Obsessing About Anchor Rodes

Buying a mattress is about as dull, and opaque, a consumer choice as any of us has to make.  You can't take your friends for a ride in (on) it, however much you like the color, it is quickly obscured, and you really can't test it out... in spite of the fact that you're going to spend a third of your life on it for a long time.

Ground tackle is like that.  You might get some cred for your choice of anchor (probably not), and every sailor loves to debate the virtues of chain and lay and scope and flukes

I purchased a new anchor, and obsessed for a time about the rest of the ground tackle.  My existing rode was a mere 200 feet of 1/2" triple strand nylon, and twice that will be barely enough for some of the more extreme Inside Passage anchorages.

Conventional wisdom suggests an eighth inch of rode diameter for each 9 feet of boat length (for a working anchor). That would be 3/8" line for Ripple, but 3/8" is hard to grip and haul, especially with 50 lbs of anchor and chain, plus the weight of the anchor line itself.  So I narrowed my choice to 7/16" and 1/2" three-stranded nylon.

These are by no means the only plausible choices.  What about braided nylon? Single braid? Double braid?  Three strand nylon offers elasticity (about 25% stretch).  Braided, about half that.  Every primer on anchor rodes extolls the virtue of elasticity to absorb energy and reduce shock loading.

Boat designer Steve Dashew alleges that less-stretchy polyester line minimizes some of the negative aspects of elasticity: with less stretch, the boat should do less sailing on the anchor in a blow.  The loss in elasticity can be accommodated by letting out more rode, which one would be inclined to do so in heavy weather in any case.  Apostasy or simply evolution of technique?

Then there is the question of strength.  How much is enough? The forces on a boat at anchor include tide, wave action (and, in particular, shock-loading from such waves), and wind.  Resisting these forces are the holding power of the anchor (modulated by the bottom characteristics of the location), the strength of the anchor chain, the strength of the anchor line, the strength of the connections between components, and the strength of the boat fixture to which the rode is attached.

The bitter-end of Ripple's rode is made fast to the sampson post, so failure there would entail disintegration of the boat itself, in which case, all else is moot.  Estimating the loads suffered by the other components is non-trivial.

I came across a formula for determining wind loading forces on a sailing vessel, and worked out the numbers for Ripple:

F = (A * Sf * Gf * V^2 ) / 390
where
F   = Horizontal force in pounds
A  = Area in square feet
Sf  = shape factor (.9 for round, 1.1 for a flat plate)
Gf = gust factor at sea surface
V  =  wind velocity in MPH (knots * 1.15)
AreaWidthheightNumbershape factorArea
Mast0.53010.913.5
cross trees50.1110.5
shrouds0.053060.98.1
halyards, lifts0.053060.98.1
house51115
hull831124
stanchions, pinrails0.12.560.91.35
Dinghy on deck41.510.95.4
dodger51.5117.5
Projected Area73.45
Fudge Factor (20%)14.69
Total Projected Area88.14
Wind Loads
Wind Velocity (knts)Force (lbs)
25205
50822
751849


Anything under 50 knots of wind is pretty clearly near or under the recommended working load limits (WLL) for triple strand nylon of 7/16 or 1/2 inch diameter (WLL is by convention 20% of tensile breaking strength load, though I have seen 'safety factors' as high as 12:1).  Assuming 5:1 safety factor, even the 75 knot winds are still roughly only 50% of the breaking strength load (for new line under test conditions).

Exceeding WLL is known to damage nylon rope, though the rarity of such events makes it unlikely that the anchor line would sustain such stress frequently.  A prudent climber replaces his or her rope after a single fall.  Perhaps the prudent mariner should do the same with an anchor rode that sustains more than a 50 knot night?

Summarizing the choices between 7/16" and 1/2":

Factors favoring 7/16" nylon triple strand:
  • greater elasticity - the ability of a rode to absorb additional energy (force) by stretching
  • weight - all that line has to be hauled up after every anchorage
  • compactness - it all has to be stored in the anchor locker
  • cost  - 7/16" is a bit cheaper
Factors favoring 1/2" nylon triple strand:
  • handling - a slightly larger size is easier to grip
  • brut strength - this may be as much psychological as anything else
  • resistance to chafe - the extra cross section of 1/2" line provides a significant additional margin of chafe resistance 
The dilemma is a common one in managing risk, cost, and convenience, and there is no perfect answer.  At the end of the day, you're asking yourself how severe an event (a storm or severe williwaw) you're trying to protect against, and how much in treasure and inconvenience you're willing to invest in this protection.

My friend Terry scoffed and said to stop fretting... the anchor set will fail long before anything else. Probably true.  I ended up with 400 feet of 7/16 rode plus the 20 feet of chain.  I will have the old anchor, the old rode, and the kedging anchor rode in reserve to deploy in the case of a big blow.

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