Effect of anchor chain elasticity

[wsmurdoch]

The anchor chain, rope rode, and snubber should be modeled by both a spring and a dashpot. Some of the energy mentioned above is stored in the stretching of the chain, rope rode, and snubber and in the raising of the chain as the boat slows in its backwards motion. That energy will reappear later pulling the boat forward increasing the load if a following gust occurs during that forward motion. Some of the energy is converted to heat from internal friction in the system and from viscous drag and is lost. The dashpot part of the equation is important and should not be forgotten. It is in the dashpot that the energy is dissipated.

As Thinwater said, the math is horrible and empirical measurement of the loads that are experienced may be the best path to a useful anchor system design.

Bill

[roland stockham]

Interesting thread but a couple of questions. First I am no sure what you mean when you refer to stretch in chain. Are you talking about the chain links settling into alignment, distortion of the links under load or elasticity in the steel? The second one is that it would be interesting to extend the calcs to show the impact of using a heavier chain. I have always seen the size of chain used as the critical factor on anchor holding so what would be the effect of replacing the 8mm chain with 10mm. I would expect 50-75% increase in the amount of energy the system could absorb for a given scope.
Have you also confirmed you calculated predictions by measurement with a strain gauge and if so how close where the result? Something I have never seen but which I think would be very interesting is to find a submersible strain gauge and actually measure the strain transferred to the anchor and the boat under different conditions. I know from practical observation on static loading that there comes a point where the chain absorbs all the stresses and the load on the anchor approaches 0, presumably due to friction between the seabed and the chain but never actually measured it.