elastic-anchor-rode / kinetic-rope paradox - SailNet Community
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post #1 of 5 Old 12-25-2008 Thread Starter
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elastic-anchor-rode / kinetic-rope paradox

(I started this particular discussion under an ongoing "snubber" thread, However, its potential implications are much wider and would seem to deserve a separate thread)

Why should sailors trust elastic nylon rodes to keep their anchors firmly set in sand or mud bottoms when offroad 4-wheeling enthusiasts use the same nylon rope as a KERR (kinetic energy recovery rope) to help yank vehicles weighing thousands of pounds out of the mud?? This vexing paradox led me to Alain Fraysse's "tuning an anchor rode" webpages in which he analyzes the static as well as dynamic behavior of anchor rodes as a function of rode type/length/ thickness, water depth, vessel shape/size/weight and wind strength. In addition to discussing the basic laws and equations with the aid of practical examples, he provides interactive forms and Excel worksheets with which the reader can test various vessel, wind and rode parameter combinations. Among the dynamic phenomena considered are the combined effects of wind gusts and rode elasticity (particularly with regard to shockloading through surging) as well as the mechanism and effects of yawing (aka sailing at anchor). Alain purposely limits himself to wind-induced rather than wave-induced dynamic phenomena. While hinting that the latter could easily be 10 times as strong, he basically summons cruisers to avoid -- or abandon -- wave-exposed anchorages.

The dynamic examples given show a single, sustained windgust producing a 400 daN (900 lb) drag force on a 5-ton boat attached to a securely set anchor in 5 m (16 ft) of water by means of a single rode, variously consisting of 55 m (180 ft) of 8 mm (5/16") chain, 55 m of 18 mm (3/4") nylon, 20 m of chain + 35 m of nylon, and 45 m of chain + 10 m of nylon. Assuming a steady wind force of 100 daN (225 lb) before the gust, he then plots rode tension, surge velocity, distance behind anchor and degree of angulation between rode and bottom as functions of time. The Excel spreadsheets also allow variable windgust rise times, rather than a (zero-rise time) step function. Since Alain's graphs for the different rodes tend to be scaled differently, making direct visual comparison difficult, I have rescaled and combined his graphs for the 4 different rode types into a single plot (attached below). For the purpose of this anchor rode vs kinetic rope discussion, I have omitted the rode angulation plots.

The composite plots only show the first surge event in response to the windgust. According to Alain's calculations, that first event is followed by a series of oscillatory surge events that dampen out relatively slowly. In fact, even the tension spike in the full chain is followed by a second maximum of nearly equal height and duration. To me this is counterintuitive as I can't recall ever having noticed oscillatory shockloading from our anchor chain as the result of a single, sustained gust. For the purpose of the present discussion I will just focus on the first surge event. Unsurprisingly, Alain points out that it is difficult to obtain an accurate value for the minute elasticity of anchor chain under the conditions used. Depicting the chain as completely inelastic, however, is not an option as it would produce infinitely high shockloads in the tension plot. Therefore he uses a small elastic stretch value (equivalent to about .07 % at 1,000 daN or 1/200 of that of the nylon rope).Because of the nonlinearity of the shockloading forces produced during minimally elastic collision events, as well as the lack of reliable anchor chain elasticity values, the tension plot for the full chain rode can only be a very rough estimate at best.

By way of an everyday, minimally elastic collision example: imagine letting a small ceramic cup drop from your hands. On a concrete floor the outcome is likely to be very different than on a wooden floor, although the latter may be assumed to flex no more than a few thousands of an inch. Yet this minute amount of elasticity ends up making a huge difference. During the inelastic collision with the concrete floor the cup's momentum (MxV; where M is mass and V is velocity) is primarily transferred into impulse (Fxt; where F is force and t is time duration); i.e. MxV = Fxt. Since the contact time of is extremely short in this case (typically in the microsecond range), the shockload force F exerted by the floor is very large, thereby shattering the plate. The minuscule elastic movement of a wooden floor, however, allows some of the plate's momentum to be preserved as floor momentum (with very small V but large M) while potentially extending the contact time into the millisecond range, thereby greatly diminishing the value of the shockloading force F.

If such minute amounts of elasticity or pseudo-elasticity (e.g. due to residual chain catenary and/or to link misalignments) can already produce such major reductions in shockload force it should come as no surprise that the inclusion of a 32 ft length of 3/4" thick nylon line in the 180 ft rode can be seen to nearly reduce the maximum tension in the rode to that of a full nylon rode. Although even the use of only 10 ft of nylon "snubber" line markedly reduces the overtensioning of chain rodes in response to wind gusts, sailors often argue that the longer the elastic nylon line section, the better it will be for vessel comfort and safety. Consequently, nylon rodes with relatively short leader chains (e.g. 20-30 ft) have become quite popular as cost and weight are low, they are easily handled and stowed and the chain leader help keeps the angle of pull on the anchor low while reducing the risk of chafing the rode on rocks or coral outcroppings.

Before we decide, however, that we have just identified the ideal anchor rode, let's take a step back and examine the dynamic response profiles of the two most elastic rodes shown in the figure, i.e. the full nylon rode and the nylon rode with chain leader, in more detail. Compared to the minimally elastic all-chain rode the highly elastic all-nylon rode allows a 2.5x higher maximum vessel velocity (and thus maximum momentum) plus a 10x larger surge excursion (and thus mechanical energy relase). Most importantly, while calculated maximum rode tension drops by a factor of 2 (assuming that the all-chain elasticity estimates were reliable enough), the duration of the gust-induced tension maximum becomes a factor of 10 longer, thus amplifying the impulse (F.t) delivered to anchor and vessel approximately 5-fold. As any sailor familar with the retrieval of well-imbedded anchors from sand or mud bottoms knows, rather than trying to break the anchor out suddenly with great force one learns to apply a steady pull over a period of 20-40 seconds, or so......

Obviously, the all-nylon rode in Alain's sample calculation behaves like a kinetic rope, first stretching to store the kinetic energy imparted to the vessel by the wind gust in the form of potential energy and then shortening again to release this energy while pulling strongly on vessel and anchor. Moreover, the dynamic behavior of a long nylon rode with short chain leader does not appear to vary greatly from that of an all-nylon rode, other than by its ability to reduce the angle off pull on the anchor (provided the tension in the rode is not high enough to fully straighten out the rode, of course; see Alain's website for in-detail discussions).

In short, while a measured amount of elasticity in an anchor rode is a highly beneficial property, there would also seem to be such a thing as too much elasticity. Based on the well-established fundamentals and applications of KERR systems there appears to be no particular reason to expect that anchors attached to highly elastic rodes with properties similar to kinetic ropes are somehow magically destined to stay firmly embedded in sand or mud rather than to be yanked out rather unceremoniously when winds and waves conspire to load the rode up with a dangerous amount of potential energy..... If so, the safe solution might be to choose rode materials with lower elasticity and/or to dimension and proportion high-elasticity rode segments in such a way as to limit the maximum amount of potential energy that can be stored and released to levels that are safe for the type and size of anchor as well as the type of bottom involved.

Have fun!

Flying Dutchman
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post #2 of 5 Old 12-25-2008
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Gee. The more I drink the more I like looking at the pretty lines......

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post #3 of 5 Old 12-25-2008
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Henk--snowed in, huh?

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1.20.09 Bush's last day the end of an error !! Hopefully we still have a constitution and economy left by then.

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post #4 of 5 Old 12-26-2008
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Surely a lot of information both in your post and the source.

The piece that is missing - not your fault at all - is test information regarding the "impact" holding power of anchors; not what they do over long pull, but how they react to brief overloads of 5, 15, or 60 seconds. I would bet they will hold far more than the static test results indicate, closing the gap between chain and nylon performance and explaining why chain rodes do not yank the anchors out more often. Showing why chains hold well in surging conditions. In part it is the catenary effect, but there are other things in play as well, some of which you mentioned.

I would love to see someone test anchors at long scope with differing rodes, pull up some slack, and take a running go at them. I bet some 2500# holding anchors would hold 3x that for 10 seconds.

The example of the kinetic rope being used to pull another vehicle from the mud: I haven't tried it, but if there is an advantage it is because the long slow pull is better in the mud than a sharp yank.

So, who has actual dynamic data to stack up? Fortress? Ronca?
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post #5 of 5 Old 12-26-2008 Thread Starter
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OK, Mitch, with the corpse still warm I won't deny that being snowed in did help provide the opportunity. However, wanting to clear the decks before closing the book on our nautical adventures provided the motive.

PDQ, with regard to (dynamic) anchor test data, thanks for being brave enough to open up that barrel of worms. Real-world test data should indeed inform this type of theoretical discussion. Much of the anchor setting and holding test data produced over the past decade is so confusing, however, as to benefit from a careful discussion of fundamentals, rather than the other way around.

For instance, Sail Magazine's and Monthly Yachting's controversial 2006 test (in which they reported to be unable to make CQR or Bruce-type anchor designs set properly in a firm, almost claylike sandy test bottom covered with several inches of softer sand) used a mixed rode consisting of 20 ft of 5/16" leader chain attached to 108 ft of 1" 12-strand nylon rope. The amount of kinetic energy that can be stored and released by108ft of 1" nylon rope is huge. In fact, some of the stronger commercially available KERRs , e.g. the Super Yanker, consist of only 30 ft of 1" rope.......

One can imagine the Bruce and CQR anchors dragging through the superficial soft sand layer while occasionally penetrating the underlying hard layer, thereby immediately starting to load up the "Super Yanker" (actually, 3-4 Super Yankers in series!). From there on it probably became a race of time between the seconds that it took the anchor to try and dig in deeper and the speed with which the rode was storing the towing vessel's kinetic energy........

Fair winds,

Flying Dutchman
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