All chain vs 100'

[noelex 77]

Originally Posted by Lodesman

I'd like to know what theoretical calculations you use?

This is a commonly used formula that indicates when the last link of chain will just lift off the bottom:

F= W(L^2 - D^2)/2D

F is the force in Kgf
W is the weight of the rode in water per unit metre
D is the depth + bow roller height (m)
L is the length of chain (m)
There is also an article here:
Scope vs catenary (Rocna Knowledge Base)
It includes this graph showing a graphical representation of the chain (note the horizontal and vertical scales are different). This visually shows there is very little sag or catenary.

[noelex 77]

Originally Posted by Lodesman

In my own observations, I've sat out at anchor in over 20-25G30 and never saw my chain go taut, with 160' of 3/8" chain in about 15' of water.

You won't see the chain lift off the bottom near the anchor in only 20-25 knots of wind but this is not the sort of windspeed where you should be concerned about dragging.

[Lodesman]

Originally Posted by noelex 77

This is a commonly used formula that indicates when the last link of chain will just lift off the bottom:

F= W(L^2 - D^2)/2D
...
This visually shows there is very little sag or catenary.

Where did you source that formula? I've seen other discussions that have used other equations, that didn't quite fit, so I'm curious if this is truly applicable to anchor chain geometry?
Assuming it's correct, then it demonstrates the progressive effect on force required to overcome the catenary when more chain is put out - ie. doubling the amount of chain, quadruples the force required to lift it off the ground. Adding essentially weightless nylon will not do that.

It's a curious graph - while a 6:1 scope (48m of chain) is certainly sufficient for benign conditions, it is in no way suitable for 50 kt winds. Even as such, the cable is not bar taut and it seems obvious that adding chain at this point will see the catenary reappear.

Originally Posted by noelex 77

You won't see the chain lift off the bottom near the anchor in only 20-25 knots of wind but this is not the sort of windspeed where you should be concerned about dragging.

You've omitted the gusts, which were fairly long and actually hits peak speeds well above 30. In an open roadstead, with a sizable chop. The point was that it didn't even pull the last 40' out of the sand. Sorry I haven't anchored in a hurricane, but if I do, I'll be putting out the other 160' of chain, not sitting at 6:1 scope.

Actually the only time I've dragged, it wasn't very windy at all. Tight anchorage, strong reversing tidal current, and fairly scoured bottom: Allans Cay in the Bahamas. Due to proximity of other vessels, only put out 60' (4:1). We rode out the turn to flood OK, but on the switch to ebb started a very slow drag. Put out another 40' and we didn't budge after that. Admittedly, the reduced angle at the joining shackle might have been the deciding factor - I don't know how much that would've been improved if it had been rode instead of chain. Certainly 4:1 scope didn't do my Rocna any favours, especially given the bottom - thin layer of sand over hardpan. Obviously in those conditions, where the anchor can't dig in, the only added hold will come from deadweight and/or friction - something chain provides. But to each his or her own.