center of flotation calculation and implications?

Discussion in 'Boat Design' started by capt vimes, Jan 7, 2010.

  1. Eric Sponberg
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    Eric Sponberg Senior Member

    Correction

    The table above got messed up because the post window does not recognize extra spaces or tabs too well. Here it is again, more clearly:

    Speed/Length ratio Cp
    1.0-------------------------0.52

    1.1-------------------------0.54
    1.2-------------------------0.58
    1.3-------------------------0.62
    1.4-------------------------0.64
    1.5-------------------------0.66
    1.6-------------------------0.68
    1.7-------------------------0.69
    1.8-------------------------0.69
    1.9-------------------------0.70
    2.0-------------------------0.70

    Eric
     
  2. TeddyDiver
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    TeddyDiver Gollywobbler

    Sectional Area Curve

    for reference..
     

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  3. frank smith
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    frank smith Senior Member

    Eric, good lessons , thanks very much.
    Frank
     
  4. fredschmidt
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    fredschmidt Naval Architect

    CF, pushing down the bow

    A interesting case is a IOM, International One Meter (radio control), a little one meter sailboat.

    The pushing down the bow occur also in downwind caused by the sails forces and studding this behavior I have other view to look this occurrence and the CF.

    The moment to face the moment introduced by the sails that causes the bow trim, is achieved by the position of CB. How quickly the CB moves forward less the trim. This is achieved with a more better volume distribution in the forward part of the hull. If I have a good volume near the forward position of CB, since the trim initiate, quickly the CB reach a position that assure the righting moment with low trim.

    The position of CF forward in this case is achieved by consequence of the volume forward. CF is a consequence not the goal, the goal is when heeled or in trim we need have forward volume sufficient to establish a quickly resistance moment.

    The great property of CF is to be the center where the transverse rotation axis pass, the axis where the trim turn.
     
  5. Leo Lazauskas
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    Leo Lazauskas Senior Member

    When is the centroid of wetted surface area an important design consideration?

    Thanks,
    Leo.
     
  6. Landlubber
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    Landlubber Senior Member

    again Eric, thank you for your considerations and time.
     
  7. Eric Sponberg
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    Eric Sponberg Senior Member

    The centroid of the wetted surface area really isn't important, except perhaps when you are trying to assess the center of any weight, such as the bottom laminates or plating, or the weight of the anti-fouling paint on the hull. Other than that, it is not used anywhere.

    Eric
     
  8. Leo Lazauskas
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    Leo Lazauskas Senior Member

    Thanks, Eric.

    I have only seen one reference to the centroid of wetted area and that was
    in equations for equilibrium conditions involving dynamic sinkage and trim.

    In those (lengthy) equations it is used in a similar way to accounting for stabilisers, e.g. a moment involving the drag of a stabiliser and the vertical lever arm to that stabiliser.

    The vertical arm for the frictional drag is measured to the centroid of
    wetted area. This factor is quite small in most cases so I'm not surprised
    it isn't used much. I thought it might be important for yachts at large heel
    or other rakish angles, like your cap.

    Leo.
     
  9. fredschmidt
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    fredschmidt Naval Architect

    centroid of wetted area

    Sincerely, I never thinked in the action of the frictional drag. I only thinked that we need a minimal wet area for a minimal frictional drag.
    But really the centroid of wetted area can be important if we think in the action of the frictional drag force.
    This force introduce moments in the boat in any condition of trim and heel and is possible that we can study a better position for the centroid of wetted area to minimize the effects of these moments.

    Very interesting.

    Thanks Leo
     
  10. Leo Lazauskas
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    Leo Lazauskas Senior Member

    I thought so too, but I usually deal with thin, slender hulls where all trim angles are assumed to be small, so I have not had any incentive to research the effect on yachts.

    I suspect it should NOT be used with the standard friction calculations using the ITTC line, e.g. RF=CF*0.5*rho*S*U^2.

    It might be better to use the local skin friction, cf (not CF) on the wetted hull surface. cf is the x-derivative of CF with respect to distance along the hull. The local Reynolds number should also be used, rather the Rn based on the LWL.

    Of course, the skin-friction estimated either way is mostly empirical, so I could be over-refining the problem. :)

    Have fun!
    Leo.
     
  11. fredschmidt
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    fredschmidt Naval Architect

    OK Leo.

    Not thinking in the amount of the frictional drag, but in his action:

    If we decompose the frictional drag in the x, y, z axis we have the three moments possibles.

    In x, we have a moment that trim the boat. The arm is the distance of the action line of the force to the CF.

    In y, we have the moment that turn the boat in a horizontal plane modifying the yaw angle.

    In z,we have the moment that also trim the boat.
    The arm is the distance of the action line of the force to CF.

    I was thinking in study the position of the centroid of wetted area to minimize these effects, like approach the centroid to CF to minimize the moments. Study this situation.

    Naturally this involve the shape of the hull.
     
    Last edited: Jan 18, 2010
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  12. Leo Lazauskas
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    Leo Lazauskas Senior Member

    I understand what you mean.

    Are all of the moments of the skin-friction necessarily bad? Can one or more of the three moments arising from friction actually be beneficial in some cases because they cancel other moments, for example from the sails, rudder, etc?

    Leo.
     
  13. fredschmidt
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    fredschmidt Naval Architect

    Leo

    The moment in x is bad because they introduce trim by bow as the drag force is directioned to stern and the wetted surface centroid is down to CF, pushing the bow down.

    The moment in z depend on the position of the centroid relative to CF and his direction to up or down.

    The y component that tends to modify yaw I don't visualize clearly, but is interesting if the drag force introduces some benefits.

    Fred
     
    Last edited: Jan 18, 2010
  14. Eric Sponberg
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    Eric Sponberg Senior Member

    I would be very surprised if the center of wetted surface drag played any significant role in boat behavior. The more important factor is the wetted surface itself--in sailing yacht design we frequently like to minimize it. We also know that long narrow hulls have less drag than short wider hulls of the same wetted area. The volumetric effects play a much bigger role, and so the centers of volume and gravity have an overshadowing effect. We don't really care where the center of wetted area is because it usually is very closely in line with the direction that we are moving and drag forces due to the wetted area will not be located at the center of the wetted area. Rather, they will be governed more by the distribution of volume and the centers of volume. Center of wetted area falls out of the problem because it is so insignificant and does not play a role in where the centers of forces actually are. You can certainly study it more, but I think that is what you will find.

    Eric
     

  15. fredschmidt
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    fredschmidt Naval Architect

    Hi Eric

    "I would be very surprised if the center of wetted surface drag played any significant role in boat behavior. The more important factor is the wetted surface itself--in sailing yacht design we frequently like to minimize it. We also know that long narrow hulls have less drag than short wider hulls of the same wetted area."

    I thought also like you. But Leo put me in doubt. Long narrow hulls, short wider hulls have different Reynolds number then different flow conditions and in consequence, different drag.

    "!The volumetric effects play a much bigger role, and so the centers of volume and gravity have an overshadowing effect. We don't really care where the center of wetted area is because it usually is very closely in line with the direction that we are moving and drag forces due to the wetted area will not be located at the center of the wetted area."

    The wetted area is a shell in 3D and on heel we do not have symmetry. The IOM design open my mind to any aspects, because is very competitive.
    I personally do not know the real direction of drag force in the sense of down or up. For astern is clear. I think will be interesting if we have a study in this level: what is the importance of the drag force in y and z directions.

    Things that are valid for great boats is not valid for IOM, exactly all that is affected by Reynolds number. as example for the keel and rudder foils we need use 6% that is more appropriated for low Reynolds number.

    The foil for the bulb also has to be different like 1/10 diameter to cord.

    Still I think like you, but, I do not would surprised if we discover the importance of these aspects. May be.

    "Rather, they will be governed more by the distribution of volume and the centers of volume. Center of wetted area falls out of the problem because it is so insignificant and does not play a role in where the centers of forces actually are. You can certainly study it more, but I think that is what you will find."

    Ok, but distribution of volume and center of volume is attached to Cp, that is a important factor to the study of punching the bow down and is more important for wave resistance.

    I agree with you, but.... Leo put me in doubt.

    Fred
     
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