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One ton of bollard pull on a tug can pull how big of a barge?

  • I'm having a hard time understanding bollard pull ratings on tugboats. One tug I looked at had a rating of about 27 tons of bollard pull. The ship itself weights over 250 tons. So how big of a load could it actually pull. A large tug can pull ships that weight thousands of tons. So, how much does one bollard ton actually pull in terms of weight of boat/barge. Can a 27 ton bollard pull handle a 500 ton barge? I'm sure this all boils down to Archimedes principle somehow....

  • Answer:

    The Bollard Pull of a tugboat is a rating of how hard it can pull against a fixed bollard on a dock. This is not directly related to the deadweight tonnage of the vessel under tow in actual conditions. As the previous posters have correctly stated, even a small amount of force will move a large vessel. The larger the Bollard Pull the faster you will be able to reach towing speed and also slow the vessel safely. The Bollard Pull is determined by the horsepower of the engines, the shape of the propeller, and the type of propulsion system. There are trade-offs with the propeller shape, a propeller with a higher top speed will not be as powerful for close in handling. The difficulty comes when you are trying to determine if a specific tug will safely and efficiently handle a specific tow. Here are some factors to be considered: --Will the tow be on flat inland bays and rivers or on the open ocean? --What currents will be encountered? --What will the winds be like? --Does the vessel being towed have a large "sail area" like a ship or is it flat like a barge? Even barges loaded with containers are significantly affected by the wind --Are you considering just the towing or the actual maneuvering encountered in docking? Will you have assistance for this? In order to make an informed decision about your future tug I would talk to several boat brokers and to as many tug captains as you can to get their opinions on Bollard Pull, horsepower of the tug, type of propulsion, and typical deadweight tonnage handled. Do a Google search for "bollard pull" and read as many articles as you can. http://www.google.com/search?q=bollard+pull&rls=com.microsoft:en-us:IE-SearchBox&ie=UTF-8&oe=UTF-8&sourceid=ie7&rlz=1I7GGIG I have listed several below to get you started. Even though Wikipedia is not always a good source in this case it has a reasonable summary of Bollard Pull.

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It's NEWTON, not Archimedes that rules.. a body at rest remains at rest and abody in motion tends to remain in motion. I would guess about 10 to 1 for a 27 tn Bollard. You can actually move a cruise ship by standing on a dock and pushing until it eventually moves.

Jazz Handz

Yes, Newtonian physics deals with bodies in motion and at rest, while Achimedes proved that if an object is placed in a liquid it will displace exactly the same mass as the object in order to float (in particular he was seeking a way of proving an item made of gold was pure or cheapened with base metals). Anyway. If you apply a force, even very small relative to the mass of the object you are pushing against, and that object is supported in a fluid medium that in practical terms means the object has mass but no weight and minimal friction - it will begin to move away from that push. One person can pull a 15 tonne boat around a dock with a pretty small effort, and two people can move an 80 tonne fishing boat with a little patience - did it last week. Wind can complicate the issue as that is a force you cannot direct - useful if that is the direction you want to go though.... A 27 ton bollard pull could pull way more than 10 times that force - maybe hundreds of times that mass - but that very much depends on the conditions and the specific task. A couple of tugs can push a bulk carrier weighing 10's of thousands of tonnes gently up to the wharf sideways - but not into a gale! In calm conditions there is little that can't be moved with surprisingly small force as the friction is so minimal, the mass of the object is largely irrelevant except that if it takes you five minutes to get something big to move, it will take as much effort to stop it again, so tug operators are often required to spend half the time pushing, and then pulling in the opposite direction to slow the vessel down again before it belts into the dock. Depending on the situation a pilot will try and use the momentum of the large vessel to approach the wharf, and utilise the tugs as bow and stern thrusters to bring the large vessel alongside - the main engine will provide the ahead and astern thrust as required. Some modern shipping utilise thrusters and pod type main drives to be largely self sufficient - but they are very much the exception given the average age of the merchant fleets on the worlds oceans.

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