Where's the greatest load?


 This is a fuller answer to the question on anchor strain that appears in the July issue of Yachting Monthly, on sale Thursday 14 June.

Thanks for the interesting article comparing C links – very useful – but I am puzzled by the statement about chain loads, that load at the anchor end will be higher than at the bow. Could you explain the theory behind this? My gut feeling is that since the bow has to carry the weight of the chain whereas the chain approaching the anchor lies on the bottom, the strain on the bow will be greater.
NSR Duffin

Vyv Cox, a marine engineer with a qualification in metallurgy, replies:
Forces experienced by the anchor, its warp, bow roller and attachment to the boat fall into two, quite distinct modes. In quiet conditions, assuming no waves, the forces are static. There will be a small load on the bow roller and cleat, as suggested a larger vertical and smaller horizontal force on the chain as it falls from the bow roller, gradually becoming more horizontal as the chain reaches the bottom, perhaps reducing due to friction at the anchor and its connector. In stronger conditions, again without waves, the chain gradually becomes straight and the links adjacent to the anchor lift off the bottom. Forces at this stage are pretty much equal along the length of the chain. In the past it has been assumed that catenary, the effect of the chain’s weight in providing some damping to the system, was effective in almost any wind strengths. In fact, this is not the case and the chain becomes straight in relatively moderate wind strengths. I have witnessed my own, all-chain arrangement lifting completely off the bottom with a 7:1 scope in about 30 knots of wind.
The situation changes when the dynamic effects of yawing and, particularly, wave action are added, with a considerable increase in force being applied at the anchor end of the rode. Although the snatching effects of yawing are more disconcerting to the occupier of the boat, the effect of waves is recognised as being more damaging to the rode. As the boat rises and falls there is no remaining elasticity in the chain to absorb the forces. Sudden and possibly violent movements of the boat are translated to shock loadings that are transmitted directly to the anchor and its connector, in a ‘whiplash’ type effect. Less-efficient anchors might break free or drag at this point but modern anchor designs are more likely to be fixed in the seabed, resulting in high peak loads at the connector and final links of the chain.
The best way to reduce these damaging effects is to add a snubber into the rode, a length of nylon rope parallel with the chain being a favoured method. This has a very beneficial effect on the connector and anchor, by introducing some flexibility and damping into the otherwise rigid chain. The force at the anchor end will still be higher but the severity of the shocks is reduced.