Define what a Surface-Piercing propeller is

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

  1. sandhammaren05
    Joined: May 2009
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    sandhammaren05 Senior Member

    You have a pitch gauge? You'd need one to do anything about it. I bought my Rundquist for about $200 in 1958. Now they 'discount' for $1500. Lack of competition.
     
  2. baeckmo
    Joined: Jun 2009
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    baeckmo Hydrodynamics

    Checked your pic, Joakim. Like Joe says, without measurements there is only guesswork. But personally I find this tip shape a bit weird. Look at the outer 10 to 20 mm's of the blade. The cupped part is a very huge part of the total chord; in the tip region there is "only cup". Without the front part of the foil, there is not much thrust generated there, ie loss of power. It may produce a lot of lift though, but that lift could be produced by other means.

    I feel modern SP props have more of a constant ratio cup/chord all along the radius. When the blade emerges out of the wet stuff, it is still carrying a substantial volume of water as a film over the pressure side. In the absense of a surrounding volume of water, this film is thrown off tangentially into the air instead of beeing pushed aft for thrust.

    To counteract this tendency, either the cup must "go around the ear" as in the chopper or the "classical" Newton-Rader prop (or the Russian SK series for that matter), or the TE must be cut forward, so there is an inward force acting in the tip region.

    BTW, do you have access to a lathe? I use mine, with a 360 degree scale on the chuck for blade shape control. A micrometer (don't get the English word for it right now) on the doll plus a length scale to the doll and that's it, plot with spline function in any CAD application. If you happen to have a stroboscope, first check the prop rotating under strobe light; you will immediately spot the out-of-shape blade. It is not unusual to find substantial pitch differences coming with new props right out of the box!
     
    Last edited: Jan 20, 2010
  3. Frosty

    Frosty Previous Member

    Below is some technical drawings of levi surface drives with diamond back Seafury 3 blade cleavers 24x24

    Top picture left is the original levi rudder. 44 cat 2x250HP

    Second pic is that of my latest modification. Removal of 1 rudder each side increase in speed and dramatic improvement in 20KT steering. Was this because of more air to the prop or less wetted surface of the cut rudder.



    3rd pic is my intended mod shortly, following on the success of above (more may be better). More air may have been the reason. Some think that the prop wash can be re directed into thrust , my opinion is that this wash is spent and has no benefit.
     

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  4. Joakim
    Joined: Apr 2004
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    Joakim Senior Member

    I think also, that it is important, that the water film is not thrown in the air. The Ron Hill in the pictures was much better at this than the 10x18 Radice, which produced a big rooster tail. When you look at boats really performing well, there is no visible rooster tail:

    http://pellinge.fi/fresegustafson.com/Snygga bilder.html

    Look also at the propeller section:

    http://pellinge.fi/fresegustafson.com/prop.htm

    Is there much of a difference between the cup of these compared to the Ron Hill?

    I don't have a pitch gauge, but a friend of mine has a lathe and many measuring devices for it. If I find some spare time, I try to do some measurements of the Radice, but it will take some time.

    What kind of efficiency (not slip) would you expect for the Ron Hill in the conditions I specified? It would be nice to compare to the predictions of my Savitsky program.
     
  5. daiquiri
    Joined: May 2004
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    daiquiri Engineering and Design

    Frosty... :D :D :D
     
  6. sandhammaren05
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    sandhammaren05 Senior Member

    I can't comment/compare cup without measuring the props, 'eyeballing' is misleading. The cleaver on top, blade shape near tip, looks like the ones I made for myself. I've posted photos of some of my raceboats, will add a few here. You don't see much rooster tail, that's only wasted power. As for predicting efficiency of a Hill or any other prop, I'd have to measure it on my pitch gauge and know the boat/motor, some details. I never used Savitsky, I always found the existing theory to be way too crude for top performance. Theory was always a qualitative guide for me: try to minimize the tip vortex, and the leading edge vortex as well. So I don't like blades that have a tip like a 'knife'. I ran a Mercury chopper on 2008, ran 12x23 OMC cleavers on 853 (I had four cleavers that I'd reworked for myself in Louis Baumann's prop shop). Leading edge was than 23" pitch, obviously! Evinrude 75 turned about 6700RPM, geared at 15/28, speed about 67mph (I could get about 1mph more by reworking a 12x25, in the end only the pitch distribution along the blade at constant radius was different with the leading edge pitch set about 1" higher than on my 12x23s). The Evinrude 235 ran a 28" chopper, turned about 6500-6700RPM (best I recall) at 85 mph. Geared 2:1.
     

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  7. yipster
    Joined: Oct 2002
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    yipster designer

    [​IMG]
    :D nevertheless interesting experiments
     
  8. sandhammaren05
    Joined: May 2009
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    sandhammaren05 Senior Member

    The blade shape is basically good, I easily could improve it with a file. The cup? Would have to measure it. Also, is the pitch along the leading edge constant as radius varies, or (like some mis-engineered 10x16-17 Yamaha cleavers) does it increase with radius along the leading edge?
     
  9. sandhammaren05
    Joined: May 2009
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    sandhammaren05 Senior Member

    Your engineering drawing is impressive, Popeye. In the local Austrian Gymnasium they unnecessarily force our son to use CAD.
     
  10. sandhammaren05
    Joined: May 2009
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    sandhammaren05 Senior Member

    Consider Baeckmo's statement: "...To counteract this tendency, either the cup must "go around the ear" ...". I agree partly from experience.

    With my small sportboat (35 hp, 350 lb Allison pad Vee) the cup must full at max radius, or else the 'sharp transition' to higher speed that I stated in my second 'problem' on this site doesn't occur (instead, there's only a bigger roostertail). If the cup goes around the tip (beyond max radius) then the transition to bow lift occurs very fast but then top speed is reduced by about 10%. I didn't find this to be so with my larger hp raceboats, where the cup could feather near to zero at the tip.
     
  11. sandhammaren05
    Joined: May 2009
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    Location: Texas & Austria

    sandhammaren05 Senior Member

    With my small sportboat (35 hp, 350 lb Allison pad Vee) the cup must full at max radius, or else the 'sharp transition' to higher speed that I stated in my second 'problem' on this site doesn't occur (instead, there's only a bigger roostertail). If the cup goes around the tip (beyond max radius) then the transition to bow lift occurs very fast but then top speed is reduced by about 10%. I didn't find this to be so with my larger hp raceboats, where the cup could feather near to zero at the tip, and here's why: with greater hp the required bow lift is otherwise dynamically generated. The Allisons had airlift built into the bow, so at a high enough speed air did the trick. The pad felt greater water pressure and also lifted. The Laser (#2008) did not have much bow lift, but had small lift strakes rear along the chines. These tunneled enough air to lift the transom, and also made the boat turn very tightly on a race course.
     
  12. hoytedow
    Joined: Sep 2009
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    hoytedow Carbon Based Life Form

    :D :D :D Frosty, I tried to give you rep points for your "technical drawings", but I must "spread repu...blah..." first.
     
  13. Frosty

    Frosty Previous Member

    My drawings seem to have become a source of amusement. They took me quite a while to do even sprinkling talcum on the touch pad to get the round curves.

    I thought they were rather good. Should I have another go and see if I can improve them?
     
  14. hoytedow
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    hoytedow Carbon Based Life Form

    No. They are perfect.
     

  15. sandhammaren05
    Joined: May 2009
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    sandhammaren05 Senior Member

    Is there a German word for 'cup'? Scandinavian?
     
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