Mathematic approach to anchoring scope

[barnakiel]

Originally Posted by MathiasW

(...)



It also shows that it needs to be a snubber that can really absorb some energy to have an effect. Not these toy versions one sometimes sees that are just 2-3 metres long. The snubber must be able to stretch by 1-2 metres, to really have an effect. A short snubber cannot do that.


(...)

However, anything that stretches 2 meters, adds 2 meters to a very unwelcome ability of some boats to sail to and fro, with their bows getting even further blown off the wind line.


Getting off the line adds loads on the rode by adding windage.


Theoretical findings from models need to be always seen jointly with in-field experience(-s) and testing.


b.

[MathiasW]

Originally Posted by barnakiel

However, anything that stretches 2 meters, adds 2 meters to a very unwelcome ability of some boats to sail to and fro, with their bows getting even further blown off the wind line.


Getting off the line adds loads on the rode by adding windage.


Theoretical findings from models need to be always seen jointly with in-field experience(-s) and testing.


b.

Yep, that is very true. But if you can manage to keep your boat steady, perhaps always slightly exposing the same side of it to the wind, then this information may help.

To aid our trimaran to have a preferred side, I made one side of the bridle slightly longer than the other.

In any case, I think it is good to know what it would take to have an effect. If that has another negative side effect, then I need to deal with that, too.

[barnakiel]

Originally Posted by MathiasW

Yep, that is very true. But if you can manage to keep your boat steady, perhaps always slightly exposing the same side of it to the wind, then this information may help.

To aid our trimaran to have a preferred side, I made one side of the bridle slightly longer than the other.

In any case, I think it is good to know what it would take to have an effect. If that has another negative side effect, then I need to deal with that, too.

Lucky U - a monohull has many advantages.



But now you have the bow on one side and plenty of stretch (some from chain, some from the extra 2 m stretch in the proposed snubber) the wind will have it easy to blow the bow off. At least this is what will happen in our boat.



Additionally, you will have the boat heading off the swell direction (if we assume the swell and wind directions coincide). The swell is serious (otherwise we would not have this exchange) and the comfort of riding the gale or storm out will be limited.



Additionally, if you are not alone, with the bow off the wind your boat may now be sitting uncomfortably close to your neighbour's boat. And what if they opted for their bridle on the other side of the bow (or have just experienced a bad blow-off event intowards).



Building mathematical models of reality is great (and most desired), living thru the experience of anchoring in very nasty conditions is another part of the same puzzle. Think, test, improve. Both activities are required to move ahead and anchor better, tomorrow.



My single data point take on the matter is (a small mono) - deploy ZERO stretch beyond the amount desired to protect the fittings. This is easy to adjust by paying out or shortening the snubber. If one desires short and stretchy snubber, just downrate the diameter (and replace before it snaps).


I do not have adequate multihull anchoring experience to have my own opinion. In a monohull, I try to keep her as close to the wind direction as I can, and I try to minimize sailing about. The best way I have found to minimize sailing about is to use pure chain rode (plus, of course, a snubber to protect the fittings).



b.

[rslifkin]

Personally, I'd go long, fat snubber, not skinny. A fatter snubber will be easier to adjust the amount of stretch. Anywhere from pulled in short with hardly any stretch to 50 feet out with significant stretch.

[MathiasW]

Originally Posted by barnakiel

Lucky U - a monohull has many advantages.



But now you have the bow on one side and plenty of stretch (some from chain, some from the extra 2 m stretch in the proposed snubber) the wind will have it easy to blow the bow off. At least this is what will happen in our boat.



Additionally, you will have the boat heading off the swell direction (if we assume the swell and wind directions coincide). The swell is serious (otherwise we would not have this exchange) and the comfort of riding the gale or storm out will be limited.



Additionally, if you are not alone, with the bow off the wind your boat may now be sitting uncomfortably close to your neighbour's boat. And what if they opted for their bridle on the other side of the bow (or have just experienced a bad blow-off event intowards).



Building mathematical models of reality is great (and most desired), living thru the experience of anchoring in very nasty conditions is another part of the same puzzle. Think, test, improve. Both activities are required to move ahead and anchor better, tomorrow.



My single data point take on the matter is (a small mono) - deploy ZERO stretch beyond the amount desired to protect the fittings. This is easy to adjust by paying out or shortening the snubber. If one desires short and stretchy snubber, just downrate the diameter (and replace before it snaps).


I do not have adequate multihull anchoring experience to have my own opinion. In a monohull, I try to keep her as close to the wind direction as I can, and I try to minimize sailing about. The best way I have found to minimize sailing about is to use pure chain rode (plus, of course, a snubber to protect the fittings).



b.

Thank you, this is very valuable feedback indeed. Frankly, I do not have a lot of SEVERE storm experience yet, luckily. So, I will have to test my theories when it happens. But as you said, a theory without validation in reality is nothing. And this is how I would like to position my thoughts for now: What I could gain from a very elastic and durable snubber, and it what situation. The model says, in shallow water I can expect the most effect from snubbers. And this should be true regardless of whether I get more issues by sailing at anchor and exposing the vessel's side more, as you said. It also seems to be in agreement with many statements in this thread. And I can tweak the snubbers to absorb more energy or less, also that should be true. The question is then when the advantage obtained thereby is getting diminished by the disadvantages you describe.

[barnakiel]

Originally Posted by rslifkin

Personally, I'd go long, fat snubber, not skinny. A fatter snubber will be easier to adjust the amount of stretch. Anywhere from pulled in short with hardly any stretch to 50 feet out with significant stretch.

Yes.


Just think stretch is a % of load, so a fat one needs that extra length you mentioned.


If I assume 10% elongation (and is this a realistic elongation of say a nylon yarn?)


then to get to 2 meters of stretch I need 20 meters of snubber (as you said 60ft).


Here comes the challenge: this amount of rope in the rode adds significantly to boat's tendency to sail about.


If you are happy with your boat sailing about a lot, this is fine.


b.

[MathiasW]

Originally Posted by barnakiel

Yes.


Just think stretch is a % of load, so a fat one needs that extra length you mentioned.


If I assume 10% elongation (and is this a realistic elongation of say a nylon yarn?)


then to get to 2 meters of stretch I need 20 meters of snubber (as you said 60ft).


Here comes the challenge: this amount of rope in the rode adds significantly to boat's tendency to sail about.


If you are happy with your boat sailing about a lot, this is fine.


b.

I am using about 13 m of bridle per leg, so it is of that order. I will report how it is performing in the next severe storm. We did have some 44 kn during one night in the past, and that was no issue at all in this configuration.

[barnakiel]

It could be very beneficial to get load data from boats anchored in various conditions.


Something is telling me there may be a quality shift rather than only quantity change as the wind goes up. So we might need one attitude for gales, another for storms and beyond.


Getting such data should be easy with some simple tooling, also remotely.


b.

[conachair]

Originally Posted by barnakiel

Getting such data should be easy with some simple tooling, also remotely.

Would cost little, esp32, broken digital kitchen scale. Make a bracket/box to use the load cell as a rope tension meter so the 0-15Kg will be proportional to the load on the snubber attachment point. send the data over wifi to something running signalk. Done it as a test, the load cells inside kitchen scales are accurate down below 1g.

And indeed would be interesting to cut through the masses of guesswork & bias on forums with real world plotted data, with signalk graph it against wind speed/boat speed/max angle of sheer if you have those sensors available.

[MathiasW]

Originally Posted by conachair

Would cost little, esp32, broken digital kitchen scale. Make a bracket/box to use the load cell as a rope tension meter so the 0-15Kg will be proportional to the load on the snubber attachment point. send the data over wifi to something running signalk. Done it as a test, the load cells inside kitchen scales are accurate down below 1g.

And indeed would be interesting to cut through the masses of guesswork & bias on forums with real world plotted data, with signalk graph it against wind speed/boat speed/max angle of sheer if you have those sensors available.

Couldn't agree more! Would love to do that, but for months now we are in Panama in lockdown at anchor and all shops where one could buy the parts needed for this experiment are closed or out of reach... Even ordering stuff online I would not know how to do that here...

[waterman46]

Originally Posted by conachair

Would cost little, esp32, broken digital kitchen scale. Make a bracket/box to use the load cell as a rope tension meter so the 0-15Kg will be proportional to the load on the snubber attachment point. send the data over wifi to something running signalk. Done it as a test, the load cells inside kitchen scales are accurate down below 1g.

And indeed would be interesting to cut through the masses of guesswork & bias on forums with real world plotted data, with signalk graph it against wind speed/boat speed/max angle of sheer if you have those sensors available.

This is a very cool idea! But can you suggest a DIY mechanical device that will make the pressure on the load cell linearly proportional at about 50 to 1, to tension on the anchor rode?

And if would have to be calibrated somehow, requiring a known load of several hundred pounds to be applied temporarily.

There are already rig tension meters that could be used if you want to somehow insert a short length of SS wire into the rope/chain anchor rode. Disadvantage of those is they are mechanical not digital. But it is fairly easy to find or make an A/D converter that would convert the mechanical reading to digital. All servo systems need them and I think I saw cheap ones on a website a while ago.

[conachair]

Originally Posted by waterman46

This is a very cool idea! But can you suggest a DIY mechanical device that will make the pressure on the load cell linearly proportional at about 50 to 1, to tension on the anchor rode?

And if would have to be calibrated somehow, requiring a known load of several hundred pounds to be applied temporarily.

There are already rig tension meters that could be used if you want to somehow insert a short length of SS wire into the rope/chain anchor rode. Disadvantage of those is they are mechanical not digital. But it is fairly easy to find or make an A/D converter that would convert the mechanical reading to digital. All servo systems need them and I think I saw cheap ones on a website a while ago.

The geometry is simple, just make a really shallow bridle next to the termination of the snubber with the apex force vertical down onto the load cell. Called a rope tension meter in rigging world. Then calibrating is easy in software, do a low load & a high load to check linearity which which should be fine, even those cheap load cells in digital kitchen scales (available on ebay much higher than 15Kg) are amazingly accurate for the cost.
What would make all the difference is logging the data in a database for plotting, just looking at a number now and then tells you nothing much about what's really going on, plotting is gold dust, more so if you can include boat speed, wind speed etc. Signalk makes this easy
A couple of 18650 lithium batteries should keep it running for a night in a waterproof box spitting data out over wifi. Influxdb database can write data received over wifi lots of time per second so should be possible to get deep into the dynamic loads. Building & fixing a little box might be the hardest bit.

[conachair]

Originally Posted by MathiasW

Couldn't agree more! Would love to do that, but for months now we are in Panama in lockdown at anchor and all shops where one could buy the parts needed for this experiment are closed or out of reach... Even ordering stuff online I would not know how to do that here...

Oh I feel for you!!! Get 2 of everything when you can

[waterman46]

If anyone is really interested, here is the easy but expensive solution to measuring tension:

https://www.hogentogler.com/dillon/a...iABEgIYTPD_BwE

[MicHughV]

I suppose if you really want to determine an anchor load capacity, you can arrange for a "tug bollard pull"......this where they strap a gauge to a large bollard, lead the bollard line out to the tug on the water, and tell the captain to let 'er rip...
these gauges can read enormous pull....big tugs have 300 tonnes capacity...Youtube has several examples...

but, I've been following this thread for a while now, anchor loads must also consider the line capacity of the anchor rode...no point doing a test with 1/4" line or chain...6mm....

anchor rode and chain typically also use a 5:1 safety factor to boot....so the safe working load for 10,000 lb capacity nylon rode is only 2,000 lbs.

interestingly enough, I was involved in righting a boat one time....the boat was on land and had fell out of it's cradle.
A nylon rode was brought out from somewhere, probably 3/4" diameter, maybe 100' long, one end attached to the boat, the other to a truck...after the line was tight, that truck still moved a long way, before all the line stretch was out. I couldn't believe my eyes at how much that line stretched. That rope looked to my eyes like a rubber band.