Just read an interesting piece on anchor snubbers in the latest Practical Sailor. What really jumped out at me was not about snubbers but rather just how little load was required to lift the last link of chain off the bottom of the ocean and affect the anchor shank angle.
If the chain is laying on the bottom, as people often assume it is, the anchor shank is parallel to the bottom or in optimum holding orientation. When the last link of chain lifts it can change shank angle to match that of the scope being used. If on short scope.......
PS used a 100' section of 5/16" chain set to a 5:1 scope to test how much load was needed to lift the last link off the ground..
To apply the load they used a chain-hoist / come-along and measured it with a calibrated load cell. They physically pulled the chain to see how much load was required to lift the last link of chain eg: the anchor shank, off the bottom.
The shocker for me was that it took just 190 pounds to lift 100' of 5/16" chain at a 5:1 scope off the ground out of the water
. The in water load calculation to do the same, when taking the density of the chain into consideration, would be just 158 pounds to lift the last link off the bottom
Seeing as I own a digital load cell and have physically measured the loads of our 36' sloop at 140 - 218 (218 was peak loads) pounds of load in 17-19 knots the idea of chain holding your anchor on the bottom is really considerably less than where I and many books and experts suggest it would be....? Just 17-19 knots on our boat is enough to lift our chain at 5:1 !!!!!
17-19 knots is not even a stiff breeze.......
I actually just ran the numbers through an anchor load calculator and it does not show the chain lifting until a load applied of 242 pounds yet based on the PS actual test data it takes just 158 pounds of load to do this... Does this mean that all the theoretical
data we've been using for years has been skewed???? It would be one thing if it was skewed in the safer
direction but it has been skewed to the unsafe
This means the wind conditions to affect anchor shank angle, with all chain, seem to be considerably lower than originally thought...
So if I am anchoring at 5:1 with 100' of 5/16" chain in approx 17' of water it will take just 17-20 knots, on our boat, based on actual measurements, to begin to lift the chain so the anchor shank angle is affected.
Interesting stuff to say the least and certainly some interesting data to ponder........
Quote: Practical Sailor
"We fixed one end and then tensioned the chain with a come-along until the last links at the lower end had lifted free of the ground. Lifting this required a load of 190 pounds, which translates to 158 pounds in the water. Based on data from last yearís test (PS, May 2012), this would be the equivalent of about 15 knots of wind on a 40-foot boat anchored in about 15 feet of water with 100 feet of 5/16-inch chain."
We've all heard the old axiom that 5:1 on chain is okay but 7:1 on rope/chain rode is needed. With this physically tested & measured data it appears that this is simply not entirely true..
It seems prudent that scope should be set irrespective of all chain or rope/chain rode because chain alone really will not help shank angle when you really need it most,
in high winds...??