Glenn Ashmore wrote:
If anyone wants to explore the effect of rope, chain and kellets on rode
centenary and maximum horizontal tension
http://www.johnsboatstuff.com/Articles/anchor.htm has a good spreadsheet for
calculating it. Scroll to the bottom of the page.


I'm extremely skeptical of this calculator for several reasons.
First, I plugged in a few numbers to see if it thought I needed more
than 50 feet of chain. The result was that if I held the chain
constant and increased nylon (with no kellet), the horizontal tension
decreased, implying that it would hit its limit sooner if you fed out
more scope. This is completely counter to all experience.

Secondly, the basic premise is that you hit some sort of limit when
the chain get lifted off the bottom so that the angle to the anchor is
no longer zero. In fact, anchors are designed to keep digging in even
with a small up angle of about 8 degrees. This means that a properly
set anchor will not try to pull out if the scope is 7:1 or greater,
even if the rode is straight.

And finally, although I didn't have enough time to look closely at the
spreadsheet, it seemed to have some problems, and nowhere does it
actually describe the math behind its claims, or even the meaning of
its "answer." Also, the sheet doesn't seem to correspond to the
examples in any way.