Effect of Trim on ship Resistance

Discussion in 'Hydrodynamics and Aerodynamics' started by Anum, Feb 7, 2013.

  1. Anum
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    Anum Junior Member

    Hi:

    I read the following paragraph in book "Ship Resistance and Propulsion" by ‘Anthony F. Molland, Stephen R. Turnock and Dominic A. Hudson’.

    "In the average merchant ship form, additional trim by the stern in rest condition usually results in an increase in resistance at low speeds and a decrease at high speeds. At low speeds the increased draft aft makes the stern virtually fuller, with a consequent increase in form and separation resistance, whereas at high speeds this is more than offset by the reduction in wave making due to the finer entrance in the trimmed condition."

    But it did not say anything about the effect of additional trim by bow?

    I mean for the average merchant ship form, what will be the effect of additional trim by the bow on total resistance low speeds?

    Is it possible that Comparing with even keel condition, there is a significant decrease in total resistance when is trimmed by bow.

    Waiting for comments suggestions and opinions.

    Thanks in advance
     
  2. Leo Lazauskas
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    Leo Lazauskas Senior Member

    It depends on the detail of the bow (and whether there is a bulb) and the
    Froude number, but I would think it has a similar effect to increasing the
    draft: the entrance angle would be larger, and hence the bow wave would
    be higher, and most likely, the wave resistance would increase a bit. But
    that also depends on the stern shape and how it might tend to cancel that
    bow wave. Transom sterns present difficulties at low Fr when they are not
    running fully dry.
     
  3. gonzo
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    gonzo Senior Member

    Trimmed bow down increases resistance at all speeds. Think about the way the water flows under the hull. If the aft of the bow is the lowest point, everything behind it is turbulence and drag.
     
  4. Anum
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    Anum Junior Member

    Thank you so much sir for your reply.

    Did you mean that trimming the ship by bow will always increase the resistance no matter what is the shape of bow or stern?
     
  5. Anum
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    Anum Junior Member

    Thank you sir for your response.

    If we take the case of KVLCC2M Hull which has a bulbous bow and a transom stern and the Froude number is 0.142.

    What type of behavior this hull will possibly have if it is trimmed by bow?
     

    Attached Files:

  6. Leo Lazauskas
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    Leo Lazauskas Senior Member

    I really don't like guessing, and I don't deal with these type of ships and
    very low Froude numbers. This hull has been investigated in many reports
    and papers, for example, Gothenburg 2000, and it is a benchmark hull for
    CFD. You should look up the papers to see what others have calculated and
    measured. I am fairly sure there are sinkage and trim measurements that
    have been published.

    Good luck!
    Leo.
     
  7. Leo Lazauskas
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    Leo Lazauskas Senior Member

    A very recent publication may be of interest:
    "A simple alternative approach to assess the effect of the above-water bow form on the ship added resistance",
    Ocean Engineering, Vol. 57, Jan. 2013, pp. 34-38.
     
  8. Ad Hoc
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    Ad Hoc Naval Architect

    Another very informative paper on the same lines:

    "The effect of forebody section shape on ship behaviour in waves", Swann WA., Vossers G., RINA Transactions 1961.
     

  9. Anum
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    Anum Junior Member

    Hi:

    I want to thank all the people who replied to this thread.

    Thanks for sharing your knowledge!
     
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