SAN

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Anchor Chain Calculator

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 / rope length based on vessel characteristics and weather / sea conditions.
  • Calculates anchor load and pulling angle at anchor shaft.
  • Works with snubbers & bridles.
  • 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.
  • New: It is also possible to calculate a mix of chain and rope.
  • 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? 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 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… 🙂

iPhone & iPad version available in the Apple Store!

Android version available in Google Play Store! (Android 4.1 with some imperfections in layout, but it works.)

Free online version available with slightly reduced functionality (e.g., no mix of rope and chain).

Video to App.

Usage

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! (Please note: The following examples are all in metric units, but if you prefer, you can switch to imperial units.)

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 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.

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.

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. Below 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 shaft does increase slightly, thereby marginally reducing the maximal holding power of the anchor, but the reduction in anchor load more than compensates for that.

Comparison of different anchor situations with a maximum chain of 50 metres. There is a lot of swell (velocity at anchor is high) and you can see that the anchor load gets extremely high in shallow water and without a snubber. A snubber does much more than just take the load off the windlass!

Main Screen

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.

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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.“

Basic Mode

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 catamarantrimaran and slimmedium 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.

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.

Fix [rope length]: The amount of rope to be deployed. If set to 0, the app will calculate the amount of rope needed for the chain to pull with 0 angle at the anchor shaft. Note that for a mix of chain and rope the length of chain needs to be set to a specific value (possibly 0), using the field Max in the chain view.

Swell energyVessel 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‘. 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 less is better / safer.

Snubber Quality with ‚custom‘ selection

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, French, Danish or Norwegian, 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 Max field. Max = 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 shaft 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 Max. 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 with zero angle. This can quickly result in a very large, completely unrealistic value if the chain is 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 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 chain length with zero pulling Angle at the anchor shank, or the Max chain length as given at input, 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 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 Max as provided in the chain view) pulls with zero angle at the anchor shank. Values can quickly get very large and unrealistic. Obviously, when only rope is used (by setting Max 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 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 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 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. If you do know the breaking load BL of your snubber / bridle, you can specify it in Expert Mode and then the colour coding 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.

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 then the colour coding 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 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 / rope cannot fully absorb the required energy. This may be due to too heavily constrained chain, due to too much swell 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, French, Danish or Norwegian, depending on your system settings).

iPhone & iPad version available in the Apple Store! Android version available in Google Play Store! (Android 4.1 with some imperfections in layout, but it works.) Free online version available with slightly reduced functionality (e.g., no mix of rope and chain).

Expert Mode

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 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 is the amount by which the snubber / bridle stretches elastically at 8 Beaufort 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 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 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 2^2 = 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 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 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 measuring scale is not powerful enough to measure the loads at 6 and 8 Beaufort, but you have, to because of non-linearity, consider using a pulley block which reduces the load at the scale, and correct the measurement data afterwards correspondingly.

4th Tip: If the measuring 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 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, 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.

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, French, Danish or Norwegian, depending on your system settings).

Concluding Remarks

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/.

iPhone & iPad version available in the Apple Store!

Android version available in Google Play Store!

Free online version available with slightly reduced functionality.

Video to App.

External Articles for the App

German: Blauwasser.de and in the YACHT 09/2021. Boote 12/21. YACHT 11/2022.

English: Article in Winter 2021 Offshore Edition of the Australian Yacht club.

English: Article in Winter 2021 Latitudes & Attitudes.

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.

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Hinterlasse einen Kommentar

Dieser Beitrag hat 29 Kommentare

  1. Dr. Gerhard Ohm

    Das ist eine App, die keine Wünsche offen lässt. Es ist auch gut, dass es einen Basic-Mode und einen Expert-Mode gibt. Im Expert-Mode würde ich den Anwendern empfehlen die Windage Area durch Messung zu bestimmen. Die Messung ist genauer als eine Berechnung/Abschätzung und einfacher als man denkt.

    Wer häufig ankert sollte sich diese App unbedingt zulegen. Es wäre natürlich schön, wenn es auch eine Android-Version gäbe.

  2. trimaran-san

    Danke! Und ja, Android ist geplant, aber einen Zeitrahmen kann ich noch nicht dafür abschätzen. iPhone sollte morgen online gehen im Shop. iPad folgt hoffentlich etwas später, dann kann ich mich mit Android beschäftigen.

  3. trimaran-san

    Android Version 1.0.2 contains two bug fixes. The first one addresses a crash at start-up, whilst the second one deals with the number format in countries like Germany, who use a decimal colon rather than a decimal point.

  4. trimaran-san

    Android Version 1.0.4 makes input from soft keyboard more convenient as it does not not require the user to hit the enter key anymore to accept the user input.

  5. Henry Li

    Hello, I just purchased Anchor Chain Calculator for iOS and find results that do not make sense. I am running iOS 13.3.1 and using the Basic mode, imperial units:
    Vessel weight: 9000.0 lbs
    Vessel length: 24.0 feet Mono med
    Chain size: 1/4 inches
    Wind strength: 5.0 knots
    Anchor depth: 36.0 + 4.5 feet
    Seabed angle: 0 degree
    Swell energy: 0.0 joule
    Snubber: none
    Calculate:
    Chain length: 46.1 feet
    Bow swinging circle: 16.3 feet
    Anchor load: 3.4 lbs Angle: 0.0 deg
    Bow load: 26.5 lbs Angle: 82.6 deg

    I find it hard to believe that a 1:1 scope would be recommended. Is there an error in the calculator? Or did I do something wrong?

  6. Henry Li

    Hello,
    Following up on my previous question/comment, I think my problem was assuming no swell energy / vessel velocity. Once I enter a vessel velocity of 0.7 knots and wind speed 6-10 knots, the calculated chain length becomes what I would expect. My apologies for questioning your calculations. I like the calculator thus far, particularly to allow me to determine chain length for different wind speeds.

    1. trimaran-san

      Hello Henry, Thanks for your feedback! You assumed almost no wind – just 5 knots, so essentially the chain is just dangling straight downwards. You can see this also in the fact that there is almost not load at the anchor at all. If you put in, say, 20 knots of wind, the results will be rather different. As you say, chain length depends strongly on the wind strength. I found some results astonishing as well at first, but eventually could always explain them with the underlying physics. For instance, if you try out strong wind, very strong swell, and shallow water, but no snubber at all. You will find out that the chain is not coping well at all with such a scenario and chain lengths go through the roof. A decent snubber will be tremendously important in such a scenario. Have fun exploring, and thanks for purchasing my app! Cheers, Mathias

  7. trimaran-san

    Android Version 1.0.6 corrects some truncation of text on Samsung Galaxy 20S. It also makes better use of the screen available on tablets and other more squarish displays. Functionality unchanged.

  8. trimaran-san

    Android Version 1.0.7: Fixed screen issue when using other default display fonts for better accessibility

  9. trimaran-san

    iPhone / iPad Version 1.03: Made more efficient use of screen size for larger devices.

  10. trimaran-san

    Version 1.1.0: Added ‚custom‘ option for Snubber Quality to store values customised in Expert mode

  11. trimaran-san

    Version 1.2.0: Made it more convenient to edit custom field for Snubber Quality

  12. trimaran-san

    Version 1.3.0: Provided localisation for English, German and French behind Info Buttons to allow for offline help.

  13. trimaran-san

    Version 1,.4.0: Provided additional localisation for Danish and Norwegian behind the Info Buttons to allow for offline help. On Android, the Back Button is now working when wanting to leave the offline help view again and return to the main screen.

  14. Roger

    Hi there,
    A great app I , I love the idea of physics not guesswork.
    now here is what i have noticed in regards to the snubber, it seems to be linear is there any way we can amend the program to read correct values of the snubber with some more detailed parameters in the future.
    using the anchor load data to give the strain on the snubber I have a 16000 kg 13.7-m mono bulky, 60-m 10mm chain my real snubber is 15-m in length, with a BL 5000 kg at 40% elongation at break therefore it stretches 6-m before breaking

    I followed your stretch and measure test to input the snubber value which at 40 Knots gave me 1.23-m , now my issue is when i input a wind-speed to correspond with the with the breaking strain of 5000 kg anchor load the snubber stretches to 14.03-m 93% of its length, more than twice of reality under the correct BL of 5000 Kg.

    Now i plotted the real data given by stretching it and the manufacturers own data and reading from the graph my 15-m length 16mm nylon rope i use as,
    the snubber at 5000kg (100% BL) it stretches 6-m (stretch of 40%)
    the snubber at 3750kg (75% BL) it stretches 4.95-m(stretch of 33%)
    the snubber at 5000kg (50% BL) it stretches 3.75-m(stretch of 25%)
    the snubber at 5000kg (20% BL) it stretches 1.80-m(stretch of 12%)
    the snubber at 5000kg (2% BL) it stretches 0.30-m(stretch of 2%)

    Now this time I set the snubber value at 40 Knots (0.53-m) to match 6-m of stretch to the breaking(anchor load) of 5000 kg and these were the values

    the snubber at 5000kg (100% BL) it stretches 6-m (stretch of 40%)
    the snubber at 3750kg (75% BL) it stretches 4.55-m(stretch of 33%)
    the snubber at 5000kg (50% BL) it stretches 3.09-m(stretch of 25%)
    the snubber at 5000kg (20% BL) it stretches 1.23-m(stretch of 12%)
    the snubber at 5000kg (2% BL) it stretches 0.14-m(stretch of 2%)

    So does this give a better version of reality in regards to the snubber its stretch and its breaking load or have i done something wrong

    1. trimaran-san

      Hi Roger, thanks so much for this great detailed feedback. You are absolutely right, of course, assuming a linear snubber is a gross simplification. What I am doing is to measure the snubber stretching on deck under load. To be more precise, I do not start with a slack snubber and try to stretch that, but I start with quite some tension in the snubber to begin with and then see how much further the snubber length increases if I increase the load from there. So I kind of linearise the nonlinear snubber curve at a working point under load.

      That is still not precise but I guess better than before.

      I will think how to include your request in perhaps a super Expert mode, so that folks who do not want to bother with this are not bothered by a more complicated input mask…

      Cheers, Mathias

      PS: Can it be that you have some copy paste errors in your comment? The 5000kg appears all over the place, even when it is less than 100% BL?

  15. Roger

    Sorry an error………………………………………………………………………..
    the snubber at 5000kg (100% BL) it stretches 6-m (stretch of 40%)
    the snubber at 3750kg (75% BL) it stretches 4.95-m(stretch of 33%)
    the snubber at 2500kg (50% BL) it stretches 3.75-m(stretch of 25%)
    the snubber at 1000kg (20% BL) it stretches 1.80-m(stretch of 12%)
    the snubber at 100kg (2% BL) it stretches 0.30-m(stretch of 2%)

    Now this time I set the snubber value at 40 Knots (0.53-m) to match 6-m of stretch to the breaking(anchor load) of 5000 kg and these were the values

    the snubber at 5000kg (100% BL) it stretches 6-m (stretch of 40%)
    the snubber at 3750kg (75% BL) it stretches 4.55-m(stretch of 33%)
    the snubber at 2500kg (50% BL) it stretches 3.09-m(stretch of 25%)
    the snubber at 1000kg (20% BL) it stretches 1.23-m(stretch of 12%)
    the snubber at 100kg (2% BL) it stretches 0.14-m(stretch of 2%)

    So does this give a better version of reality in regards to the snubber its stretch and its breaking load or have i done something wrong

  16. Roger

    Hi thanks so much for the quick response… yes my snubber when measured was already under tension and I „pulled“ the current tension plus the „load“ tension to get the result as you described. 🙂
    I look forward to the super advanced version…. I wonder if snubber length, snubber breaking load, and snubber max elongation % should be included?

    1. trimaran-san

      I need to give this some thoughts… 🙂

      So far I had been trying to avoid having to specify the snubber length itself, as it is not relevant as such for determining the contribution the snubber is doing. One only needs to know by how much it stretches – if it is linear, at least. This is easier to measure than other characteristics some snubber rope vendors might offer – and which for sure will not be consistent across all vendors. As to breaking load – yes, important to know, but so far I have left it to the user’s judgement and I support this judgement by changing the colour of the snubber elongation, if it seems to get very excessive. And finally, often the breaking load for a snubber is even not known.

      I need to check your data, but if they can be reasonably fitted by a parabola, then it might be an idea to simply add a 2nd data point to derive the fitting parameters for that parabola. So, I would measure not only the stretch at 40 kn, but also the stretch at, say, 25 kn. One would simply use the same method for measuring it that I have already described. Just for a different tension.

      Cheers, Mathias

  17. Roger

    Hi sounds good, from what I can figure out a lot of snubber ropes seam to have the same or at least very similar curve noting the steepness as it takes load and flattening as it gets closer to breaking load. So maybe a way including way to have the user input breaking load stretch manually as almost all reputable rope manufacturers have elongation at BL data.that would give us three points which is more than suffice I think 🤔

  18. Roger

    Hi
    Sounds great. From what I can figure out a lot of nylon snubber ropes seam to have the same or at least a very similar curve in regards to BL and elongation noting the steepness as it takes load to a flattening as it gets closer to braking load.
    So perhaps a way forward is to have the user input the upper end being breaking load elongation manually, as almost all reputable rope manufacturers have elongation at BL data. This would give us two measured points at the beginning and a „top end“ to the curve which would be more than suffice.

    The other question i have is in regards to swell energy and SOG, without the ability to measure SOG is there some guideline in regards to swell height at anchor to give us for example the 0.1-m is dead calm and 0.7 Knot, is that a 1.0-m swell? or is it possible for me to use a vector between m/s up with swell and m/s back with the swell and given an average wave/swell period and then converting to Knots?

    1. trimaran-san

      Thanks for this feedback, Roger. It is all put into a cooker in the back of my head to stew something out of it… 🙂

      As to the question of SOG. I use my plotter to read off that value. I would not know how this relates to wave heights and I suppose it would depend on the vessel. For my trimaran I find that 0.1 kn is indeed very calm sea with maybe 30 cm of waves or so. 0.7 kn would correspond to more than 1 metre of waves. But then again, are these short choppy waves, or is it long running swell? Doing this maths better is rather complicated, I am afraid. I did see a research paper some time back on wave dynamics acting on vessels, but I have misplaced the pointer to it… 🙁

      Cheers, Mathias

    1. trimaran-san

      So, in a first instance, I have created a new pull down menu in Expert mode where one can select between snubber breaking load and stretch @ 8 BFT. The breaking load information is only used to colour-code the snubber stretch result according to bow load < 20% BL black, < 70% BL orange, and beyond that red. The release 1.5 is just to establish this new layout. I will then start working on adding snubber stretch @ 6 BFT as a new entry in this pull down menu, which then gives 3 data points for the snubber stretch, allowing me to establish a non-linear relationship. I will use something like ln(1+x) to make sure the function is always rising with x... 😉

      1. trimaran-san

        So, almost there. Modelling a non-linear snubber is working in the app. Now testing it… 🙂

      2. trimaran-san

        Non-linear snubber is now implemented. A little difficult to use, perhaps. But it is a start…

  19. trimaran-san

    Version 1.7: Different configurations can now be stored by name. Snubber can be non-linear.

  20. Stuart Hall

    Love the calculator….use it on iPad. Please can we have the option of working in metres rather than feet for depth…..would be so useful for tired brains!!

    Stuart

    1. trimaran-san

      Thanks Stuart! There is a 3-way button at the top right with which you can select which metrics to use. The calculator tries to be smart and at first startup determines which country your iPad is set up for. If it is US or UK, it will select feet and pound. Any other country it will use metric units. But you can change it at any time with the three buttons at the top right.

      Cheers, Mathias

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