If you like to know the anchor load and minimal chain length... here is the tool to do it...

[MathiasW]

Some of you will know already my fascination for the physics and mathematics of anchor chains. Knowing when a chain will work well (deep water), and when it does not (shallow water), and what one can do about it to improve it (good snubbers).

I have now created a free web-based online calculator with which you can calculate the anchor load and minimal chain length for various vessel characteristics (including snubbers), sea and weather conditions. No guarantee it will work well in all browsers - that was quite a nightmare, actually (just fixed Android mobile), but still, you may find it useful:

https://anchorchaincalculator.com

What I like in particular is that it allows you to perform what if scenario plays. Before bad things start to happen. Feedback is always welcome.

Cheers, Mathias

 

[fredrussell]

“The system is unstable with a negative elasticity! Something has gone utterly wrong”

Describes fairly accurately my sailing style.

 

[MathiasW]

Is that what you get when trying it out? What parameters did you use? It should only happen in extreme borderline cases...

Thanks

 

[jdc]

I prefer to have a graph over a spot value. see this.

 

[AntarcticPilot]

Didn't work at all for me - sensible parameters for a Moody 31, Mozilla Firefox (up-to-date). Just gave all zeroes.

 

[dunedin]

Sorry. It didn’t work for me either - Safari on recent iPad. Again just all zeros

Also, on the input screen, didn’t understand
- Depth. X + Y. What was the second figure (I bunged in 1 just to test)
- Vessel speed What does this mean when at anchor?

 

[sarabande]

Worked for me, using data from a windy picnic two years ago.

Though the snubber extension at 1.5m was a bit of a surprise and IRL impossible to achieve.



I like the principles involved Mathias. They give a good handle on what is normally a "suck it and see" process.

 

[dgadee]

Works but no idea what daN and Kp are.

 

[sarabande]

International units for decaNewtons and Kilopascals. Strength of a push or pull, and amount of pressure.

 

[dgadee]

International units for decaNewtons and Kilopascals. Strength of a push or pull, and amount of pressure.

And? I was always top of the class at physics but bottom at maths.

 

[MathiasW]

International units for decaNewtons and Kilopascals. Strength of a push or pull, and amount of pressure.

dekaNewton indeed, but Kp bis Kilo Pond. But if you do not like these units, switch to imperial units...

 

[MathiasW]

Sorry. It didn’t work for me either - Safari on recent iPad. Again just all zeros

Also, on the input screen, didn’t understand
- Depth. X + Y. What was the second figure (I bunged in 1 just to test)
- Vessel speed What does this mean when at anchor?

it would be most helpful if you could provide the numbers you have punched in.

I have tested it on iPad with Safari and it was working fine there.

Depth X + Y: This is just for convenience. For instance, you have your depth sounder set to refer to the bottom of your keel as zero. Put the distance bow roller to bottom of keel into the X field. Then you can simply put the reading of your depth sounder into the Y field.

But it is fine to just use one field and keep the other at 0.

Cheers

Mathias

 

[MathiasW]

Worked for me, using data from a windy picnic two years ago.

Though the snubber extension at 1.5m was a bit of a surprise and IRL impossible to achieve.



I like the principles involved Mathias. They give a good handle on what is normally a "suck it and see" process.

It really depends what snubber you are using. Mine can easily stretch to 1.5 metres...

Use the custom field to customise for your snubber. But you will need a load cell to measure it...

 

[MathiasW]

Didn't work at all for me - sensible parameters for a Moody 31, Mozilla Firefox (up-to-date). Just gave all zeroes.

Please give me the data you used as input. Thanks!

 

[pandos]

I tried it and found it gave sensible results...I took the two depth figures as, water depth plus freeboard. Why not just say "max deck to seabed" ?

I ignored the speed of vessel etc. But I wonder can this be used to input the effect of a river stream or a tide.

I did have to consult with my son (Mech Eng) and then Google to get a handle on what numbers in daNs meant in real life....

It would be especially useful if the holding of your anchor in various substrates could be factored in.

 

[MathiasW]

I tried it and found it gave sensible results...I took the two depth figures as, water depth plus freeboard. Why not just say "max deck to seabed" ?

I ignored the speed of vessel etc. But I wonder can this be used to input the effect of a river stream or a tide.

I did have to consult with my son (Mech Eng) and then Google to get a handle on what numbers in daNs meant in real life....

It would be especially useful if the holding of your anchor in various substrates could be factored in.

Thanks for this feedback. The split of anchor depth was done on previous requests by other users. If you are using a depth sounder like in a chart plotter, you are unlikely to have calibrated it to the height of the bow roller. More likely, it is calibrated to sea level or bottom of keel. So, to save you from having to add this offset mentally, I created a 2nd entry form.

speed of vessel is at anchor the maximum speed over ground you can observe pointing AWAY from the anchor. This is then used to estimate the impact of swell and waves. It is not meant to describe a current.

You can switch from daN to lbf/lbs with the toggle at the top right. Perhaps I should try to detect from which country people are landing on this page and set the default correspondingly...

And finally, yes, the holding in different grounds would be nice to add. But this would add a lot of uncertainties and opens a whole new can of worms...

Cheers, Mathias

 

[AntarcticPilot]

Please give me the data you used as input. Thanks!

Sorry - don't think I can recall all the values I used. They were sensible ones for a Moody 31, anchoring in 5m of water with 1m from stemhead to the water, no snubber, 30 knots wind. The only value that I wasn't sure of was speed through the water - I put 5 knots.

 

[Tradewinds]

Some of the values required were a little obscure to me. One of those little click-on ( i )nfo buttons with an explanation of what's needed would have helped (in my instance).

Some of us (ie me) have simple minds and need guidance.

 

[pandos]

Thanks for this feedback. The split of anchor depth was done on previous requests by other users. If you are using a depth sounder like in a chart plotter, you are unlikely to have calibrated it to the height of the bow roller. More likely, it is calibrated to sea level or bottom of keel. So, to save you from having to add this offset mentally, I created a 2nd entry form.

speed of vessel is at anchor the maximum speed over ground you can observe pointing AWAY from the anchor. This is then used to estimate the impact of swell and waves. It is not meant to describe a current.

You can switch from daN to lbf/lbs with the toggle at the top right. Perhaps I should try to detect from which country people are landing on this page and set the default correspondingly...

And finally, yes, the holding in different grounds would be nice to add. But this would add a lot of uncertainties and opens a whole new can of worms...

Cheers, Mathias

The issue with units was just my lack of education...I equate everything to kgs...as it is easier for me to visualise.

Best of luck with the app

 

[Neeves]

Mathias - great work - you are getting there, or are there.

I'm a bit time short currently and will look into the link later. I note that in post 16 you mention that the speed refers to speed, when yawing? Yawing speed is inordinately difficult to measure as our mechanical paddle wheel logs are insufficiently sensitive and our GPS are also insensitive.

You can derive speed using an accelerometer - part of every modern phone and tablet - what do you use to measure speed?

Antarctic Pilot - there are a number of Apps, free, available to measure boat speed have a look at VibSensor, Accelerometer and I think the best one SCraMP (these are all free). They work on tablets and phones, though not necessarily the phone you have. These 3 all work on an iPad. I think 5 knots might be a bit high. AP - if you look and then use one of these Apps, or any other you might find, I would be interested in your experiences (and the data) as (I am sure) would Mathias.

My granddaughter, finishing first year engineering, is building me one to affix to the rode itself (the problem being it needs to be fairly robust). The one she is building will measure movement of the rode, vibration, rather than speed and horsing (in chop). I will get the rest of the data from SCraMP. I have only played with SCraMP but when the new device is built I'll sit down and record everything at once.

Many people are still using snubbers that are too short and too beefy and a 1.5m amount of elasticity will be a surprise (and impossible).

If you have a load cell it would be easy to rig up to measure actual tension, for tension on a snubber you could simply use a suitcase 'balance' (cheap as chips) ,would need a bit of faffing about to set it up. Crane 'balances' are also quite cheap now - and adequate for the rode itself. But none of these are waterproof.

Thanks Mathias

Stay safe, take care

Jonathan