MadSci Network: Engineering

Re: Why do tug boats push large barges instead of pull them?

Date: Fri Sep 29 11:12:19 2000
Posted By: John Metcalfe, Staff, Computing and Information Services, Texas A&M University at Galveston
Area of science: Engineering
ID: 968046142.Eg

Good question Phil,  and it's interesting that it got forwarded to me, as I'm in 
the merchant marine. 

Anyhow, It really depends most of all on where the tug is...  In the wider parts 
of the Mississippi, they'll tow them alongside the tug.  At sea, where the 
barges are much larger, they tow them behind the tugboat at the end of a fairly 
long cable.

However, the main reason that they are pushed is because of control.  When you 
see a tug and barge, you really have to think of it as a single unit like any 
ocean-going vessel or even a small boat.  A ship is controlled in much the same 
way as an airplane, in that the movement is in direct response to a medium 
acting on the control surfaces (water flowing past the rudder).  Actually, the 
rudder does not give that much control without the help of the water that has 
just pushed against it by the propellers, hence the reason why most ships are 
difficult (at best) to control in reverse.  

Another control factor is that the rudder is as far aft (the back of the vessel) 
as possible to provide leverage (not the most technical of descriptions, but it 
works).  In fact, if you watch a boat or ship turn, you will notice that it 
turns around a point about 1/4 of the ships length forward of the stern.  If the 
tug was rigidly attached to a barge being towed, it basically could only slide 
right or left and not actually turn.  The oceangoing tugs get around this by 
using that long cable to their advantage and dragging the front of the barge in 
the direction they want to go (at the expense of fine control).  Obviously that 
wouldn't work though in a river or the Inter-Coastal Waterway.  

Hopefully, I've answered your question well enough for you,
Fair winds, and following seas,

John Metcalfe
3rd Mate, Unlimited Tonnage (Ocean)

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