Of Interest To Professional Mariners, Admiralty Expert Witnesses, and Lawyers
THE GREAT NAMAZU EXPLAINS RECENT EVOLUTION IN DETERMINING THE SAFETY FACTORS FOR TOW LINE (SYSTEM) STRENGTH
The Great Namazu, 3,000 years of nautical experience in one analyst
GREETINGS SAILOR BIPEDS
Back in the day when the Great Namazu was a pup mariners talked of "tow lines". Today with the advances in cordage technology and innovations in chafe protection, and winch technology several different types and sizes of line or wire might be utilized in a "towing system". Back in the day, Tow Masters, Mates, Boatswains often considered the "safety factor" when arranging for a tow. There was usually only one line, the "tow line" or hawser to consider, now "towing systems" rarely lend themselves to one size fits all "safety factors". Each system component line must be considered separately such as pendant, mainline, and backer"
The towing systems we used to simply call the tow line or towing hawser or wire is perhaps the single most critical component in a towing operation. Once connected, the towing vessel, her officers and owners usually have responsibility for the towed vessel or vessels. Tows can result in disaster for a variety of reasons. Miscalculated stability resulting in the capsize of either the towing vessel or the towed vessel and failure of the "tow line" (towing system) are two of the most common causes of casualties in the marine towing industry. In this post we will deal with avoiding the broken towing line(s) (system). Such avoidance is tougher than it used to be and fixing liability in the wake of such a failure is more difficult than ever. The expert witness in admiralty who cites to the "custom of the Industry" is probably skating on thin ice today and may often be impeached by an expert who understands codes, standards, standards of instruction, and the ongoing evolution in standards in the towing game today.
Image Courtesy French Ministry Of Defense / Marine Nationale
Back in the day we learned certain concepts like "breaking strain" and "safe working load". The key to safety in most marine operations involving "rigging" (line, wire, blocks and tackle , etc.) from mooring to towing, and working men aloft was insuring that you used the right strength of line for the job at hand. Most cordage and wire came with a labeled "breaking strength or strain" and a frequent rule of thumb in simpler applications such as mooring was to select cordage with a breaking strain half again the weight of the load, For towing often twice the weight or "strain" of the load", for working with men aloft we never went up without a boatswains chair supported by a line that could bear seven times our weight and tools. Things have changed faster than the authoritative literature.
When an American Merchant Marine Officer takes takes the various Coast Guard administered professional license exams the questions are based on certain publications considered classic and reliable such as KNIGHTS MODERN SEAMANSHIP, and such works are also considered in determining the standards of instruction for nautical training institutions. But often such classics are twenty years or more behind the times in certain areas. Not all aspects of the nautical arts and sciences are classic and few are static. Applying an obsolete "rule of thumb" to a particular rigging problem and then losing your tow is not a legally acceptable excuse. When you suspect that the classic textbook computations and recommended safety factors may not apply we suggest that you look to the towing system's manufacturer's literature. If you are applying and operating a towing system within the designer's and manufacturer's safety parameters it will be difficult for a board of inquiry, or admiralty court to find liability or negligence in your action. Unfortunately an all too common "custom of the Industry" is to lose or never have delivered to the tug the manufacturers manuals. Fortunately it is becoming more common to have access to a computer or smart phone on board. Some of the best cordage manufacturers and distributors have their rigging manuals available on line. One Such is SAMSON ROPE USERS MANUAL:
( http://www.samsonrope.com/Documents/Rope_Users_Manual_WEB.pdf )
Over the years as I have observed sailor biped behavior I've noted that many responsible bipeds such as Masters, Mates, and Boatswains have attempted to explain away accident causation with the equivalent of "there wasn't time to do it right", yet some how there always seems to be time to clean up the resulting mess and do it over. Maybe we should suggest a couple of twenty first century "Rules Of Thumb" while many of the standards of the nineteenth and twentieth century still around and all too uncritically followed.
FIRST : If your "towing system" does not consist of some type of right laid three strand line of a single type from towing winch or deck fitting to towed vessel, suspect that the old rules of thumb are not going to work for you. Now think twice before you act.
SECOND: If you do not know the manufacturer's breaking strain and safe working load, try to obtain it before proceeding. If the manuals are not on board and you have access to the internet, see if the information is posted on the manufacturers web site such as : SAMSON ROPE USERS MANUAL:
( http://www.samsonrope.com/Documents/Rope_Users_Manual_WEB.pdf )
THIRD: If you feel you must operate and you can't find the pertinent data to accurately calculate safe working load remember two all important things. (1) Required safe working loads as a ratio of load to breaking strain for the latest types of cordage are much higher than in the past. If in similar but older applications you were safe with a multiple of 1.7 times over bollard pull or weight to be moved consider that 2.2 to 2.5 is probably the recommended safety factor today. In the past prudent tug operators purchased their new towing hawsers with safety factors of at least 5:1 over their continuous ahead bollard pull rating. With some line today a 7:1 rating may be recommended. (2) In the past we always reduced safe working load by age and condition of the line based on known criteria of visual inspection. Some news cordage gives no reliable and predictable visual indication of wear. In the old days when safe working load was reduced based on visual inspection to 50% of original (which was often 5 times the breaking strain of the new line) we replaced the towing line and the old one became fenders . Today some hawsers must be replaced based on service times, and a 30% reduction in hawser strength is grounds for replacement. In short ,service times are becoming shorter and required retained line strength as a percentage of original breaking strain is becoming higher. It is probably asking for an accident and the subsequent liability to operate a towline assembly at less than 70% of original strength ratings.
So in an industry where suits ashore buy the cordage and don't know what to ask for at delivery to assure that crews have the information to properly calculate towline assembly strength what do you do? First ask for the manufacturer's manuals and contact information. Second start your own note book on your rigging, I mean you Captain, Mate, or Boatswain. Don't assume that any notebook you create won't grow legs and walk away after the next crew change. Dig for the information and learn to do the calculations, get the help you need and write everything down. Be sure to write the authoritative source of your information. When the day comes to ask the suits to replace that towing system after only 30% of strength loss which comes much faster than budgets typically anticipate you will have to defend your position. Learn how to use and inspect and compute safe working loads of modern cordage. Remember if the tow system cordage components you are looking at aren't right laid three strand, it is more probable than not that anything you had to regurgitate on the seamanship or marlin spike seamanship portion of your Coast Guard license or rating exam actually applies.
When ordering new line ask for:
* The exact cordage composition, lay, manufacturer,usage requirements, breaking strain. safety factors, any inspection hints.
* Copies of certifications of approval by Classification societies, any test certificates, and manufacturer's handling, storage and maintenance requirements.
* Material Safety Data sheets, these should accompany the spools of line when they are delivered aboard and contain inspection tips to aid you in knowing when you should refuse delivery of aged, discolored, or damaged line.
Start educating the suits. High tech cordage has fewer indicators of wear and has to be replaced more often. It also will require more crew training and time to handle safely.