Pulling Things - by Mike Creasy
With interest in model tug & barge events growing, it seems like a good time to look at some of the components of a towing rig and think about how to plan your maneuver as a towboat captain.
Beginning with your boat, its critical to have the tow point (main bollard or tow winch) set ahead of the rudder so that you can turn your boat while maintaining forward pull. The greater the distance between rudder and bollard, the greater the turning moment, and the better your control capability. This is less of a concern with Z-drive or cycloidal drive boats, which can direct their thrust and turning moment in almost any direction. It's of almost no concern to the latest tow style, which uses something called Dynamic Oval Towing, wherein the tow point is mounted on a rail around the deckhouse - essentially, the tug rotates under the tow point, giving it the capability of pulling in any direction. Conventional propeller and rudder equipped boats, even those with nozzles and other fancy gear - such as thrusters - have to pay attention to tow point location to avoid being caught "in irons", or unable to steer. So, let's limit this discussion to conventional power sets.
Next, your boat should have a clear sweep from the tow point across the stern to abeam the tow point on either side. Its important to have nothing that will snag your towline when you maneuver either way, so nice smooth gunwales and clear decks are critical. Most tugs have removable pins that can be fitted to limit towline sweep during steady state conditions, but these are not used for turning maneuvers.
Most full-size tugs these days will have automatic towing machines or other devices to control towline tension, but most of our models will have only a single hard point attachment so control of line tension is completely up to the judicious use of rudder and throttle. Fortunately there are a few things that we can do with the towing rig to make this task a little easier.
The rig for towing a ship's hull is generally not the same as that for a rectangular barge, so we'll just look at a barge rig.
A typical barge-towing rig will use a Y-shaped bridle to connect to each forward corner of the barge. The bridle (which may be rope, wire or chain) will lead to a connection (often called a fish plate or flounder plate) with the tug's main towing wire. Each leg of the bridle should be the same length, and should be sized to form about a 60º angle with the front of the barge. The fish plate will distribute the pulling power of the tug; when tug and barge are in line, the fish plate pulls evenly on both sides of the barge. When the tug turns left, the left hand leg of the bridle will slacken applying more pulling power to the right hand side of the barge, helping to begin the turn. Of course, the opposite is true as well.
Another problem for tow boats of all sizes is the need for some "give" in the towline. Because the barge will usually outweigh the tug by a good margin, the tow boat operator must avoid the "slack and tight" phenomena caused by waves (or poor throttle control!). This can result in failure of the towline, the winch, the tow bollards, etc or even the capsizing of the tug - model or full-size. A full-size tug with a long wire line will get a "catenary" or sag in the line between itself and the barge, so that a sudden increase in line tension will flatten out the catenary and ease the shock load. Model tugs can add a little artificial catenary sag by adding a bit of extra weight to the model towline. This will have the added benefit of taking the model towline almost straight down when the tug and barge are stopped, keeping the line safely out of the model's propeller.
Next month we'll talk about sheering, yawing and how to influence the direction of your tow!
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US Navy Towing Manual, US Navy, 2002
Knight's Modern Seamanship, 1988