Define what a Surface-Piercing propeller is

Discussion in 'Propulsion' started by tom kane, Jul 12, 2009.

  1. sandhammaren05
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    sandhammaren05 Senior Member

    Here's an exercise for you. The manufacturers couldn't answer it correctly in 1980, I doubt they can answer it now. We have a high speed, high-planning hull (very little wet surface or 'bow wave' from the bottom near the transom), the outboard is jacked high enough that the prop has at best 1-2 blades in the water at one time during a revolution. the gear ratio is 14:27, and the diameter has been chosen optimally for that gear ratio. The top speed should be 41 mph at 6100 RPM. What is the optimal pitch at the leading edge, and (hydrodynamically) why? Pitch is measured via a Rundquist pitch gauge, i.e., over approximately a 3/4" chord.

    Send me your answer and I'll have a good idea whether you might be able to teach me something, or vice-versa.





     
  2. gonzo
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    gonzo Senior Member

    There is no such thing as a "pitch at the leading edge" You are trying to change the subject and not answer a straightforward question: What outboard manufacturer did you work for? Also, we have to believe that propellers are surface piercing because no one can steer and take photos of a prop. Have you ever heard of camera mounts? Too much BS.
     
  3. sandhammaren05
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    sandhammaren05 Senior Member

    It's clear that you haven't the slightest idea what you're writing. I have no further time to waste.



     
  4. gonzo
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    gonzo Senior Member

    You haven't answered the question. What outboard manufacturer did you work for? It is interesting how upset people that make wild claims get when asked for evidence.
     
  5. baeckmo
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    baeckmo Hydrodynamics

    Sorry Gonzo, but Sandhammaren is completely on track here, and you are way off. The "leading edge pitch" is used to define the local angle of attack on a cambered or cupped blade, particularly when we talk about ventilated or supercavitating propellers. For these, the inflow factors normally used for fully wetted props, are not applicable. At the design operating point, the angle of attack to the leading edge on a SC or SPP prop is more important than the average pitch in order to control cavity size and thus performance.

    If you check the pic on the outboard on the transom, you will see that it is adjusted vertically, so that the flow leaving the step is meeting the rig far below the antiventilation plate. This is not an uncommon view in racing circles.

    So back off Gonzo, you don't know what you are talking about here!
     
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  6. gonzo
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    gonzo Senior Member

    Pitch is a nominal measure. That is industry standard. I asked about his claims that it is not possible to take a photo of the propeller and that he worked for an outboard company. Show me a photo
     
  7. Frosty

    Frosty Previous Member

    Merc Mod VP V6 fuel injection were used, no warranty-- 101 octane had be got from the airport. The solid hubless props would shatter the gearboxes so starting in gear was necessary. The exhaust would go into the blades and almost Max RPM was used to take of until they would bite. The foot controled throttle was necessary also as jabbing throttle op to full and off again was needed to get them up.

    The gearboxes were so high they could not pick up water for the engine, various adaptors were made but a separate pick up scoop on the transom did the trick. 2x3 inch holes were drilled in the leg approx 6 inches down, there were also mods on the exhaust trumpet, i forget if that was on the mod VP.

    26x30 I think they were, rabbit eared 3 bladers. If I remember correct cleavers were for rough water. All a long time ago.
     
  8. sandhammaren05
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    sandhammaren05 Senior Member

    Good to hear from you. I discovered the importance of camber by accident in Louis Baumann's prop shop in Houston in 1978. His old pitch gauge measured over about a 3" or more chord, I wanted a 22" down from 23" but the stainless was still too thick so I bent mainly the leading edge. Was an eye opener for me how it accelerated and ran on top end. That was before I'd done any theory homework in Newman or Landau-Lifshitz to help me to understand what I'd done. Paul Allison independently discovered 'cup' in a worse way: he pulled his trailer up his driveway with the prop dragging. He took out the damage and lost speed. That was 1955, river racing in Knoxville with a 25 Johnson on a homemade boat. Three of the photos shown by me are Allison boats, the other is a Laser, all made 1975-1981. Thanks again for adding to the discussion.



    QUOTE=baeckmo;333386]Sorry Gonzo, but Sandhammaren is completely on track here, and you are way off. The "leading edge pitch" is used to define the local angle of attack on a cambered or cupped blade, particularly when we talk about ventilated or supercavitating propellers. For these, the inflow factors normally used for fully wetted props, are not applicable. At the design operating point, the angle of attack to the leading edge on a SC or SPP prop is more important than the average pitch in order to control cavity size and thus performance.

    If you check the pic on the outboard on the transom, you will see that it is adjusted vertically, so that the flow leaving the step is meeting the rig far below the antiventilation plate. This is not an uncommon view in racing circles.

    So back off Gonzo, you don't know what you are talking about here![/QUOTE]
     
  9. Frosty

    Frosty Previous Member

    Altering pitch is difficult infact altering leading edge pitch and trailing is all you can do, as you will know no more than 1 inch is enough.

    An old cylinder block with all its nooks and crannies and cylinder holes are great for tweaking props.

    I always altered my own props after driving miles to take a brand new prop for cupping to the manufacturer and he just hammered the hell out of it,---I though right well I can do that.
     
  10. sandhammaren05
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    sandhammaren05 Senior Member

    We had to start the 49.9 c.i. Evinrude 75 loopers in gear too. It was 1981 at Havasu that Mod-VP boats were allowed to run a non-stock undergearcase-hub water pickup. The Mod-VVP tunnel speeds went from about 85mph to about 100mph because of the drag reduction because they could run with the gearcase hub above the water in typical tunnel style. You couldn't raise the motor that high on a V bottom and still turn, or have any control at all. I ran boat #2008, a Laser/Evinrude 235 that year, finished 13/50 and enjoyed the fine handling of the boat (I turned inside of everyone) although the speed was but 85 mph. It was that year that I noticed that Hendricks at OMC had picked up on my adding camber to leading edge of props, but we still ran a 28" pitch Mercury Chopper, not his prop. There was another round0eared Mercury prop that wasn't bad, but I've forgotten the name. The cleavers did not provide enough bow lift on the heavy boats.



     
  11. sandhammaren05
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    sandhammaren05 Senior Member

    A first rate photo of outboards running surfacing props can be found on pg. E14 of the following well-known reference:

    Robert F. Kress, High Speed Propeller Design, The Society of Naval Architects and Marine Engineers, paper E, presented at the Spring mMeeting, Lake Buena Vista, Fla., Apr. 2-4, 1973.

    I have the paper on hand in case anyone wants a copy, belongs to my stack from ca. 1980.:p
     
  12. PetterM
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    PetterM Senior Member

  13. baeckmo
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    baeckmo Hydrodynamics

    Place a note on the reference in Rudi's thread on surface prop litterature in the "Surface drive section". And yep, I would like a copy as well.
     
  14. sandhammaren05
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    sandhammaren05 Senior Member

    Give me your email address and I'll fax a copy. I'll also list other lit on Rudi's thread that I have if it's not already there.

    jmccauley@uh.edu



     

  15. sandhammaren05
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    sandhammaren05 Senior Member

    Will scan and send to the email address on your website.


     
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