Anchoring Systems: Refining A Tried and True Approach

We have been having an internal dialogue about the ever-critical issue of anchoring systems, and the fact is that there is nothing like a real blow with a lee shore off your stern to focus your attention on the subject. It will come as no surprise that we like to sleep well at anchor, and by traditional definition this requires substantial holding power. It’s a given that it takes weight to achieve security at anchor, but beyond this simple postulate there are a plethora of choices. What our experience has led us to evolve into may surprise you.

First some load considerations: windage, hull shape, mass, waves, and shearing all create load. And in spite of our desire for stout anchors, anchoring loads are for the most part surprisingly low (at least that has been our experience with our sailing and power designs). Before you roll your eyes and wonder if we’ve lost it in old age, try this little test on your own vessel.

The next time you are anchored in sub-optimal weather, put the engine into gear at minimum revs and note the fuel burn required to hold station without moving. We tested this with FPB 78-1 Cochise, and found that at 35-40 knots of breeze in a protected anchorage, one engine at idle burned about 1.05 GPH with alternators and hydraulics off. Assume you get about 19 HP/gallon, if you have an efficient prop you are putting 25 lbs of thrust into the water for each HP. So, 1.05 x 19 x 25 = 570 pounds of thrust or drag. Before waves or shearing back and forth that’s the load on our massive anchor.

Here is another example that Steve Parsons related from FPB 64-3 Iron Lady during a short but intense blow at anchor in Fiji. The breeze was a steady 45 to 50 knots and gusting, in a protected anchorage, so chop of a couple feet at most. He had the engine turning over just in case. During the gusts he would put the engine into gear for a few seconds to unload the chain, but would have to quickly go to neutral or he would override the anchor. Idling on the John Deere 6068 SFM 50 in the 64s burns about 7/10ths of a gallon per hour, let’s call it 13 HP (being generous now). 13HP x 25 lbs/HP = 325 pounds of thrust.

The minute you begin to sail at anchor the load jumps dramatically, at the same time the anchor shank is being worked back and forth.

If the loads are so low why the massive anchors and related gear? Several reasons:

  • Holding in the tropics is often poor with a thin layer of sand over hard coral pan. The anchor cannot really dig in (see photo above). In which case bigger is always better!
  • River deltas frequently have very soft mud where large fluke area is needed.
  • It might be blowing 75 knots, and wind force goes up by the square of its velocity. With only 570 pounds of drag on Cochise at 40 knots, at 75 knots that jumps to 2000 pounds. Let’s throw in shearing and four-to-six foot waves, and for fun double the peak loads to 4000 pounds. And of course this is in an anchorage with poor holding.
  • We can anchor on much shorter scope.
  •  We really like BFAs. They make us feel all warm and fuzzy.

Sail magazine did a story on tests run in 2006 by West Marine. One of the anchors they tested was a Manson Supreme, all 35 pounds of it. The FPB 78 has a similar anchor in design, just nine times the mass at 330 pounds. Before we tell you what the 35-pounder tested at, know that our experience is that holding power goes up in a nonlinear fashion with weight. The increase in surface area and mass gives an increase greater than just the ratio of the anchor weights. The 35 pound Manson Supreme tested at 5000 pounds.

Now let’s consider scope. Old time anchors require five-to-one scope to attain good holding. But the modern types can get by with less. The Manson, Rocna and Fortress showed little difference between three and five-to-one scope. The Manson hit the max test load of 5000 pounds at both three and five-to-one.

Now we get to the free lunch, or lack thereof. There are firm rules against excessive weight in the bow. Penalties for transgression depend on sea-state, length of voyage, and your tolerance for discomfort. This can also become a safety factor in heavy weather, particularly when trying to fight your way off a lee shore.

For the past 35 years we have used big anchors and heat treated alloyed steel chain, what Acco calls System 7. The FPB 83 and the FPB 64s as well as the 78′ ketch Beowulf and all the Sundeer 64s used 3/8″ System 7 chain. The weight we saved here was put into the anchor where it does the most good. On the FPB 78s we switched to 1/2″ high test chain, or what Acco calls G43. The weight of this chain is 2.69 pounds per foot compared to the 3/8″ chain at 1.6 pounds per foot. Safe working load goes from 6600 pounds for the 3/8″ Grade 7 to 9200 pounds for the 1/2″ G43. Of course we are more interested in what the breaking strength is than some bureaucratically determined SWL. In our litigious society break strength data is hard to come by, but the implications seem to be that five times SWL is about where minimum break comes into play.

OK, we’ve got our BFA, our hell for stout chain, and the last question is…how much chain do we carry? Remember the free lunch, or lack thereof? Here is where it comes into play. Let’s start by looking at anchoring risk factors. Poor holding, hurricane or stronger gusts, a bit of sea sweeping in, we’ve got this covered with the BFA. So we don’t worry about dragging. And besides, we can always offset extreme anchoring load scenarios coupled with poor holding with a judicious application of power. What we do worry about is fouling something on the bottom and not being able to recover our main anchor.

Now a bit of exotic math. The area of a circle, or more aptly stated the area of the bottom swept by a given length of chain, is 3.14 times the square of the radius. So a little extra chain on the bottom geometrically increases the swept area and thereby the risk of fouling the anchor. Hence our theorem: (BFA+heavier chain) X (reduced scope)² =  a lot less swept bottom and significantly reduced bottom fouling risk.

How does this exotic math work out in the real world? On Wind Horse and Cochise, with the Rocna on the former and Manson the latter, we have dragged a single time in close to 100,000 nautical miles of cruising. We typically carry about 240′ of chain, although Cochise has a little more at roughly 275′. In shallow anchorages our norm is to set the anchor on three-to-one scope, including the height of the bow roller in the scope equation, and then once set bring the chain back to where we have a two-to-one scope. As the water gets deeper this approach begins to bring catenary into play. We have seen situations where we got reasonable holding in 180’+ of water using 300′ of chain.

Our biggest anchoring issue occurred in Greenland, where one bay had a thick weed on the bottom.

Trying to set on three-to-one scope resulted in the anchor skating through the grass, picking up a huge clump of same and requiring an hour of tedious labor with bread knife carefully lashed to the boat-hook. We reset at five-to-one and then after getting a good bite, shortened to three-to-one. We feel that with the heavier chain relative to loads we are now using, 240-250′ of chain is what we really want or need.

We hope to see you at anchor in the not-too-distant future.

Posted by Steve Dashew  (May 30, 2018)

5 Responses to “Anchoring Systems: Refining A Tried and True Approach”

  1. Scotto Says:

    some familiar faces perhaps?

  2. Arch Fonken Says:

    Do you use a snubber? If so, how does it figure into the equation.
    Thanks, Arch

  3. Steve Dashew Says:

    Howdy Arch
    We do use a snubber on occasion, primarily to reduce noise. If we are expecting a real blow we will fit one as well. We typically carry a short snubber for normal conditions, and a long one that we used when conditions are expected to be difficult.
    Our snubbers are typically made from three strand nylon for stretch.

  4. marcus Says:

    your thoughts on the use of alloy aluminium anchors to take a bit of weight out of the bow?

  5. Steve Dashew Says:

    Hello Marcus:
    There is a section in Offshore Cruising Encyclopedia discussing aluminum anchors. We usually carry two, for use as kedges and primary anchor back up. The Fortress anchors have tested well and in soft mud in particular they do very well.