This conversation reminds me about DP. When DP became prevalent on modern vessels, the new generation of mariners relied on the technology so much that boat handling skills deminished. Its still like that today, minus a few exceptions.
With modern tow winches, you have brake band load cell tonnage feedback while on the brake, pressure transducers on the hydr. motors to give you force when off the brake, and this all correlates to the initial winch setting wire length and size which are counted/tracked via prox sensors on each tooth of the ring gear as the drum rotates. Yoy even have dynamic and towing modes. Now that you have all that information, you may fall in the trap that tugsailor mentioned. But, if you couple that technology with the practical experience, you will know whatâs the most important. Think Aiviq. The experince wasnt there, but the technology was.
Thereâs no substitute for experience of course but to gain maximum benefit it helps if the experience is put in the right context. Having an estimate of the catenary by calculation (or table etc) might provide helpful context to evaluate observations made while towing.
Sometimes it helps to have two methods for comparison, for example the DR vs a fix. The DR being abstract (lines on paper) and a fix is by actual measurement. Except of course in this case the actual distance the wire is off the bottom is difficult to directly measure.
Mermaid Consultantsâ table for Towline catenary vs. tension at winch:
Useful on modern Tugs/AHTs with static and dynamic winch tension display and alarm:
If there was too much tension or if you saw a lot big fluctuations in the tension on the wire (shock load) you would either pay out more tow wire or slow down.
As other have mentioned you can do something similar to this visually by looking at the tow wire, but it is a lot more scientific actually having something that will tell you the actual tension on the wire.
The big factors for tow length are obviously water depth and weather.
The reality is the US is that 99% of tugs do not have any fancy winches with tension meters or computer controls.
Itâs still the tried and true 1950s here.
The job requires a tugboat operator with experience, not just any video game player off the street.
Of course experience matters, but that kind of attitude towards better equipment and is harmful to the industry.
So often I see people doing things in a way that is âthe way it is has always be doneâ when there are clearly better ways. Having actual information to inform decisions is both useful for new people who are still developing the needed experience and for those with lots of experience who should always be verifying the optimal methods.
Sometimes that information with be a new tool, sometimes it is a document, something you own systematic data collection.
There reality is 99% of tugs do not have tension meters. Would they be nice to have? Sure. Are they necessary? Obviously not. They are a very expensive frill.
Iâd rather see that same amount of money spent on other things, for example better electronics, a good crane, quieter, more comfortable crew quarters etc.
Not sure about the concern for accurate measurement between tug and barge. When towing in rough weather the distance varies quite a bit. For example, in a following sea the tug will surf down the front of a big swell until the wire barely has any catenary at all (if you have the right length of wire out) before being checked up by the tow, letting the wave pass under. Then the wire will sink down until the next swell repeats the sequence. Given that reality the tug master would be better served by paying attention to the catenary rather than distance. As long as there is some catenary there is still some shock absorbing capability left. You donât want to get into having no shock absorption left at all unless you have reduced power to take load off the wire that way.
I worked many years ago for Moran. Their policy was no bridles, just a single towing point on the centerline. When the barge was loaded there was a 200â synthetic for surge protection. When empty the barge was connected directly to a wire pennant with our tow wire. The tow winch was a simple affair without load measuring and a manual brake. (I didnât like it, but it wasnât my decision as master. Thatâs how the tugs and barges were rigged.) As you can imagine we paid a lot a lot of attention to surging and catenary.
For a rule of thumb⌠if the wire is close to coming out of the water put more wire out - as measured by layers of wire on the drum. We would start with three layers of wire in normal weather. If itâs calm and there is little surge bring the wire in half a at a layer a time and watch it to see if you can bring in more wire. Less wire means more speed since catenary is drag. I once crossed the Gulf with an empty barge in a perfect calm with only a few wraps (not layers) of wire out. The wire was out of the water but had a gentle sag in it and never came tight (just to be on the safe side the brake was lightly set. We had a simple alarm in the wheelhouse to let us know when the wire was paying out). Upon arrival we just steamed on in since we were already shortened up to harbor tow length. The judgement call becomes more nuanced as you approach port and the weather is not so great. Shallow water is only one issue. Being in close proximity to idiots in ships means you want to keep the barge closer than you do in open sea. Shorten up as necessary. If the wire starts to come out of the water lengthening the wire is no longer an option. Reduce your power to protect the connection since youâve lost the protection of the catenary. As you approach the sea buoy you might be crawling along in clutch but the wire will still be jerking up and down. When this happens you might tell the deck hand on the winch to ease off on the brake so that the sudden jerks pull a little wire off rather than break it. It is nerve wracking, but doesnât last very long.
The real test is with an empty barge in storm force winds or greater. I hope this never happens to you. We hove to head up to the wind and let out almost all the wire we had. The strain on the wire came not from the seas but from the force of the wind on the unmanned barge which was 400â long with only 6-8 foot draft (no ballast tanks). The wire was still coming out of the water with a shock felt on the tug. We had reduced speed to clutch from the beginning when the decision was made to heave to, so that option was gone. When I came on watch at midnight (I was mate then) the barge was behind us lying in the trough perpendicular to our course. We werenât using enough power to keep her head up to the wind. The tug had begun to have difficulty steering and the satnav indicated we were being pulled backward. We decided we had to increase the revs on the engines reasoning that would give us more steering ability and reduce the wind load on the barge by holding her bow to the wind. It worked, but a big heavy bridle would have given us a lot of peace of mind. It was a long night watching that distant barge in the beam of a searchlight, adding rpm when the head began to fall off and pulling back the throttles when we could.
We would have had even more peace of mind if we had doubled or tripled our moorings and stayed at the dock. There is another rule of thumb for towing captains. If it makes your butt pucker, donât do it.
Tow line length counters, tension meter for static and dynamic measurement has been standard equipment on Brattvaag winches for Trawlers, AHTs and AHTSs for many years.
Kongsberg Marine offer Towcon System that can be fitted to any brand of winches:
https://www.kongsberg.com/maritime/products/deck-machinery-and-cranes/deck-machinery/anchor-handling-vessels/towcon-winch-control-system-ahts/
PS> Brattvaag Hydraulic was part of Roll-Royce Marine and now Kongsberg Marine for the last 30 yearsâŚ
The OP asks about a good way to estimate the depth of the catenary. Iâve never seen it done in practice but according to the book it can be done by comparing the amount of wire out and the distance between tug and tow.
I think it can also be done using the tension on the tow wire but not familiar with that either.
I agree with this, a prudent mariner will always be on the watch for a mismatch between what is expected and what is happening. The better the information the closer the expected will be to the actual. Of course the inexperienced and incompetent might get confused if they are not paying attention but thatâs risk with or without the tools.
'Donât let mariners have good tools because a moron might misuse them." is not good argument.
We all saw how state of the art towing equipment with tension meters did not provide any benefit whatsoever when AIVIQ put KULLUK on the rocks.
All the basics were overlooked:
Hire competent captains and crew with the necessary knowledge for the job
Wait for weather
Acquire local knowledge
Plan an appropriate route
Stay in deep water and put out all the wire
Donât remove chain bridles; they are there for good reason.
Use appropriate surge gear
Keep an eye on the tow wire
Avoid unnecessarily crossing shoals where strong currents and rougher seas can be expected.
Donât ignore crewmen with local knowledge
If you have a tension meter, monitor it
When you hear tension meter alarms ringing, slow down and find out why.
Itâs nice to have all of the best hi tech tools for the job, but they are no substitute for the basics.
Yes I agree, experience is important. Any number of nice gadgets doesnât help if they are not used, or not used right.
Some âexperiencedâ persons that think. âI donât need none of all that sh*tâ may be the problem. (Aiviq/Kuluk MAY be a proof of just that)
Any truly experienced person use the information available to make better decisions.
If you have access to the ACTUAL tension and length of wire out, put that into a reliable computer software that is pre-program with the correct tow configuration and you can get out the actual catenary, both as a curve and a figure for deepest point.
Set an alarm to activate if the catenary get too shallow, or too deep and you donât have to spend all your attention on looking at the tow wire.
Yes I know; that is cheating. âAnybody that is good at playing computer games can do itâ.
NO, adding your experience to the info is what get you out of trouble.
âNo dramaâ so nobody are going to thank you, but you have the satisfaction of knowing that you saved the insurers a bunch of money by using your experience, knowledge and information available to AVOID trouble.
Iâve used Brattavag winches on trawlers that had all of that plus automatic pay out / pay in to maintain correct tension of the towing warps. We also had suitcases full of replacement computer boards in the crawl space under the wheelhouse.
A large trawler needs this equipment to be competitive. Clearly, the typical tugboat does not.
OneEighteen, Your replies to this subject displays your experience of the pucker effect towing an unballasted, large , light barge in heavy wind. I agree totally. you must keep enough speed to keep the barge headed into the wind, the effect of wind load on the rig multiplies if you go too slow and rig gets sideways⌠I had one 250k barge with a single point chain bridle. It was a friggin bear.loaded or light. Most of the rigs I handled in the fleet were double chain bridles, a slight bit easier to manage in heavy weather⌠I transited with quite a few Moran rigs, you cats had your hands full with that setup. A good man at the winch entering or departing jetties was priceless. We didnât have any tension alarms or special shit. Just good guys on the winch. .
Smart to keep a good supply of spares. 
Did you ever have to use any of them??
PS> Brattvaag winches on AHT/AHTS appears to hold up pretty good.
For those not familiar with Auto Trawl systems, here is the latest from Kongsberg:
Itâs going to be driven by the same logic in every sector. The tugs I worked on were 2000 or 3000 hp, minimal equipment as youâd expect. An ocean-going tug with 200 or more tons of bollard pull sailing world-wide is going to to have a lot more sophisticated equipment.
I think of it in terms of fuel.
If you are burning 2000 gallons a day, something that costs $10,000 is not very expensive, but something that costs $100,000 is.
If you are burning 20,000 gallons a day, something that costs $100,000 is not very expensive, but something thatâs costs $1 million is.
Iâm way behind the times on prices, but a Norwegian auto trawl system for the typical Alaska factory trawler probably costs $20 million today.
A typical new tow winch for a 3000hp tug is probably about $1 million.
A winch for a 30,000hp tug might be $30 million.
A factorytrawler has lots of spares and a full time, highly qualified electrician. Probably a 30,000hp tug does too.
A 3000hp tug has few spares and the chief maybe a minimally qualified Deckineer.
Yes, thereâs a term for this.
To think at the margin is to assess the impact of a decision by considering the effects of spending an additional, or marginal , unit of resources. Since resources typically have diminishing marginal impact (i.e., each additional unit is less impactful than the previous unit), thinking at the margin is critical for properly assessing the impact of a decision.
Interesting reply sir. Could have used âAdvancedâ towing equipment. As a few posters including yourself, it was not monetarily feasible. In my career, lost one large ,appx 600 ft empty barge. Not because of speed or wire length, but mate not shifting the cable on the âDoughnutâ on a regular basis. Broke right at the wear spot. Same storm Nw heavy winds rounding Tortugas. A notable early ATB tug sank a bit north of us. . Cmakin knows a bit about that much larger and incompetently designed rigâŚMost of the rigs I operated later in my career were 5600 to 7000 hp., The captains I worked under were were passing knowledge in many different ways. To this day, and retired now, that advise was welcomed⌠Main one, GET A GOOD CHIEF ENGINEER! Folks, I listened to that one. for sure.