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#1
03-05-2013, 11:17 AM
 mitch10284 Join Date: Mar 2013 Rep: 10 Posts: 2 Location: St Louis
Drag Forces on a Flat Face Barge

Hello. I'm working on constructing an anchoring system for Barges on the MississippI River in St Louis and need to determine the force the river will impose on an anchored barge. I'm using the Drag Force Equation

F = 1/2 x velocity^2 x density x Area x Cd

Assumptions

Drag Coefficients
Cd = 1.98 for the bow
Cd = .005 for bottom and sides

Density = 62.4 lb/ft^3 for water

Barge is 35'x195' and sits 11' in the water

I've been told by a few Barge Captains the river can reach 26 ft/s during a strong flood. I'm designing for worst case so I've used this number.

The problem I'm having is that the drag force numbers I'm getting seem to be way high.

Bow - 16,576,329 lb
Sides - 466,433 lb
Bottom - 742,053 lb

Hoping someone can confirm these numbers are incorrect and can help me out.

Thank You
#2
03-05-2013, 11:30 AM
 Submarine Tom Previous Member
Isn't water density 1.0?

Don't discount windage.
#3
03-05-2013, 11:36 AM
 mitch10284 Join Date: Mar 2013 Rep: 10 Posts: 2 Location: St Louis
Using US Customary Units and the Mississippi is fresh water in St Louis.

Regards
#4
03-05-2013, 12:00 PM
 TANSL Senior Member Join Date: Sep 2011 Rep: 300 Posts: 3,901 Location: Spain
Can be Cd bigger than 1?
#5
03-05-2013, 12:39 PM
 DCockey Senior Member Join Date: Oct 2009 Rep: 1485 Posts: 3,694 Location: Midcoast Maine
Quote:
 Originally Posted by TANSL Can be Cd bigger than 1?
Yes. Nothing special about a Cd value of 1. The table in my old fluid mechanics textbook shows a drag coefficient of 2.00 for an infinitely wide flat plate normal to the flow, 1.20 for a rectangular flat plate with an aspect ratio of 5 normal to the flow, and 1.10 for a square plate normal to the flow. Those are drag coefficients for an object completely immersed in an "infinite" flow.
__________________
David Cockey
#6
03-05-2013, 12:44 PM
 DCockey Senior Member Join Date: Oct 2009 Rep: 1485 Posts: 3,694 Location: Midcoast Maine
Quote:
 Originally Posted by mitch10284 Hello. I'm working on constructing an anchoring system for Barges on the MississippI River in St Louis and need to determine the force the river will impose on an anchored barge. I'm using the Drag Force Equation F = 1/2 x velocity^2 x density x Area x Cd Assumptions Drag Coefficients Cd = 1.98 for the bow Cd = .005 for bottom and sides Density = 62.4 lb/ft^3 for water Barge is 35'x195' and sits 11' in the water I've been told by a few Barge Captains the river can reach 26 ft/s during a strong flood. I'm designing for worst case so I've used this number. The problem I'm having is that the drag force numbers I'm getting seem to be way high. Bow - 16,576,329 lb Sides - 466,433 lb Bottom - 742,053 lb Hoping someone can confirm these numbers are incorrect and can help me out. Thank You
You need to divide by the "density" of water of 64.4 lb/ft^3 by 32.2 ft/sec^2 to obtain the mass density of 2.0 slugs/ft^3. That will reduce the drag force numbers by a factor of 32.2.
__________________
David Cockey

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