Anchor chain catenary is real

[BjarneK]

Let us look at an example of how anchor chain behaves in strong wind conditions. The results are based on a program I wrote for this, but you can get much the same results from using the software at: Tuning an Anchor Rode

Consider a 42' boat anchored in 6 meters of water with 1.2 meters of freeboard. Let us assume that it uses 8 mm Maggi Aqua4 chain and a 5 meter long snubber of 5/8" Yale Double Brait for about the same working loads of the two components. According to Robert Smith's "Anchors - Selection and Use", the load in 60 knot winds on a 42' boat is 1,499 lbs or 681 kgs. I will assume that this is the horizontal force on the boat. The force includes dynamic loads from wind variations and quite a bit of fetch for waves. The value is, however, extrapolated from lighter winds.

With a 7:1 scope, 45.4 meters of chain is used along with the 5 meter nylon rope (I include the rope in the scope). In this situation, the bow angle of the rode is 10.26 degrees while it is 5.62 degrees at the anchor. This give an effective scope at the anchor of 10.20:1, so much better than the 7:1 overall scope.

So in some situations, catenary is relevant. I would probably use 10 mm chain and 3/4" rope a 42' boat and that makes the catenary even more pronounced: 11.58 degree bow angle and 4.06 degrees at the anchor (14.13:1 effective scope). Even in 90 knot winds (1,532 kg force), the effective scope of the 10 mm chain setup is 9.14:1 with a true scope of 7:1.

Depth is very important for catenary. In 3 meters of water, the original setup of 8 mm chain in 60 knot winds and 7:1 scope gives a bow angle of 8.89 degrees and an angle at the anchor of 6.66 degrees (8.62:1 effective scope). There is still an effect of the catenary curve of the anchor chain. In shallow water, extra scope is often suggested and going to a 10:1 scope in this scenario, the effective scope at the anchor is 16.09:1 (angles of 7.38 and 3.56 degrees).

[conachair]

Are the forces from the boat sheering from side to side included? F=ma. These can be huge with an undersized snubber in addition to the loads already exerted from the wind/waves and proportional to velocity squared, lot to be said for riding sails.

[BjarneK]

Originally Posted by conachair

Are the forces from the boat sheering from side to side included? F=ma. These can be huge with an undersized snubber in addition to the loads already exerted from the wind/waves and proportional to velocity squared, lot to be said for riding sails.

Unfortunately, I don't have Robert Smith's book, but from my understanding, the forces are based on measurements made in the Columbia River with four miles of fetch. So I assume they include yawing and other wind related effects as well as wave effects.

[Dsanduril]

No disagreement on what catenary does, even with "tight" chain. I think the argument in some of the other threads on this subject is that a recent design, relatively heavy (in comparison to older recommended sizes) anchor doesn't need these tiny angles at the shank in order to perform its role. To me the question isn't whether or not catenary exists, it is what angle is required for best performance of the anchor?

[Cheechako]

On my catamaran in 30-35 steady winds We were in about 13 ft of flat water with white sand bottom. I snorkeled the anchor which was buried too deep to see. What surprised me is there was still a good curve to the anchor chain.

[Dsanduril]

One more item, should be easy for the program to calculate/report - the uplift force on your anchor shank. For your first two cases I get ~106lb and ~147lb. At least for me, this is an easier to visualize value/change than the angle at the anchor.

With that value you can also then start to look at fluke area/shape and soil mechanics to calculate the stress/strain in the seabed.

[double u]

I am convinced that even at 95% breaking load 50m of chain will have some sag. Agree though: weight is better invested in the anchor than the chain.

[BjarneK]

Originally Posted by Dsanduril

One more item, should be easy for the program to calculate/report - the uplift force on your anchor shank. For your first two cases I get ~106lb and ~147lb. At least for me, this is an easier to visualize value/change than the angle at the anchor.

With that value you can also then start to look at fluke area/shape and soil mechanics to calculate the stress/strain in the seabed.

Thanks for that input. You are right, it is easy enough to calculate.

[RichMac]

Yacht yawing makes a big difference to chain loads, and it’s not easy to calculate or simulate the results.
Modern yachts all yaw a lot in strong winds. The boat tacks across the wind with quite low chain loads, and it forms a big vertical catenary, and also a horizontal ‘catenary’ on the bottom(assuming enough chain out). At the end of the tack, the chain takes up, but the load is smoothed out by the takeup of the two catenaries. We need to consider the inertia of the chain, and also the drag across the bottom and the water drag while the chain is straightened.
My observations are that the chain jerking is reduced by the action of the chain during yawing, but I have no figures to back that up. But I don’t think that calculating a simple straight pull on the chain is very useful, because it doesn’t happen in real life.

[Dockhead]

Originally Posted by RichMac

Yacht yawing makes a big difference to chain loads, and it’s not easy to calculate or simulate the results.
Modern yachts all yaw a lot in strong winds. The boat tacks across the wind with quite low chain loads, and it forms a big vertical catenary, and also a horizontal ‘catenary’ on the bottom(assuming enough chain out). At the end of the tack, the chain takes up, but the load is smoothed out by the takeup of the two catenaries. We need to consider the inertia of the chain, and also the drag across the bottom and the water drag while the chain is straightened.
My observations are that the chain jerking is reduced by the action of the chain during yawing, but I have no figures to back that up. But I don’t think that calculating a simple straight pull on the chain is very useful, because it doesn’t happen in real life.

Well, at the very least, you need to start somewhere.


People (like, I think, Thinwater) have calculated wind loads based on the yacht lying obliquely to the wind. Or you can just use the ABYC loads which are about 3x typical experienced loads but are considered by many to be good design loads accounting for some dynamic effects.



And in real life we will stop this yawing by using a spring line, which is where the yacht lying obliquely comes from. The yawing comes from the center of pressure above being ahead of the center of resistance below the waterline, but that can be dealt with in most cases with a spring line.

[Dockhead]

Originally Posted by BjarneK

Unfortunately, I don't have Robert Smith's book, but from my understanding, the forces are based on measurements made in the Columbia River with four miles of fetch. So I assume they include yawing and other wind related effects as well as wave effects.

ABYC design load for a 42' boat in 60 knots of wind is 4989lbf or 2263kgf.


According to Alain Fraisse, this is about 3x the typical loads. But most certainly does not consider any wave action. Significant wave action in a storm could lead to dynamic forces way out of proportion to all of this.



So if you are unable to get decent shelter, all bets are off. Nor would it be reasonable to assume that dynamic loads from uncontrolled yawing would be included. This can be controlled with a spring line and/or riding sail, and must be, in a really bad storm.

[Auspicious]

Originally Posted by Dockhead

And in real life we will stop this yawing by using a spring line, which is where the yacht lying obliquely comes from. The yawing comes from the center of pressure above being ahead of the center of resistance below the waterline, but that can be dealt with in most cases with a spring line.

This is a good technique.

On my boat I found attaching the snubber (ha! *grin*) to my bow cleat which is over a meter back from the bow roller is enough to prevent most tacking back and forth. There is still some yaw and sway but the boat sits off to the side of the anchor relative to wind at a fairly small steady state angle. I've applied this observation to other boats with less helpful cleat arrangements using a longer spring line to good effect. Reduced oblique angle to the wind reduces windage and therefore loads.

[Dockhead]

Originally Posted by Auspicious

This is a good technique.

On my boat I found attaching the snubber (ha! *grin*) to my bow cleat which is over a meter back from the bow roller is enough to prevent most tacking back and forth. There is still some yaw and sway but the boat sits off to the side of the anchor relative to wind at a fairly small steady state angle. I've applied this observation to other boats with less helpful cleat arrangements using a longer spring line to good effect. Reduced oblique angle to the wind reduces windage and therefore loads.

Indeed, and this can be a matter of survival in a big storm.


I grew up on my dad's boat hearing just that "she dances at anchor"; my dad didn't understand nature of the phenomenon or that all boats do this or that it can be prevented. Then we got into one quite big storm at anchor and just about lost the boat because of the stresses of violent yawing which broke one snubber after another, and I understood I had to figure it out.



I did so and have never had this issue since.



I think a riding sail might also be prudent in a really big storm. Another advantage of a ketch rig is how easy it is to rig one.

[Auspicious]

Originally Posted by Dockhead

I think a riding sail might also be prudent in a really big storm. Another advantage of a ketch rig is how easy it is to rig one.

The challenge of a riding sail is that you are balancing increased windage with better weather vaning.

Anchoring out in heavy weather, including named storms, I have taken reducing windage to an extreme. I strip my dinghy, let some air out of the tubes, and pull the plug. The mostly sunken dinghy acts as a sea anchor and reduces yaw substantially. It takes me about an hour to get the dinghy empty, pumped up, and clean with the engine and other gear back aboard.

[Auspicious]

I left out, get the jib off the furler. That has a huge impact on yaw.