Does it really make sense to use "kinetic ropes" as anchor rodes ??
- Thread starter henkmeuzelaar
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What you need is a 'damper' that quickly retards the amplitude of the elastic 'vibration'/strain - like an automotive 'shock absorber' or the recoil damper on an artillery mount. You cant build that sort of thing into an 'elastomer/polymer'..... ya needs a piston, mon.
Agreed. I've got 100 meters of chain in my bow, and would not have it any other way. But before I switched to an all-chain rode on my previous boat, I had serious issues with charging the anchor. It seems that I was often abeam of where I'd dropped the hook, first to starboard, then to port. I'm wondering whether it's possible that the reason for the charge forward was the release of kinetic energy from the nylon.
Drag is certainly part of the equation here, part of how chain works to keep the boat in place. But....
Guys, first I want to thank everyone for an interesting and pertinent discussion. But one point bothers me. The discussion seems to assume that the forces, stored kinetic energy in the rode and the wind/wave forces on the boat are summed together. It looks to me like its an equilibrium system such that the total force felt by the anchor cannot exceed the greater of either the stored kinetic energy in the rode, if wind/wave forces are smaller, or the wind/wave forces less the energy being absorbed by the rode as it stretches (until it hits equilibrium or its elastic limit) if wind/wave forces are the greater of the two. I reach this conclusion because the rode is elastic - it will respond to increased wind/wave force by stretching and to decreased forces by contracting, eventually to its resting state. While this may result in a longer time at higher loads when the wind/wave forces drop below the stored kinetic energy, the total force at any one time is limited as stated above.
The other point that strikes me is that as a sailor, my concern is the practical one of whether the total forces, applied in the direction felt by my anchor (yes, I know:"vectors") exceed my anchor's ability to hold.
Since my education in physic and calculus is limited, I would like to hear from anyone with a better understanding. Thanks.
Jim Kolstoe, h23 Kara's Boo
The other point that strikes me is that as a sailor, my concern is the practical one of whether the total forces, applied in the direction felt by my anchor (yes, I know:"vectors") exceed my anchor's ability to hold.
Since my education in physic and calculus is limited, I would like to hear from anyone with a better understanding. Thanks.
Jim Kolstoe, h23 Kara's Boo
Quite often when we set the anchor and shut the engine down we find that we have been pulled forward by the elastic rode that we had just stretched while setting the hook.
300 feet of nylon rope will stretch nearly 90 feet before breaking. If it tears a deck fitting out when stretched like that it can be lethal at quite a distance.
300 feet of nylon rope will stretch nearly 90 feet before breaking. If it tears a deck fitting out when stretched like that it can be lethal at quite a distance.
Might be. But, you'd want to be careful if you're likely to rotate a bit (eg. in a place with a strong reversing current) because if the stern kellet get tangled with the bow anchor rode unfortunate events might follow...
--Tom.
Sometimes. It's a horses for courses kind of thing. If the current is strong enough to turn your stern to the wind a riding sail will just make things worse. In the spot where I'm typing this (La Paz) the ebbs can be in the 6 knot range and are opposed to the prevailing winds... With winds in the 20 knot range as they are now the current is strong enough to turn most boats stern to the wind. Then the gusts push them over their anchors or cause them to jibe and spin and the lulls let them fall back. At which point a drogue might be the answer but only if you could convince your neighbors to use one too... At the change of the tide everybody goes every which way, and a boat with a riding sail out would be a real hazard.
--Tom.
All I can say is that 300' of 3/4 3 strand with 20' of chain and another 200' of 5/8 3 strand with 50' of chain withstood 8' waves pounding the stern for a few hours in 100 knot winds. The rodes were streached tight the whole time.
The 200' rode was two years old and used about 20 times. The 300' rode was brand new. If I remember correctly, the 300' rode's breaking strength was about 15,000 lbs.
I estimate that the constant pull on the two rodes was combined about 1,000 lbs. Both rodes were attached to the stern via a bridal of 5/8" dock lines (one per cleat).
There must have been additional pull on the rodes as the waves hit but I couldn't tell when I was trying to un-do them so I could turn the boat around. I was able to undo one side of the bridal but not the other as the force was too great and the cleat knot was too tight and I couldn't generate any slack to loosen it.
My anchors were one size bigger then the boat called for. One was a fortress and the other was a bullwagga.
Depth was 22' dropping the anchor and 37' during the storm. Bottom is gumbo type mud.
So there are your real numbers and all I feel I need to know. It works. It doesn't yank anything out. If I ever need to anchor in anything worse, I'll add another rode and anchor.
The 200' rode was two years old and used about 20 times. The 300' rode was brand new. If I remember correctly, the 300' rode's breaking strength was about 15,000 lbs.
I estimate that the constant pull on the two rodes was combined about 1,000 lbs. Both rodes were attached to the stern via a bridal of 5/8" dock lines (one per cleat).
There must have been additional pull on the rodes as the waves hit but I couldn't tell when I was trying to un-do them so I could turn the boat around. I was able to undo one side of the bridal but not the other as the force was too great and the cleat knot was too tight and I couldn't generate any slack to loosen it.
My anchors were one size bigger then the boat called for. One was a fortress and the other was a bullwagga.
Depth was 22' dropping the anchor and 37' during the storm. Bottom is gumbo type mud.
So there are your real numbers and all I feel I need to know. It works. It doesn't yank anything out. If I ever need to anchor in anything worse, I'll add another rode and anchor.
Ross' Bungee Image...
After sifting through all these thoughtful replies and analysis, I'm still taken by the images created by Ross' suggestion of the different results anticipated by using a bungee, Stayset X or a chain. One has been proven to work, one will probably cause injury (but not death) and the chain, well, it'll win some unfortunate soul honorable mention in someone's Darwin Awards.
After sifting through all these thoughtful replies and analysis, I'm still taken by the images created by Ross' suggestion of the different results anticipated by using a bungee, Stayset X or a chain. One has been proven to work, one will probably cause injury (but not death) and the chain, well, it'll win some unfortunate soul honorable mention in someone's Darwin Awards.
Bungee cords operate in a ballistic, gravity-forces dominated regime. The forces, momentums and energies involved are orders of magnitude higher than when operating in the drag- and friction-forces dominated regime of the anchor rodes discussed here.
For example, if we were to drop a 5-ton vessel on a bungee cord the gravity force generates ~1,000 horse power, as opposed to the ~3 horse power generated by the 900 lb drag available during the wind gust.
Remember, the kinetic rope is basically just an energy storage device, just like a battery. You can use it both as a power amplifier (by loading it up slowly and then letting it discharge quickly) or as a power reducer (by loading it up quickly and discharging it slowly).
After just 3 seconds, a 5 ton vessel on a bungee cord has a kinetic energy equal to about 612 Watt-hr (or 51 Amp hr in the case of a 12V battery). In other words, the loading up is going so incredibly fast that in the rebound, when the gravity force is no longer accelerating but decelerating, the bungee rope is going to be releasing its energy slower than it accumulated it.
In the case of the highly elastic anchor rode behavior calculated by Alain Fraisse, the elastic cord is clearly acting as a power amplifier.
This would all seem to be pretty straightforward, so I am going to stop keeping the mechanics 101 discussion alive.
I am grateful for the many thoughtful and informative responses received to this topic and, by way of a wrap-up, was hoping to focus on a few practical implications. For now, however, the local sewer line here in Summit Park (Wasatch Mountains, Utah) appears to have decided to start backing up into our home which requires my undivided attention. Do I smell a few puns coming in?
Happy flushes,
FD
Flying Dutchman
For example, if we were to drop a 5-ton vessel on a bungee cord the gravity force generates ~1,000 horse power, as opposed to the ~3 horse power generated by the 900 lb drag available during the wind gust.
Remember, the kinetic rope is basically just an energy storage device, just like a battery. You can use it both as a power amplifier (by loading it up slowly and then letting it discharge quickly) or as a power reducer (by loading it up quickly and discharging it slowly).
After just 3 seconds, a 5 ton vessel on a bungee cord has a kinetic energy equal to about 612 Watt-hr (or 51 Amp hr in the case of a 12V battery). In other words, the loading up is going so incredibly fast that in the rebound, when the gravity force is no longer accelerating but decelerating, the bungee rope is going to be releasing its energy slower than it accumulated it.
In the case of the highly elastic anchor rode behavior calculated by Alain Fraisse, the elastic cord is clearly acting as a power amplifier.
This would all seem to be pretty straightforward, so I am going to stop keeping the mechanics 101 discussion alive.
I am grateful for the many thoughtful and informative responses received to this topic and, by way of a wrap-up, was hoping to focus on a few practical implications. For now, however, the local sewer line here in Summit Park (Wasatch Mountains, Utah) appears to have decided to start backing up into our home which requires my undivided attention. Do I smell a few puns coming in?
Happy flushes,
FD
Flying Dutchman
Dutch, I think that you are letting theory get in the way of common sense. All materials have a modulus of elasticity and an elastic limit. A bungee just has a much greater elastic limit then does nylon rope or chain but the equation doesn't change. Only the factors change exactly the same forces and reactions occur whether you use bungee or chain only the time curves change.
The acceleration of gravity is 32 feet per second squared and the distance traveled is one half that value. Horse power is 550 foot pounds per second. Using that you can work out the potential energy in a falling weight. The peak power output will be determined by the rate that the power is extracted. That is why lead bullets penetrate steel plates.
The acceleration of gravity is 32 feet per second squared and the distance traveled is one half that value. Horse power is 550 foot pounds per second. Using that you can work out the potential energy in a falling weight. The peak power output will be determined by the rate that the power is extracted. That is why lead bullets penetrate steel plates.
May I remind you that none other than Albert Einstein concluded that "common sense is the collection of prejudices acquired by the age of eighteen?"
(Even Einstein was to fall victim, however, to the all-too-human desire to rely upon one's intuitive feeling of "common sense" when he later rejected quantum theory because of it's nonintuitive theoretical predictions, quite a few of which are now widely accepted facts)
Have fun,
FD
I am getting ready to purchase a new anchor, chain, rode, etc. so this is a pretty interesting discussion. Almost everything that I've read indicates that nylon rode and snubbers are the way to go. I am interested in the use of dacron. Seems that it doesn't have quite enough stretch.
In the end, I think that the models and physics are useful to point to issues and problems but the overall problem is so complex (as has been pointed out in other posts) with eddys, currents, hull drag, etc. that rules of thumb are more useful than detailed physics calculations. As several anchor designers and manufacturers have noted...it's about half physics and half art.
In the end, I think that the models and physics are useful to point to issues and problems but the overall problem is so complex (as has been pointed out in other posts) with eddys, currents, hull drag, etc. that rules of thumb are more useful than detailed physics calculations. As several anchor designers and manufacturers have noted...it's about half physics and half art.
Elastic is good
My understanding of the analyses posted is that an elastic rode is good because it minimizes the peak stress on the rode, fittings, and anchor for a given disturbance of wind or wave. I can't think of a scenario where the elastic rode increases stress. I do think waves will often drive the highest stress in a storm.
So chain provides shorter scope, and protection against coral or other dangers on the bottom, but not better holding in rough conditions. And a long nylon snubber will significantly reduce peak stress (shock loading) from gusts or waves with a chain rode.
For the mostly clear sand or mud bottoms and big anchorages of our sailing area, a nylon rode (with some chain by the anchor) seems a good solution.
By the way, Practical Sailor has run excellent tests on the holding power of different anchors in different bottoms.
My understanding of the analyses posted is that an elastic rode is good because it minimizes the peak stress on the rode, fittings, and anchor for a given disturbance of wind or wave. I can't think of a scenario where the elastic rode increases stress. I do think waves will often drive the highest stress in a storm.
So chain provides shorter scope, and protection against coral or other dangers on the bottom, but not better holding in rough conditions. And a long nylon snubber will significantly reduce peak stress (shock loading) from gusts or waves with a chain rode.
For the mostly clear sand or mud bottoms and big anchorages of our sailing area, a nylon rode (with some chain by the anchor) seems a good solution.
By the way, Practical Sailor has run excellent tests on the holding power of different anchors in different bottoms.
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