move props towards the mid hull section?

Discussion in 'Boat Design' started by sdowney717, Jun 25, 2015.

  1. sdowney717
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    sdowney717 Senior Member

    I have been wondering why are props sitting so close to the back?
    Convenience of the powertrain design?

    If props were centered under the middle hull, I was thinking the bow would not be pushed up, the part of the hull rearwards from the props would act like trim tabs and the boat would stay more level with the water surface.

    Even moving props forward only a few feet, I would think would eliminate the need for trim tabs to keep the bow down?

    Any thoughts?
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    You are obviously talking about non-steering propellors, a sterndrive or outboard would lack the ability to steer if placed centrally.
     
  3. Mr Efficiency
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    Mr Efficiency Senior Member

    If you move the drive line forward, but with the same angle to horizontal, the bow will be pushed up more, not less.
     
  4. TANSL
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    TANSL Senior Member

    Figure explains why I believe that nothing differently happens in both cases.
     

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  5. Rurudyne
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    Rurudyne Senior Member

    Though in some shallow draft boats operating at displacement speeds it may help to keep the prop in the water as big waves roll by.
     
  6. gonzo
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    gonzo Senior Member

    With an inboard installation, the shaft is not horizontal so the thrust is shifted forward which makes the bow lift more.
     
  7. TANSL
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    TANSL Senior Member

    Is it more clear now?
    R does not change, T does not change, h does not change, will change M?
     

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  8. Kailani
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    Kailani Senior Member

    What about the force * distance from the green arrow?
     
  9. baeckmo
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    baeckmo Hydrodynamics

    Tansl, you have to consider the sum of moments from both the horizontal and the vertical composants and their levers, when you calculate the trim moments. In your example you only consider the horizontal forces and their levers!

    Put in another way, you take the moment from the resultant times the lever from its action line to the center of resistance.

    This gives an increase of the nose-up moment in your lower picture, just as Mr E and Gonzo have noticed.

    Now these effects are not the main issues with a midship propulsor. Instead it is the combined effect of increased friction loss (more hull area subjected to increased velocity from propeller jet) and loss of propeller efficiency when it is working in the zone of increased velocity where the hull is "fattest".

    The propulsor (be it a propeller or a jet) has a higher efficiency when it is "ingesting" fluid that has collected some forward movement, as in the boundary layer or in the wake region.
     
  10. TANSL
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    TANSL Senior Member

    Kailani and Baeckmo, you are quite right, one should not forget that moment. But the vertical component is so small (the angle should be about 3-7 degrees) which, in my opinion, its effect varies very little.
    Green force = between 0.052 and 0.12 times R
     
  11. baeckmo
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    baeckmo Hydrodynamics

    The vertical composant depends on shaft angle as you noticed and also on the velocity gradient under the hull surface. But the lever connected to the vertical composant is far greater with propellers aft than for the midships arrangement. Again, this is NOT the significant issue, it is a secondary consideration.

    Btw, it is interesting that "Mother Nature" has developed propulsion systems working around these principles during thousands of years of evolution. If you look at penguins or cormorants, ie birds that excel in high speed hunting on and in the water, you will see that their "surface propulsion"; i.e. their paddling feet are working as far aft as possible in the wake. But when they dive, they use their "midship propulsion"; the wings, which have low thrust loading and operate outside the hull flow region.

    The low thrust loading stems from an increased working area, and results in a far lower velocity increase over the body surface (ie lower friction losses), than you would have with a highly loaded midship propulsor.

    If we now compare with a generalist, the seagull, we can see that it is using "midship propulsion" in surface mode; it works, but a cormorant can easily race in circles around a swimming seagull, due to the cormorants better efficiency and lower friction losses.
     
  12. PAR
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    PAR Yacht Designer/Builder

    Getting a reasonable shaft angle, preferably below 8 degrees is the primary reason inboards, place the prop well aft. If you move the wheel forward, you'll have to move the engine forward and likely have to accept more shaft angle, because of the increase in deadrise in the forward sections of the hull, forces the engine mounts higher. Of course, some hulls will permit a much more forward placement and mullet skiffs, here in Florida commonly used a midship, well mounted outboard. These boats don't plane though some over powered versions, driven in light ship conditions, can certainly press well past displacement speeds. Unfortunately with this configuration, once the boat starts to trim up as speeds increase, the prop vents and kills any additional speed potential.

    Mechanical advantage and practically are the main reasons a fixed shaft has the wheel well aft. Vectored thrust is a different animal, requiring the most advantageous location, such as on, or aft of the transom.
     
  13. gonzo
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    gonzo Senior Member

    TANSL: you need to consider the thrust vs CG position in your calculations.
     
  14. TANSL
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    TANSL Senior Member

    GONZO : frankly, I do not see why.
    To evaluate the effect of changing the position of that force, in the trim of the ship, just consider its position in relation to the center of gravity of the waterplane.
    But if I'm wrong, any clarification will be helpful and will be greatly appreciated. Thanks
     

  15. Mr Efficiency
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    Mr Efficiency Senior Member

    It's in the angle of a shaft drive, that the lift comes from. You could have a non-steering (or steering if very well forward) "leg" well forward that had a line of thrust parallel to the waterline, but to what advantage ? It is probably going to become aerated with severe pitching.
     
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