Crouch Constant

Discussion in 'Hydrodynamics and Aerodynamics' started by DogCavalry, Jul 6, 2021.

  1. DogCavalry
    Joined: Sep 2019
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    DogCavalry I aim to misbehave.

    From Propeller Handbook

    "(See Dave Gerr's Propeller Handbook, pages 15-17)

    knots/hour =(hp/pounds)^0.5*C

    Dave gives the following constants

    150 - average runabouts, cruisers, passenger vessels
    190 - high speed runabouts, very light high-speed cruisers
    210 - race boat types
    220 - three-point hydroplanes, stepped hydroplanes
    230 - racing power catamarans and sea sleds"

    George Crouch, probably close to a hundred years ago. Still quoted, and apparently still in use. Does anyone know about Crouch's original work or publications? Dave Gerr treats it like gospel, but Propeller Handbook was written long enough ago that there would be some continuity of collective knowledge. But the guys Gerr knew, that knew Crouch, are long gone.
     
  2. Alik
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    Alik Senior Member

    The way I use Crouch constant is:

    Step 1 - find a prototype - similar size, Fn, shape boat(s), with similar propulsion system, with reliable speed data.
    Step 2 - define Crouch's constant
    Step 3 - carefully apply it to new the design, for speed estimates at early design stages only.

    All other ways are not reliable and misleading.
     
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  3. DogCavalry
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    DogCavalry I aim to misbehave.

    Hi Alik. To be honest, I'm surprised you use it even to that extent. After all, Crouch was beating the world in 1923. The science has advanced since then. But where did you learn it from?
     
  4. wet feet
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    wet feet Senior Member

    If you don't like Crouch's formula,you could always use Keith's.
     
  5. DogCavalry
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    DogCavalry I aim to misbehave.

    I neither like nor dislike. I'm interested in the history. The Crouch Formula has been superceded by better knowledge. Crouch had been dead for years before Savitsky wrote his seminal work. But for generations it was as good as it got.

    Eric Sponberg uploaded a nifty pdf of a guide he wrote, back in 2012. Gives some history for Crouch's Formula.
     
  6. DogCavalry
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    DogCavalry I aim to misbehave.

    Had a look at Keith's formula. Very dependent on LWL. Savitsky says LWL is irrelevant in a planing boat, since it could easily be 30" in a 30' boat. Or 25' or anything in between.
     
  7. DogCavalry
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    DogCavalry I aim to misbehave.

    Y'know what else amazes me? Folks been looking at that table of Crouch Constants for a minimum of 70 years, and hardly anyone goes
    "230...?! what the heck is a sea sled? I want one of those."
     
    Last edited: Jul 6, 2021

  8. BlueBell
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    BlueBell "Whatever..."

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