Anchor Chain Catenary. Anchor Test Video by SV Panope

[Captain Larry-DH]

I follow this YouTube poster, and find his real world anchor testing interesting. I think this episode is more interesting than most.

This video illustrates quite clearly that the benefits of chain catenary are dramatically reduced as load increases. It also illustrates why sailors shouldn’t be without rope rode (or long rope snubber) for at least a segment of the rode.
If you think about it, it also illustrates that the best use of a kellet is in low winds (near calm) to keep the boat from meandering around the anchor (and avoid possibly fouling the rode or bumping into nearby boats). People tend to think that a kellet would provide improved holding in high wind conditions, but one can clearly infer from this video that it would take a VERY heavy kellet to make any significant difference in such conditions (you'd be much better off, and it would be more practical, to have that weight in the anchor, not hanging from the middle of the rode).

(Edit) PS - I think the tester underestimates the amount of tree bend in his test. I think the measured shock absorbing would be even less at high pull loads if the tree didn't deflect.

 

[jssailem]

I too watch and like the work presented by Steve and his practical testing. While he applies scientific thought into his tests he is doing demonstration of various equipment in similar conditions and reporting the observations. It is up to me to evaluate the validity of the demonstration to my conditions.

 

[thinwater]

Tree bend is a good question. Let's look at that.

Pounds/Extension/max bend per 100 pounds/energy
1000/0 inches/0.34/40
900/0.5 inches/0.3/70
800/1.5 inches/0.27/62
700/2.5 inches/0.23/81
600/4.0 inches/0.2/92
500/6.0 inches/0.17/131
400/9.5 inches/0.13/190
300/16 inches/0.1/207
200/26 inches/0.07/190

Elastic tree bend would be expected to be proportional to force. Chain stretch will be 1.6 inches at 1000 pounds, or about 0.16 inches per 100 pounds.

At 1000-900 pounds, the maximum bend would be 0.34 inches/100 pounds force, but probably less because there is some catenary stretch.
At 900-800 pounds, the maximum bend would be 0.3 inches (90%). this continues, until the tree bend is only 0.1inch/100 pounds at 300 pounds. The total tree bend cannot exceed about 2 inches and in fact was probably closer to 1 inch, based on the data. Based on tree felling expereince, I'm pretty sure it was no more than 1-3 inches. That's about 200 ft-pounds of energy. On the other hand, the total energy absorbed by plotting the data is about 1200 ft-pounds (estimating the missing low force entries). Thus, tree bend adds about a 15-20% energy absorption, most of it at the high force end, where tree bend may be most of the energy absorption. Significant.

Given the limits of his equipment, I though it was a good and useful test, for those who have to see it rather than just do the math.

For comparison, a 3/4-inch nylon rope would stretch about 4 feet, absorbing about 2000 ft-pounds, or about twice as much. Also, at 1000 pounds, the chain is done. No more absorption to speak of. Nylon keeps stretching.

Chain works really well for absorbing energy in light to moderate winds, with lots of stretch. But it can get tight when you really need it. It fails in high winds, when the waves come up. His angle data (and the math), of course, suggests it keeps helping regarding angle at the bottom. As long as the water is relativly deep and the shelter is OK, the waves are not big and things will be OK. In shallow water, with steeper waves and less chain, it can be a jackhammer. Multihull sailors get schooled on this early and often, because of a habit of anchoring in shoal water. Monohull sailors more rarely, when they get surprised in a shallow cove.

Panope only tested ~ 100 feet of chain. More will give more stretch etc. It's not just the length, it's the pounds in the water. Just sayin'.

A snubber can be thinner than a nylon rode and will give different math, according to length and size.Different conversation.

I've got add that it seems... obvious... to anyone that has worked with chain and cables in rigging that when you put real tension on them they straighten out. There's no magic.

Please check my math. I was not careful.

 

[Tom J]

Interesting data. I use 100' of chain as my primary anchor rode, and I prefer to use 5:1 scope for anchoring overnight. I set the anchor at 3:1 scope, and then fall back to 5:1 and install a 10' three strand nylon snubber. I would use a 3:1 scope for a lunch hook, but I don't feel comfortable leaving it for very long at that scope.
Steve's view of the catenary angles match my experiences. I once anchored in 12' of water and deployed 100' of chain with 10' of nylon snubber when high winds were predicted. When the winds reached 55 knots, the chain looked like it was in a straight line from the bow to the anchor, with most of the chain out of the water.