Large keel to displacement power boat?

Discussion in 'Stability' started by ScottA, Jul 6, 2014.

  1. ScottA
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    ScottA Junior Member

    I am researching stability to displacement powerboats.
    It seems lateral stability(rolling) to sea going powerboats is a concern that can be improved with stability devices such as paravanes flopper stoppers ect. Why not just fix a large keel like a sailboat to the powerboat displacement hull for stability- As this gives the stability to the sailboat when being operated under power? as they quite often are, or is it a rolly ride until the sails can be used to (I assume) to use the wind to exert force to the sails to stop a roll, just one side lean?
    Thanks Scott
     
  2. gonzo
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    gonzo Senior Member

    It's been done since powerboats existed. A large keel is not a good solution because of the extra drag; a pair of bilge keels is the usual arrangement.
     
  3. TANSL
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    TANSL Senior Member

    What are you really talking about, to increase stability or increasing the period of balance?.
    While this may be debatable, increase stability is achieved by increasing the righting arm, GZ value. In this, bilge keels have no influence.
     
  4. ScottA
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    ScottA Junior Member

    Thank your for your replies,
    Thought the large keel would be associated with drag.
    So why is the drag issue, not an issue on sailboat?
    Sorry about the elementary questions just trying to learn something
     
  5. gonzo
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    gonzo Senior Member

    A sailboat can't point into the wind without a keel. It also needs the ballast to stay upright. Neither of those are necessary in a powerboat.
     
  6. daiquiri
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    daiquiri Engineering and Design

    It is a big issue in sailboats too. They need keels to balance the side force created by the sails, so a lots of time is spent in the design stage to find the optimum sail/hull/keel shape and configuration.

    Your question is indeed elementary but the answer to it can be of extremely complex nature, depending on how much one wants to go into details. The keel design involves aerodynamics, hydrodynamics, hydrostatics, mechanical resistance, stability, course stability and boat handling. All of it at the same time. :)
     
  7. Milehog
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    Milehog Clever Quip

    Generally a sail boat without a mast has a snappy, miserable motion. The polar inertia of the mast eases the motion.
    This needs to be considered when contemplating a ballasted power boat.
     
  8. TANSL
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    TANSL Senior Member

    This is a completely new idea to me.
    I understand that the polar inertia Ip of the mast is
    Ip = Ix + Iy
    Can someone knowledgeable about it, give an explanation of this effect?. Thank you.
     
  9. gonzo
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    gonzo Senior Member

    A longer arm has a larger but slower movement which is more comfortable.
     
  10. TANSL
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    TANSL Senior Member

    Thanks for trying, Gonzo, but I do not understand it. What is the "longer arm"?, what movement you mean, and what does that have to do with the polar moment of the mast?. Is the polar moment of mast something completely different to what I think?. Maybe I'm not prepared to understand the obvious; if so, please, do not waste your time with me.
     
  11. gonzo
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    gonzo Senior Member

    A longer arm has a larger moment of inertia without increasing the weight. For example, a clock pendulum has a longer period if the weight gets moved further out.
     
  12. daiquiri
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    daiquiri Engineering and Design

    TANSL,

    I just don't believe you when you say that you do not understand the concept of the moment of inertia and how it is related to the roll period. I think that you are just checking out if the others are able to explain it in scientific terms... :) So, for the sake of general discussion I am biting the bait. ;)

    The angular speed Omega of the rolling boat is given by the:
    Omega = 2Pi / T = sqrt (∆ GM / Ix)
    or
    T = 2Pi sqrt( Ix / ∆ GM)
    where:
    T = period
    ∆ = displacement
    GM = metacentric height
    Ix = boat's moment of inertia around x-axis.

    A mast can be approximated as a homogeneous rod which rotates around its base. Its moment of inertia is hence given by the following equation:
    Ix,m = 1/3 m L^2
    where m is the mass of the mast, L is its length (height).

    So, if the moment of inertia of a boat without the mast is Ix,0 and if we assume (in order to simplify things) that the CoG of the boat coincides with the base of the mast, the boat complete with the mast will have a moment of inertia equal to:
    Ix = Ix,0 + Ix,m

    The roll-period equation becomes then:
    T = 2Pi sqrt[ (Ix,0 + Ix,m) / ∆ GM]
    The equation shows how the term Ix,m (the addition of the mast) increases the roll period - just like any other mass placed high above the CoG would do.

    Cheers
     
  13. ScottA
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    ScottA Junior Member

    Thank you all who participated, I understand this topic at lot better now
    Regards Scott
     
  14. TANSL
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    TANSL Senior Member

    daiquiri, thanks for your explanation. In my post # 3, I asked if he was talking about rolling period. So yes I had an idea of where things were going.
    But , when I read this :
    many doubts arise to me and I do not intent to find out what others know or do not know, but I do not understand what does the polar moment of the mast with the boat moving.
    I fully understand that the moment of the mast with respect to the longitudinal axis of rotation of the ship influences the balance period. Also be taken into account that the weight of the mast up the position of the center of gravity of the ship.
    It is clear that what you all call polar moment of the mast is not what I meant. So I asked if Ip = Ix + Iy. To calculate the moment you call "polar", do not apply my formula, but Steiner's theorem.
    With all sincerity, I thought it was escaping to me a concept that was obvious to others.
    I can understand things in general, with little explanation.
    Thank Slavi, for your time. No, I do not intend to belittle anyone.
    Cheers to all you
    Ignacio
     

  15. daiquiri
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    daiquiri Engineering and Design

    Scott, you don't need that math to get the answer to the question. It was for TANSL. :)
    Your question was most succinctly answered by Gonzo in the post #4.
    Fitting a large keel on a powerboat can be done technically, but is not being done practically for several reasons. Most importantly because of the added drag, structural weight and because of the significantly increased draft. A retractable keel reduces the useful internal volume. So the cover is always too short, whichever way you pull it. :)
    One way to reduce rolling motions which uses hydrodynamic principle of sailboat's keel are fin stabilizers. You can find some examples here:
    http://www.rolls-royce.com/marine/products/stabilisation_manoeuvring/stabilisers/aquarius/index.jsp
    https://www.mhi-global.com/products/detail/mme_prod_fin_stabillizer.html
    http://www.krosys.com/product/product_view.php?menu_idx=3&part_idx=55&idx=19

    Cheers
     
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