: Measured from the tip of the anchor shank to bow roller, so at least the length of the vessel needs to be added to get the true swinging circle.
Snubber stretch: Tells you by how much the snubber / bridle stretches under this load. Do not overstretch it! A stretch by 25% of the original length may be ok for a good snubber / bridle, but 50% is not. It may overpower it and make it snap. All this will depend on the material chosen for snubber / bridle, but as a rough guide, we colour-code a snubber stretch of 100-150% of the nominal stretch at 8 BFT in orange, and anything beyond that in red. (Tip: Adjust the length of the lazy chain segment between bow roller and attachment point of snubber / bridle to limit the maximal stretch.) Also, snubbers / bridles wear out over time and have only a limited number of stretch cycles. So do replace them at regular intervals!
‚%‘: States how much of the swell energy is absorbed by the snubber / bridle – as opposed to the chain. To be clear, this percentage refers to energy absorption, not to stretching. In shallow water, a good snubber / bridle will take on 50% and more. In deeper water the chain is more effective and will take a bigger share.
Errors will be generated when the chain + snubber cannot fully absorb the required energy. This may be due to too heavily constrained chain, due to too much swell without snubber / bridle and / or seabed angle too large. Anchoring will not be safe!
There is an Info button to the right of the output parameters, where most of this information is also made available within the App.
The Basic Mode avoids having to deal with two vessel parameters of the Expert Mode that are somewhat tricky to determine accurately. The first is the windage area – so the effective cross section of the vessel facing the wind – whilst the second ‚difficult‘ parameter is the elasticity of the snubber / bridle. How effective is this snubber / bridle? In what follows we restrict the discussion only to those two input parameters that are different in Expert Mode compared to Basic Mode: The parameters Vessel Length and Snubber [Quality] disappear, and instead you have the following new parameters:
Windage area: This needs to be done only once! To estimate the windage area, roughly calculate the frontal cross section of your vessel facing the wind – possibly at an angle to account for veering! Don’t forget the mast and the lazy bag. This area (shaded in the illustration above) then needs to be multiplied by a factor 0.7 to 1.1, depending on how streamlined your vessel is, to yield the windage area. For other, more precise ways of measuring the windage area, like measuring the tension in the rode when hooked up to a pole in the water, please visit my home page: https://trimaran-san.de/die-kettenkurve-oder-wie-ein-mathematiker-ankert/. It is safer to err on the high side with this parameter.
Snubber stretch @ 8 Beaufort / 40 kn: Again, this calibration needs to be done only once! It is the amount by which the snubber / bridle stretches elastically at 8 BFT wind for your vessel – measured in the absence of swell and in shallow water. A value of 0.05 metres would be a pretty poor snubber / bridle (likely because it is simply too short), and 1 metre a pretty good one. Rubber dog bones do help, but not as much as you might think. If your snubber / bridle has a maximal stretch of 25%, you should adjust the lazy segment of your chain between bow roller and attachment point of the snubber / bridle such that the chain will take over the load once a 25% stretch has been reached. Finally, please be aware that assuming such a linear relationship between elongation and force applied to the snubber / bridle is an approximation to reality. You can improve on this by applying a suitable base tension to your snubber / bridle before doing the measurements below.
Tip for how to measure this elastic stretch: When the weather is calm, run a ’non-stretchy‘ reference line parallel to the snubber / bridle and mark them both at the bow’s side with red tape. Then, when wind is at 8 BFT (40 kn), measure how far the two red tapes have come apart. Should you decide to measure rather at 20 kn instead, so half the wind speed, you need to multiply your measurement by 2^2 = 4 to obtain the value for 8 BFT, since the wind force depends quadratically on the wind speed.
2nd Tip for how to measure this elastic stretch: Run the snubber / bridle (one leg) along the deck, with one end fixed somewhere. At the other end attach a hanging scale and connect the scale with another short line to a winch. Example: You have a hanging scale for luggage that can do 40 daN max. The anchor load at 8 BFT (40 kn) will be too much for this scale – remember the load depends quadratically on the wind speed – so let’s measure at 40/3 kn = 13.3 kn instead, and correct with a factor 3^2 = 9 later. For your normal-built monohull of 12 m length, in the absence of swell, using this App you get an anchor load of 28.7 daN. Next make sure the snubber / bridle has no slack and mark the scale’s position on deck with a red tape. Now you turn the winch handle until you read 28.7 daN on your scale. Then you measure the distance between the red tape on the deck and the scale, and multiply the result by 9 to get the Snubber stretch @ 8 BFT. If it is a bridle, divide the result by ~1.5 to account for the 2nd leg. If you want to be more precise, you need to take into account that bridles usually pull at an angle at the chain.
There is an Info button to the right of the input parameters, where most of this information is also made available within the App.
Once you have played around a little with this App, you will have a much better understanding of how the parameters interact with each other, and this understanding alone will allow you to make better decisions for safe anchoring.
Surely, not all scenarios are covered by this App and as always you still need to apply common sense and good seamanship for safe anchoring. There are vessel and weather conditions that this App currently does not account for – like currents in the water, or violent ’sailing at anchor‘, to name but a few. And the App also does not make any statements what size or type of anchor to choose, or whether the anchor you have will be up to the job, or whether the anchor load will exceed the breaking load of the chain or snubber. But the App does give you an idea when things are starting to get closer to the limits.
If you are interested in knowing more of the technical details and the physics / modelling behind this App, I encourage you to visit my more technical web page: https://trimaran-san.de/die-kettenkurve-oder-wie-ein-mathematiker-ankert/
The home button on the main screen of the App in the top right corner will lead you back to this page – https://trimaran-san.de/anchor-chain-calculator/.
Feedback for improvements, new features, or bug reports are always welcome! Please leave it as a comment on this page. Mark it as ‚private‘ if you do not want others to see this feedback.