Froude and planing

Discussion in 'Hydrodynamics and Aerodynamics' started by sandhammaren05, Feb 26, 2017.

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  1. sandhammaren05
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    sandhammaren05 Senior Member


    Your understanding of the history of relativity is false. Prior to special rel. Einstein was dis-satisfied with the asymmetry between classical mech. and electrodynamics. He was also motivated an asymmtry in the discription of the effects of moving magnets in Maxwell theory. H. Lorentz discovered the Lorentz transformations but could not interpret them. Einstein interpreted them and added a single postulate: that the speed of light must be the same in all inertial frames. This was all theory, experiments came as a result of the predictions.

    GR was not motivated by any defect in the description of the motion of the planets. Here, Einstein was motivated purely by the Equivalence Principle: that no experiment can detect the difference between an acceleration relative to an inertial frame, and a gravitational field. The description of his Gedankeneksperiment is called in English 'Einstein's elevator'. Again, there was no experimental motivation, pure thought based on the Equivalence Principle.

    I have not found Savitsky's work to be helpful, albeit more helpful than your off the wall remarks. Blablabla, that's all you know.
     
  2. Joakim
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    Joakim Senior Member

    This just shows it's a small sport with very limited budget for developing models needed for simulations. If you are referring to surface piercing cleavers, they are not much used outside racing boats and there is quite limited effort to simulate them. And it's not easy with two phase flow, sprays etc. and would need to be done for each boat separately for best accuracy, like it is done for ship propellers.

    Try to do the same for F1 cars (or any high series production car), America's cup sailboats or ships. All designed using simulations and no-one can get the same speed or drag without using simulations.
     
  3. sandhammaren05
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    sandhammaren05 Senior Member


    Not only is there no effort to eliminate sp props, you can't run above about 40 mph without them. Maybe Finland is backward, but every fast boat sold here
    is set up for surface piercing. With modern Evinrude outboards and the Mercury Optimax speeds of 60--100 mph are common.This includes bay boats, bass boats, etc. All modern boats. So when you talk about 'eliminating them' you're apparently speaking from a boating backwater.
    Ship propellers are relatively boring, fully submerged.
     
  4. gonzo
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    gonzo Senior Member

    Science is basically theories and methods. Theories must explain observed phenomena or they are considered invalid.
     
  5. CT249
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    CT249 Senior Member

    How do you know your work is correct when videos, photos and experience with windsurfers prove that you can develop lift with an immersed transom (contrary to your theory) and that your claim that a "a hull with no transom cannot plane" is completely and utterly false?

    You've tried to wave these facts away but provided no evidence for your claim that they are special cases, so it seems that they disprove your theory and you just can't admit it.
     
  6. CT249
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    CT249 Senior Member

    Surely that's incorrect. Ships like the French "contre-torpilleurs" ran at 45 knots with fully immersed propellers. And any look at an older Australian-style inboard skiboat shows fully immersed propellers (in flat water, obviously not when the boat is jumping over waves) and that's what it looked like looking over the transom (which is of course proof....).

    [​IMG]

    The angle of the shaft strut in this old boat can be seen here; hard to see how it could be surface piercing.

    [​IMG]

    Here's a similar hull jumping over a wave - again the angles show that the prop seems to clearly be fully immersed when the boat is in normal trim, as the boat in the background is. This boat achieved 78 mph and similar boats got around 90 mph. so it seems that boats can go over 40 mph without a surface piercing propeller.

    [​IMG]
     
  7. Ad Hoc
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    Ad Hoc Naval Architect

    Great..
    Can you list them please.

    Nope, that just sounds like what it is..sour grapes.

    And that's my point - very disingenuous - it is sour grapes.

    Great.
    Can you list them please.

    Sorry, just sounds like sour grapes again.

    Nope. A good paper does not need a 'subject matter expert' to recommend it. It should be able to stand on its own merits.

    Engineering and science has no emotions - only facts.

    Looking fwd to seeing/reading your papers.
     
  8. Ad Hoc
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    Ad Hoc Naval Architect

    So, if it is not a theorem, what is it?

    So, you have a lift coeff. ok. So, can you post your data and findings so show/demsotrate whatever it is - as it is not a theorem - you are suggesting/proposing.

    Your lack of intellectual rigour does that for you...
     
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  9. Ad Hoc
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    Ad Hoc Naval Architect

    Is this your example of quantitative intellectual rigour ...? Subjective sound bites?!

    Or do you have actual data of before and after and by many other independents doing the same to support this statement?
     
  10. Joakim
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    Joakim Senior Member

    When did I say you should eliminate sp props? I just said they are not used outside of very limited marked with no one willing to spent millions on simulations.

    I had a boat in mid 80's with completely standard Yamaha 40 with the standard aluminium prop (17"). It's certainly not surface piercing and it did 42 knots, which is quite much more than 40 mph. Then I bought a Radice cleaver (10x18, bronze) and lifted the engine. The speed went to 44-45 knots. Then I bought a Ron Hill cleaver (12x22, stainless steel) and got to 48 knots with propeller shaft a bit higher than the edge of the transom. Far from optimal, but this was done just for fun and with a very limited budget (e.g. we built the boat using another one as a male mold). A friend of mine did 52 knots with a similar boat and a bit modified Yamaha 25 back then.

    If you knew the history of international race boating, you would know Finns have won F1, Offshore and many other class world championships. Also Finns have made several speed records with engines and propellers modified locally. At the moment three UIM records are held by a Finn. We have a lot of boats with one 200-350 hp OB doing 45-55 knots with. Often with the standard propeller that comes with the OB. Although AV plate may be a few cm above the edge of the transom, I would not call these surface piercing propellers and they are very far from cleavers. E.g. this is quite common.

    A friend of mine just bought a Baja 226 with stern drive leg deep in the water. He claims it can do 60 knots, but I haven't seen it yet.
     
  11. Joakim
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    Joakim Senior Member

    That's your problem. I know e.g. how to implement Savitsky method in a way that gives accurate speed prediction for a variety of planing boats, including several racing boats. And I'm far from the only one who has done that.
     
  12. sandhammaren05
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    sandhammaren05 Senior Member

    I have no problem. I have a simple trig formula for lif that works very well. Planing speed depends on boat bottom design, weigh, power, propeller submersion, whether there's a keel or other obstruction in front of the prop. Etc. There is no universal formula for planing speed. You may as well look for the tooth fairy.
     
  13. sandhammaren05
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    sandhammaren05 Senior Member

     
  14. sandhammaren05
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    sandhammaren05 Senior Member

    I made a slip there. The world outboard speed record 1935-38 with a step plane was nearly 80 mph with a Baumann tractor gearcase. That required submersion, I think. I should have stated that you will have problems above about 40 mph with full submersion. To decrease the drag in order to run faster the gearcase and prop must be raised, that requires cupping and camber. The actual blade shape is less important. The Italian overdrive gearcase that moved the outboard record to over 100 mph in 1938 was a surface piercing design. That's an over 20 mph speed gain. The speed would have been higher had they had a prop designed for surface piercing. Baumann built a cleaver in 1935 (I hve photos of one, Baumann Propeller is in Houston where I live). Hi Johnson (OJ propellers, Oakland, Calif.) built surface piercing props in the late 1940s. None of this was done by marine engineers. Today's fast props are not designed or built by marine engineers. Photo 1 from Louis Rothermel collection, photo 2 by Marc H PB120206.jpg IMG_1275.jpg amer.
     

  15. sandhammaren05
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    sandhammaren05 Senior Member

    Some of you want to rely on Savitsky so let's go through Savitsky's paper starting with eqns. 8-10. The lift coefficient for an airfoil always comes out of hydrodynamics as proportional to a (=alpha=angle of attack). Savitsky starts by claiming that this is only true for large aspect ratios. What's true that it follows from a thin wing approximation. Second, he asserts that the lift coefficient is proportional to a^2 for the limit of zero aspect ratio. Maybe this follows from slender body theory (or not), I don't know. Third, he asserts point blank that (and this is his eqn. 8) we can write C=Aa+Ba^2 (he uses tau instead of alpha for attack angle). This make no sense mathematically/hydrodynamically because a boat has a definite aspect ratio. Third, he asserts that we can rewrite the polynomial in a as C=ca^1.1. This is nonsense: what is true is that we can write a^1.1=aa^.1 =ae^.1lna≈a(1+.1lna) for a≈1. Savitsky's eqn. (10) would be close to my half theoretical/half empirical prediction that (for zero deadrise) C≈acosb (b=deadrise) for properly designed v-bottoms if we take his fudge factor c to be c≈1 because his other factor of (l/b)^1/2≈1 for properly designed v-bottoms with firm, straight bottoms well above clean planing speed. Clean planing is when the speed is high enough that the trim angle falls to a few degrees and remains there. Underpowered hulls may never plan cleanly.

    In his fig, 18 (with no data points or error bars!) porpoising can set in much earlier if the bottom is rockered. In fact, a rockered bottom may fail to porpoise only at extremely low speeds.

    Savitsky did not discuss the origin of lift or the onset of lift.
     
    Last edited: Apr 26, 2018
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