Planing hulls that are Semi-displ.

Discussion in 'Boat Design' started by rustybarge, Nov 21, 2013.

  1. rustybarge
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    rustybarge Cheetah 25' Powercat.

    Moved.....
     
    Last edited: Nov 22, 2013
  2. groper
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    groper Senior Member

    You still don't get it do you?

    The key difference between a displacement and planing hull is as follows;

    The displacement hull relies on buoyancy alone to support the vessel whilst underway.

    A planing hull utilises a combination of buoyancy AND hydrodynamic LIFT to support the vessel whilst underway.

    It doesn't need to be anymore complicated than that as far as a definition goes.

    Semi-displacement is a side show, it's not worth discussing the differences or whether its even a valid term or not, it's more of a marketing term than anything else.

    So asking about hulls that are displacement AND planing is absurd, they are mutually exclusive in their physical form, i.e. they CAN generate hydrodynamic lift or they can NOT. It's that simple.
     
  3. rwatson
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    rwatson Senior Member


    That doesnt make sense to me.

    All hulls generate hydrodynamic lift when power is applied. Even fully displacement hulls lift their bow at speed for example, either from prop or sail power.

    A planing hull will show a distinct drop in drag at certain speeds as it gets up on top of the water, while a displacement hull just gets out of the water to a lesser extent.

    There are plenty of semi-displacement hull designs.
     

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  4. Mr Efficiency
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    Mr Efficiency Senior Member

    Groper's response was "semi-hostile". :p
     
  5. nzboy
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    nzboy Senior Member

    The only obvious difference between the s/d and planing craft is a deep forefoot on a semi displacement to give better directional control at low speed and prevent lift at low speeds .A modern take is seen in deepish fine entrys ,plumb bows with a straight run aft essentially a planing hull but good at low speed similar to designs in the 1910-30 era which simply didn't have access to the horsepower required to plane. As to the argument of seaworthiness .A planning hull will unlimited power and directional steering combined with active piloting is the boat of choice for lobster boats, pilot and rescue .So a definition of seaworthiness could be if you lose power and steering could your boat remain floating and upright in an 8 metre swell . Our skipper in the said article wasn't that confident
     
  6. groper
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    groper Senior Member

    No, not all hull generate lift rw, most full displacement hulls will generate negative lift or Sinkage when more power is applied... your photo shows a planing hull....

    The only worthy distinction of a s d hull would be the degree if lift it could generate or where its lift is limited or not maximized. But it gets into a very grey area and isn't worth the semantics IMHO. Hull shapes can be more varied than sand granules on a beach....

    Lastly, just because a planing hull form is underpowered doesn't make it semi or full displacement hull form- its just that its operating in a disp regime...
     
  7. rwatson
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    rwatson Senior Member

    The photo is a semi-planing hull. Because its also designed to sail. It cant get over the hump, but its hull bottom flat enough to derive some increase in performance at a certain speed.

    According to you
    "displacement AND planing is absurd, they are mutually exclusive in their physical form"

    Is my area of disagreement.

    They are NOT mutually exclusive. ALL hulls will generate lift under power, whether you can see it or not, its just physics. Even heavy commercial ships generate the lift - its just not enough to offset the huge weight of the hull.

    Lots of hulls will generate significant amounts of lift, enough to improve performance, but not enough to support the full displacement of the boat. These are semi-planing.

    You are just being dramatic trying to push the 'black and white' scenario, and confusing the issue for people trying to understand the phenomena.

    ALL hulls
     
  8. groper
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    groper Senior Member

    Again, you're wrong rw... Most full displacement hulls will generate negative lift or sinkage... Do a search on the term "sinkage". Then come back and we can continue...
     
  9. rwatson
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    rwatson Senior Member


    What I said was - "all hulls with any velocity generate lift"

    I didn't expect to have to explain in detail the concept of NET lift, based on the combination of positive lift V negative lift.


    The amount of negative lift can exceed positive lift

    eg: http://en.wikipedia.org/wiki/Squat_effect

    but the lift component is always there - it cannot be removed, only negated or exceeded.




    If you look at a post I made on the subject recently, you will see I understand about the effect of negative lift - Post 33

    http://www.boatdesign.net/forums/bo...oday-javelin-14-9-mph-48787-3.html#post662157




    Even if a hulls NET lift is negative - IT STILL HAS A LIFT COMPONENT PRESENT.

    http://www.academia.edu/1117516/HIG..._RESISTANCE_PREDICTION_METHODS_AND_COMPARISON

    When the vessel speed is equal to zero (V = 0), the force applied by water to the hull structure is hydrostatic pressure, and will be equal to the floating weight. Conversely, as the body begins moving (speed is greater than 0), water particles are put into motion by the force applied to them. The effect of the force in the opposite direction creates another force known as hydrodynamic pressure. Hydrodynamic pressure forces can cause two different drags. The first one is known as viscous pressure drag, the second is known as wave drag. The component of a pressure through the body results in frictional drag, and the vertical component of the pressure leads to an elevation of the hull...... and trim. As the speed increases, the vertical pressure component, which is known as lift force,


    BUT - This is completely irrelevant to the thread topic. We are not discussing the effects of hull shapes on lift or sinkage - we are discussing whether there is such a thing as Semi-Planing hulls.

    Rather than argue with me, why don't you take it up with a world expert, Daniel Savitsky

    http://www.staatsgeheim.com/wp-content/uploads/Savitskyreport.pdf

    he spends a lot of time talking about Semi-Displacement Hulls, which according to your black and white vision, dont exist at all.

    For other readers, the concepts and terms are discussed in a lot more detail at

    http://www.boatdesign.net/forums/hydrodynamics-aerodynamics/definition-planing-45248-2.html
     
  10. Richard Woods
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    Richard Woods Woods Designs

    Can we agree that to generate lift we need a lifting surface, an angle of attack plus velocity?

    So for efficient planing we need a big flat surface area with a good AR (so a waterski is inefficient), a high speed and a low angle of attack, say 5deg. If a boat exhibits those attributes then probably it is a fast planing boat (of course stability and control are different subjects).

    So if we redefine "semi displacement" as "inefficient planing" the discussion might be moot

    Richard Woods of Woods Designs

    www.sailingcatamarans.com
     
  11. PAR
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    PAR Yacht Designer/Builder

    This seems to be a discussion about generalities that don't really exist. Many displacement hull forms can be propelled well past theoretical speed limits (Vk/√LWL ). Conversely, planing hull forms can be under powered enough to not reach full plane mode. Shapes and approaches to design considerations can and do vary, but essentially S/L ratio governs what mode any particular hull form is performing. As to arguments about sinkage and lift, well any object, generally boat shaped, regardless of type is generating some lift underway. The same can be said of suction or sinkage, though clearly some shapes are better at some things than others.

    The Mac 26 pictured in up on full plane, which with this set of shapes is a patch farther forward then seen on a typical powerboat, where it would be much smaller and farther aft. Most Mac 26 owners report their 50 to 60 HP outboards drive them to 15 to 18 MPH, which is well within the accepted full plane S/L range (2.8 - 3.0).

    Semi planing hull forms aren't especially common, particularly now, but they certainly do exist and some have specialized in them. Atkins and others made a good living with powerboat shapes that exceeded theoretical speed limits, often with modest power. I can think of a few dozen or so of hooked bottom and tunnel sterned boats, that just can't do any better then S/L 2.5 and commonly cruise at 2.1 - 2.3 S/L. These hulls really can't get to full plane (comfortably), but do manage a reasonable speed with hull form tricks. Atkins reverse deadrise, box keels designs come to mind, using modest power, yet exceeding their LWL imposed limits and displaying true semi displacement ability.

    With power available at quite low HP/pound ratios now, these types of hulls are all but disappearing, but you can design for 1.5 - 2.5 S/L speeds, if you want to.
     
  12. daiquiri
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    daiquiri Engineering and Design

  13. groper
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    groper Senior Member

    Sure RW, what attributes are required from a hull in order to perform well in its desired speed regime was a point we had not discussed, but were coming to. This also covers the attributes which lead to the different hull classifications, planing or displacement s/d if you like the term.

    In order to understand why the attributes are required in a hull form, its important to understand why and how they effect the dynamic lift particularly - as anything in the water has a displacement, but not all shapes are efficient at generating lift. Therein lies the key to the distinction. But I think you get my point now i hope?

    PAR touched on the issue i was getting to, and that is the semi displacement hull forms are generally those which do generate some positive dynamic lift but it is limited or done inefficiently, compared to a full planing design. The reasons for doing it can be varied, no need to go into that for now.

    How its acheived can also be varied, but we can make some generalisations at this point. Full planing designs generally must utilize the following;

    1.Hard chines.
    2. Dead flat buttocks for a significant portion of the aft part of the hull.
    3. Statically Immersed Transom sterns.
    4. Have a sufficiently low enough Length to beam ratio in order to provide a sufficient aspect ratio for improved lift generation efficacy.

    Indicators of a semi displacement hull are similar to the above, except;
    1. May use rounded chines or no chines at all - reducing lift efficacy.
    4. May have a high length to beam ratio and thus low aspect ratio - reducing lift efficacy.

    Indicators of a full displacement hull differ from both of the above in that;
    1. Buttocks are much more curved aft, or hull has significant total rocker.

    If one must seek generalizations of what makes a hull plane, s/d or full disp, then the above is the best i can offer for now.
     
  14. Mr Efficiency
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    Mr Efficiency Senior Member


  15. rwatson
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    rwatson Senior Member

    Groper seems to have neatly sidestepped my peeve - - him stating that

    " asking about hulls that are displacement AND planing is absurd, they are mutually exclusive in their physical form, i.e. they CAN generate hydrodynamic lift or they can NOT. It's that simple."


    I take that as an agreement that it really isn't the case, that in fact that boat hulls utilise both hydrodynamic lift and displacement lift when power is applied.

    The fact that the hydrodynamic lift can be negated or overcome by other phenomena is another subject.

    As far as planing definitions, I don't really care - I just wanted to correct that assertion.

    heaven forbid that we have anything wrong on the internet. :rolleyes:
     
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