Wednesday, June 24, 2015

Is an Anchor Ever Really Set?

Anchor testing always raises as many questions as it settles. Too many important variables that simple cannot be controlled, and if they are,  cruising will throw something different at you. But one thing was very clear; in soft mud it takes a long distance for anchors to really set.

Other anchors took similar distances, generally 35-50 feet. When we tested 2-pound mini-anchors we got scaled down results; setting took 5-10 feet, sometimes longer, and there was never a hard stop.

What does this mean? Let's look at the green curve showing a good set (most were worse):

27 foot boat, 13-pound Danforth (assume 3x less holding than FX-37 and 1/2 drag distances)
  • Setting force 250 pounds (750 equivalent) = 15 feet setting distance.
  • ABYC storm force = 1200 pounds (3600 equivalent). Drag
  • If 2 anchors set with some allowance for single leg loading, probably move another 25-35 feet during extended setting (see comments in prior post). 

50-foot boat, FX-37
  • Setting force 750 pounds = 30 feet setting distance
  • ABYC storm force =  3200 pounds. Drag unless 2 anchors are set. May stay with good snubber or deep water, but probably not.

34-foot PDQ (me--I've anchored in the area too). FX-16 + Manson Supreme 35.
  • Setting force 250 pounds = 15 foot setting distance on both.
  • ABYC storm force 2600 pounds. With both anchors set in a shallow V. Theory says I should drag, but...
With a good snubber or nylon rode, the force should be less than 1000 pounds (see previous post), and with a asymmetrical V the force is well spread on 2 anchors (400 pounds on the Manson, 600 pounds on the Fortress), explaining why I don't move that far. However, in practice I move ~ 15 feet, which is hardly noticeable (a few thousandths of a degree on the GPS). Additionally, the PDQ does NOT sail around the anchor when on a bridle, and the legs are joined to the rode downstream of the bridle apex; there is no single leg loading.Thus, my single leg loads do not exceed 500 pounds, which is well within the holding capacity of the anchors.

The Delta Anchor moves around even more in mud, which is why I changed to a Mason Supreme (I have also tested Rocna and Mantus anchors--all very similar, which is better seems to depend on the bottom, though I'd buy a Mantus if I had it to do over) and up-sized from 25 pounds to 35 pounds. Mass always helps.
As you can see, a 25-pound Delta is only good for about 227 pounds in soft mud (400 x 25/44=227), and my boat generates about 240 pounds force in a 20-knot breeze (prior post, 20 knots, 3-strand snubber), which is just about when I would experience dragging if I didn't set a Fortress too.

But everyone moves in soft mud. Don't let them tell you otherwise.


Note that twice the Fortress dis not really set at all.

And a couple more, just to think about...

1 comment:

  1. Great post Drew. Your research is always good reading...