Lift loss with increasing deadrise

Discussion in 'Hydrodynamics and Aerodynamics' started by DogCavalry, Jul 8, 2021.

  1. DogCavalry
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    DogCavalry I aim to misbehave.

    In Savitsky's 1964 work, he describes loss of lift in a planing form with increasing deadrise in a semi-linear relationship.
    ClD=Cl0-0.0065D*Cl0^0.60
    Not hard to see why. A zero deadrise bottom makes the most lift. A 90° bottom makes none. (That would be a hull side). Other bottoms are in between. How sophisticated is the description now? Has the formula been improved, or did Savitsky nail it 57 years ago?
     
  2. Ike
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    Ike Senior Member

    Well, As far as I know, designers are still using Savitsky's formulas.
     
  3. DogCavalry
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    DogCavalry I aim to misbehave.

    Thanks Ike. It really shows why flat bottom boats are great. On flat water, that is. A flat bottom boat, on rough water is a recipe for a bad back.
     
  4. BlueBell
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    BlueBell "Whatever..."

    Wouldn't a 2 degree angle of attack (AoA) produce more lift than zero AoA?
    Like a hydrofoil.
    Isn't 3 - 4 degrees generally agreed to be an optimum planing angle (of attack).

    The lighter and faster the boat the less angle needed to plane.
     
    Last edited: Jul 9, 2021
  5. fallguy
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    fallguy Senior Member

    I am glad you said something.

    The amount of hull to lift/drag is the obvious issue.

    Technically, even the seasled has forward deadrise, no?
     
  6. Mr Efficiency
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    Mr Efficiency Senior Member

    A technical argument that has limited usefulness, deadrise looked at isolation from weight, speed, beam etc is information that doesn't avail you of much, and then there is the complication of strakes and chine flats and planks. It is an "all else being equal" parameter. Higher deadrise boats can have better ability to reduce wetted area, at higher speeds, so can go faster with the same power, even though the speed of planing cleanly may be higher. It's complicated !
     
  7. DogCavalry
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    DogCavalry I aim to misbehave.

    All things are compkicated.
     
  8. Mr Efficiency
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    Mr Efficiency Senior Member

    Not necessarily, otherwise the word "simple" would be out of a job. A lot of boats would benefit from more deadrise, but for the fact it affects static initial stability quite strongly.
     
  9. DogCavalry
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    DogCavalry I aim to misbehave.

    In the slower ranges, there's a huge advantage to no, or low deadrise, if the ride qualities aren't a problem. But that's like saying your grocery bills will be less if you stop eating. Baeckmo and I are discussing sea sled performance. He's built a dozen! Trying to understand the amazing claims made for them.
     
  10. Saqa
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    Saqa Senior Member

    I am curious, have you done any performance simulations on your hull? Scale model testing?
     
  11. Mr Efficiency
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    Mr Efficiency Senior Member

    I can well understand that sleds carry weight well, but so did the cathedral hull, and they all but disappeared, people could not hack the ride in lumpy water
     
  12. DogCavalry
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    DogCavalry I aim to misbehave.

    Mr Efficiency, your observation about reduced wetted area in a V hull at higher speed is pure gold. I've been wondering about Crouch's constants, and how he got a constant 30% faster for sea sleds than anything comparable, but they have been superceded now by other forms for supreme speed. Your point illuminates it perfectly. Par Savitsky, a 0° deadrise hull has 35% more lift, and simultaneously less drag than a 20° hull, at 4°trim. That is a fantastic advantage! That alone would easily explain the empirical observations he made. Because... he made them in the 1920's! Getting up to the kind of power to weight ratio that allows planing on the corner of your wallet, so to speak, simply couldn't be done for all the money on earth. He built a famous world beating boat called Baby Bootlegger. It was 29' long, with a 220hp engine.
     
  13. DogCavalry
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    DogCavalry I aim to misbehave.

    Yes, the cathedral hull was garbage. A naked ripoff of Hickman's work, grossly inferior in every way, except that Hickman didn't own the patents.

    I'll say it again. Sea sleds don't pound. Not in lumpy water. Not otherwise. Ask baeckmo. He built a dozen. His don't pound. Ask Marcus Lee. He built a dozen too, and they don't pound, off the pacific off Alaska. I'd say ask Hickman. He built around 10000. He's dead. His blew everyone's minds, because they expected them to pound, but they didn't. And we are way off topic. This was about Savitsky.
     
  14. Mr Efficiency
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    Mr Efficiency Senior Member

    Obviously with an inverted vee, deadrise is not the same animal
     

  15. latestarter
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    latestarter Senior Member

    Deadrise and angle of attack are 2 different properties.

    "The deadrise of a boat is the angle measurement between the boat bottom and a horizontal plane on either side of the center keel."
     
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