1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Required force to move mass at a certain velocity in water

  1. Sep 7, 2014 #1
    I'm trying to figure out how much force is required to move a 300/350lb object across the top of water and maintain a certain velocity. My goal is to do a minimum of 20mph but depending on the size of the engine and weight considerations 30-35mph would be ideal.

    From some of my research I realized water has 3 different drag coefficients until you are planing on the waters surface.

    300lbs and 30mph converted and if I've applied f=ma right then:


    converted to 5502.196lbs of force required to accelerate to 30mph. This doesn't take into account drag created by water. I would say that obviously that much force is not required to maintain that velocity. So I suppose the question should be, how much force is required to maintain 30mph in a fluid environment?
  2. jcsd
  3. Sep 7, 2014 #2


    User Avatar
    2017 Award

    Staff: Mentor

    You cannot convert a velocity to an acceleration, those are completely different things (in the same way you cannot convert a mass to time: how many hours are 1kg?). A force that is sufficient to maintain a fixed velocity will also be sufficient to (slowly) reach that velocity over time. This force depends on details of the object shape, there is no easy answer for it if you have a "ship-like" object.
  4. Sep 7, 2014 #3


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    Unless we know more about this 'object', it is difficult to give you an answer on how much power will be required to move it at 30 mph. Is this a boat, a box, a rock, what? Is it floating on the surface of the water, submerged, what?
  5. Oct 2, 2014 #4


    User Avatar
    Gold Member

    Look for the book
    Fluid Dynamic Drag - S.F. Hoerner
    where you can find all sorts of good, empirical information about fluid drag.
  6. Oct 2, 2014 #5
    Hadn't seen the older reply. It's floating on the surface of the water. Surf board like object.

    Thank you for the book recommendation, I'll check it out.
  7. Oct 7, 2014 #6
    This is entirely related to the shape of the object (pressure drag) and the skin friction (related to wetted area), and thusly if the object has planed or not (surf board type craft at 35mph should definitely be planing). And of course drag from the incoming air. So have your fluids set at your desired velocity, and find out what the drag is. This will be your force required, since they are what you need to overcome.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook