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.
- 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