Tow length

I was wondering if anyone knew of any resources giving guidelines on optimal tow length. Obviously there are many rules of thumb, customary practices, and observing real world behaviour, but is there any systemic guidelines/references out there?
Also any good ways of estimating how deep under the water a centenary is going?

Thanks

I used to to figure 10% of tow wire out length to be the “draft” of tow wire.

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Outside of what you mentioned in terms of customary practices, is gets pretty technical without much benefit to the typical mariner, unless you just love complicated dynamic calculations for the hell of it. But, DNV RH-H-103 (towing section) and to a lesser technical extent the US Navy Towing manual are good resources. I’m pretty sure The Naval Architect published a calculator based on RH-H-103 (but, I could be wrong). None of this replaces the experience you typically get by learning the ropes and applying customary practices.

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Thank you, I am certainly interested in digging around in unnecessarily technical calculations, as well as more practical things.

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Towed for many years before ATB’S. Didn’t need a calculator to figure out the length of tow wire. A visual of the tow and along with boat surge and catenary will talk to you whether to let out more wire or shorten up. It ain’t that hard to figure out. The weather talks to you if you listen. Tug speed is also an important consideration to help with catenary in bad weather .Payed special attention to the bridles on the tow.

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Some modern tugs I worked on had a load cell that would tell you the tension in tons that was on the tow wire, you would just try and keep the tonnage within a certain target range, not too much or not too low.

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.

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I’d start with the Navy Towing Manual.

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If you have a load cell and know what winch tension you’re towing at you can calculate the catenary pretty easily. Otherwise it’s a guess.

If doing an ocean tow with chaffing gear on I prefer to put out however much tow wire I would want in a storm because by the time the weather is crap I don’t want guys on the back deck removing the chaffing gear so I can put out more wire. If you’re doing short tows without chaffing gear you have more flexibility and can put out less wire in good weather and lengthen the tow is conditions change.

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In deep water, use all of the working length of your wire. Some companies have this as a policy

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This is the heart of the issue.

It takes time and effort to get information reliably down to the mariner on deck. I’ve found that it’s a process of trial and error. If an experienced and careful mariner says a document, instrument, guidelines or whatever is useful it probably is.

If you don’t have a load cell to tell you the tension, it is possibly to approximately calculate centenary by using the radar to measure the distance between the tow vessel and the tow compared to the length of tow wire out. Can set up a VRM so that if the tow gets too closer you know that the catenary will have increased.

True. And a person good at the math could probably work out a way to closely approximate the wire tension based on the range to the tow.

I have always been skeptical about the accuracy of the radar range to the tow. Therefore, I would be skeptical of any tow wire draft calculations based upon radar range.

I have wanted to experiment with a laser range finder, but haven’t actually tried it. This could be useful to accurately and consistently find the distance to a target on the bow of the barge.

It might also be useful for finding distance off to dock and bridge fenders.

I have also wanted to experiment with “trawl eggs” (remote echo sounders designed to show net head rope opening and distance off the bottom or surface) that could be attached at various points along the tow wire. These could give the actual draft of the tow wire at various points at various speeds, currents, and weather conditions.

As a practical matter, I know from experience towing in shallow Bering Sea waters and observing the polishing and wear on the tow gear, that the end of the chain surge gear and tow shackle receive the most shine and wear. The end of the tow wire receives more shine and wear than the rest of the wire.

It’s common to wear out one or two shackles during the year. Much less common, but sometimes the socket on the end of the tow wire becomes thin and needs to be replaced. Nearly all of the shine and wear on the tow wire is within the 200 feet closest to the barge. It’s not uncommon to cut a 100 feet of wire off once during the season.

All of this depends upon the type of work being done and how shallow the tug must go. If you’ve only got a few feet of water under the tug, you are going to be dragging a lot of gear.

The chain bridals on the barge are typically about the beam of the barge. At least they are supposed to be. (60’ to 100’ feet- with one full shot (90 feet) being common). Typically, there will be a chain “pigtail” perhaps 20-40 feet long. There is usually a half shot (45 feet) or one full shot of surge gear (another 90 feet). If the weather is good and there is no plan to lay on the wire, sometimes no surge gear will be used.

If you assume that the bridals and gear hangs straight down (it doesn’t): 90 + 30 + 45 (half shot) = 165 feet or about 27.5 fathoms.

If the angle of the gear under tow were about 45 degrees (it should be deeper than that) the draft of the chain surge gear would be about half as much, or 14 fathoms. It will usually be deeper, especially at slower speeds.

I generally assume a full length tow wire draft of 25 fathoms. This works very well.

The actual tow wire draft is probably closer to 15 fathoms at standard speed. There are many reasons why speed can drop off quickly, and tow wire draft can increase quickly.

If you only have 10 fathoms of water, you will be dragging on bottom unless you shorten up, if you can. If you have 10 feet under the tug, you will be dragging the gear.

In more than 25 fathoms, I generally use all the wire. In less than 25 fathoms, I usually shorten up.

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That’s where that practical experience comes in. Different for how you get off the hip in pilot town and short hawse out of SW pass, or shortening up on the east coast of Florida going south to get inside of the gulf stream axis. You cannot replace what you described in any manual. You just need to learn the ropes from someone like yourself with that local knowledge, experience, and proven methods.

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The book “Tugs, Towboats and Towing” by Brady (1967) says: “The distance between the tug and tow can be measured by using a stadimeter.” It goes on to say a comparison with the length of the towline can be used to approximate the catenary which in turn can be used to estimate the strain on the hawser.

The book only mentions radar once, saying only that tugs “should” be equipped with one.

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Coming from a hawser boat the rules are clear; when rough always put it out to the chain.

If its calm or you are “inside” you can shorten up or more commonly not let it all the way out.

In theory you could shackle one hawser to another but I have only ever done that once. So in practice you are bound by the length of your hawser.

Ideally you want your hawser to lay flat in the water, when you have it all out.

I did work on one ship that towed with a combination of hawser and wire, but there we were towing a submerged submarine, so the desired depth of the sub dictated the length of the tow.

I use the Vortex Ranger 1000 more at work than I do for hunting. I think their newer version of this one is the 1500. Probably around $300

What I’ve seen in practice is that the length of the tow wire is measured using the number of wraps out. IIRC 1/2 wrap leaving the breakwater, pay-out to 3 wraps then 7 or 8 wraps in deep water.

If a ranger finder is used than there probably should be a conversion chart available somewhere in the wheelhouse showing wraps to feet (or meters).

Assuming such information is useful the catenary information should be given in a table or a diagram. SImilar to what is used in the various references except probably specific to the situation (the actual tug and barge).

All the information needed should be set up such that a calculator is not needed.

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Tow wire tables should be standard, but I’ve never seen one that attempted to estimate catenary.

I’ve seen tables that showed the quantity of wire in each layer. I’ve also seen it marked on the bulkhead near the winch. Good things to do.

The amount of catenary changes with the load on the barge, sea conditions, currents, tide changes, engine rpm’s, course changes, and often changes in water depth.

There are often stronger and changing currents around shoals, shelf’s, sea mounts, and in passages.

The tug will usually lose (or gain) speed much faster than the barge in currents. This usually increases catenary. In some passages, or coming up on a steep shoal it’s very common for the barge to close on the tug, increasing catenary, just when you don’t want it to.

There are passages where the barge might come alongside or even pass the tug. Active Pass comes to mind; I normally won’t take that route.

Towing into the wind, a light barge or a container barge will slow down much faster than the tug in gusts.

Towing off the wind, especially jogging, a light barge gain on the tug in gusts and produce a lot of catenary. I’ve shined the wire up in 50 fathoms.

Catenary tables and calculations, even if they were generally accurate are no substitute for experience and good judgment. They might provide a false sense of security.

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Yes, that’s what was saying upthread. If someone with experience says something like that is useful it probably is, otherwise maybe not.

The towing book I have here has some drawings showing a typical catenary, than some other scenarios; one with the hawser being payed out, coming up taut and a tow on a long hawser. A table or diagram could show something similar maybe.

I think a table or some such might be useful as a benchmark. For example if the table shows some clearance but the gear comes up with evidence of having dragged that would be a data point to help calibrate the table. After a while someone with experience would have good idea of how accurate it is and under what conditions it’s not.