Every grower, from the seasoned old hands to the folks who have just had their grants approved, will absolutely need to work with rope and have a basic understanding of its properties.  A competent level of rope crafting skills using various cordages is needed to succeed in this industry.  Archaeologists date the use of rope from between a quarter million and two and a half million years ago, and some modern apes even have very basic knotting and rope-work abilities.  So, like the club, cordage may pre-date mankind.

 

Modern ropes used in today’s aquaculture are nearly all synthetic fibers, some with very different properties that could make the improper handling or mixing anything from unprofitable to downright lethal.

 

The Big Three Fibers Today

 

Polypropylene / Polyethylene are the fibers that encompass the floating ropes.  Straight Polypro is losing favor to the newer technology of the Co-polymer fibers that are a blend of the two types extruded in such a way to add about 30% to the strength values approaching those of nylon and polyester.  The float ropes are about 30% to 50% less costly than nylon and polyester fibers which both sink.  The co-polymers can go by a variety of trade names like PolySteel, UltraLine, DanSteel, Steel-TEC etc.  They are all pretty similar and use the same fiber. Compared to the polypro ropes, the blends have far superior abrasion and UV resistant properties.  A simple way to tell them apart is the individual fiber strands of polypro will be a larger diameter, say equivalent to 15 to 30 pound test monofilament fishing line, the copolymer will be finer than your hair.

 

Polyester, also known as Dacron, which is Dupont’s trademark for the fiber, is a high strength, low stretch fiber with the highest specific gravity of the three.  It is often twisted into a combination rope with the above fibers, with low stretch to make an economical sinking rope for commercial use. These “Combo” ropes have been in use for decades by fisherman, farmers, and arborists becoming the standard for commercial ropes.

 

Nylon is a little stronger than the others with the unique property of being very elastic, with the ability to stretch 30% or more in a twisted configuration.  This makes for an ideal anchor or mooring rode especially in heavy weather when the elastic qualities will absorb a portion of the shock loading and take strain off the ground tackle and deck hardware. Many vessels inadvertently lose some or all of this advantage when the skipper chooses too heavy of a nylon anchor rode.  Charles Chapman’s Piloting Seamanship and Small Boat Handling may be the best guide for choosing wisely.  Nylons ability to stretch and store energy has had fatal results.  People have been killed towing with twisted nylon when the anchor point gave way on the towed vessel, and the stored energy in the rope contracted yanking the hardware forward, fatally striking the helmsman of the towing vessel.  Nylon makes a poor combo rope with the two above fibers because it will stretch away from the load.  Combo ropes need to be made of yarns of similar elongation characteristics.

 

Knots can weaken your lines by up to 50%, as can using a pulley wheel that is less than 4 times the diameter of the specific piece of cordage being used.  What happens in the too-tight radius of the undersized pulley, or the turning of the rope to form a knot is that the load is not distributed evenly.  The fibers on the outside of the radius take most of the load and those on the inside bear little to nothing. This inequitable radius loading is what causes knots to weaken your lines.  Short splicing is generally the strongest practical method for joining two lengths of rope. The short splice can also make an eye to go over a dock piling, or attach lines to your cages with about 85% retained rope strength.  A bowline retains about 70%, and a square knot only half.  If you need to join two lengths permanently and can’t splice them, learn the double fisherman’s knot.

 

Bridling the corners of lobster traps and aquaculture gear with a sling for hoisting is common practice but could cause problems if done improperly.  When using slings or bridals on aquaculture cages there is an angularity factor that can result in extreme rope loading forces that could cause failure, injury at sea or crush your gear.  When lifting a load of, say, 2,000 pounds with a single rope then the load is 2,000 pounds, and using 2 evenly spaced vertical ropes the load is halved at 1,000# each. Now switch to a long bridal so the two ropes instead of being at right angle to the lift are now tilted to an inside angle of each leg being 60 degrees up to the center of the bridal.   The load on each leg with the angularity factor is increased to 1154#.  Flatten the bridal angle to 30 degrees the load per leg is 2000#. Continue to flatten the bridal to 10 degrees and each leg is loaded to 5759#, putting an inward crushing force of more than eleven and a half times the weight of the vertical load on your gear.  At Ketcham’s we have had irate customers bring us traps crushed by extremely flat bridals complaining of “weak” steel, in the end it became a lesson in physics and leverage.  Don’t be penny wise trying to save a few cents in rope costs with too short bridals.  Generally in the rigging business an inside angle any flatter than 30 degrees is considered dangerous and should be avoided.

 

Bob Ketcham, is a retired ECSGA director, he founded and ran Ketcham Supply in New Bedford, Mass from 1975 thru 2016. The chandlery division of the business became, and still is a principal supplier of cordage, in all sizes, to the commercial marine industry.

Leave a Reply

Your email address will not be published. Required fields are marked *