Bold claim for new propellors

Discussion in 'Outboards' started by Mr Efficiency, Nov 7, 2013.

  1. tom kane
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    tom kane Senior Member

    When you put a boat into forward gear the prop turns and digs a hole under the transom so the boat sqats.
     
  2. michael pierzga
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    michael pierzga Senior Member

    Yes...thats the way I see it... but how would a prop push the bow down ?

    I still cant get my head around this concept. It may be possible on a very high speed craft that has no hull in the water at speed , but the promotional literature for the mercury prop states..cruising speed ? At lower speeds.

    And why would this special bow depressing prop be useful when every outboard or stern drive allows the operator to trim the engine to fine tune the hulls angle of attack while underway.

    Im skeptical of the product.
     
  3. FMS
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    FMS Senior Member

    Different props are being talked about here. The Mercury Enertia press release talks about increased bow lift at cruising speed.
    "To maximize the fuel economy improvement provided by the Enertia ECO, Mercury Marine engineers designed a new propeller with a broad 16-inch diameter, large blade area and a high progressive rake, a combination made possible only through using Mercury’s proprietary X7 stainless steel alloy. While the Enertia ECO design results in reduced hull drag through increased bow lift at cruising speeds, it still matches the performance of other Mercury propellers such as the Enertia and Revolution 4."
     
  4. powerabout
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    powerabout Senior Member

    Hi Tansl
    if thrust/bollard pull was an issue I guess you would need to calculate it if the prop blast goes along the hull as that will reduce the net thrust
     
  5. TANSL
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    TANSL Senior Member

    Hi powerabout,
    I guess you know that what you said is beside the point. I'm talking about an effect that did not know and, without ulterior motives, ask for something that, although it may be of little importance, I'm sure it happens. If you know the answer, and you fancy, you answer me. If you do not know, for my part, I do not need you to write anything.
    In post # 33 it is said : "... it still matches the performance of other Mercury propellers such as the Enertia and Revolution 4." I can deduce that, despite what I say happens, the performance of the propeller does not decrease.
    Thank you for your attempts to help.
    Cheers
     
  6. PAR
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    PAR Yacht Designer/Builder

  7. TANSL
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    TANSL Senior Member

    This is "rake".
    I think all the propellers have "rake." But I can be wrong, I'm no expert.
     

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  8. PAR
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    PAR Yacht Designer/Builder

    At the far left of your drawing is rake. No, not all props have rake. Most props do have some aft rake, but not all and there are instances where forward or neutral rake are required/desired.

    Again, this clearly shows rake.

    [​IMG]
     
  9. TANSL
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    TANSL Senior Member

    PAR, that has been explained perfectly by baekmo and daiquiri. The truth is that your comment does nothing to clarify my doubts. Anyway, thanks for trying to explain it. Study a bit and try again.
    Cheers
     
  10. powerabout
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    powerabout Senior Member

    Hi MP
    You need a prop in your hand with rake and then think about the effective pitch of the blades relative to the forward movement ( direction fo boat not the shaft) when it rotates, the blades going down are doing a different job than the blades going up.
    This is why typical an outboard steering pulls one way trimmed in and the other trimmed out
    Grasp that and you have it!
     
  11. TANSL
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    TANSL Senior Member

    The operation of the propeller is one of the things that are really complicated when dealing with vessels. Since I do not know anything about it, I dare say nonsense so I will not say anything else. Now, I can understand if what is said about it is foolish or wrong. Many things are just common sense.
     
  12. DCockey
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    DCockey Senior Member

    Does the rake cause an vertical force component due to shaft angle, due to the propeller operating near the surface. or both?

    If a propeller of any shape is deeply submerged with the shaft parallel to the oncoming flow I wouldn't expect any vertical force. If the propeller is deep submerged and the shaft is at an angle then I would not be surprised by a vertical force component which is different than thrust * sine of shaft angle. This difference in vertical force component would be due to the angle of the oncoming flow as seen by a blade on the descending side being different from that on the ascending side.

    If the propeller is operating near the surface then several different phenomena could cause the propeller to have a vertical force component even when the shaft angle is zero relative to the oncoming flow. Is such a phenomena responsible for a raked blade causing a vertical force?

    Any insight into the physics of this phenomena would be appreciated.
     
  13. TANSL
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    TANSL Senior Member

    David, apparently there are two effects mentioned by baeckmo and daiquiri and which can generate a vertical component in the thrust developed by a propeller. I just discovered these phenomena and still do not know anything about it. At the moment I do three questions:
    1. - Why there is a vertical component and that component is not cross, from axis outward?.
    2. - I guess this is only interesting for inboards with shaft + propeller . In other cases the power trim can do the same effect?​​??
    3. - Why is preferable to provide an additional racking to the propeller than changing shaft angle?.

    I suppose the raking changes the components lift and draft occurring in sections of the propeller´s blades. But I dare not say anything else.
     
  14. baeckmo
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    baeckmo Hydrodynamics

    David, this is what I am saying. The inflow velocity is varying with operating depth (when propeller disc within, say one disc diameter depth from surface). The result is, that the lower half of the propeller disc is producing more thrust than the upper half.

    When the blades are set 90 degrees ("transversally") to the mean flow direction at the disc plane, the net thrust from both halves is directed more or less straight aft in the shaft direction. Since both upper and lower flows are aiming in the same direction, the difference in thrust from the two halves will not generate any vertical forces. Note that the local flow at the actuator ("disc") plane is slightly concentric, due to the acceleration of the fluid.

    However, when blades are raked rearwards ("aft") in relation to the local flow direction, the blades in "low position" will generate an upwards acceleration (vertical thrust vector) that is stronger than the corresponding "down vector" from the blades in the upper half of the disc, due to the difference in impulse, or "blade loading". And vice versa, of course.

    It is all depending on the asymmetry of the inlet flow; at the surface, no inflow acceleration is possible (no pressure difference, the prop cannot create any sub-atmospheric pressure at atmospheric conditions, only when submerged).

    The presence of a hull with its boundary layer above the propeller disc will have a similar, but considerably lesser effect on the inflow; still talking about a shaft directed in line with the flow. With an inclined shaft, there is the additional effect of the transverse acceleration, of course.

    In this context it may be appropriate to note what happens to the inflow velocity field when the flow is retarded due to the development of free vapour volumes, caused by cavitation, or the situation with a surface piercing propeller, where the total inflow mass cannot be accelerated at all. In both cases, the impulse change mechanism is completely different from the fully/deeply submerged propeller.

    ....And, Mr Kane: the propeller is not digging any holes. The squat is the effect of the accelerating thrust beeing applied at the propeller level, while the accelerated boat mass is located at a substantial height above the thrust line, causing a torque (force times lever) around the center of mass! Basic Newtonian mechanics.

    ...."hole in the water....jayseees, where do you get all your stuff from?"
     

  15. baeckmo
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    baeckmo Hydrodynamics

    TANSL: Hope you find the first two q;s answered above (to get a stern lift corresponding to transom flap lift, the rake should be inclined forwards). The interest in "bow lift" (in effect "transom dive") is due to the fact that most planing hulls tend to operate at too low trim angles at high speeds and too high angles at low speeds. The most efficient way to create the required "bow lift" is to use a raked propeller. The reason is fairly simple: the propeller blade has a dramatically higher Lift/Drag ratio than a hull with an upward slope in the aft portion of the bottom (needed to create a downforce).

    To nr 3 the answer is that with a propeller shaft parallell to the inflow, the relative inflow velocities (read angles of attack) onto the blade leading edges are roughly constant over the full circle, which means that the blade profile can be operating at its optimum Lift/Drag ratio over the complete 360 degrees.

    Again a sideview to the SP prop: here an inclined shaft will produce a low incidence angle (angle of attack) during the downstroke, and increasing angle during the upstroke. Due to the special behaviour of the blade-profiles used, and the varying degree of ventilation during the operating sweep, this often gives a better overall efficiency in these circumstances, than an inline shaft. This does not apply in general to the supercavitating propeller though!
     
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