Safer anchoring using an anchor chain length calculator! What scope is really needed?
Anchor scope calculator: Calculates required minimal anchor chain and/or anchor rope length as well as anchor load – tailored to your vessel: Anchor Chain Length Calculator / Anchor Rope Length Calculator
Diese Seite ist auf Deutsch hier zu finden: https://trimaran-san.de/ankerketten-rechner/
Esta página se puede encontrar en Español aquí: https://trimaran-san.de/calculadora-de-cadena-del-ancla/
Cette page se trouve en Français ici: https://trimaran-san.de/calculateur-de-chaine-dancre/
In addition to these languages, the Apple and Android apps also support Danish, Swedish, Norwegian, Dutch, Turkish, Italian and Portuguese for offline help and error messages.
A complete list of all the languages in which a (possibly short) description is available is found here: https://trimaran-san.de/anchor-apps/
How much anchor chain do I need?
- Calculates the required minimal anchor chain / anchor rope length based on vessel characteristics and weather / sea conditions.
- Calculates anchor load and pulling angle at anchor shank.
- Works with snubbers & bridles.
- Anchor chain or rope not long enough? The Anchor Chain Calculator will tell you what effect this has.
- Different vessel configurations can be stored under different names.
- It is also possible to calculate a mix of anchor chain and anchor rope, or just rope only.
- Video how to use the App.
- Video on the effect that strong gusts and swell have in shallow and deep water.
In a severe gust, an elastic snubber will reduce the anchor peak load substantially. Particularly so in shallow water, where in this example the load is almost halved. In all these 4 cases the anchor chain length is kept the same! Interestingly, in this example, the anchor peak load in deeper water is less than in shallow water, despite a substantial stretch of chain still lying on the seabed in the latter case. (Contrary to popular belief, a chain lying on the seabed is not reducing the anchor load by a lot.) The chain is simply not used in that case and is wasted! See the red and green arrows labelled ‘last link’ indicating how much chain is still lying on the seabed. In deeper water the chain can store much more energy and thereby efficiently reduce the impact of gusts and swell. The old sailor’s wisdom of “a lot of anchor chain helps a lot” is thus not quite correct. More accurately, it should say “a lot of anchor chain helps a lot – provided it is given enough anchor depth to work with”. We will dig deeper into this topic further below.
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 – you 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 for 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 forecast 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 coral reef. How much can you reduce the chain length and still be safe? Or should you use a mix of chain and rope?
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 / rope that you need to pay out, based on a number of weather conditions and vessel parameters. With this minimal length of chain / rope 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 / rope, for one reason or another, the App will tell you by how much the anchor load increases, and at which angle the chain will pull on 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 an elastic line connecting a cleat on the foredeck with the chain using a chain hook, and which is essential to absorb swell but only too 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 11.0 and Android 5.1! So, it should still work on this very old Android / iPhone / iPad of yours… 🙂
Free online version available with slightly reduced functionality (e.g., no mix of rope and chain).
Video to App.
On the right 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! (Please note: The following examples are all in metric units, but if you prefer, you can switch to imperial units.)
Far right: 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 Fix field. As a consequence, the chain pulls at almost 5º on the anchor shank, and the anchor load has increased from 733.7 daN to as much as 1130 daN. (One daN roughly equals one kp – kilo pond, which in turn is very roughly 2 lbs.) This is a substantial increase, and if the seabed is not good, it may be too much for the anchor to bear.
A snubber is a piece of rope that is attached to the chain on one end and on the other end to a cleat on the foredeck. Often, its main purpose is claimed to be strain relief for the windlass and to reduce the noise the chain would otherwise make on deck. But as we will see here, its real purpose is to provide elasticity and with that temporary energy storage to absorb gusts and swells – in particular when the chain does not work well in shallow water. The longer the snubber is, the more energy it can store – see the table further below. A short snubber of just a metre of length cannot absorb enough energy. On a catamaran or trimaran the equivalent is the bridle. The length of the slack chain segment should normally be chosen such that it can limit the maximal stretch of the snubber / bridle to within their working load, if there is a risk that those limits may be reached or even exceeded. On the other hand, if this is not a worry, providing plenty of slack in this chain segment does add further elasticity for absorbing shock loads and also lowers the point of pull at the anchor. However, do make sure this chain segment is not touching the seabed, as it must not get get caught in a rock or something else.
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 49.3 metres. Even better the anchor load has come down to 336 daN, which is a little more than a quarter of the load we had in the absence of this very 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 in the bottom right corner we see the snubber takes on as much as 64.3% 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 @ anchor 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 @ anchor for calculating the swell energy. It should also be noted that severe yawing or sailing at anchor not only increases the wind load, but will also lead to a substantial reduction of the anchor’s maximal holding power. Measures to reduce yawing are thus highly recommended.
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 on the anchor shank, but it will be bad in terms of how much swell or gusts the chain can absorb.
Talking about swell and gusts: When you play around with the snubber / bridle you will soon realise how important this gear is for minimising the anchor load when swell or gusts are strong. In particular in shallow water swell and strong gusts are 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. To the right is a table with different anchor scenarios which clearly demonstrates the importance of snubbers in shallow water. What a difference between anchoring at 5 metres without a snubber and anchoring at 9 metres with an excellent snubber: 1322 daN versus only 162 daN anchor load! Even without any snubber it is advantageous to relocate from 5 metres to 9 metres depth when using a chain of fixed length, here 50 metres. Yes, the pulling angle at the anchor shank does increase slightly, thereby marginally reducing the maximal holding power of the anchor, but the reduction in anchor load more than compensates for that.
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 (one daN roughly equals one kp – kilo pond), kp again for metric SI units, but using kp for forces, and finally lbs/lbf when you want to use imperial units such as feet and pounds. Note, some folks get confused with all these units: kp or kilo pond, is a unit of force, whilst kg or kilo gram, is a unit of weight. On Earth both have the same numerical value ;). Same applies to lbf and lbs in imperial units, same concept.
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.
Vessel Name: Click the trimaran SAN logo at the top right to provide a vessel name and select/edit a configuration with vessel-specific parameters. (Alternatively, you can also click on the app name / vessel name at the very top.) For instance, you can save here the different parameters for your various snubbers / bridles. To prevent accidental editing, it is possible to lock configurations using the little lock icon in the top left corner. In this case input fields that cannot be changed are greyed out. Please note that you need to unlock the configuration, before you can change its name again.
In Basic Mode, the following inputs need to be made:
Vessel weight: The vessel’s displacement – in kg (or lbs, if you are using imperial units), 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 to convert vessel length to windage area. 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.
Wind strength: Include gusts here as well, but remember that wind speed at the top of the mast is higher than at deck level, where the bulk of the windage area is. So, more relevant is the wind speed at deck level. For a wind strength less than 18 kn a warning will be issued. An increase of wind strength by merely 5 kn has a much more dramatic effect on the pulling angle at the anchor shaft (and hence the anchor load) at low wind speeds compared to high wind speeds when only the minimal length of chain is used. It is therefore prudent not to press one’s chances and add a healthy safety margin when little wind is forecast.
To be more precise: The duration of a gust is also relevant when deciding whether to use the gust or the base wind as input for wind strength. When the gust is short compared to the time it takes the vessel to oscillate away from the anchor and back again, then using the base wind as input is a good choice. However, if the gust is longer, then a better choice is to use the gust as input for wind strength. In any case, using the latter is always safer.
+5 kn, -5 kn: With these two buttons you can quickly increase or decrease the wind strength in steps of 5 kn, whilst keeping the length of the chain and / or rope the same. (If you did not fix these lengths yourself, the app will do this for you.) This simulates the situation that the wind strength increases after the anchor has already been deployed and the lengths of chain and / or rope are thus fixed. As the wind strength increases, watch how the anchor load as well as the pulling angle at the anchor increase. You can use this feature to judge whether an unexpected increase in wind strength will become a problem or not.
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 or gusts are excessive and the snubber / bridle is poor or – worse – not existent. Remember that swell and gusts are 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. There are two possible ways to provide this parameter: Either as angle in degrees, or as gradient in percent, e.g., 2 metres per 100 metres. Play around with this parameter to see what effect it has. Do mind changes in wind direction as they affect the relevant 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.
Fix [anchor 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 Fix to restrict the anchor chain length. Please note that constraining the anchor chain length does come at the expense of a reduced maximal holding power of the anchor.
Fix [anchor rope length]: The amount of anchor rope to be deployed. If set to 0, the app will calculate the length of anchor rope needed for the chain to pull with 0 angle on the anchor shank. Note that for a mix of chain and rope the length of chain needs to be set to a specific value (possibly 0 – if you are deploying only rope), using the field Fix in the chain view.
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 for instance your chart plotter or a GPS app 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. Generally, this value is larger for multihulls and small boats where the ratio of windage area to vessel weight (for gusts) or underwater surface area to vessel weight (for swell) is very large. These vessels move strongly at anchor. A compact heavy displacement vessel, on the other hand, will hardly move at all for the same swell or gust, and will therefore record much lower values. This has a great influence on the behaviour in shallow water, when anchoring with chain only.
Snubber [quality]: In Basic Mode, you only need to select one of a few preset choices for the elasticity of your snubber / bridle, ranging from ‘none’ to ‘excellent’. A snubber is a piece of elastic rope that is on the one end attached to the chain and on the other end to, e.g., a cleat on the foredeck. Strain relief is the keyword here. A bridle is the equivalent for a catamaran or trimaran. If you have the feeling that the snubber stretch predicted by the app is more than what you actually observe, then this is a sign that your snubber is not as good as you may think and you should give it a poorer rating until the calculated stretch matches with what you observe. You can always switch to Expert Mode to see what these preset choices correspond to in terms of Snubber stretch @ 8 Beaufort / 40.2 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. In fact, if you do know your snubber’s data sheet and in particular its stretch at a certain load, you can and should enter it either in Expert Mode, or as ‘custom’ value in Basic Mode. The steps to perform the required conversions to what this app needs as input are described in a Tip in the Expert Mode section.
Rope elasticity: In Basic mode, you can specify the elasticity of your anchor rope in qualitative terms. What those mean you can check by temporarily switching to Expert mode. Please note that in very strong winds / swell too much elasticity may overpower the rope and cause it to stretch too much. In this case less elasticity is better / safer.
There is an Info button to the right of the input parameters, where most of this information is also made available offline within the App (in English, German, Spanish, French, Swedish, Danish, Norwegian, Dutch, Turkish, Italian or Portuguese, depending on your system settings).
Mix of Rope and Chain
With the chain / rope symbol on the right edge of the screen you can switch between two modes: Chain only, or chain + rope. When the chain symbol is visible, a chain + snubber / bridle is calculated. If the rope symbol is visible, a combination of chain and rope is calculated. In this latter case, there is obviously no snubber / bridle. The length of the chain is set in the chain view in the Fix field. Fix = 0 means here that only rope is used, but no chain at all. Obviously, the specifications for the snubber / bridle are irrelevant, if a rope is used instead.
The properties of the rope are entered in exactly the same way as for the snubber, the only difference being that the information is now given in % of the length of the rope and not in absolute length units. Stretch @ 8 BFT is therefore the percentage stretch of the rope at 8 Beaufort. If you do not know this stretch at 8 Beaufort, but only the percentage stretch S at a nominal working load WL, for example, then you can use the rule of three to determine the stretch at 8 Beaufort as follows: S * AL / WL, where AL is the anchor load at 8 Beaufort. The latter is obtained by entering anchor depth 0, velocity @ anchor 0, and 8 Beaufort (40.2 kn) as wind load in the app.
In the rope view, you can specify the rope length to be used in the Fix field. In this case, it may be that part of the anchor chain remains on the seabed, or that too little chain / rope has been paid out for the chain to attach to the anchor shank with zero angle. Whether and if so how much chain remains on the seabed can be seen when comparing the calculated minimal chain length with the value provided in Fix. If it is less, then the difference will lie on the seabed.
If you enter Fix = 0 in the rope view, then the length of the rope is calculated that is needed for the chain to attach to the anchor shank with zero angle. This can quickly result in a very large, completely unrealistic value if the chain is very short. If it turns out that no rope is needed at all and the chain is sufficient, a rope length of 0 will be shown.
Otherwise, a rope is treated in exactly the same way as a snubber / bridle and therefore all descriptions of the latter can also be used analogously for the rope. Like a snubber / bridle, the rope is a linear or non-linear energy storage for buffering peak loads. In Basic Mode, some values are preset, but you can also define your own values in the custom field or in Expert Mode, which you have either measured or taken from the data sheet of the rope. Again, use common sense to double-check the results. If the rope is supposed to expand by 50%, but still has not reached its breaking load, then something is wrong with the specification / characterisation of the rope.
Last but not least: It is quite easy to choose parameters for the app that overload the ground tackle. E.g. a too large swell and too small snubbers / bridles or an insufficiently elastic rope will lead to a situation where the energy cannot be completely absorbed any longer. In this case, the app gives an error message — usually also with a hint as to what the problem might be. In borderline cases, the app also struggles to find a solution and it will take a moment before a result is available.
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 anchor chain length with zero pulling Angle at the anchor shank, or the Fix anchor chain length as given in the chain view, if that is shorter. 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 6° may still be ok. Please note that the anchor chain length calculated by this app is anchor to bow roller.
Rope length: The length of the anchor rope when using only rope or a mix of rope and chain. When you leave the length of rope unspecified by setting this value to 0 in Fix, the app will calculate the length of rope required so that the chain (of length Fix as provided in the chain view) pulls with zero angle on the anchor shank. Values can quickly get very large and unrealistic. Obviously, when only rope is used (by setting Fix to 0 in the chain view) the pulling angle will aways be larger than 0. When you want to pay out a fixed length of rope, set this value in Fix in rope view.
Anchor load / Angle: The load the anchor needs to be able to bear / the angle at which the chain pulls on the anchor shank (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 in the absence of a decent snubber or rope. 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 on the anchor shank at an angle, the maximal holding power of the anchor will get reduced. Commonly accepted rough values for this reduction 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. If you do know the breaking load BL of your snubber / bridle, you can specify it in Expert Mode and the colour coding then is as follows: Up to 50% BL black, up to 75% BL orange, and beyond that red.
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. Please note that an entry in the Fix input field is always taken into account when calculating the swinging circle. It is then assumed that this amount of chain has actually been paid out.
Snubber stretch: Tells you by how much the snubber / bridle stretches elastically 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, by default we colour–code a snubber stretch of 100–150% of the nominal stretch at 8 BFT in orange, and anything beyond that in red. If you do know the maximally allowed stretch MS of your snubber / bridle, you can specify it in Expert Mode and the colour coding then is as follows: Up to 50% MS black, up to 75% MS orange, and beyond that 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!
Rope stretch: Quite analogous to the elongation of the snubber / bridle, this indicates the elongation of the anchor rope in percent if a combination of chain and rope, or only a rope was used. If you have put in the maximally allowed stretch in Expert Mode, its colour coding is an indication of how much strain has been applied to the rope: 50 to 75% of the maximally allowed stretch is orange, beyond that is red.
‚%‘: States how much of the swell energy is absorbed by the snubber / bridle / rope – 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 60% and more of the swell energy. In deeper water the chain is more effective and will take a bigger share.
Error messages will be generated when the chain + snubber / rope cannot fully absorb the required energy. This may be due to too heavily constrained chain, due to too much swell or strong gusts without snubber / bridle / rope 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 offline within the App (in English, German, Spanish, French, Swedish, Danish, Norwegian, Dutch, Turkish, Italian or Portuguese, depending on your system settings).
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 yawing, veering or sailing at anchor! Don’t forget the mast and the lazy jack with main sail. This area (shaded in the illustration to the right) 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 mooring rope when the vessel is 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 breaking load: The breaking load of the snubber / bridle. This is only used for colour-coding the bow load results.
Rope breaking load: The breaking load of the rope. This is only used for colour-coding the bow load results.
Snubber maximal stretch: The maximal stretch allowed for the snubber / bridle. This is only used for colour-coding the snubber stretch results.
Rope maximal stretch: The maximal stretch allowed for the rope. This is only used for colour-coding the rope stretch results.
Snubber stretch @ 8 Beaufort / 40.2 kn / 20.7 m/s: Again, this calibration needs to be done only once! It characterises the elasticity of your snubber for your vessel. More precisely, it is the amount by which your vessel’s snubber / bridle stretches elastically at 8 Beaufort wind — measured in the absence of swell and in shallow water. A value of 0.05 metres would be a pretty inelastic and hence 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 a linear relationship between elongation and force applied to the snubber / bridle (meaning twice the force will result in twice the stretch) 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. Better still, take a 2nd measurement at 6 Beaufort and let the app find the best non-linear fit.
Tip if you have the data sheet of you snubber rope available, so breaking load, working load, and stretch per metre at working load is known, then you can use these data to set Snubber stretch @ 8 Beaufort as follows: Start by calculating the stretch of your snubber at the working load given. For instance, when the snubber stretches by 15% at a working load of WL = 1000 kp, and your snubber is 12 metres long, then you get a snubber stretch at this working load as WS = 12 * 0.15 m = 1.8 m. Next you use the app to calculate the anchor load AL of your vessel at 8 Beaufort (40.2 kn). First you need to provide all relevant data for your vessel, so either windage area (Expert Mode) or vessel length and vessel type (Basic Mode). Then you specify 40.2 kn of wind, 0 anchor depth and 0 vessel velocity @ anchor (so no swell) and hit the Calculate button. Assuming the snubber has a linear characteristic, your snubber stretch @ 8 Beaufort is finally given as WS * AL / WL. This is simply the Rule of Three and it works in any physical units.
Snubber stretch @ 6 Beaufort / 26.8 kn / 13.8 m/s: Same as @ 8 Beaufort, but at roughly half the wind load. With this 2nd data point it is possible to characterise a non-linear snubber / bridle. If set to zero, a linear snubber will be modelled. If larger than zero, its value needs to be between (26.8/40.2)2 = 44.4% and the full value @ 8 Beaufort, otherwise the app will correct the respective other (hidden) value correspondingly. For this reason it is always very prudent to check the other value again when making any changes. In the non-linear case, the snubber stretch x as a function of force f will be modelled as x = b*ln(1+c*f), where b,c>0. With this choice twice the force will cause at most twice the stretch. In particular, it is impossible for twice the force to cause, e.g., three times the stretch. This would not be a realistic scenario. Finally, please note that non-linearity is degrading snubber performance!
For a linear snubber, just a single measurement is required — at 40.2 kn. Luckily, in this case you may apply the scaling trick described below and perform the actual measurement at less wind. For a non-linear snubber, this trick does not work and two measurements are needed — at 26.8 and at 40.2 kn.
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 (40.2 kn), measure how far the two red tapes have come apart. Should you decide to measure rather at 20.1 kn instead, so half the wind speed, you need to multiply your measurement by 22 = 4 to obtain the value for 8 BFT, since the wind force depends quadratically on the wind speed. Of course, this 20.1 kn trick only works if the snubber is linear!
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 (about 40 kp) max. The anchor load at 8 Beaufort (40.2 kn) will be too much for this scale – remember the load depends quadratically on the wind speed – so let’s measure at 40.2/3 kn = 13.4 kn instead, and correct with a factor 32 = 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 29.2 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 29.2 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. Of course, this trick only works if the snubber is linear! 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.
3rd Tip: If the hanging scale is not powerful enough to measure the loads at 6 and 8 Beaufort, but you have, to because of non-linearity, then consider using a pulley block which reduces the load at the scale, and correct the measurement data afterwards correspondingly.
4th Tip: If the hanging scale is not powerful enough to measure the load at 8 Beaufort, but can do so at 6 Beaufort, take the 6 Beaufort measurement and a second one at, say, 4 Beaufort. You then calibrate the app for this snubber by setting anchor depth and vessel velocity to zero, and then key in the snubber stretch measured at 6 Beaufort. After that you change the snubber stretch at 8 Beaufort until the snubber stretch calculated for 4 Beaufort matches what you have measured. Please do check during the process that the value for the snubber stretch at 6 Beaufort did not change!
5th Tip (Nerds only! ;)): If you want to calibrate your non-linear snubber using manufacturer’s data, here is how to do it: Suppose the manufacturer has a table for your snubber where at 5000 kp it gets stretched by 6 metres, and by 3.75 metres at 2500 kp. You start out by setting the anchor depth as well as the vessel velocity to zero. Next you set the wind strength to 40.2 kn (8 Beaufort), and the snubber stretch at 8 Beaufort to 6 metres. Then, in Expert Mode, you modify the windage area until the anchor load is (close to) 5000 kp. In the next step you reduce the wind strength until the anchor load reaches (close to) 2500 kp. Then you modify the snubber stretch at 6 Beaufort until the snubber stretch calculated by the app is 3.75 metres. Best is to start with a value only slightly smaller than the value at 8 Beaufort and then keep reducing it in small steps. Please do check during the process that the value for the snubber stretch at 8 Beaufort did not change! (The app will automatically correct the hidden value, if there is a too large mismatch between the 6 Beaufort and 8 Beaufort values.) If the manufacturer has provided even more data — for instance at 1000 kp: 1.8 metres and at 3750 kp: 4.95 metres — you can now check how closely these are approximated by the app. With this you have calibrated the snubber, but not yet for your vessel. To do this, you first need to calculate the square root of the ratio of the windage area that led to 5000 kp / 6 metres stretch, and the actual windage area of your vessel. Then, in the next step, you revert to your vessel’s windage area and divide the wind speed at 6 Beaufort, so 26.8 kn, by the square-root factor you have just calculated and put that into the app as new wind speed. Take a note of the snubber stretch calculated at this wind speed, as later you will use it as the snubber stretch at 6 Beaufort parameter — but only put it into the app once you have repeated this procedure for 8 Beaufort (40.2 kn) wind speed as well. At last you can replace the snubber stretch at 6 and 8 Beaufort parameters with these new values just calculated and you have finally calibrated your snubber for your vessel. If all has gone well, for the example of a trimaran of 16 metres length and a windage area of 29.07 square metres, the values for 6 and 8 Beaufort snubber stretch should now be 0.69 m and 1.43 m, respectively. It is a good idea to test this calibration by increasing the wind strength such that 5000 kp are again reached (but now because of the wind strength, and not because of the windage area). The calculated snubber stretch should then be close to 6 metres again. Because of rounding errors due to having only two decimals behind the comma, there may be larger deviations, in which case you may want to adjust the snubber stretch parameters slightly. In the example above, you will find that a snubber stretch at 8 Beaufort of 1.44 or 1.45 metres gives a slightly better fit. Complicated, but you only need to do it once. A similar approach can also be used if two measurements have been made, but not at 6 and 8 Beaufort.
6th Tip: When you anchor in strong current and you need to include this in the calculations, for now all I can offer is an approximate calculation according to the force diagram shown to the right. In this case one needs more parameters in addition to the frontal windage area, w||, such as the lateral windage area, w⊥, when viewing the vessel from the side, as well as the angle β between wind and vessel heading, the angle γ between current and vessel heading, as well as the angle δ between anchor rode and vessel heading. In a first instance, one calculates the effective windage area w(β) = w|| cos2(β) + w⊥sin2(β) which the wind sees as a weighted average of the two. In a second step, a fictitious windage area f = w sin(γ+β)/sin(γ-δ) that includes the effect of the current as well. This fictitious windage area f finally needs to be entered as the actual windage area in the Expert Mode. f is not really a windage area, strictly speaking, but it is rather convenient to include the effect of a strong current in such a way. Please note that the angle δ of the anchor rode may actually also be negative. Whether this is the case will depend on the properties of the vessel and, in particular, on the relative positions of the points where effectively the wind and current push the vessel. If — without sails — the bow tends to be pushed out of the wind, so leewards, this angle δ will be negative. Admittedly, those are many parameters and it is rather cumbersome to work it all out, but this is at the moment the only way, I am afraid. It is probably best to work out some values for a couple of scenarios and save them as configuration parameters. Then you will have them at your finger tips for later use. For this it is useful to know that sin2(60°) = cos2(30°) = 0.75 and cos2(60°) = sin2(30°) = 0.25 and cos2(45°) = sin2(45°) = 0.5. For normal vessels one will always find that w is larger than w||. However, whether f will be larger or smaller than w will depend on whether the current will work with or against the wind. Example: The wind is at β = 30° with respect to the vessel’s heading and its lateral windage area is three times the frontal one, w⊥ = 3 w||. With this one gets w = 0.75 w|| + 0.25 w⊥ = 1.5 w||, so an increase by 50%. Next we assume the angle of the current with respect to the vessel’s heading as γ = 45° and the angle between anchor rode and vessel’s heading as δ = 15°. With this we have f = w sin(75°)/sin(30°) = 2.9 w||, meaning that the fictitious windage area that needs to be entered in Expert Mode has almost tripled. In the case of multihulls, one also needs to consider the effect that one leg of the bridle will be much more stretched than the other one, which is not accounted for here. This will have an effect on the dynamical anchoring.
There is an Info button to the right of the input parameters, where most of this information is also made available offline within the App (in English, German, Spanish, French, Swedish, Danish, Norwegian, Dutch, Turkish, Italian or Portuguese, depending on your system settings).
Once you have played around a little with this anchor chain / rope length calculator 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. And it certainly is much more than a simple anchor scope calculator… 😉
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/.
Free online version is available with slightly reduced functionality.
A summary document with the analysis of different anchoring scenarios.
External Articles for the App
English: Article in Winter 2021 Latitudes & Attitudes.
Feedback for improvements, new features, or bug reports is 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.