Saturday, October 15, 2011

How Quickly Things Can Change...

I've been following the adventures of SV Footprint for some time... since I first started looking for a cruising cat. Just a few days ago, after crossing oceans and cruising for years, they made a gamble on an exposed anchorage that turned out badly. Fortunately, no one was injured and the insurance company came through like a champion. The expereince would leave me in shock. They are looking inward, I'm sure, wondering where life will or should point next.

The blog of SV Footprint

In a nutshell, the cove turned into a shore break, the anchor somehow failed, and the boat was lost. I believe they had a Spade 80 (33 pounds), all 1/4-inch G43 chain rode, and a bridle, but I'm working from memory and a cursory search or older blog posts. If we consider the case of a catamaran on an all-chain rode in exposed conditions (based upon the this calculation) we must quickly conclude that there was simply no way the anchor could hold--the strain peaks at 7,550 pounds, well over the breaking rating of 1/4-inch G43 chain. This is before the effect of breaking waves is considered. Any veteran of ocean beaches has felt the difference between the gentle vertical undulations of a wave in deep water and sharp impact of a breaker, which would throw the boat up and backwards with a force exceeding the displacement of the boat.

They might have survived this on a combination rode--20 feet of chain plus 1/2-inch nylon 3-strand--for much the same reason a big fish can't escape light tackle in open water--he can't get a good solid snatch on the line, it just keeps stretching. But we can't truly know.

 I've just converted to an all-chain rode, for reasons of convenience--the windlass handles it more easily--but now will need to find a solution for this scenario, should I (...when I...) face it. One solution would be a 50- to 100-foot snubber secured to the chain with a prusik knot or locking chain hook... but how would it be deployed in quickly worsening conditions? It couldn't. Would it be more practical to lengthen the bridle with extensions made of softer stuff, perhaps 7/16-inch nylon, perhaps 70 feet long, capable of stretching 4-6 feet in extreme conditions? I carry a retired climbing rope meeting just these requirement, designed to absorb climbing fall energies in exactly this range. Yup, I think that's the plan.

What can we take away from this expereince, and this discussion?
  • Shallow anchorages are very bad when waves move in. There may have been no effective mooring solution in the waves they saw.
  • All-chain may be a mistake in some circumstances. We all need to consider a way to ease the impact load.
  • They had a bridle and it didn't matter. Two 15-foot lengths of 5/8-inch nylon simply don't stretch enough to change the math. Longer, undersize bridle lines would help, but most sailors over-size them for better durability. Unless they were 75 feet long and more slender, I doubt the outcome would have been any different.
  • Jackline Insurance gets top marks.

And by all means read their blog. They've had some wonderful adventures and tell the story well.

Strengths of all-chain rodes:
  • Excellent chafe resistance, where rocks and coral are present.
  • Quiets the motion of the boat in shifting winds. Thus, better holding in moderate conditions.
  • Matches the swing to other boats on chain.
  • Can help the anchor re-set on a shift by slowing the rate at which the direction of pull changes (the chain has to pull across the bottom and through the mud if deeply set, making the change more gradual and allowing the anchor to slowly redeploy to face the new strain).
  • Easier to handle with a windlass.
  • Generally longer life, though this is only true in areas with abrasive bottoms. In the Chesapeake, corrosion often destroys the chain of  the weekend or occasional sailor before abrasion gets the rope. The rope-to-chain splice should be remade every year or two, depending on chafe.
Weaknesses of all chain rode:
  • Greater surge forces in gusts than a mixed rode--about double.
  • Greater surge forces in waves--about four times--if combined with strong winds, which serve to straighten the chain and eliminate the shock absorbing potential of the catenary. The chain itself cannot absorb the surge as it has no stretch, and thus, has reduced holding in extreme conditions.
  • Expensive. Not really important, in the grand scheme of things.
  • Harder to handle without windlass. Need a chain stopper or equivalent, in addition to bow cleats.
  • Harder to handle as kedge or secondary rode.

Another detailed treatment of anchor rode loading, by Don Dodds, author of Modern Cruising Under Sail. This is rather long and convoluted, but arrives at very similar conclusions; anchoring in shallows on all-chain rodes is fatal if the waves pick up.
 Don Dodds Anchor Rode Calculations Part I
Don Dodds Anchor Rode Calculations Part III

      And a summary of some on-line discussion:
      Compuserve Group Anchor Discussion


      1. This is a very sobering read. Things went to hell in an instant.

        One of the problems with cats (and shoal draft keel/centerboard boats) is that there is a strong temptation to anchor close in, in shallow water, where less rode will be required. The problem with this is that, after all, you are close to shore, where the time available to react to events is very short.

        s/v Eolian

      2. Exactly, and that is one reason for my post; many of my friends sail cats, and for us anchoring in 5-7 feet is normal. Generally, that lets us get further up a little creek and into very safe water. But off a beach, the very opposite is true.

        The other reason for the post is to explore the strength and weaknesses of an all-chain rode. I will edit the post with some more thoughts.

      3. There is nothing on the Footprint blog that elaborates on the anchor, and whether it dragged, or pulled out, or broke, or if the rode broke, etc.

        The break strength of 1/4" G43 chain is closer to 8,000 #. But, your figures are way over-the-top, the Fraysse spreadsheet gives peak numbers more appropriate for severe surge peaks and not allowing for snubbers - Footprint do say they had a bridle out. And I am having a hard time figuring how you got your #s as they don't seem to have mentioned wind speeds.

      4. Anonomous:

        No, we don't know what failed. However, given the strain involved, any part of the system could have failed if worn. The object lesson doesn't change.

        Are the forces over stated. I think the model overstates the surge force a bit because it neglects certain friction effects.However, several respected authorities place wind plus wave strikes in that range; please read the post on "drogues" in this blog and read the references. However, if you immagine a 9,000 pound boat plus the water on deck being trown up and back at 10 feet per second and stopped in a few feet, the deceleration component need only be 1/3 G (the rest is wind and wave slope).

        The purpose of the post was to remind us that a shallow lee shore remains as dangerous as it has been for the past centuries.

      5. I think this kind of analysis is very helpful and something that can be overlooked when sailors choose ground tackle. In this instance (as probably in most situations like this) it sounds like the anchor dragged and although this may have been influenced by the reduced 'spring' of an all chain rode my bet is that the anchor simply wasn't sufficient for the conditions. Personally I've always wondered why more don't choose much larger anchors than is typical. When you consider the overall weight of a boat is in the multi-ton range why quibble over a 25 ~ 35 pound difference. In other words, instead of carrying a 33 pound anchor why not a 66? The usual reasons given are: 1) too much weight in the 'ends' of the vessel, and 2) hard to handle. But seriously, an additional 33 pounds of weight wouldn't really be noticed on a cruising boat and a good windlass doesn't know the difference between a 33 and a 66. Even assuming that this extra 33 pounds would cause a slight bit of extra trouble, since the entire safety of the vessel is relying on this single item virtually all the time, surely it would be worth it.