Eric,
While I certainly respect your opinion and appreciate your posts, we are going to have to respectfully disagree on this issue. I have listened to your points, but disagree with your assumptions.
I approach this matter not as a marine engineer, but as an aeronautical engineer with a flyboy mentality with training in mathematical analysis including vector analysis and physics. No doubt there are flaws in my analysis or description, but I feel the basic foundation of this argument is solid.
1. I feel you are confusing the issue in thinking of 2 forces being applied to the rode. Think of it as 1 force being applied to the anchor. This single force applied from the boat end acts through the rode to the anchor. The manner in which the anchor reacts is dependent on the weight and mechanical properties of the rode and the anchor. Weight, elasticity, drag resistance and strength all come into play as to how the force applied at the boat end will be transmitted to the anchor shank. The reaction of the anchor is a result of the DIRECTION of the forces being applied to the anchor shank.
2. When the tension force is applied on the rode, the anchor presents a restriction to that force in the way of anchor 'friction' or drag which we refer to as 'anchor hold'. Anchors present the greatest hold when the tensile force from the boat pull is applied horizontally, or parallel to the bottom (assuming a level, flat bottom).
3. The tensile force on the line and anchor are presented from the boat in 3 dimensions; up/down, right/left and forward. Since the rode is not a rigid connection like a rod, there is no aft push in the anchor or line.
A theoretical non-elastic rode of infinite strength and zero diameter/water drag like a piano wire will apply the load immediately, directly and without resistance from the water or medium (i.e. mud) in which the rode lies. This rode would tighten like a…well…piano string and apply all the force to the anchor exactly as a sum of the directional forces being applied to the boat end. In other words, if I stood on the bow and applied 20 Lbs of force at a 45 degree vertical angle, 0 left/right force, then 14.1 lbs would be applied horizontally and 14.1 lbs vertically at the anchor shaft. The rode would be straight as an arrow at a perfect 45 degree angle. (Pythagorean’s theorem: A2+B2=C2) If we can keep the transfer of force in the horizontal plane for as long as possible, we can maximize the effectiveness of the anchor.
In the real world, we all get the benefit of two principles that help us keep this force horizontal:
a. Catenary which, thanks to gravity, forces the rode to the bottom and applies much of that force in the horizontal plane. (more from chain, less from thin, floating poly line)
b. Rode drag though the bottom mud which provides additional resistance, much like a well set anchor, to the horizontal (and to a lesser extent the vertical) forces being applied to the boat end of the anchor line. (more from chain, less from piano wire)
For the purposes of this discussion, let’s leave out the following:
1. the anchor design and its resistance to the boat’s pull since we’re really talking about the transmitter of the boat’s forces…the anchor rode,
2. abrasion issues…chain resists abrasion better than nylon rode, and
3. the bottom is level and perfectly consistent medium of mud. No rocks, no rebar and no lobster traps to foul the discussion.
Let’s focus our brains on the DIRECTION of pull and the resulting components of that force…the lateral (left/right) and vertical (high/low angles) components.
With a light, yet somewhat elastic rode like a 3-ply nylon braid, the rode will apply the load in the vertical plane sooner than a heavy chain rode would since it lacks the weight and mechanical properties to resist the vertical component of the force applied. But its elastic properties will provide shock absorption to the vertical and horizontal components of the load as it is applied.
A chain, by comparison, will lie along the bottom longer due to its greater weight and bottom “stickiness” and apply that tension in a more horizontal plane for a longer period of time than the nylon rode. But, without gravity, its lack of elasticity would apply that load more immediately and directly, without the same shock absorption. The saving graces of chain are its catenary caused by gravity which serves as a lateral and vertical shock absorber until it becomes (theoretically) piano wire tight and its “stickiness” which resists primarily the horizontal tensile force component. It’s mechanical property of acting as independent segments is also a benefit to be discussed.
An anchor provides its best resistance when the forces applied are in the horizontal plane. If all forces were horizontal, we’d all be carrying much smaller anchors. The problem is that they are not just horizontal forces. Start pulling vertically and it releases easily, like when we retrieve our anchors. Keep the forces horizontal as long as possible and we retain the greatest resistance to the tension on the rode.
If we were to create a backyard comparison to test the principles on a smaller yet more observable scale, I’d set up two wagons as the boats. Behind the wagons I’d set up two rodes: one of ¼ nylon cord and another of swingset chain. Each rode would be set 3 inches deep in wet sand and the anchor on each would be represented by a single red housing brick buried perpendicular to the rode in that sand. Each wagon handle would be attached to a scale to measure the pull (tensile) force being applied. In the first experiment, we will apply the force of the wagon smoothly, slowly and consistently to observe the differences.
Wagon A with the poly cord rode would be pulled and the first thing we’d observe is the poly cord being pulled tight out of the sand in one fluent motion as one piece since it is a single-element component and its mechanical properties limit its hold on the sand. Once the cord was tight, it would stretch ever so slightly, providing a moment of shock absorption. Additional forces applied would be transmitted directly to the brick in the sand. The vertical component would lift the brick out of its shallow grave and drag it along the surface of the wet sand at a force measured on the wagon handle scale.
Wagon B with its swingset chain rode would be pulled and the first thing we’d observe is the chain being pulled in segments out of the sand. The lifting would occur slower than the poly cord since each chain segment weighs more than its corresponding poly cord length and each segment acts independently of the other. As segments closer to the brick encounter the pull from preceding segments still buried in the sand, they apply that force in the horizontal plane until that segment begins to lift from the vertical force component. Only then does THAT segment of chain begin to lift, hinging freely on its anchor end which delays the following chain segment from lifting. This additional weight, resistance to horizontal pull and segmented lifting action postpones the transmission of the vertical component of this force, allowing the brick to remain embedded in the sand for a longer period of time. The force measured on the scale at the moment of brick release would be greater than that of Wagon A.
Now let’s look at it as a force applied not smoothly over time, but as a shock in a very short burst of force:
Wagon A rode releases from the sand and stretches, delaying the full transfer of force to the brick. When the stretch reaches its maximum, the rode then applies the force in a linear manner to the brick, pulling it out of the sand.
Wagon B rode resists the pull ever so slightly, but the rapid application of wagon pull minimizes its segmented transfer of forces and causes the chain to rip violently out of the sand in a very short period of time. There’s still a bit of catenary in the chain providing minimal shork absorption, but the effects are not pronounced. The brick pulls out of the sand in a shorter period of time with a similar force on the wagon handle. While the catenary helped in this example, its effect was reduced with the rapid application of tensile force – a shock load.
How do we regain the advantage of the chain weight and catenary when the force is applied rapidly? Add a bungee (elastic) cord to Wagon B to spread the transfer of rapidly applied forces over time. From a force analysis standpoint, it doesn’t matter if the bungee is inserted at the anchor end or the boat end. But from a rode abrasion standpoint, it makes sense to insert the stretchable bungee at the boat end.
Now Wagon B with its unstretchable, but heavy chain has a mechanical component to deliver the force over a period time which is precisely what gave the poly cord the advantage when the force was applied as an immediate, shocking tension. The advantages held by the chain rode during the normal, gradually-applied, give-and-take forces experienced in most anchoring situations is fully retained with the addition of the bungee cord. The bungee itself becomes a wear item with a limited finite lifespan, but its minimal cost and ease of replacement make it an easy component to add and replace as needed.
Wagon B’s performance can be improved with the addition of a kellet, but the added complexity to deployment and retrieval prevent its frequent use. It also is not something that can be added easily during rapidly changing conditions without hauling the anchor and starting the anchor set procedure all over again.
Another option is to utilize components of each system, rope and chain, to achieve a balance in your anchoring system to meet the needs of your waters, and style of boating. For my purposes usually anchoring in less than 20 ft of water, I find 120 ft of 5/16 chain and 240 ft of 8-ply Brait to meet my needs quite nicely. Of course, it helps to have a windlass that can handle the combo rode. YMMV.
I’m sure there are holes in my simplistic analysis and look forward to hearing about them all. But in the meantime, I’ll continue to seek solace in my (primarily) chain rode and kellet-free Claw anchor system.