SEA­MAN­SHIP

These are the care­fully con­sid­ered de­sign el­e­ments of an ef­fec­tive an­chor­ing ar­range­ment

Soundings - - Contents -

Know­ing the in­di­vid­ual com­po­nents of your ground tackle sys­tem and how they each work is an es­sen­tial first step in a peace­ful night’s rest while at an­chor.

Tether some­thing heavy to your boat and throw it over the side. That is the essence of an­chor­ing. And yet, given the ar­ray of an­chors that ex­ist, not to men­tion the breath­tak­ing quan­tity of re­search, thou­sands of years of ex­pe­ri­ence and count­less sailors with opin­ions on the mat­ter, there is ob­vi­ously more to an­chor­ing than that.

An an­chor does not hold by weight alone, nor is the func­tion of the rode solely to con­nect the an­chor to the boat. Col­lec­tively, the an­chor, rode and as­so­ci­ated hard­ware are known as ground tackle. Since we can­not see ground tackle at work on the seafloor, it’s im­por­tant to vi­su­al­ize what is go­ing on down there, how it all works and why it some­times doesn’t.

Most an­chors have flukes. They pro­vide the broad, flat sur­face that gen­er­ates re­sis­tance when em­bed­ded in the bot­tom. But if a broad, flat sur­face were the only re­quire- ment, then an­chors would look like shov­els. They would skip across the bot­tom, gen­er­at­ing no hold­ing power what­so­ever. To over­come such fu­til­ity, an­chors have a stock or a trip­ping palm that points the flukes down­ward. The flukes them­selves are ta­pered to pen­e­trate the bot­tom.

The first ef­fect of a strain com­ing onto the an­chor is to straighten out the rode. Next, the an­chor drags un­til the flukes get ori­ented down­ward. Fur­ther strain (hope­fully) causes the flukes to bite into the bot­tom. Ad­di­tional strain buries the flukes un­til the an­chor is well set.

The strain that a boat places on an an­chor has two com­po­nents: hor­i­zon­tal (par­al­lel to the seafloor) and ver­ti­cal (per­pen­dic­u­lar to the seafloor). The hor­i­zon­tal com­po­nent is the good part. That is when em­bed­ded flukes pro­vide max­i­mum re­sis­tance to the strain com­ing from your boat. Like nails in a plank, they make it dif­fi­cult to re­lease the grip by pulling side­ways.

But the rode even­tu­ally leads up­ward to the boat, pro­duc­ing a ver­ti­cal com­po­nent to the strain. Like draw­ing out a nail with a claw ham­mer, the ver­ti­cal force will over­come the hor­i­zon­tal force, break­ing the flukes out of the bot­tom—no mat­ter how big the an­chor. This is pre­cisely what oc­curs each time you shorten up the rode to re­trieve your an­chor.

The ob­vi­ous pur­pose of the rode is to con­nect the an­chor to the boat. But the rode, es­pe­cially chain rode, is also in­te­gral to the per­for­mance of the an­chor it­self. In­stead of the line from the boat to the an­chor be­ing straight, the weight of the rode causes it to sag. This sag has a name: cate­nary. Cate­nary acts as a shock ab­sorber be­tween the boat and the an­chor. It pre­vents the full force of the boat from be­ing trans­mit­ted to the an­chor be­low, while en­hanc­ing the hor­i­zon­tal force near­est to the an­chor. Cate­nary is key in the tow­ing in­dus­try for re­duc­ing the strain on a tow­ing wire and the points of at­tach­ment.

As you can imag­ine, cate­nary rises and falls with puffs and lulls. The stronger the wind, the less cate­nary, and the greater chance that the ver­ti­cal com­po­nent of the strain will dis­lodge the flukes. In a per­fect world, the chain near­est to the an­chor would lay on the bot­tom all the time, but that is not al­ways pos­si­ble in strong weather.

Heav­ier chain pro­duces more cate­nary than lighter chain. Ves­sels equipped only with fiber rode are at a dis­ad­van­tage in this re­spect, although the chain leader, in­tended to pre­vent chafe on the bot­tom, does help to a de­gree. If you are

com­mit­ted to fiber rode for rea­sons of space and weight, then a longer and/ or heav­ier chain leader likely will en­hance the per­for­mance of your an­chor.

Some­times, peo­ple at­tach weights called kel­lets to the rode. Kel­lets as­sist in main­tain­ing cate­nary, as well as the hor­i­zon­tal com­po­nent of the strain. Some re­search sug­gests that kel­lets are not worth the trou­ble, but you can pic­ture how the ad­di­tional weight on the rode will help to keep it down.

On one oc­ca­sion when a gale was fore­casted, I shack­led a sec­ondary an­chor to the rode and slid it down to the bot­tom on a sep­a­rate line, like a tram­car on a wire. Mother Na­ture blew stink, but we did not drag. I’ll never know what role the sec­ondary an­chor played, but it could not have hurt. A fur­ther ben­e­fit was that this ap­proach sidestepped the pos­si­bil­ity of en­tan­gling two sep­a­rate an­chor rodes at a lo­ca­tion where the tidal cur­rent re­versed every six hours. In all cases, the best pol­icy is to have a re­ally great an­chor, sized right for your boat and the wa­ters you fre­quent.

Cate­nary is only part of the story when dis­cussing ground tackle ef­fec­tive­ness. A big­ger fac­tor is scope. Scope is the ra­tio of the length of rode to the depth of the wa­ter, plus the dis­tance from the wa­ter­line to the chock or the hawsepipe. A ra­tio of 1:1 scope means the an­chor rode is equal to wa­ter depth, straight up and down, with the an­chor just touch­ing the bot­tom.

The Amer­i­can Mer­chant Sea­man’s Man­ual rec­om­mends scope of 5:1, with even more in high winds or in a lo­ca­tion where sea and swell add to the strain on the an­chor. The greater the scope, the more hor­i­zon­tal strain there is on the flukes. With chain rode, greater scope also means more weight con­tribut­ing to the cate­nary, ob­vi­at­ing the need for kel­lets.

In gen­eral, I find that a 5:1 scope pro­vides peace of mind up to 20 knots or so of wind. The suc­cess of that cal­cu­la­tion de­pends on many things: the suit­abil­ity of the an­chor, the bot­tom type, the size of the chain, the sea state and the amount of windage ( sur­face area) that the ves­sel presents, along with past ex­pe­ri­ence in any given an­chor­age. Peo­ple get away with less scope, and I have too, es­pe­cially un­der mild con­di­tions or with crew on deck keep­ing an eye on things.

It is re­mark­able to ob­serve the care­fully con­sid­ered de­sign el­e­ments that pro­duce an ef­fec­tive an­chor. If you are re­ally cu­ri­ous to see how an an­chor works, take a small one to the beach, tie a line to it and drag it along un­til the flukes bury. Bet­ter than any­thing, this will help you vi­su­al­ize what is go­ing on down there when you drop the hook. Turns out, an an­chor is much more than some­thing heavy to pitch over the side.

Calm con­di­tions al­low for a shorter an­chor scope.

Your an­chor should be matched to your boat and your cruis­ing grounds.

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