Tugboats that use a cable hawser to make tow are known as wire boats.  They are used on open water and oceans for towing large barges and ships.  The large cables used in this towing require an engine powered winch to lay out and haul in and they have to be protected from the abuse of the weather, waves, and steel of the stern.  It's a very efficient towing technique but the random movement of so much heavy gear on a rolling deck makes it more dangerous than other methods.

To tow on the wire, a winch or winches, a huge cable, some towing gear (bridles, pendants, large shackles, and etc.), and a system to hold the cable to the stern are all that are needed to make a long distance tow.

The typical order of operation is simple.  The captain backs the tug's stern up to or near the tow.  Someone on the tow will pass a messenger line (pick-up line) to the crew on the tug's stern.  They will put the messenger on the large capstan and use it to heave the heavy towing arrangement up and over the stern railing and onto a clear part of the deck.  The arrangement is secured so it won't fall over the side prematurely and the thimble end of the cable is made up to it used towing shackles.  After the gear is secure, the tug will move away from the tow, allowing the gear on deck to pass over the side and into the water.  When the gear is clear of the side, the captain moves away until the cable and towing gear is up and tight.  Then he will use the winch to pay out cable to get the desired towing length for the conditions and waters.  "In close" for rivers and then "strung out" once they are at sea.

A complete set of tug controls on the left and winch controls on the right.

This is the after steering station.  The captain can steer the tug from here and the console shares the winch controls.  Sometimes, this console is in a protected booth called the dog house.

The towing winch has one or two drums of cable.  Here is the capstan side of the winch.  Most winches have a capstan built on and controlled by the winch station.  If not, there is one nearby to haul up the towing gear and get it on deck where the crew can work with it.

 

The cable is very large.  The average diameter of a cable used in towing on most tugs is greater than 2 inches.

Here is the thimble end of the cable that will be attached to the towing gear using large shackles. 

After the tow is connected, the towing gear and a bit of cable are sent back over the side for the ride out the open water where the cable will be strung out.

A great concern to the crew of a wire boat is the care of the towing wire.  Miles of towing across rough seas can chafe the cable to a point where it will have to be replaced.  This is extremely  expensive.  There are two common ways to relieve the wear stress on a large cable.

Depending on where the boat is from, this is a Texas bar or a Dutch bar.  The two wheels in the middle there are called doughnuts and they can slide separately the entire length of the bar.  A doughnut is slid down to the extreme end and the captain steers the boat so that the stern runs under the cable, which is by now hanging over the side.  As the cable tries to slide across the stern rail, it falls into the doughnut and is trapped.

Now the cable and the doughnut will continue together to the middle of the bar.  The crew pass a small chain around the doughnut and cable to keep the cable from jumping out.  On long trips, the cable is paid out a few feet every so often to keep the same place from riding in the doughnut for too long.

Here is another example of gear to hold the cable in place.  These rollers on the stern of this tug are able to be retracted.  With one roller up, the cable is steered into the center of the stern as described before and then the second roller is raised up to hold the cable to the middle of the stern rail an on the horizontal roller seen between the two roller there.

Here is a shot of the cable in place between the rollers.  You'll notice a piece of pipe has been used for chafing gear.  This kind of gear is made by cutting a section, length-wise out of a two or three foot long pipe.  The slot allows the cable to slip into the pipe.

Cable clips like this one have been welded to the inside of the pipe, attached by the curve of the u-bolt (top of image). The u-bolt is secure with the two threaded ends ready to accept the cable.  The saddle (bottom of image) is fastened down and the device clamps tight around the cable.

Here is another shot of a towing cable over the stern with no roller system.  The chafing gear can be seen more clearly and a hogging line has been added to hold the cable more firmly in place.