Category Archives: Anchoring Reference Reading

Anchoring: Reference Materials

I strongly recommend two books on anchoring and ground tackle:  They are

  1. The Complete Book of Anchoring and Mooring, Second Edition, Earl Hinz, Cornell Maritime Press, 2001, and
  2. The Complete Anchoring Handbook, Alain Poiraud, Achim and Erica Ginsberg-Klemmt, International Marine/McGraw Hill, 2008.

Both of these books are primarily experiential, backed up with theory and math.  Hinz’ presents more conclusions and relatively little math.  Poiraud et. al. include a very thorough Appendix of the mathematical analysis of both static and dynamic anchor and rode behavior authored by Alain Fraysse.  This analysis is heavily mathematical, for those interested, and can be skipped by those dis-inclined toward the math.  Hinz writes in US units of measure, Poiraud et. al., in metric units.  Conversion of the units back-and-forth is possible and fairly easy with today’s Internet conversion tools.  Poiraud is a Hylas designer and inventor of the spade anchor.  Fraysse maintains a very useful, if technical, web site, here: http://alain.fraysse.free.fr/sail/rode/anchor/anchor.htm.

Both books cover the same general material, but each presents “pearls” of their own.  Both books cover the subject primarily from the point of view of blue-water sailors cruising remote parts of the world and occasionally encountering severe to extreme conditions.  That profile of prolonged offshore cruising greatly exceeds the lifestyle and preference criteria of Sanctuary’s crew.  Sanctuary and crew are US near-coastal, coastal and river cruisers.  We do occasional offshore passages of less than 200 miles, such as crossing from Florida to the Bahamas, or crossing the Gulf of Mexico from Carrabelle, FL, to Tarpon Springs, FL.  We sometimes run off the Georgia and South Carolina coast, offshore around New Jersey, and the New England coast.  We select these trips as carefully as possible for “acceptable” sea-state conditions.  If the weather forecast looks at all questionable, we stay in-shore or seek marinas, for cover and the possibility of extended weather delays.  That said, I nevertheless wanted a ground tackle system that would handle the “what if” times when we just got caught out.  We have several times found ourselves in short term storm-force conditions in summer thunderstorms.  That is never pleasant.

Key points I took from the above-mentioned books are:
1.  All anchors perform better in some bottom materials than in others; no anchor is a “universal fit” for all bottom grounds.
2.  To ensure secure holding and prevent/avoid dragging, adequate scope is by far the single most critical routine and controllable variable every captain must consider.
3.  All-chain rode is not the best rode for conditions above 35 kts; at that point, chain sizes found aboard pleasure craft loose all catenary.  Once catenary is lost, chain rode cannot absorb or cushion heave and surge motions.  The addition of kellets or additional lengths of chain makes surprisingly little difference.  When catenary is gone, heave and surge loads can unset an anchor and/or damage deck fittings/gear.  Furthermore, the instantaneous forces in the chain will far exceed the Working Load Limits (WLLs) of BBB and G4 Chain, which can result in parting the chain.
4.  In light of item 3, above, experts generally recommend use of a hybrid rode consisting of a chain leader attached at one end to the anchor’s shank and spliced at the other to a length of 3-strand nylon line.  The chain leader at the anchor shank resists abrasion as the anchor buries itself and works into and against the bottom.  The 3-strand nylon section provides elasticity, so absorbs, cushions and disperses the energy of both working load and shock forces.  Poiraud, et. al., suggest that the length of the chain leader should be equal to the “average depth of the water” in which one typically anchors.  Other experts advise a length of chain leader equal to the weight of the anchor.  Others recommend a length of chain leader equal to the length of the boat.  Whatever your choice, in calm conditions, the chain will lay on the bottom, and the nylon 3-strand component of the hybrid will hang vertically from the bow roller to the bottom.  As wind forces increase, up to the point where the rode is fully taught, the weight of the chain will help reduce vertical loads on the anchor shank that might tend to lift the shank and unset the anchor.  When winds and seas reach the point at which the rode is fully taught, the only game in town is the shank-to-seabed angle.  Shank-to-seabed angle is a controllable variable which is directly related to the amount of scope deployed.  As scope is increased, elasticity (shock absorbency) is consequently also added to the system.  Increased elasticity accommodates worsening wind and sea state conditions by absorbing increasing instantaneous heave and surge loads.  For the US Mid-Atlantic and Southern coastal regions, Ontario and the Inland Great Rivers, all of these mixed (hybrid rode) chain formula seem fine.
5.  With chain rodes, both authors discuss and recommend the use of bridles to buffer the attachment point of the chain-to-deck hardware in light-to-moderate conditions and snubbers in moderate-to-severe conditions.  These accessories relocate anchoring loads from the windlass to hard points on the hull, accommodate chafe reduction in more severe conditions, and increase crew comfort by reducing mechanical noise in the forepeak in calm conditions.
6.  Chafe can be an enormous problem in conditions as little as 15 – 20 knots.
7.  Neither author recommends swivels in the anchor rode.  If used, they offer several suggestions to increase safety.  Sanctuary does have a swivel installed.  Ours is not attached directly to the anchor itself.  We installed a 6 inch length of chain from the anchor shank to the swivel, then placed the swivel in-line, then attached the rest of our chain.  This approach reduces the chance of side loading and failure at the swivel’s pivot pin.  But, it does add to the total number of connection points in the rode, and each connection point is its own potential point-of-failure.
8.  Both authors strongly caution the use only high tensile-strength shackles, such as thse made in the USA by Crosby (http://www.thecrosbygroup.com/).  These shackles have their safe WLLs cast into them.  Both authors strongly caution against the use of the Chinese-made utility hardware found at big-box and hardware stores across the US.  In the Annapolis metro area, the only place we know of that stocks high tensile shackles is Fawcett’s Boating Supplies (http://fawcettboat.com/online.htm).  Otherwise, source these items from the Internet.

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Our main anchor is attached to its rode with a toggle, not a shackle.  The design of the toggle allows it to slip over the end of the anchor shank, where it fits well very little excess clearance for undesirable movement.  The inherent design of a toggle is stronger and passes more smoothly through our bow roller assembly than does the largest possible shackle the chain can accept.

Our rode is fit with a 5500# WLL swivel.  The design of the attaching ends of the swivel allows it to slip the ears directly over the end of the anchor shank.  The swivel is made of up two attaching ends connected together by a large diameter swivel pin.  The swivel pin is essentially a large diameter bolt.  The nut is welded to the bolt so it can not turn off.  The pin is oriented axially along the pull of the rode.  The bolt is very strong in tension, but substantially weaker in shear caused by side-loading pulls.   To avoid shear forces caused by side-loading, we have added a 5-link length of 3/8″ chain.  That short piece of 3/8″ chain is located between the toggle and the swivel.  The swivel and connecting chain is visible in the above pictures.  Our configuration eliminates any possibility of side loading and allows for larger connecting hardware than would be possible with a conventional shackle.

Related detail:

1)  https://gilwellbear.wordpress.com/category/boat-technical-topics/anchoring/anchor-rode-loads/

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