Higher Cp at higher speeds equates to less resistance?

Discussion in 'Hydrodynamics and Aerodynamics' started by dustman, Dec 1, 2020.

  1. dustman
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    dustman Junior Member

    So the higher cp basically increases waterline length and pushes the high resistance hump to a higher speed by increasing the effective waterline length. It seems like once you pass this hump the higher cp would have dramatically higher wavemaking resistance.
     
  2. DogCavalry
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    DogCavalry I aim to misbehave.

    Well, apples and aardvarks. Past that limit high cp forms do increase drag, but not dramatically, and the low cp forms could not possibly reach even the speeds that the high cp forms easily reach, so it hardly matters. Only at very low speeds do the low cp forms have any advantage.
     
  3. dustman
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    dustman Junior Member

    This sounds backwards to me. A low cp hull would seem to reach higher speeds more easily, and have much less of a hump to climb to get over "hull speed" due the smaller wave that it has to climb over in the first place, not to mention piercing the wave more than going over like with a fuller bow. Further, it seems like a fuller hull would have less wetted surface area so less resistance at very low speeds where wavemaking is minimal. What are you defining as "very low speeds"? What am I missing here?
     
  4. DCockey
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    DCockey Senior Member

    Why?
     
  5. dustman
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    dustman Junior Member

    Because it's a blunt end pushing water ahead of it, causing higher wavemaking resistance(bigger waves, more energy dissipation).
     
  6. DogCavalry
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    DogCavalry I aim to misbehave.

    I'm just going to have to go with, your feeling is wrong. This was proven before we were born. It's over.
     
  7. dustman
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    dustman Junior Member

    I want to understand the fundamental physical principles of what's going on.
     
  8. DogCavalry
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    DogCavalry I aim to misbehave.

    Okay. There's a visualising error you seem to be making. Two hulls with completely different CPs can only be compared meaningfully if they have the same displacement. So the high CP hull has a much narrower waterline. Picture 2 of them mkving at the same V. The low CP hull, with maybe almost twice the beam, is pushing water aside much further. Much larger waves, that can not possibly rebound in the remaining waterline length. So hull speed is a merciless wall. KE of those waves is proportional to v². Further distance of displacement loses more energy to waves.

    The high cp hull is overall much narrower. Small waves form close to the bow. Water rebounds well before the transom. Hull speed is not really a wall at all. Also consider that the high CP hull has less surface area.

    I hope thhs makes it clearer.
     
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  9. JRD
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    JRD Senior Member

    My understanding below and very happy to be corrected...

    A low cp hull has less volume in the ends and more amidships.

    At a velocity where the length of the wave train equals the waterline length the hull squats and to travel any faster would need to climb over its own bow wave to escape the hollow between the bow and stern wave. Remember the hull still needs to displace its own weight in water and when traveling between two waves it is literally behind a wall of water going at the same speed.
    The higher displacement/length the bigger this barrier is.

    Having more volume at the ends (high cp) allows the squat to occur at a slightly higher velocity.

    Low cp shape can be a little easier through the water at low speeds due to comparatively slender shape. All other factors being equal low cp may experience lower drag or better relative speed up to "hull speed" after that the drag increases exponentially and higher cp can stretch a little further before hitting the hump.
     
  10. JRD
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    JRD Senior Member

    Which is most extremely observed with comparison between a monohull and multi..

    @DC, I'm enjoying following your build, very inspiring
     
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  11. DCockey
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    DCockey Senior Member

    This is only true when a vessel is at rest and the only pressure acting on the hull is hydrostatic. When the vessel is moving the pressure acting on the hull will have both hydrostatic and hydrodynamic components. The hydrodynamic component is the direct result of the motion of the water releative to the hull.

    When a vessel is moving the weight of the water displaced by a ship may be less than or greater than the weight of the vessel. At slow speeds most vessels tend to sink deeper into the water and the weight of the water displaced is greater than the weight of the vessel. The weight of water displaced by a vessel planing is considerably less than the weight of the vessel. Whether the displcement of a vessel moving is less than, greater than or the same as the static displacement of the vessel depends on the shape of the vessel and the speed.

    The resistance above "hull speed" depends on multiple factors including the shape of hull, displacement/length ratio, location of CG and line of action of the propulsion. Vessels which plane will typically see resistance decrease above the "hump" speed until the speed is high enought that total resistance starts to increase due to increasing skin friction. Cp is irrelevant for when vessels are planing.

    For vessels which do not plane the resistance will frequently increase above hull speed for lighter hulls (lower displacement/length ratio). Resistance for heavier vessels may intially decrease above hull speed. In general the change of resistance above hull speed can be a bit complicated.

    Ad Hoc has posted this image several times in threads discussing resistance and speed. I believe it is for a particular family of non-planing hulls and should not be taken as universal. [​IMG]
     
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  12. dustman
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    dustman Junior Member

    Does all this apply to extremely slender hulls. The cat I plan to build would have hulls 24' long and 12" wide with a semicircular cross section.
     
  13. DCockey
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    DCockey Senior Member

    The hulls in isolation would probably be similar to the curves labled 8.0 or 10.0 in the chart above.

    With catamarans there can also be interference effects between the hulls. The resistance vs speed effects of the the interference depends in large part on the distance between the hulls.
     
  14. DogCavalry
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    DogCavalry I aim to misbehave.

    Yes, this is a good example of a high cp hull.
     

  15. Ad Hoc
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    Ad Hoc Naval Architect

    What is your target speed and displacement?
     
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