How to calculate drag coefficeint for a small toy boat

Discussion in 'Boat Design' started by bhanu prakash, Dec 16, 2021.

  1. bhanu prakash
    Joined: Dec 2021
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    Location: India

    bhanu prakash New Member

    As a part of the final year project, we made a small toy boat (20cm x12cm), the main goal is to calculate the drag coefficient for this small toy boat, we used Holtrop Mennen empirical formulas to calculate the drag coefficient and wave-making resistance (although wave-making quite small for this toy boat), But our supervisor told us that those formulas were experimentally determined for big ships, this created doubt among us whether we are right or wrong. Please suggest to us how to calculate the drag coefficient for a small boat (like an RC boat controlled using a small DC motor and propeller)
  2. Olav
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    Olav naval architect

    bhanu prakash,

    I doubt that the Holtrop-Mennen resistance estimation is applicable here. Without having any idea on the shape of your boat (maybe you can post some drawings or pictures?) I see at least a problem with the L/B ratio which is way outside the valid range for H-M (L/Bmin = 3.9; your boat has L/B = 1.67 under the assumption that the dimensions stated above are waterline length and beam). I suggest you reconsider the resistance estimation method for your application.

    Your supervisor is right, the H-M formulae will give questionable results for a small toy boat as yours (apart from being outside the L/B range and probably in other aspects, too). My suggestion is the following:

    - Estimate the resistance of a "scaled up" version of your boat (i.e. assumed as a boat with 20 m length and 12 m beam or the like) using an appropriate method for your hull form
    - Calculate the model resistance using something like the ITTC '57 or ITTC '78 method "in reverse"

    The result then should still be taken with a grain of salt (to say the least!), but may be in the right ballpark though.

    Apart from any number crunching I reckon the easiest and most accurate way would be to just tow the model boat in a swimming pool and measure the resistance with a (very) sensitive spring balance.
  3. bhanu prakash
    Joined: Dec 2021
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    bhanu prakash New Member

    Thank you for your reply.
    Our toy boat was made through stainless steel sheet metal which was having a smooth flat bottom (a very basic one, our goal was to find the drag coefficient for this semester, for next semester, will consider all the designing parameters for the boat's hull). Can you please elaborate on that easiest and most accurate method you mentioned about towing the boat in the swimming pool. How to perform that experiment?
    ~our boat has an A2212, 1000kv DC brushless motor, by controlling PWM we are able to control the RPM of the motor and the speed of the Boat.
    ~ we have one dynamometer which measures up to 30N.
    ~ Some suggests running the boat at constant velocity and stopping the motor and noting down the distance and time till the boat stops moving.
    how to calculate the drag coefficient of a toy boat which was having a flat surface or (for the next experiment we were trying to use PVC pipe, cylindrical shape)

    Please elaborate on that towing experiment?
  4. HJS
    Joined: Nov 2008
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    HJS Member

    Attached file shows how Edmond Bruce tested very small models.

    Attached Files:

    Olav likes this.
  5. Heimfried
    Joined: Apr 2015
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    Heimfried Senior Member

    Search for "Bruce Tank". For example AYRS booklet No. 30 (Pages 26 ...), free download here:
    Booklets – Amateur Yacht Research Society

    The measurements of resistance were made 60 years ago and time measurement was far more complicated than now. But the article contains a lot of very useful information.

    Edit: While I was looking for the source and typing my answer, HJS got it.

  6. bhanu prakash
    Joined: Dec 2021
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    bhanu prakash New Member

    Thank you very much HJS,
    The attachment really helped us to go in the right direction.
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