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.

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 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.

6 comments:

  1. Interesting... perhaps we have all been using too large anchors for all these years...

    It has been my observation here in Puget Sound that lightweight anchors are difficult to set in our hard mud - they are simply too light to get a bite. Is your mud soft? Did you have any setting difficulties?

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  2. Talk about a long subject....

    The gist of the articles for Practical Sailor (3 parts) is:
    * Dingy anchors. Since I had to develop baseline data, why not? I've used a Guardian G4 for years and never really liked it.
    * Tandems for soft bottoms. The Chesapeake mud is generally bottomless ooze, the stuff you will sink into right up to your crotch. That is the focus. Many creeks are like that, and staying one place can be a challenge. The testing is mostly with small anchors, since dragging full size anchors as tandems is very difficult.
    * Full size field testing. Since few are familiar with setting 2 anchors, this is mostly about the practicalities of not getting tangled.

    Hard bottoms are interesting, but they are not reproducible and are very difficult for smaller anchors. A different problem. Mostly, only mass helps. We have some of these in the Chesapeake, but not in places you spend the night. This is probably the only place where in-line tandems make sense, as 2 hooks increase the odds that one of them will find a soft spot.

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  3. Drew, I've been giving a lot of thought to anchoring lately since in the last 3 years I've drug several times. I set my anchor carefully, backing down on it slowly and gradually increasing rpm to 85 percent. I have two 13 # danforths, each set on 20 feet of chain and 150 feet of half-inch twist. The best holding I get is with both anchors set in a vee. Recently found that this isn't guaranteed secure either: when anchoring in St Marys River (Solomons), a line squall blew through and we dragged 30 feet. This area is notorious for difficult holding - like anchoring in kleenex - so I was actually satisfied that the set didn't fail completely.

    I supposed there is no way to stop the cyclic/alternate loading of anchors in a vee set up. I attach both rodes to the bow cleat. . . and thought if I attached them both to a snubber of sufficient length that would give me an even load on both hooks, but the more I think about it, the more I think this won't stop the cycling.

    While two anchors are certainly more secure than one, it is a pain in the neck to do it. Which makes me think one of the new generation anchors is a better answer - like a Manson or Rocna. However, anchoring in ooze 6 feet deep may defeat the best anchor on earth. Any thoughts?

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  4. It would really help if I knew the size of the boat.

    a. I don't know what your setting force is (inboard or outboard, HP?).
    b. 30 feet may not include much actual dragging. In sand you can expect a Danforth anchor to reach full power in 6-10 feet, but the resent testing at Solomons Island (should be on-line somewhere) showed that most anchors take about 30 feet to fully set in the ooze you are talking about. Even the little 2-pound anchors took over 10' to develop full power in ooze.
    c. 13 pounds is pretty small for a Danforth. I have a 32' cat (equivalent to a 40' mono) and find a FX-16 more appropriate, paired with a 35-pound Manson. If there is any trash (sticks and shells) a 13-pound anchor just can't get through it. Do they come up with trash in them?
    d. The key to avoiding some of the cyclic loading is to NOT attach the anchors at the bow. I attach the second rode some distance forward.
    e. Do everything you can to prevent sailing around the anchor (cyclic loading). I use a very long bridle (35'). However, with wind shifts their is always some.

    And yes, 6' ooze is tough. No matter what you do, you may need to accept that in very soft materials, anchors are going to move a good bit. Not too reassuring, when the creek gets crowded. A part of the problem is that your engine is not strong in reverse (you aren't well set.)

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    Replies
    1. Hmmm. . . well, here are the numbers anyway:

      Watkins 27: 24.5 LWL
      27 LOA
      10 Beam
      9200# displacement (actual, not design)
      4 draft
      Yanmar 2gm20f 16 hp inboard diesel

      I set the anchors at 3k rpm, but max rpm is 3600

      You're right - the 13# danforths may not be enough. I guess bigger is always better wrt ground tackle.

      Not picturing what you mean by not attaching both anchors at the bow, but some distance forward. Do you mean in line with each other?

      Your information on the set distance in deep ooze for all anchors is enlightening.

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  5. The setting force is probably about the same as mine, since inboards are more efficient in reverse. Say 250 pounds.

    The wind load on your boat is a LOT less than mine, perhaps 1/2. The anchors should be good, but you would LOVE a 25-pound Mantus or similar.

    By attaching both anchor rodes to a single point you can reduce cycling, and for those with chain, provide a single snubber location. I simply use a fixed length rode for the second anchor and attach it to the main line with a soft shackle or prusik.

    http://sail-delmarva.blogspot.com/2011/07/laying-second-anchor.html

    Also, if the wind does a 360 during the night, I will unhook in minutes, while you have to figure out how to unwind the mess. I once watched my boat spin 4 times in 10 minutes in Taylor's Creek (wind against tide); that is when a single scoop anchor is nice.

    I will post some of the Solomons data in a minute.

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