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Drag vs Depth

  1. Nov 7, 2007 #1
    Hi, I have been trying to understand the formula for the drag of an object vs the depth of the object in Water. The variables are Velocity, Mass, Weight. Velocity being in Miles per Hour. Mass taken from the cross section of the weight as it moves through the water and the mass of the line used to drag the weight (diameter x length). Weight being comprised of lead ( the line is made of spectra and does not factor into weight ).

    Velocity 3 mph
    Line length ( diameter being .009" )
    Weight 5 ounces
    Mass ( line diameter x distance ( weight diameter .5" )

    How do I calculate depth given a water temperature of 60 degrees constant.

    Here is the closest I have found on the web, unfortunately they do not allow for line diameter of spectra

    http://www.geometrics.com/TowDepth.htm [Broken]

    This formula has eluded me and I am hoping someone can help.

    I am a new member and this is my first post!

    Last edited by a moderator: May 3, 2017
  2. jcsd
  3. Nov 7, 2007 #2
    i am cofused! what do you want to know?
    the drag force on an object is independant of its mass, it depends on its geometry?

    looking at your link, i'm thinking your towing an object through water using a tow line and you wish to know what angle the tow line makes with the horizontal??

    you need to balance forces in the horizontal and vertical.
    for verticle you'll have weight of the object, the bouyancy, and the verticle component of tension in the tow line

    for the horizontal you'll have the horizontal component of the tension in the line, and the drag force working against it!

    the drag of an object is .5*density*v^2*Cd*A
    density is the density of the fluid your in [water?]
    v is its velocity, and A is the FRONTAL area (the area of its shadow would cover)
    Cd is a drag coeficient which can be looked up from tables and will depend on the SHAPE of the object.
  4. Nov 8, 2007 #3
    Hi what I would like to know is a formula to know the depth attained by the sinker given the following variables

    Velocity 3 mph
    Line length 100 feet ( diameter being .009" )
    Lead Sinker being 5 ounces in weight
    Mass ( line diameter x distance ( weight diameter .5" )

    How do I calculate depth given a water temperature of 60 degrees constant.
  5. Nov 9, 2007 #4
  6. Nov 9, 2007 #5
    I very much appreciate your responses.

    The general problem is dragging a towfish through the water doesn't produce a straight angle. the line curves similar to that of the curved line when flying a kite. Initially the weight of the cylindrical object causes a direct angle from the source to the weight. As the line extends further out the mass of the line starts to have an effect and thus produces a curve in the line from the source to the weight. A line that is .01" may seem insignificant however at 100" that line is causing one square inches of mass and at 900 inches of line out there is now 3 square inches of drag. This is what causes the curved shape in the line to the weight being towed.

    I am still unsure of the formula to show

    line out (causing line mass drag)
    frontal area of the weight (causing weight mass drag)
    Water temperature of 60degrees
    = what depth

    Thank you for your responses!
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