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...

Tuesday, June 23, 2015

So You Think You Don't Need a Snubber....

rev. 1/2017

I've still got data to collect and average, but the bones are pretty clear...

Data collected on 324' catamaran anchored in 4.5 feet of water at 10:1 scope including freeboard. Fetch was 5 miles.

  • The ABYC Tables are worst case but absolutely correct.
  • Short snubbers don't do much for peak loads. Obviously.
  • The peaks are MUCH higher than the average--it may be possible to haul the anchor in by hand, but that does not mean the peaks are not severe (I hauled in by hand just after recording a 700-pound peak).
  • A nice long snubber really takes the load off  the anchor, and that means less dragging. Think of how it will reduce fatigue on the chain and bowsprit.
  • Hauling the boat up to the anchor using the windlass is dicey as the wind comes up; the snatch loads can be severe.

Sunday, June 21, 2015

Tandem Anchoring V

I had another good day testing in nice firm sand at a nice comfortable beach today. Much more pleasant
than mud... though less important to me, since everything works in sand.

Strangely, no real surprises, though it is still impressive what force you can generate with no stretch and some chop. Over the last few weeks...
  • I stopped at 500 pounds pulling the 2-pound Guardian solo. It would have taken much more, but what does it matter? Scaled-up that is about 2 tons.
  • The Guardian easily rotated 90 degrees to meet the force every time, so long as the Mantus in the asymmetrical V allowed transition time. It just carved right around, smooth as anything, defying the conventional wisdom that pivoting fluke anchors can't handle a wind shift. What they do not tolerate so well is a strong shift with no transition.
  • We anchored in 10 knots just to get a wind load number. Only about 13 pounds with a supper elastic rode and the surges averaged out. When we made that all Amsteel the peaks went to 115 pounds!!
  • We anchored in 4.5 feet, 10:1 scope, in a sustained 17-20 knots with 10 mile fetch for several hours (Mantus + Guardian). Nothing moved, loads peaking at 180 pounds with a 35' climbing rope snubber.
  • I then switched to an Amsteel bridle and all chain. Wow! Twisted a shoulder, lost a load cell (it was a cheap one), and recorded loads GREATER than the ABYC values (over 500 pounds). The Mantus moved several feet, as predicted, and the Guardian went deep. While the Mantus came up easily, the Guardian was as difficult to recover as my 35-pound Supreme would be after a 35-knot night using a snubber. Scaled up, this test is equivalent to ~ 7000 pounds on an all-chain rode; a storm load with plenty of surging. I'm going to have to practice this some more. It was fun... in a way, and is certainly an interesting test.

If you click on the picture and look closely, you can see 2 small orange floats marking the 2 x 2-pound anchors set in an asymmetric V. The connecting rode would typically be about 1 boat length, but since this is small scale, it is only 8 feet. Full scale, the chain will break before the anchors give.

It's strange being anchored in a good solid breeze by a couple of paperweights! And interesting seeing thunderstorm loads in a moderate breeze, simply because there was no stretch in the system! I will NOT be testing this full scale in a real breeze; that's got breakage written all over it.


 And for what it is worth...
  • 2 x 9.9 hp high thrust Yamahas in reverse = 260 pounds
  • 2 x 9.9 hp high thrust Yamahas in forward = 500 pounds
At typical 2/3 throttle, about 220 pounds.

Sunday, June 14, 2015

Safe Through Hulls

Most boat sinkings happen at the dock. The most common reason is a failed through hull or the attached hose. The nightmares scenario is a failure at sea, with no way to quickly install  plug. In fact, during our delivery trip home we experienced a small failure.

 The head bulkhead is at top, the holding tank bottom. There is a crash tank, also sealed, in front of the holding tank. It was the speed sending unit, lowermost in the picture, that leaked due to a faulty o-ring.

While not always practical, why not put the through hulls in a bulk headed compartment? On a monohull this could be practical either forward or aft, or by running the bulkhead up the side. You do have to hang down to service valves, and opening them is not very handy (extensions can be fitted), but it is safe. During the homecoming trip we flooded this compartment right to the the water line without ill effect.