Tugs and Tows

 

 

TOW TUG

 

Tugboat Diagram

 

A Tow Tug is a tug boat used to tow a barge. It may also be used as an anchor handling tug by a derrick or lay barge. In harbors and restricted sailing areas, ships are usually assisted by one or more tugboats. The ship and the tugboat are firstly connected by cable. The tugboat sails with the ship and positions itself in such a manner that it can tow the ship in a specific direction by means of the towing cable. During these manoeuvres, it is also possible for a tugboat to be manoeuvred against the ship in order to be able to push it.

A tugboat provides propulsion by means of one or more screws. Many ships are equipped with two screws positioned next to one another. In older models, these screws are positioned aft beneath the ship by means of a propeller shaft. In this case, the thrust is produced predominantly in the longitudinal direction of the ship. This direction is also directed partially sideways by means of rudders. Modern tugboats are often equipped with so-called thrusters. In this case the entire screw/propulsion unit can turn in the horizontal plane and thrust can be produced in any desired direction. In a number of models, these thrusters are arranged beneath the stern (a so-called azimuth-stern-drive tug), and in a number of models the thrusters are arranged roughly 1/3 of the length from the forward part of the ship (a so-called tractor tug).

 

 

 

 

 

 Towing a Boat

 

A smaller towing vessel with a line connected to a single stern cleat will have little, if any, maneuverability. The pull exerted by the vessel being towed will either rip the cleat off of the stern, whipping it back towards the vessel being towed at high speed, or will dip the stern of the towing vessel under water, swamping it. You'll notice that commercial towing vessels have a tow bitt located at the centerline of the boat at least a third of the way forward, if not further forward. The tow bitt allows the towing vessel to pivot while under a strain. The tow bitt also provides a strong point of connection for the tow line and allows the helmsman to quickly lengthen or shorten the towline to compensate for changing conditions.

We are assuming that you, someday, will either tow another boat or will need to be towed by a non-commercial tower. Because towing is inherently dangerous, you want to ensure that your tow will be as safe as possible. An improperly rigged tow boat can cause damage and injury. Let's review some basic towing procedures and rigs:

1) The best point of connection to the towed vessel is the bow ring down by the waterline. However, most boats over 30' don't have one. You, therefore, will need to construct a bridle. The purpose of a bridle is twofold: 1) to spread the strain of the tow over two or more attachment points and 2) to minimize yawing. The easiest way to make a bridle is to take two of your dock lines, tie the tow line to the spliced eyes using a bowline and run the ends through your bow chocks or hawseholes (with chafing gear) to your bow cleats. A second option is a single line with a " bowline on a bight" tied in its center. You can, of course, use a simple overhand knot to make the loop but you may never get it untied. The length of the bridle legs should be at least equal to the width of your vessel plus the distance from the chocks to the bow. A word of caution here: if your bow cleats are not fastened to the deck with through-hull bolts and back plates, they are liable to rip off during a long tow or in rough seas. If they don't have back plates, then you must take a couple of turns on the cleats and continue the lines aft to your stern cleats or, if on a sailboat, to the mast. The additional connection point will help absorb some of the shock. The bridle rig itself should be stronger than the tow line. If anything is going to break, you want it to be the tow line.

 

2) The tow line itself should be strong enough for the job at hand and be capable of some stretch. Double-braided nylon is the traditional choice. There are, however, a number of synthetic ropes now available that combine high strength with some elasticity (and they float). If possible, avoid twisted nylon. If twisted nylon breaks while fully stretched, it will whip back and hurt somebody. The tow line should be long enough so that there is Catenary (dip in the line) during the tow. The length should be adjusted so that both vessels are "in step" - both riding up and down waves at the same time, not one going down a wave while the other is shouldering up a wave. The longer the tow line, the easier the ride. The easier the ride, the less stress applied to the hardware. Three or four hundred feet of tow line is typically used when towing a thirty foot to fifty foot vessel in from offshore - sometimes more. Very few boats keep five hundred feet of synthetic tow line stored on board. You probably will need to use your anchor line as a tow line.Anchor line is typically twisted nylon - the worst stuff to use. If this is the case, use as long a towline as possible and keep your head down.

3) A bridle should be used on the stern cleats of the towing vessel as well if there is no tow bitt.

4) Plan the tow before connecting. Communication is critical during hook up and during the tow itself. Pick a channel on your VHF to be used for primary communications. Make sure everyone understands what will happen. Establish the person-in-charge on each vessel and the person in charge of the tow. Plan the transfer and connection of the lines. If the seas are anything other than calm, don't try to come directly alongside the other vessel to toss them the towline. There are enough problems already. The safest way to transfer line is to attach a fender on the end and another about 50 feet up (or use life jackets), let out about 150 feet of line, make a run behind the disabled boat and then run parallel to it. The line will come up to the disabled vessels stern where it can be snagged with a boat hook. Just be sure that the towline doesn't get entangled in the props of either boat and be prepared to let out slack.. The second choice is the use of a heaving line tied to the towline.

5) Periodically check the lines for chafig and check the hardware for signs of excess stress. When checking the rigging, ensure that no one stands in direct line of, or straddles, the tow line. As mentioned, if the line snaps, it will whip forward and backward, severely injuring anyone in its way.

6) Watch your speed. The tow should not exceed seven knots. A higher speed will put excessive strain on both vessels and the towing apparatus.

7) When you get in protected waters, shorten the tow line for maximum maneuverability. Be careful, however, as the vessels will not both slow at the same rate. You don't want the disabled vessel overrunning the towing vessel. The towing vessel should control the rate of speed decrease and must ensure that the tow line doesn't get wrapped in his prop(s).

8) Once near the dock, go very slowly. This is usually where the damage occurs. Work with, not against, the wind and current.

 

Try to use a face dock for landing.

All active boaters know the power of the sea. She can go from friend to foe very quickly. By following the above few steps, by using common sense and by being prepared, we can minimize the risk of being another Coast Guard statistic.

 

 

  • SOURCE: 
  • http://www.boatingsafety.com/tow2.htm
  • http://www.globalsecurity.org/military/systems/ship/tow-tug.htm
  • http://www.answers.com/topic/towing-1&