Shock load on mooring/anchor chain & fittings

[Neeves]

As to our whereabouts - we are currently in Puerto Vallarta on the Pacific side of Mexico, but heading out today. All spare parts have arrived, the dental surgeries are completed, we are good to go!

Cheers

Mathias

Good to hear you are moving and that the clutches of Covid missed you.

Are you to go, north, south or west? Might we expect to see you in Oz or might you be looking at Patagonia (where anchoring introduces a whole new ball game). Sadly Fortress Oz remains internationally isolated and we are not going anywhere (in fact locked down to max travel of 10km currently . There is talk of opening up in 2022 - but that's, in Covid terms, in dream time.


Take good care, stay safe

Jonathan

 

[Neeves]

Mathias,

I might misunderstand something, somewhere.

You mention that the weight of the yacht is not of much significance.

If the yacht is moving, you are using 0.6 knots (why 0.6knots? - it seems an unusual number for a German - who not 0.5 or 1.0 knot?) then energy is a function of mass and velocity/acceleration and a bigger (heavier) yacht will develop more energy that needs to be accommodated by the snubber - so how come you are minimising the impact of mass.

It maybe that speed is a function of mass and windage and if you increase windage you also increase mass and maybe the velocity is less for the larger mass - so its all 'self cancelling'. I don't know - but I guess there is not a simple one size fits all answer.

Jonathan

 

[MathiasW]

Good to hear you are moving and that the clutches of Covid missed you.

Are you to go, north, south or west? Might we expect to see you in Oz or might you be looking at Patagonia (where anchoring introduces a whole new ball game). Sadly Fortress Oz remains internationally isolated and we are not going anywhere (in fact locked down to max travel of 10km currently . There is talk of opening up in 2022 - but that's, in Covid terms, in dream time.


Take good care, stay safe

Jonathan

Yeah, it was good to escape from one year lockdown in Panama...

As OZ and NZ are both closed, we decided not to venture out to French Polynesia - it would be a dead end road. Perhaps next year. For now we are heading for the Sea of Cortez and then, should we get the US visa, which is unlikely, we continue to California. Another dead end road... If not, ok, then reassessment... (blog on www.trimaran-san.de)

All the best

Mathias

 

[MathiasW]

Mathias,

I might misunderstand something, somewhere.

You mention that the weight of the yacht is not of much significance.

If the yacht is moving, you are using 0.6 knots (why 0.6knots? - it seems an unusual number for a German - who not 0.5 or 1.0 knot?) then energy is a function of mass and velocity/acceleration and a bigger (heavier) yacht will develop more energy that needs to be accommodated by the snubber - so how come you are minimising the impact of mass.

It maybe that speed is a function of mass and windage and if you increase windage you also increase mass and maybe the velocity is less for the larger mass - so its all 'self cancelling'. I don't know - but I guess there is not a simple one size fits all answer.

Jonathan

Jonathan,

You are quite right, of course. The mass enters the equation for kinetic energy of the vessel and this kinetic energy needs to be absorbed by the anchor gear. So, yes, it does matter. I was a bit careless when stating that so vaguely. What I had meant was that a different mass can be 'compensated for' by a different velocity to give the same kinetic energy, and it is only relevant with regards to swell and gusts, not the steady anchor load, which is determined by the windage area mostly.

Why 0.6 kn and not 0.5 or 1.0 ? Well, I was once told by my superior I am not a typical German, more an Austrian or Italian. That was when he was looking at the mess on my desk... He himself was Austrian, by the way...

Cheers

Mathias

 

[MathiasW]

From memory Mathias has an app available for anyone who wants to plug in data for their specific yacht (I don't recall if he linked to it) - saves extrapolation and you will have specific data for your vessel.

Take care, stay safe.

Jonathan

I did not - for risk of getting a slap on my fingers...

But here we go, the description in English. German and French is available as well... It is available for iOS and Android.

Anchor Chain Calculator - SAN

Cheers, Mathias

 

[Neeves]

Jonathan,

You are quite right, of course. The mass enters the equation for kinetic energy of the vessel and this kinetic energy needs to be absorbed by the anchor gear. So, yes, it does matter. I was a bit careless when stating that so vaguely. What I had meant was that a different mass can be 'compensated for' by a different velocity to give the same kinetic energy, and it is only relevant with regards to swell and gusts, not the steady anchor load, which is determined by the windage area mostly.

Why 0.6 kn and not 0.5 or 1.0 ? Well, I was once told by my superior I am not a typical German, more an Austrian or Italian. That was when he was looking at the mess on my desk... He himself was Austrian, by the way...

Cheers

Mathias

Thanks Mathias,

I was not being picky with the 0.6 knots just interested - there are so many unknowns or uncertainties that if you are precise it is then thought the science is exact. The science is exact - except we don't include all the science (swell for example, there is swell and swell, long or short). But the more precise you are with data, like 0.6 knots or 1 knot the more likely some will assume the data is all accurate - rather than simply being a convenient figure. 1 knot is convenient as, from memory (which maybe flawed) its 2m/sec.

I find your work invaluable - keep it up! I put it all down to your time in Oz - nothing to do with being German with an overlay of Austrian or Italian personality - more 'It'll be right mate!'

But I have wondered if there is some 'self cancellation' light skittish yachts tend not to be heavy and though they might rush around like scalded cats they don't weight as much as the long keeled heavy displacement that are more sedate - but as I cannot measure the velocity of anything other than our cat - its all a bit 'academic'.

On the size of snubber, I believe its not size anyway but weight per unit length, it tends to be a bit 'suck it and see' take your 1.6m stretch, or extrapolate from there, and see how close you can get to it. We had initially worked with retired kermantle dynamic climbing rope and when we bought our 'final' bridle (snubber) we bought 2 x 30m x 12/13mm but then found it did not stretch as much as we had thought - and replaced it with 2 x 30m x 10mm (keeping the 12mm as spare).

Jonathan

 

[thinwater]

Do you guys actually measure the snubber stretch (for example, monitor the movement of a marked spot and extrapolate, correcting for friction around pulleys if present)? I recall measuring it a few times, but mostly I focused on load readings, having measured rope stretch earlier. But now that I think of it, since rope stretch varies with rate of pull, I should have done both.

 

[Neeves]

Do you guys actually measure the snubber stretch (for example, monitor the movement of a marked spot and extrapolate, correcting for friction around pulleys if present)? I recall measuring it a few times, but mostly I focused on load readings, having measured rope stretch earlier. But now that I think of it, since rope stretch varies with rate of pull, I should have done both.

You have a mechanism to measure rate of pull?

I have fixed marks on the deck and a sewn mark on the snubber (one snubber of the pair) near the bow that allows me to monitor stretch. I have not tried to compensate for friction. However I have not monitored stretch vs load and windspeed. I can sit at the chart table and measure load and windspeed but not measure stretch at the same time - different location. I was primarily interested in correlating wind with tension, and looking for worst case under certain conditions, variation in scope. Thus I recorded both wind speed and tension continuously the maximum were really the only data of interest to me - who cares what the tension is in 20 knots - its the 35 knots, and +, you need to plan for.

Tension under simple windage was not interesting - its the veering that is the killer.

The maximum loads were as a result of a snatch load and the snatch load was caused by veering - I suspect all the maximum loads were under similar rates of pull

I confess I chickened out when snatches reached 650kg - it was a bit too. traumatic for me and I was not prepared to break something for the sake of the few in the yachting community interested in such esoteric data. If you want to know what happens beyond 35 knots either extrapolate or set up your now experiments.

I have measured stretch in situ - but not correlated it with wind speed nor tension. I have not measured maximum stretch as that means sitting on the bow simply measuring a snubber - life is too short. I do know the elasticity of rope from which my snubber is made vs tension but on the basis of loading a length with one end tied to a tree, the other end tied to a tow bar with a load cell in between and measure a fixed length in the 'middle'

I do have the intent to better quantify veering, measuring the speed of the veer, Mathias 0.6 knots. As in the absence of 'speed' you cannot measure or calculate energy. It was not a priority and Covid has upset plans as in order to measure I have to anchor somewhere that is sheltered from swell but exposed to wind and have good holding. Round Pittwater shallow water is full of moorings - and I have to travel beyond permitted limits for the ideal location....... I'm not ignoring swell - I just want to be subject to one variable at a time, I can have a swell time later

And whilst on the subject how to best measure swell height?

Jonathan

 

[thinwater]

You have a mechanism to measure rate of pull? This only matters in lab testing, where you pull at higher rates to save time.

I have fixed marks on the deck and a sewn mark on the snubber (one snubber of the pair) near the bow that allows me to monitor stretch. I have not tried to compensate for friction. However I have not monitored stretch vs load and windspeed. I can sit at the chart table and measure load and windspeed but not measure stretch at the same time - different location. I was primarily interested in correlating wind with tension, and looking for worst case under certain conditions, variation in scope. Thus I recorded both wind speed and tension continuously the maximum were really the only data of interest to me - who cares what the tension is in 20 knots - its the 35 knots, and +, you need to plan for.

Tension under simple windage was not interesting - its the veering that is the killer. Windage-only is interesting as the baseline.

The maximum loads were as a result of a snatch load and the snatch load was caused by veering - I suspect all the maximum loads were under similar rates of pull. Where in the swing? Most of my testing was with rope rode, and there is no "snatch" from the catenary coming out. The peak load is a combination of being most side-on to the wind and wave/gust impacts. The question is, was it actually the chain snatching tight, or some other factor?

I confess I chickened out when snatches reached 650kg - it was a bit too traumatic for me and I was not prepared to break something for the sake of the few in the yachting community interested in such esoteric data. I feel that !! I had a load cell explode once when a big power boat wake hit. If you want to know what happens beyond 35 knots either extrapolate or set up your now experiments.

I have measured stretch in situ - but not correlated it with wind speed nor tension. I have not measured maximum stretch as that means sitting on the bow simply measuring a snubber - life is too short. I do know the elasticity of rope from which my snubber is made vs tension but on the basis of loading a length with one end tied to a tree, the other end tied to a tow bar with a load cell in between and measure a fixed length in the 'middle'

You might try varying the yawing angle by adjusting the bridle length. I was able to adjust my tri from 20-120 degrees bearing swing. I can imagine 120 degrees will get weird before 35 knots! You will pull anchors out. I did in 20 knots, and sometimes less.

I do have the intent to better quantify veering, measuring the speed of the veer, Mathias 0.6 knots. As in the absence of 'speed' you cannot measure or calculate energy. It was not a priority and Covid has upset plans as in order to measure I have to anchor somewhere that is sheltered from swell but exposed to wind and have good holding. Round Pittwater shallow water is full of moorings - and I have to travel beyond permitted limits for the ideal location....... I'm not ignoring swell - I just want to be subject to one variable at a time, I can have a swell time later

And whilst on the subject how to best measure swell height? No good idea on chop. With real long swell, the depth sounder can give a clue.

Jonathan

See comments above.

 

[MathiasW]

Thanks Mathias,

I was not being picky with the 0.6 knots just interested - there are so many unknowns or uncertainties that if you are precise it is then thought the science is exact. The science is exact - except we don't include all the science (swell for example, there is swell and swell, long or short). But the more precise you are with data, like 0.6 knots or 1 knot the more likely some will assume the data is all accurate - rather than simply being a convenient figure. 1 knot is convenient as, from memory (which maybe flawed) its 2m/sec.

I find your work invaluable - keep it up! I put it all down to your time in Oz - nothing to do with being German with an overlay of Austrian or Italian personality - more 'It'll be right mate!'

But I have wondered if there is some 'self cancellation' light skittish yachts tend not to be heavy and though they might rush around like scalded cats they don't weight as much as the long keeled heavy displacement that are more sedate - but as I cannot measure the velocity of anything other than our cat - its all a bit 'academic'.

On the size of snubber, I believe its not size anyway but weight per unit length, it tends to be a bit 'suck it and see' take your 1.6m stretch, or extrapolate from there, and see how close you can get to it. We had initially worked with retired kermantle dynamic climbing rope and when we bought our 'final' bridle (snubber) we bought 2 x 30m x 12/13mm but then found it did not stretch as much as we had thought - and replaced it with 2 x 30m x 10mm (keeping the 12mm as spare).

Jonathan

Oh yes, my brief sabbatical time at the UNSW was an experience for a lifetime!

I do not have any opinion on self cancellation effects for these types of vessels. My trimaran is certainly very stable at anchor as I am using a bridle, of course, having one leg sightly longer than the other, yielding a preferred direction towards the wind. ("Breaking the symmetry" would a theoretical physicist say... )But this means I do not have any first hand experience.

As to size of snubber: I measured mine on deck, using a winch to stretch them and measure the force it took to extend them by 50 cm. But as you said, you start with what you hope is the correctly dimensioned snubber and then improve from there...

Cheers

Mathias

 

[Roberto]

What coatings?

I've tested several on polyester (Yale Maxijacket was best), and saw huge improvements, but not so much on Amsteel. I assumed this was because it comes with a coating, and because the coatings can't really bond to Dyneema. I gather they use specific polyurethanes on industrial Dyneema assemblies.

I would also think a heavy coating would gunk up the working of the SS. It looks like you are only coating the chain contact area, which makes sence.

I'm interested!

Hello thinwater,
I first knew these thick coatings existed from this video

as I use/test these things for personal curiosity only, buying any of their products from here would have been too expensive, you might have more luck or perhaps already tried that.
So I first tried a few different "liquid rubber" compounds, they also sell a similar one like "liquid electrical tape", but it took a lot, lot of coats to get a sensible thickness, also they were not that resistant I could damage and eventually break the coating by rubbing by hand against some stones.
I showed the video to an industrial chemist friend and he gave me a few different "potions" to try, my organic chemistry class goes back 30years so no idea what they are, sure one is urethane based (a lot like the "Aquasure" glue).
Apart from soft shackles, I am also trying them with normal splices, we'll see what turns out

regards r

 

[Roberto]

Re stretch, there is also the ancient "tattletale" method, taken from a naval seamanship handbook.
The thin line length/anchorage points to be chosen depending on material of course.
For a full electronic version, two three tattletales of different lengths with buzzers activated when they are completely stretched

 

[thinwater]

Hello thinwater,
I first knew these thick coatings existed from this video

as I use/test these things for personal curiosity only, buying any of their products from here would have been too expensive, you might have more luck or perhaps already tried that.
So I first tried a few different "liquid rubber" compounds, they also sell a similar one like "liquid electrical tape", but it took a lot, lot of coats to get a sensible thickness, also they were not that resistant I could damage and eventually break the coating by rubbing by hand against some stones.
I showed the video to an industrial chemist friend and he gave me a few different "potions" to try, my organic chemistry class goes back 30years so no idea what they are, sure one is urethane based (a lot like the "Aquasure" glue).
Apart from soft shackles, I am also trying them with normal splices, we'll see what turns out

regards r

From what I can gather, they function not so much by covering the rope, but by somehow holding the frayed bits in place so that wear stops right there. The two that impressed me most were:

• Yale Maxijacket
• Flexdel Rope Dip

I've used these for years on dock lines, as well as other assorted uses. Very reliable. They extend the wear of nylon and polyester >10 times. But little of the testing was on Dyneema.

 

[thinwater]

Re stretch, there is also the ancient "tattletale" method, taken from a naval seamanship handbook.
The thin line length/anchorage points to be chosen depending on material of course.
For a full electronic version, two three tattletales of different lengths with buzzers activated when they are completely stretched

Funny. I sent Neeves a note about that method several hours ago. The thread can also be embedded in the rope so that it pulls inside (done on some modern industrial safety tethers). The thread should be non-stretch and slippery.

 

[JumbleDuck]

Our catamaran is 38' long with a 22'6" beam, fully laded with fuel, provisions and water and 2 crew it weighs in at a hefty 7t (the 7t is worst case). We have the same windage as a Bav 45 - I've measured them both.

How did you measure them? Do you have CdA values for the two?

I've measured the snatch loads on our cat using a dymeema bridle and at short rode and with a 2.5: scope I measured a peak snatch load of 650kg in 35 knots. Under these condition its like running your yacht into a brick wall.

A snatch load of 650kg on a 7,000kg boat is less than 0.1g, which doesn't sound much, and it's hard to imagine that it feels like hitting a brick wall. Are you sure about these figures? And how did the loads arise - as a result of the boat being blown back and then beeing pulled up short, by wave action or by something else?

As I am sure you know, the dynamics of systems like this are very complicated, and what applies to one boat may not apply to another.

 

[JumbleDuck]

I am enclosing a table which details the maximal loads at anchor in the presence of a strong swell, and at various anchor depths. It also shows the large effect that a well dimensioned snubber has, in particular in shallow water, where the chain just sucks...

As you can see, the loads can be enormous! Way beyond a metric tonne!

More can be found here: Catenary Anchor Chain Length - Die Kettenkurve - Fun Facts - SAN

PS: The table was created using my AnchorChainCalculator App available on Apple / Android. Swell is entered via the maximal velocity at anchor away from the anchor position. In this case 0.6 kn.

View attachment 118892

Interesting. How did you measure the dynamic response of the hull to an increased downward load at the bow. It seems clear (or possibly likely) to me that a traditional hull like mine with a curved stem will respond very differently to a modern vertical stem and horizontal forefoot.

 

[JumbleDuck]

Centigrade is a recognised unit. The other three you've quoted aren't, although schoolteachers seem to use centimetres for some reason.

centi (like deci, deca, and hecto) are all perfectly valid and recognised SI prefixes, although they are not the preferred ones for some purposes. Not that it really matters, because real engineers and scientists use scientific notation with the base units for everything: 1.234 x 10^3 m and not 1.234 km, for example.

The official temperature scale which goes from 0 to 100 is Celsius, not centigrade, which is unofficial. In any case, one degree celsius is defined to be the same as 1 kelvin, which is the recognised unit for temperature.

 

[JumbleDuck]

Centipoise. The most common SI unit of viscosity.

The poise (dynamic viscosity) and the stokes (kinematic viscosity) are CGS units, not SI ones. We teach them as curious relics, sometimes found in American specs and texts, but anyone who has trained as an engineer here for half a century will expect Pa s (or equivalents) and m^2/s.

 

[thinwater]

...A snatch load of 650kg on a 7,000kg boat is less than 0.1g, which doesn't sound much, and it's hard to imagine that it feels like hitting a brick wall. ...

Like an abrupt 20 pound shove on the shoulder every few seconds, it will make it hard to stand up. The load was likely 3-4 times lower with a nylon snubber. That is the point.

I've done the same testing, and the difference is very noticeable.

 

[thinwater]

The poise (dynamic viscosity) and the stokes (kinematic viscosity) are CGS units, not SI ones. We teach them as curious relics, sometimes found in American specs and texts, but anyone who has trained as an engineer here for half a century will expect Pa s (or equivalents) and m^2/s.

Oh dear. I offended your sensibilities.

By the way, most UK specs for engine coolants and automotive fluids are from the ASTM, since the British standard societies folded in about 20 years ago. Redundancy, inadequat funding, and most of the makers and users were US companies. There were many UK members on the committees I worked with. The standards that use CGS/SI units provide the conversion factors as needs. Trivia, of course, since the users know all the unit systems.