Catenary?

[noelex]

Here are some examples where fellow cruisers have anchored in smooth rock. This type of substrate is dangerous as the anchor can develop quite high levels of grip if it snags into a fissure, but with a slight change in the direction of pull the anchor can break free and drag very rapidly. Also the chain or anchor can become stuck.

Unfortunately this is the one type of substrate that a larger anchor does not offer any more security.

It can be a difficult substrate to pick even in clear water, as looks not dissimilar to sand. The best indication is the anchor behaviour when dropped. The anchor will take longer to set than normal. This is just one reason why anchors that set in short distance are helpful - the change in behaviour is obvious. The other giveaway is vibration/rumbling from the chain as it is dragged along the rock. This can be heard (as a rumble), seen (the chain will vibrate) and felt (if you put your foot on the chain the vibration can be felt).

 

[noelex]

Even relatively small rocks can sometimes be deceptive.

In this weedy anchorage this convex plough anchor could not obtain any grip. If you look at the top of the photo you can see the furrow through the weed. The anchor has come to a small clearing and jammed itself under a small rock. No doubt the skipper felt the anchor had a solid set, but if a change of wind direction occurred the grip offered by this anchor in this substrate could be low.

 

[MM5AHO]

I've always though that Catenary is most useful when you don't need it, and all but non-existent when you do need it.
In other words, there's no point in discussing its usefulness.
I use a 10m nylon snubber (rope and hook on the chain) as shock absorber.

 

[rogerthebodger]

When we anchor what we are trying to do is to prevent our boat from moving from a chosen location.

The forces involved are the forces on the boat trying to move the boat and th forces on the anchor and rode resisting this movement

The forces on the boat are the static force due to wind or current acting on the topsides and hull.

There is also a dynamic force or snatch which is stopping any movement by decelerating the boat back to zero velocity over the movement allowed.

Once we have determined these forces er can determine the static forces and the dynamic forces on the anchor on the rode.

We can then calculate the force pulling out the anchor under static and dynamic loads

we and then also calculate what if any the catenary or what ever you decide to use as a snatch resistor to slow down the rate of declaration as the slower the deceleration the lower the snatch force will be.

We know that windage force increases by a factor of the square of the wind force so if we can determine the wind area we can estimate the static force for various wind speeds.

We can then work out what the catenary droop would be under the various static load conditions. w can then determine the distance the boat would move under catenary extension of any other elastic stretch

Once we have the loading on the rode we and then chose an anchor to resist that load in various sea beds

This will allow us to the choose of the correct type and size of anchor to resist the forces we have calculated under various wind speed conditions

 

[geem]

When we anchor what we are trying to do is to prevent our boat from moving from a chosen location.

The forces involved are the forces on the boat trying to move the boat and th forces on the anchor and rode resisting this movement

The forces on the boat are the static force due to wind or current acting on the topsides and hull.

There is also a dynamic force or snatch which is stopping any movement by decelerating the boat back to zero velocity over the movement allowed.

Once we have determined these forces er can determine the static forces and the dynamic forces on the anchor on the rode.

We can then calculate the force pulling out the anchor under static and dynamic loads

we and then also calculate what if any the catenary or what ever you decide to use as a snatch resistor to slow down the rate of declaration as the slower the deceleration the lower the snatch force will be.

We know that windage force increases by a factor of the square of the wind force so if we can determine the wind area we can estimate the static force for various wind speeds.

We can then work out what the catenary droop would be under the various static load conditions. w can then determine the distance the boat would move under catenary extension of any other elastic stretch

Once we have the loading on the rode we and then chose an anchor to resist that load in various sea beds

This will allow us to the choose of the correct type and size of anchor to resist the forces we have calculated under various wind speed conditions

Na, I just throw the anchor over the side and hope for the best

 

[sarabande]

For those of us without snorkelling or scuba skills, it would be handy to have a mini underwater drone to make a visual check.

Any devices out there in the wild wide world?

 

[boomerangben]

[

For those of us without snorkelling or scuba skills, it would be handy to have a mini underwater drone to make a visual check.

Any devices out there in the wild wide world?

Loads. How much do you want to pay?

 

[Neeves]

With a bottom covered in seagrass, an anchor with a sharp toe, capable of penetrating the tough seagrass is very desirable. I don't think an oversized anchor would give us any advantage

It seems that you and I are both wrong and we must have been lucky over the last few decades - as we would agree with you, a sharp toe and no roll bar seems to work well (but we have both been lucky as neither of us have big anchors - so take care - our luck might run out). Now that I think about it - if I am so lucky, I should visit a casino or three .

Finding a sharp toed anchor without a roll bar with a good reputation is easy - Spade, Excel and now, maybe Epsilon. The former 2 anchors also come as aluminium models if you need light weight (and we have the aluminium versions of both Spade and Excel).

Its interesting - the market place became (still is?) besotted with roll bars - and they are a killer in grass and weed (which is why Morgans Cloud, rightly or wrongly, became disenchanted).

We need to sail, approximately, 1,000nm miles north to have the need to worry about coral.

Jonathan

 

[Neeves]

For those of us without snorkelling or scuba skills, it would be handy to have a mini underwater drone to make a visual check.

Any devices out there in the wild wide world?

Most, all, underwater drones need an umbilical cord which introduces some restrictions. But google 'underwater drone' and you will be spoilt for choice. As Boomerangben implies - the ones you want are not very cheap.

Jonathan

 

[Neeves]

When we anchor what we are trying to do is to prevent our boat from moving from a chosen location.

The forces involved are the forces on the boat trying to move the boat and th forces on the anchor and rode resisting this movement

The forces on the boat are the static force due to wind or current acting on the topsides and hull.

There is also a dynamic force or snatch which is stopping any movement by decelerating the boat back to zero velocity over the movement allowed.

Once we have determined these forces er can determine the static forces and the dynamic forces on the anchor on the rode.

We can then calculate the force pulling out the anchor under static and dynamic loads

we and then also calculate what if any the catenary or what ever you decide to use as a snatch resistor to slow down the rate of declaration as the slower the deceleration the lower the snatch force will be.

We know that windage force increases by a factor of the square of the wind force so if we can determine the wind area we can estimate the static force for various wind speeds.

We can then work out what the catenary droop would be under the various static load conditions. w can then determine the distance the boat would move under catenary extension of any other elastic stretch

Once we have the loading on the rode we and then chose an anchor to resist that load in various sea beds

This will allow us to the choose of the correct type and size of anchor to resist the forces we have calculated under various wind speed conditions

You can buy a crane scale fairly cheaply - it will tell you exactly the tension in your rode. I have one, uses AA batteries, and can place it 'in' the rode - but the display on mine is in the scale. I have a 2t load cell (costs a bit more), runs off 12v, with a digital display on a very long lead and I can read the tension and the wind speed simultaneously (which is decidedly more convenient). I have not looked but suspect a crane scale with a display on a lead might be available - try Aliexpress. Both of our devices are over 10 years old and still offer stalwart service.

When reading the two displays - the wind gust comes first and the maximum tension developed by that gust comes 'later'. If you are correlating wind with tension you have plenty of time to record the gust peak and then later the maximum tension. I did my rode tension and scope correlations in a developing Seabreeze which are fairly accurately forecast. Our seabreeeze can develop to slightly less than 40 knots at its 'best' or 'worst' - but once you get to 30/35 knots you have sufficient data to make a sensible extrapolation. A Seabreeze is advantageous as it means the weather is pleasant, sunny. I chose a location where I was exposed to wind but sheltered from any significant effects of seas - so very sheltered from chop and swell. A sea breeze is also useful as it develops in intensity with time - so you get the full gamut of data from light to strong winds. It might be suggested you could complete the same exercise under 'bad' weather - say once a gale forecast is confirmed. Winter gales are not the weather to be prancing around on deck setting up a load cell.

Jonathan

 

[rogerthebodger]

Hi Jonathan

Yes I m aware of load cells and used then to weigh several boats. I have access to one from a friend that was used to weight boats for class measures and buoyancy calculation on surveyed boats

I have a hydraulic press with a calibrated pressure gauge that I used to weight a 20 ton boat

I have also part made a weighing system for a 9 Kg gas bottle by weighting the software to set the tare to 9Kg and to count down to zero.

This allowed for differences in the weight of the steel bottle.

My University project was using electric strain gauges to work the loading and stresses on a structure

So Load cells to me are well known.

I would rather use a non electric load cell on the front of my boat I would go fr a hydraulic cylinder and pressure gauge to measure the force in the cylinder and thus the loads on the chain and anchor

 

[doug748]

That is obvious. It must be obvious to so-called "catenary believers". Are you sure you understand what they think they are claiming? As I've never heard anyone suggest that applying a force (wind/tide/engine etc) to the end of a catenary didn't move the curve closer to the straight line? The only way it could not would be if the anchor dragged or the force was so low it was opposed by the drag of the chain through the water.

Indeed.

I have never heard anyone claim that anchor chain does not straighten out. That's what catenary is.

Here is someone who has actually looked at he matter in a practical fashion:

.

 

[Neeves]

Hi Jonathan

Yes I m aware of load cells and used then to weigh several boats. I have access to one from a friend that was used to weight boats for class measures and buoyancy calculation on surveyed boats

I have a hydraulic press with a calibrated pressure gauge that I used to weight a 20 ton boat

I have also part made a weighing system for a 9 Kg gas bottle by weighting the software to set the tare to 9Kg and to count down to zero.

This allowed for differences in the weight of the steel bottle.

My University project was using electric strain gauges to work the loading and stresses on a structure

So Load cells to me are well known.

I would rather use a non electric load cell on the front of my boat I would go fr a hydraulic cylinder and pressure gauge to measure the force in the cylinder and thus the loads on the chain and anchor

Sadly I don't have your expertise nor access to a hydraulic press - but I do see the logic.

As I mentioned I did choose my weather with care and measures during the development of a Seabreeze. The best see breeze develops when the weather is good - sunshine - and I chose to take the measurements where the water was sheltered from a long fetch. I wanted to remove any influence of chop. Our cat has a brdgedeck clearance of 1m - I was not worried about the kit getting wet.

I have used our crane scale, which only measures to a 500kg limit, to measure the effect of a snubber (or bridle) by interposing the scale at the transom (where our bridle terminates) and I have taken measurements in chop - its remarkable how much tension is 'removed' by a snubber - 100s of kg at 30 knots reduced to less than 70kg (and 70kg is about the tension a fit man could hold - and you could not hold a chain rode (without snubber) at 30 knots!

Using a good snubber, its needs to be long, elastic, ideally variable and you do need to carry a spare (or spares). You need the variability as most times you simple need an 'everyday' snubber, once in a while you will be caught out and need something a bit more robust (or longer). A good snubber means your anchor is less stressed and can be minimalist. We have the windage of a Bav 45 - we use the aluminium versions of Spade and Excel and the aluminium models we carry are the same size as a 15kg steel model. How many with a Bav 45 use a 15kg anchor (and 6mm chain)?

The reason for the spare snubber is because cordage fails as a result of the number and intensity of stretch cycles. The whole basis of a snubber is that it takes the cycles - so you are working it - it will eventually fail. To engender length - start the snubber at the transom then you have a deck length of snubber and only a short length forward of the bow. If you 'commence' at the transom you can 'terminate' at a sheet winch and if you plan the length with excess you can deploy more from the sheet winch if conditions are, or are going to become, questionable.

If you get all of this right - and its not difficult - you then open up the options of smaller chain (which removes weight and volume in the bow). If you are in the position to opt for a new windlass (new yacht or old windlass) - then is a good time to consider. I don't minimise the 'size' of the decision - its a big step to take as it goes against everything that has been said numerous times on the 'advantages' of catenary, it goes against everything you see in a marina (big chain is king!) - but you would not regret the change to smaller.

Its interesting the change in attitudes. Today an increasing number of reputable individuals are vocally condemning the idea of 'Bigger is Better' in anchors - these same people and more are now openly critical of the concept of catenary. Go back, maybe 5 years and such ideas were laughed at (and are still laughed at by some - you still see posts saying, off anchor choice, 'buy one size bigger than that recommended' and if one size is bigger is good then surely 2 sizes bigger is better etc). Just think of those who retired their 20kg CQR, replaced it with a 25kg Rocna and marvelled at how much better was their anchoring experience - bigger must be better. ).

If you want evidence of changes in attitude check out this old thread on CF

Anchors - Bigger is Better ? - Cruisers & Sailing Forums

Some of the changes in attitude are as a result of 'better' anchors, led by Spade and Fortress, and it has taken time for the increased performance to become accepted. We are still seeing 'improvements', and apologies but I do think Viking have taken the bull by the horns with their widespread use of HT steels. Chain is also enjoying some change in attitude as we do have better chain now (when was the last time you read of a chain failure?).

But change remains glacial - people still recommend 'bigger is better' still laugh at smaller chain.

The rode should be considered as one matching component, not as a series of independent decisions. Get it right, you will save money, be safer and be more comfortable.

Jonathan

 

[geem]

Indeed.

I have never heard anyone claim that anchor chain does not straighten out. That's what catenary is.

Here is someone who has actually looked at he matter in a practical fashion:

.

I watched the video. At 1000lbs of pull he measured 11deg angle (at the anchor) between 'seabed' and chain. Allowing for chain buoyancy, that should lift the chain a little more in saltwater you could expect the chain to be maybe 12degrees?. By simple trig, the actual angle of the chain pulled perfectly tight is 15 degrees. Pretty straight at 1000lbs of pull.
Interesting that the actual length of catenary shock absorbing was so tiny. Measured in inches rather than feet. 26" of useful shock absorbing. We achieve more in our snubber by a good margin.

 

[thinwater]

... I would rather use a non electric load cell on the front of my boat I would go fr a hydraulic cylinder and pressure gauge to measure the force in the cylinder and thus the loads on the chain and anchor

Why? I have used both kinds for this purpose. I'm curious re. your reasoning.

• Analog is easier to read when the numbers are bouncing around.
• Waterproofing is less critical.

The devil in the math is that there are so many variables in the real world.

• The wind changes constantly.
• The boat yaws.
• Waves push.
• Boats can hobby horse in shallow water.
• Slack comes in and out of the rode.

So the peaks happen when all the bad things happen at once, but they can't all happen at once, because some reduce the effects of others. It really is simpler to do some measuring than to puzzel through math with too many variables. The testing also shows you where the peaks are. For some boats it's the hobby horsing. For others it's yawing.

 

[noelex]

I watched the video. At 1000lbs of pull he measured 11deg angle (at the anchor) between 'seabed' and chain. Allowing for chain buoyancy, that should lift the chain a little more in saltwater you could expect the chain to be maybe 12degrees?. By simple trig, the actual angle of the chain pulled perfectly tight is 15 degrees. Pretty straight at 1000lbs of pull.

Changing the chain angle from 15° to 11° is not a huge difference, but is still significant.

It is the same as reducing the scope from 5.2 : 1 to 3.8 : 1 assuming the rode was bar tight. Most sailors would feel reducing the scope by this amount would have a significant effect.

 

[geem]

Changing the chain angle from 15° to 11° is not a huge difference, but is still significant.

It is the same as reducing the scope from 5.2 : 1 to 3.8 : 1 assuming the rode was bar tight. Most sailors would feel reducing the scope by this amount would have a significant effect.

Less if you allow for buoyancy of the chain in water

 

[rogerthebodger]

Why? I have used both kinds for this purpose. I'm curious re. your reasoning.

• Analog is easier to read when the numbers are bouncing around.

I preare an non electronic as I would attach the cylinder to the chain in front of the bow roller by using inline chain hooks like a rubber chain snubber

Working in that location I would be worried about dropping the borrowed electric scale of it getting wet from any bow splash

May be I just being over protective

 

[noelex]

Less if you allow for buoyancy of the chain in water

Even if you allow 1° as you suggest to allow for chain buoyancy, the scope increase caused by the catenary is from 3.8 : 1 to 4.8 : 1.

This is not an insignificant improvement.

 

[Neeves]

Even if you allow 1° as you suggest to allow for chain buoyancy, the scope increase caused by the catenary is from 3.8 : 1 to 4.8 : 1.

This is not an insignificant improvement.

On a modern well set anchor with a sensible snubber - it will make no measurable difference.

In any event it is not the scope that is critical but the tension at the shank end. This angle is determined by the size of the shackle (larger increases the angle), whether there is a swivel (increases the angle), the shear strength of the seabed, stronger increases the angle, how much chain is buried, more buried chain increases the angle.

No-one worries a jot about shackle size, swivel (and/or size), chain size - all of which are chosen by the owner - why would they worry about the angle difference between 4:1 and 5:1 when by changing the size of various components they would make a more significant change.


If you prod - members here will admit to having deployed their anchor at what they thought was, say, 5:1, but was actually 3:1. They then sat though a strong wind event - oblivious. And the well set, well designed modern anchor did not budge. It would be a different story with a Delta/CQR etc.

Jonathan