Why do tug boats use very long tow lines?

Question

I've noticed that when tug boats tow in large container ships near the shore, they use extremely long tow lines that appear to be 1/4 mile long. It would seem that using a long tow line gives them less control to steer/pull/direct the container ship, so there must be a reason why the tow line is so long.

Answer 1

The term you need to understand is "CATI-NARY". It is the dip in the tow hawser that actually supplies the power to tow the boat. It acts as a spring and as it dips deeper it allows the towing vessel to move forward and the towed vessel to spring toward the towing vessel. I have misplaced the calculation formula, but the rule of thumb was 10 times the length of vessel being towed divided by the speed required of the tow vessel ( I think this was for 10 kts). You also need to take depth of catinary into consideration when reaching port and adjust tow distance and speed accordingly.

Answer 2

The tow line should be long for two reasons; first a low hawser dampens the shock of towing. Second, the tow should be long enough to have both vessels in sync with the wave action. Towing thousands of tons tends to stretch wire rope or synthetic hawsers. A long tow lessens vibrations. As a line stretches a lot of heat is generated through friction. Tow lines are not cheap, so all is done to prolong its life. When towing both ships should ride the waves together. Otherwise one ship will be high and other ship low. Thousands of tons falling 20-30 feet puts a heavy strain on the tow ship.

Answer 3

If the tow is a dead ship(no power), or is a barge, and the tug is towing off shore, the towline(generally steel cable on modern tugs), is payed out at a length proportionate to sea conditions, water depth, force applied by the tug, and sea room. Where there is plenty of sea room ie the open ocean, a certain amount of wire is payed out to create a fair amount of catenary, but not enough to ground the wire. In a seaway, the catenary acts as a shock absorber when waves hit the two vessels independently, and the vessels surge. On the west coast, heavy chain, called surge gear is also connected between the tows towing pendant, and the tugs Hawser or tow wire to help this action. A tow Hawser is a synthetic fiber line usually having a fair amount of stretch, and the abiltity to float for easier retrieval. When the tow enters a harbor, the tug shortens up the wire(it will be out of the water), for greater controll in confined water, and narrow channels, eventually changing tow mode to get the tow to the dock. A container ship or tanker proceeding under her own power when entering a harbor will pick up docking tugs. If they are tractors or SDMs(Ship Docking Modules), they will generally be made up on the bow and stern to help steer around tight turns, maneuver off the dock, and or stop the ship. These types of tugs generally have their tow hawser on a winch, and are highly maneuverable. Their hawser length varies depending on their orders and can be easily adjusted. Traditional tugs are generally made up alongside a ship, and must shift around to get a desired effect.

Answer 4

The cryptic answer from Phone E is actually correct. A towing hawser (a heavy tow line) will stretch a bit with the stress of the waves acting on the tugboat and on the towed vessel. A short hawser will put tremendous stresses on the two vessels, and it is quite possible for the line to break. A long hawser multiplies the stretching capability of the line, at the same time adding its own weight (hanging in the water) to absorb the shocks of the seas.

Answer 5

All they do is tow, the ship does it's own steering. When towing something that has no brakes, use a long tow rope. In this case though it depends on the size of the ship and how much space is required to stop by reversing the engine.

Answer 6

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I tried towing a 18 ft. boat with a 4 cylinder ford ranger.Every time I tried to shift out of third gear it would bog down.When I read the owers manual, it said towing was not recommended.

Answer 7

Will probably overwork motor , trans , brakes. Forget about pulling it up a launch ramp . I've seen too many guys with full size 2wd try to pull an 18' out of the water . Only way to go is 4wheel . Unless you want to be the main clown on the boat ramp circus .

Answer 8

they tug big boats and the lines as long as they are give them the power to pull (it is an equation of geometry) the angle of the rope is equal to the weight of the big boat or so (not one of my better sub..)

Answer 9

Short lines break.

Answer 10

if those big ships get to swinging out of control for some reason, they won't get hit with it.