Adjustable propeller pitch for surface pircing propeller

Discussion in 'Surface Drives' started by Christian Lucas, Jun 27, 2020.

  1. Christian Lucas
    Joined: Dec 2018
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    Christian Lucas Junior Member

    Hi,
    something different, how can you make the propeller pitch adjustable for a semi-submersible propeller without the propeller blades having to be movable?
    Here are a few pictures, in the case of semi-submersible propellers, only the blades that are immersed in the water are relevant for the drive. If two opposing propellers are placed next to each other, you can lift the propwalk on the one hand, but you can also adjust the angle of the valley to the straight line of travel as viewed in the angle of attack. Since it would be a bit cumbersome to adjust the whole propeller shaft, it is best to use two Arneson Drives. You have to remove the Skeg fins and put a rudder blade between the propellers.
    Now you can adjust the propeller inflow at an angle and thus have the option to start with flatter propellers for better acceleration and then to switch to a higher propeller pitch to be able to reach a higher top speed. In order to relieve the joints at maximum performance, the drives are installed at an angle so that the couplings have to achieve as little angle compensation as possible on a steep incline. See the pictures of the drive in the boat.

    Happy Amps Christian
     

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  2. gonzo
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    gonzo Senior Member

    You are not changing the pitch, only the direction of the thrust. As a result, the forward thrust will be reduced and the drag increased.
     
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  3. DogCavalry
    Joined: Sep 2019
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    DogCavalry I aim to misbehave.

    Well, that is not entirely correct. Certainly with a submerged prop at an off angle, the performance is less than ideal. But then most inboards run props at a bad angle. Here only part of the prop is in water, and the vector addition gives a very low angle of attack. If the assembly can be adjusted on the fly, it has an effect similar to a CPP. But possibly at greater cost and complexity.
     
  4. gonzo
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    gonzo Senior Member

    What vectors are you adding. I see in the photo shafts that are not parallel to the centerline, so the thrust is not parallel to the centerline. That creates less forward force and more drag.
    Controllable pitch gives a different rate of advance at the same RPM. This setup changes the vector direction only.
     
  5. BlueBell
    Joined: May 2017
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    BlueBell "Whatever..."

    Greetings Christian.

    This is an intriguing concept.
    Pages will fill here as arguments are made.
    I look forward to your sea-trials and the performance report.
    Perhaps you've already done that, if so, do tell,
    what were the results?

    Regards, BB
     
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  6. DogCavalry
    Joined: Sep 2019
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    DogCavalry I aim to misbehave.

    I am adding the vectors that are present. With a side angle on the shaft, the travel of the blade with respect to water flow has a cross flow component, and a flow direction component. The OP describes orienting the shaft such that the blade in the water is moving rearward as well as across the flow. This definitely has the effect of reducing effective angle of attack. This phenomenon was completely understood in aircraft in the 1940's. It is why a puller propeller has a negative effect on aircraft stability, and a pusher has a positive effect. The changing angle of attack as a blade goes through 360° moves a blade through a state of low lift, low drag, to a state of near stall 180° later. Drag is very high at that point, and it prop walks the nose of the AC sidewise, or the tail of a pusher. I digress, but the effect is real, and very well understood in the aerodynamics family.
     
    Last edited: Mar 2, 2021
  7. BlueBell
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    BlueBell "Whatever..."

    Well, he's changing the angle of attack
    if he's able to isolate the water flow from the other half of the prop-disks.
     
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  8. baeckmo
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    baeckmo Hydrodynamics

    Nothing new under the sun..... In 1869 a C. Sharp of Philadelphia filed a patent for a yawed shaft, surface piercing propeller with cambered blades, which aimed at directing the asymmetric thrust from the surface prop directly aft, and he correctly identified the blade camber as a necessary requisite for good L/D ratio with a ventilated profile.
    In 1914 a twin-shaft surface prop arrangement was patented in the US by a W.H. Farber (not clear if the shafts were yawed though)
    The practice of yawing the shaft has been used for decades in racing "prop-riders", and there are research papers on the subject. But like all ventilated props, there are problems with low thrust in the hump speed range.

    Edit: It turns out that the 1914 guy was not a "Farber", but William Henry Fauber (Frenchman?), who patented quite a number of radical inventions for high speed water transport. "Honor to he who deserves it...."
     
    Last edited: Mar 4, 2021
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  9. DogCavalry
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    DogCavalry I aim to misbehave.

    That is so damn cool. Thanks baeckmo!
     
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  10. DogCavalry
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    DogCavalry I aim to misbehave.

    What is written, baeckmo, about surface piercing props at relatively low speeds? I saw an abstract of a paper a few years back, but the link was broken.
    C. Sharp's patent must have been for a displacement vessel. Not much planing going on in 1869.
     
  11. DogCavalry
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    DogCavalry I aim to misbehave.

    With regard to the original suggestion. I think it might be relatively practical. Start with shafts submerged and parallel. Low pitch props for high initial thrust. As the boat lifts onto the plane and the shafts clear, they can be splayed or converged to increase the effective angle of attack. An extra trick for an existing arneson type setup. Although, per my original impression, not necessarily cheaper than just fitting CPPs as god intended.
     
    Last edited: Mar 3, 2021

  12. Christian Lucas
    Joined: Dec 2018
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    Christian Lucas Junior Member

    Hi,
    Thank you for all the answers. Due to the current health situation I had not the time to complete the test boat. So I can not give a answer from running this idea. Doing this with a small outboard motor will only work if the lower gear has not skeg fin, as this will always direct the water flow in the direction of the propshaft. This surface drive, a very basic version of a Arneson drive I designed for model company Graupner. For this test I removed the skeg fin on both stearable drives and add a separate ruder for steering. Maybe also a different trust line setting of the propshaft will give a stearing possibility. I will see. Hope next month I can fix all parts and do some test.

    Happy Amps Christian
     
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