a prop's rake

Discussion in 'Props' started by Sindel, Aug 7, 2009.

  1. Sindel
    Joined: Jun 2009
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    Location: Ohio

    Sindel Junior Member

    Interesting discussion here bring up cavitation...

    In the 30's and 40's it was commonplace to do.
    Miss England II made by the british had 2 "secret modified" rolls royce aircraft engines that turned its props like 12,000 rpm...

    So decreasing the area of the blades (say 13" to 12.5") will create more cavitation and perhaps a better trim alone or is the negative rake creating the additional outward force of water bouncing off the bottom of the boat a necessary component?

    And say I was to spend the money to do a prop modification, how close would it be to make the newest 13 x 13 into a 12.5 x 13 in comparison to a 12 x 14 prop?

    Ya, I know it's a Bronze prop, but lots of people refer to them as brass because they are so soft...
    I thought I said that I live on a river full of debris:
    [​IMG]
    ...and with that; shaft material and general drive train strength are another issue I should address, but that's another discussion...

    The shaft angle is supposed to be 12* according to the drawings I have.

    The props:
    12 x 14
    [​IMG]

    and the 13 x 11
    [​IMG]
     
    Last edited: Aug 13, 2009
  2. Guest625101138

    Guest625101138 Previous Member

    Sindel
    Under cavitating conditions the force on the front side of the blade is reduced. The back face has to do a higher proportion of the work.

    The factors that contribute to cavitation are the velocity of the blade through the water, the area of the blades and the shape of the blade section. A flatter section will have less tendency to cavitate than a thicker section at the low angles of attack that the blades normally operate at.

    You will get increased cavitation by reducing blade area through reducing diameter. Whether this achieves the same result as you get with the 12 x 14 prop depends on the blade section and the chord length of the blades.

    The larger diameter prop seems to have the blades skewed more as well. This can affect cavitation.

    You could spend a fortune on analysis to try to determine what is going on with some precision in the complex flow field around your prop to get better understanding. However the most economic way is to modify and test. You already have a prop that satisfies your requirements. The easiest way is to try to copy it as close as you can. Taking the 13" down to 12.5" will be a good start. Also compare blade profiles and blade plan forms. A single section at the 70% radial position will be useful for comparison.

    Rick W
     
  3. Guest625101138

    Guest625101138 Previous Member

    If you are able to measure the blade sections I am able to provide values for the pressure coefficients that will allow comparisons of the blade velocity that promote cavitation.

    Also a propeller of modern design is likely to be better profiled to prevent cavitation, which is not what you want.

    Rick W
     
  4. baeckmo
    Joined: Jun 2009
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    Location: Sweden

    baeckmo Hydrodynamics

    Ad Hoc, thanks for your kind remarks! In your #45 thread, you intuitively note that the magnitude of the vertical force fom the prop may not be enough to create the behaviour. I think that's a correct observation. Due to the asymmetry with this propeller, we have a high speed, low pressure jet blowing almost parallell to the bottom. This, we know, has the effect of reducing the surface pressure over a significant portion of the bottom, resulting in a noticeable trim change!

    As for this Willoughby character, I may have overestimated his qualifications. Now that he has revealed that he is not even familiar with basic hydrofoil theory, I believe you are right; his nonsense is best left unattended, hoping that people don't loose orientation in his endless fog!
     
    1 person likes this.
  5. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    baeckmo
    I'm always very interested in your more detailed hydrodynamic assessment/analysis than I "usually" do when i design (don't have the time); i just check the basics and the "what if" scenarios, and focus on the remaining design of the vessel. Your expert diagnostic skills are very clear and much appreciated. It is easy to spot the professionals on this site.

    As you've already notice Rick is not one, doesn't have the education, diagnostic skills, comprehension nor experience of real design save for his computer playstation and 5m peddle boat. But sadly those that come on here who need help get lost in his fog. Very sad, as you noted, more ego than help..

    Pity really, his practical knowledge on small 5m peddle power boats and what wire needs to go where electrically, could be of good use to many, but he can't help himself. As you noted, the ego doesn't allow for the "oh hang on, I've never designed a real boat before perhaps i should listen and learn from professionals who do this for a living"...oh well. I know it stops many from wishing to contribute where their professional advice would be very useful, since I've had many emails from others stating as such...very sad!

    He still hasn't said how many real full-on high speed prop boats he has designed....i'm not holding my breath!

    PS..i have my stopwatch on now...seeing how long it takes for the very predictable and childish neg. points!...ergo, not a professional!
     
  6. apex1

    apex1 Guest


    Just as a aside Bronce is´nt soft!!! And it has nearly the strength of steel! If yours are "soft" they are probably "Marine Bronce" which is just brass!
    Richard


    And I like to say, that I fully concur on Baeckmo´s and Ad Hoc´s statements about Mr. Willoughby and his (limited) knowledge! It is a shame to see his endless fights against professional knowledge. He knows quite much, but does not know his limits, neither his softwares limits.

    Richard
     
  7. Frosty

    Frosty Previous Member

    Sindel ,--the damage on those propellers shown are not from driving over river debris but from reversing back over something like a rock.

    The damage is on the trailing edge.

    No propeller of marine brass or stainless steel will protect you from careless driving.

    By the way that is so easy to repair.
     
  8. baeckmo
    Joined: Jun 2009
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    baeckmo Hydrodynamics

    One thing Sindel, before we dig into the details on propeller: You are running direct drive 1:1, and have a right-hand screw, how come? Is the engine really rotating this way, or is there a V-gear or something along the line? Or have I missed this info somewhere?

    And: are 3200 rpm the correct speed for max power for this engine?
     
  9. Frosty

    Frosty Previous Member

    I asked this question too. if you look at the picture the engine it is far aft meaning there must be a V drive.

    The shaft looks like 15 degrees meaning the prop is trying to push the boat in the air meaning trim will be sensative to weight distribution.
     
  10. Ad Hoc
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    Ad Hoc Naval Architect

    Exactly....hence post #19
     
  11. Sindel
    Joined: Jun 2009
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    Sindel Junior Member

    Hold on...

    I didn't realize we had smart___ rocket-scientist here?

    [​IMG]

    Damaged caused me to lose about 1.5mph and probably contributed to shaft braking later with 13 x 11 prop...

    Cause of damage above...
    Set screw points sheared off... (possibly caused by bad 12 x 14 prop shaking?)
    [​IMG]

    Upon arriving at the damn to let the dogs out, the shaft popped out and the prop hitting the rudder caused the trailing edge damage...

    And yes, it's a very easy fix... Just cost about $100 (I wish santa or one of his 'friends' would give me the money...)

    The 13 x 11 prop is damaged too...
    [​IMG]

    That's another $70...

    [​IMG]

    And look... The shaft is broken, another $200 (aqualloy)
    That's close enough to call $300, the repair shop is 80 miles away...

    When can I expect my new friend to send me some money?

    Okay now that I got that out of the way...

    The drive (1:1)
    [​IMG]

    The engine turns the same direction as the prop, it was just designed that way...
    [​IMG]

    And yes that IS a lot of motor...
    Contributes to 1700 lbs of the boat's weight...
    And yes the "book" says 3200 peak rpm...

    Now before weight distribution is mentioned, might I suggest examining the "Miss America X"
    It held the water speed record for nearly 5 years going 124.86 mph
    The old boats like mine were designed based on winning race boat designs, before computers and all the math...

    [​IMG]

    [​IMG]

    How is that for weight distribution?
    Can they move those engines any further back?

    I also might add...
    Years ago, I went to a lake with a nice marina and was able to test drive several of the props they had...
    Tried 12 x 16 first... (nose high trim...)
    Next tried 13 x 12... (nose high trim...)
    Even tried 12 x 14 new out of the michigan wheel box (nose high trim...)

    The rake is what I have always thought was the key, but everyone says that it would make the prop not work as well...

    I don't really care if the prop is working at its best...
    The one I have now (12 x 14 with its unoptimized rake) allows me to go out for 2 to 3 hours pulling someone on a tube or kneeboard and use less than 10 gallons of fuel...
    It works go enough for me...

    I just want a spare like the one I've already got...
     
  12. baeckmo
    Joined: Jun 2009
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    baeckmo Hydrodynamics

    Nice piece of machinery; certainly deserves some care! Now, one problem with installations from this era is that the engine is mounted stiff to the boat and propshaft is flanged stiff to the transmission. On one of your photos we can see a shaft broken within the coupling hub. The fracture is a typical fatigue breakage, caused by the combination set screw dents plus key recess. A few more blows to your prop, and the shaft bracket may be at risk as well!

    Thus I would strongly recommend that you install a flexible shaft coupling, rubber "donut type" (Centaflex, Bullflex......). It is a deviation from the original, but a reasonable compromize to save your beauty. It may require sliding the engine slightly forward on its girders, but it will save you a lot.

    First, they come with a clamping hub for the flange, so there are no dents or key recesses to initiate a shaft fracture. Second, the rubber "donut" will take much of the energy when your prop hits the unavoidable log, saving both propeller, shaft and gearcase. And third; the costs you have had for propeller and shaft repairs are a fart in the wind, compared to the costs for repairing a mashed gear, wich is taking all the punishment at the engine side of the propline.

    So, for propellers: There are several varieties on the market; a 12"x14" from one series is not equal to a 12x14 from another series. Take Michigan for example; if the 12x14 you tested was from their "M" series, it has a blade section shape that is totally unsuitable for the cavitating performance you need (but it is perfect in ordinary, lowspeed, submerged applications).

    This difference in shape may boil down to a few mm:s deviation in blade camber; it is not possible to distinguish one from the other without measuring the section. It is therefore not possible to say other than that the 12x14 you now have, is probably of a high-speed type, most certainly with a cambered pressure side, as opposed to most of the others you have tested.

    I suggest that you contact a propsupplier that is familiar with props for skiing; these boats often use direct drive. Modern engines often reqiure 13 or 14 inch diameters, but there are 12:ers around. For similar operation we have good experience from "Mikado propellers", type E, which is a 3-bladed prop with area ratio 0.73.

    On top of this, I would like to see the actual running trim angle of your boat (no rocket science,just a cheap inclinometer, mounted longitudinally, base parallell to keel aft), to get a better idea of what your "nose-up" actually means!
     
    Last edited: Aug 14, 2009
  13. Sindel
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    Sindel Junior Member

    Thanks for the compliment baeckmo...
    Mines the little 18 footer you know?

    Bent the "strut" 2 years ago and had to replace it...

    In the picture of the engine from above; can you see it? (drivesaver coupling...)

    Got 4 of those gearboxes behind the garage? Couple engines too...

    So back to my original question, so to speak...
    How can a prop be measured to determine its components?
    And what do I need to measure? (point to point from where to where...)
    Where are the various angles originate?
    etc.

    I'll see about getting a inclinometer, but I usually ride 1 hour before sunset to 1 hour after dark... (kind of hard to see)
     
    Last edited: Aug 14, 2009
  14. baeckmo
    Joined: Jun 2009
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    baeckmo Hydrodynamics

    Hah, so I can refer to you about spares then.......? The Drivesaver was noticed, but unfortunately it has far from the torsional flexibility you get with the rubber couplings I mentioned; there we are talking about several degrees of torsional flexing. The main virtue of your drivesaver is to allow a veeeery slight difference in shaft inclination (fraction of degree), plus a valuable resistance to Galvanic currents through the shaft.

    I understand you have access to a lathe? If yes, then I will prepare a message to you on the measuring topic. I guess your driving habits are not exactly reducing your repair budget......, but would they really stop you from reading an instrument?
     

  15. mark775

    mark775 Guest

    Damn. The dam where the dogs are dropped is spelled "dam". Hate to be a dissenter, you guys are having so much fun by yourselves ( "need a ruler?" LOL). Getting mighty technical here and you guys are way over my head with your forces generated, analyses, etc. but didn't the gentleman say that the boat doesn't work with a 13" diameter prop and does work with a 12" diameter prop? I would think about (not to start any trouble - maybe I missed something!) maybe using a 12" diameter prop (rake...What rake?).
    Now, that is one ugly rudder but true to original, I'm sure (and good for shallow water). That is an uglier shaft coupling. This beautiful boat deserves (as Frosty somewhere pointed out) the repeatable, sustainable accuracy in alignment afforded by a matched tapered shaft/coupling. A split-coupling is in a distant second place. What you have there is an Obamanation (one of my new favorite words! don't need to start that here - just a joke). Take that coupling and throw it and its safety-wire away. Set screws have no application here. Through bolt and use a nylock, if you use a split-coupling. There is no need for keys on this set-up. Don't use them. However, the coupling and prop must be lapped to the shaft. Do I see never-seize(?) on the tapers you have - don't use that either. The cotter-pin is superfluous, as well. Stick with the bronze prop. I, personally, feel that any kind of drive-saver, etc. is just something to go wrong and not too clean. Simple is elegant. Orange plastic thing down there is anachronistic and ugly.
     
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