Posted by: towmasters | June 21, 2009

WIRED: How & When To Inspect & Replace Your Wire Rope Hawser

Almost all of the conventional tugs use it, but how do you know when to replace it? Where do you go for information if your company has no policy or offers no guidance, or the policy/guidance sounds sketchy, is incomplete, or comes from what appears to be a questionable source? Don’t worry, the government is here to help you! Well, at least the United States Navy is.

I can hear the groans already: what the hell does the navy know about commercial towing?! Quite a lot, actually. Many decades of trans-oceanic dead-ship towing and salvage experience, and beaucoup engineering expertise, means that on a technical level they have as much or more institutional know-how than anyone else out there, civilian or otherwise. Consequently, the bible for such knowledge is the U.S. Navy Towing Manual (warning: this is a 520-page/ 14MB file, so it’ll take a while to download if you’re not on broadband). More specifically, Appendix B – Wire Rope Towlines has all of the information you’ll ever need for inspecting wire rope and determining when a tow hawser is due for replacement due to wear or damage. Furthermore, no matter how much you think you know already, anyone can gain a lot of good knowledge from this manual. To top it all off, it’s free. So save it on your computer for future reference.

The key points are as follows:

For individual broken/protruding wires or fish hooks (broken wires bent back into a hook) the following rules apply:

  1. 6 wires are found broken in any rope lay length* (counting all of the strands).
  2. 3 wires are found broken in any strand lay length* (an individual strand).
  3. 1 wire is found broken within 1 rope lay length* of any end fitting.

*Note: a rope or strand lay length is, on a practical level, the same. It is defined as “the distance measured parallel to the axis of the rope (or strand) in which a strand (or wire) makes one complete helical revolution about the core.” In simple terms, a full wind or turn. On our 2″ wire that distance is exactly 13″.

If item #’s 1 or 2 are met the hawser must be replaced asap. If it is item #3, however, you can just cut it back past the broken wire and put a new spelter (poured zinc*) D-socket on the end. I recommend making a guage of the correct length for your wire size from some small flat or round bar stock, whatever you have handy, with a handle in the center. This will allow you to quickly and accurately determine if your hawser meets the broken-wire standards.

*Note: Chapter 4 – Towline System Components, page 4-23, says that Epoxy-type poured sockets are not suitable for towing purposes”, although it doesn’t explain why. Well, maybe the Navy doesn’t know everything after all. Poured epoxy resin is all we use nowadays in the Northeast and I’ve never heard of any failures due to a wire pulling through the socket. Anyone else out there heard of one? American Waterways Operators’ Responsible Carrier Program – 2009, on page IV-7, states that “The towing end of the tow wire should terminate in a spelter or thermo-set resin poured socket, or a spliced eye with thimble, and should be sized to exceed the breaking strength of the tow wire.” These sockets have, as far as I know, a proven track record of not ever being the weak link, so maybe the navy has other reasons that they choose not to explain, or else they’re just plain wrong on this one. Who knows, it’s the navy…..

Next, break out your vernier calipers. As per figure B-7 – Measuring Wire Rope, if the maximum allowable nominal diameter reduction is exceeded then the hawser must be replaced. The 2″ diameter wire on our tow winch straddles two size groups (1 & 9/16″ to 2″ and 2″ to 2 & 1/2″)  and can be abraded down by up to 1/8″ or 5/32″ according to the table. My company chose the more conservative (read: safer) figure of 1/8″ for its policy standard.

In addition, if the original diameter of outside individual wires is reduced by one-third replacement is again called for. The wires in a typical hawser are of differing diameter, and finding out which is which can be difficult. Our wire’s ABS Miscellaneous Survey Report, that we were issued for our vessel document book, says that the nominal diameter of the wires is 3.24, 2.40, 1.80 & 2.90 mm, but it doesn’t indicate which measurement is for the outside wires. If the wire has some real mileage it may be tough to get an accurate measurement so it would be a good idea to do it yourself in several places whenever you get a new wire installed.

The other conditions that warrant replacement are:

  1. Pitting due to corrosion.
  2. Heat damage.
  3. Kinking, crushing or any other damage resulting in distortion of the rope structure.

Beyond the standards for wear or damage listed above, what other standards are there, and what about the tow wire log? The Coast Guard has its NVIC 5-92: Guidelines For Wire Rope Hawser Inspection. The information contained in it was generated by TSAC and it says “Based on hawser usage, an operator should be able to develop an inspection schedule. Providing no significant damage is observed during use, a good time interval for removal of the hawser from the winch drum for a complete inspection is between 25,000 to 40,000 sea miles, depending on the severity of use. This inspection should check for damage, wear, and interior lubrication. It should also include caliper measurements of the hawser’s diameter to determine where the hawser has deteriorated. From this inspection, the operator can decide how many more sea miles the hawser should be used before the next complete inspection or when it should be retired from service.” Assuming that the hawser passed inspection this is when end-for-ending normally should occur. As for tow wire logs, I’ve seen some that specified the recording of either miles or hours, and sometimes both. NVIC 5-92, however, makes no mention of service hours for the log. Instead it says “On a monthly basis, records should show the hawser usage in sea miles.”

That’s pretty cut and dried, but you could argue for doing it either way. Both miles or hours alone can be misleading as every mile or hour isn’t the same. What were the sea conditions like during those miles or hours? On any given hawser, 10 hours with an empty barge in Long Island Sound in 3-foot seas isn’t the same as 10 hours with a loaded barge in 8 to 12-foot seas out in the open ocean. I prefer to record both. My company also stipulates that whenever the wire is connected and used (whether towing astern, alongside or pushing) that mileage must be logged. Some people think that only astern towing time counts, but if the wire is taking a strain it is also wearing so all of it should be accounted for.

Another authoritative text, Tug Use Offshore (which is listed in our Library page), states the following on page 13: “Generally, with proper maintenance, a 2.25-inch tow wire should have a useful life of about 8,000 hours or 65,000 miles.” I don’t see why it matters what size the wire is. It’s the type and severity of service that the hawser’s subjected to and the quality of the maintenance, or lack thereof, that ultimately determines how long it will last. Those numbers should probably be good baselines for any size of hawser.

So you’ve diligently kept an accurate tow wire log, carefully inspected your hawser, and found that it’s time to retire it. Pages B-3 thru 5 in the USN Towing Manual’s Appendix B have the details on how to do it properly. It’s also specified that when installing the new wire on your tow winch it must be under tension of not less than 5% of it’s breaking strength. Page B-6 has a schematic diagram for the Wallis Brake, a device that any decent in-house machine shop should be able to easily make, which is used to properly tension the wire as it’s fed onto the winch drum.

It’s also important to make sure that the wraps are firmly snugged up against one another so that each layer is tight and the wire won’t cut into the layers between wraps and get crushed or jammed. This usually happens on either end of the drum when the levelwind reverses direction. The manual says “Protect the wire as necessary during any hammering by using soft-faced hammers or wooden blocks.” Typically this gets done by tapping, gently or not so gently, with a regular sledge hammer. Seldom is there room for using wood blocks while still being able to take a good swing, so a soft-metal sledge sounds like a good idea.

Many common problems with out-of-sync levelwinds, wire not wrapping properly on the drum, and the unecessary crushing and abrasion damage this causes can be traced directly back to taking shortcuts or not paying attention to detail during the critical installation process. A new wire, even the cheaper types, can easily cost $20,000.00 or more. Getting the maximum safe service life out of it is imperative. Don’t let a sloppy installation job start you out with avoidable damage before you’ve shackled even once into a barge’s pennant or surge chain.

Please see the Unchained? post for chain/shackle/detachable link inspection and replacement guidelines.

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Responses

  1. Very good coverage on inspecting and determining condition of tow wire. Thank you.

  2. Well done, if I know how to do a trackback from (to?) my blog this would be one.

  3. With many of us having wire anchor rodes with a short chain pennant these days, this information and advice should be considered and included when keeping track of our ground tackle. The gear should be subject to inspection each and every time it is dropped or pulled.
    The poorly aligned level winder is probably one of the greatest threats to the wire’s service life. The damage can become substantial after only a few sets and recoveries.
    Well written and informative Joel, nice job.

  4. I towed meny a dump scow to the seadump and back in the last 40 somthing yrs..
    I think that the most of the ware takes place when
    you payout or heave it in,.

    you know keep the wire off the bottem at alcosts,
    but some times that s impossable.


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