Join Date: Oct 2009
Location: Gloucester, MA
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Re: Reduction in anchor scope with chain use
In the end, the goal is to have the anchor stay in the same place, we can all agree that dragging is bad. For a given bottom and boat, the only 2 things that you can control are the angle of pull on the anchor and the shock loads. The lower the angle of pull, the more holding power the anchor will have and if the shock loads are lower, you need less holding power.
Line effectively provides no catenary so the angle of pull on the anchor is determined by the scope. In calm weather, chain provides catenary and the angle of pull on the anchor will be parallel to the bottom. However, in conditions that actually test the holding power of the anchor, the chain will be stretched out straight and the angle of pull will be determined by the scope, there is effectively no catenary in these conditions. Therefore, the holding power of the anchor is determined by the scope and is independent of what the rode is made out of unless you have ridiculously heavy chain (I am thinking of captaaron and his 5/8" chain on a <30' boat).
Keeping the loads down means providing some shock absorption in the system. Line is a good shock absorber because it is relatively stretchy. In calmer conditions, chain is an okay shock absorber because it has catenary. In stronger conditions, the chain is effectively a steel rod and it has virtually no shock absorption meaning that the loads are higher on the anchor. When you are in conditions bad enough to be testing the anchor's holding power, line will yield lower loads on the anchor.
In conditions that test your anchor, you do not need any less scope with chain than line. Since chain is a terrible shock absorber, you really need a good snubber in bad conditions. Chain still has other benefits such as chafe resistance, easier setting, etc so we shouldn't go out and ditch all of our chain but this rule of thumb is totally bogus.