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.