a prop's rake

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

  1. Sindel
    Joined: Jun 2009
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    Sindel Junior Member

    The part number on that rudder matches what they used in production.
    Everything chris-craft related has been saved at the mariners museum in wv (specs, blueprints, original hull cards, etc)

    The boat's trim and steering are perfect with the 12 x 14 brass prop... (single right-hand prop)

    The historic nature and value of owning one of these classic boats is to maintain its originality... (she's a 1951)
    I've been to 30+ boat show and very rarely have seen one with an extended keel or forward fin/rudder.
    Have seen one with a forward rudder for steering though...
    I personally know 5 other owners with the same model boat and have ridden in 3
    One had been modified with a modern v8 and required the addition of trim tabs, but it rides rough and drinks gas like crazy.
    My boat with its 12 x 14 only drinks 1 to 2 gallons per hour...
    Another that I rode in had a KLC model engine and got up and went around 42 mph with good trim and handling...
    Mine runs a KL model engine with 105 vs the KLC's 135 hp and they weigh about the same (different pistons and carb setup...)

    So, as stated before, there will be no modification, additions, etc. to the bottom of this boat.

    ...with the 12 x 14 prop, the boat handles and trims out well even with an additional 700 lbs of passengers in the back. (do it a lot...)

    I'm not trying to better what I already have, just duplicate it! (in nibral instead of brass)

    Everyone I know with one of these old wooden boats (that care about going fast and handling) uses an old brass prop.
    One guy I ran across had purchased over 50 props from e bay before settling on one. (I don't have this kind of loot)

    Are there prop manufacturers that could duplicate mine? (in the us)

    Again I am not trying to optimize the prop or boat, rather I'd like to get a nibral prop just like my brass 12 x 14 prop...
     
  2. baeckmo
    Joined: Jun 2009
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    baeckmo Hydrodynamics

    If this was my boat, I would NOT go for a stronger propeller material. It is far better to fix a dented propeller blade now and then, than to have the "expensive" pieces (shaft, bearing bracket, bottom.......) destroyed. Btw, are you certain that the old one is BRASS? I would have expected bronze!

    And, Willoughby, we agree on the source of the behaviour, but your analysis is completely wrong; in fact you are upside down! (Result from living down under.......?)
     
  3. Guest625101138

    Guest625101138 Previous Member

    baeckmo

    I have not shown you my analysis. Your simplistic calculations give the incorrect direction of the forces.

    If you want to have a more realistic stab you need to consider the velocity profile over the prop disc. You also need to remember the the lift force on a foil is perpendicular to the direction of flow onto the foil.

    If you do the analysis correctly you will determine that the steering moment forces the boat to port and the moment is considerably higher with the 13" prop compared with the 12" prop.

    Rick W
     
  4. baeckmo
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    baeckmo Hydrodynamics

    On that we agree, the "paddling" works two ways; first the wash from bottom half of disc is directed to port, with no straightening from a rudder surface, giving a stern reaction to stbd, ie turning the boat to port. Second, the wash from the upper section is straightened by the rudder area, equally pushing the heck to stbd. Running straight fwd, both side-forces are counteracted by a slight stbd rudder.

    Now, Sindels problem was that his boat was running nose-up (=stern trimming down) with the 13" propeller, not vise versa! In dealing with rotodynamic turbomachinery, like propellers, it is essential to understand the in- and outflow combination of velocities; the vector sum of W+U equals Va. In this case it is not necessary to go further to give an understanding of the phenomenon. In my world of trouble-shooting, knowing wether a force is directed downwards or up is not trivial........!

    An additional factor is that the 12" prop is running with heavy cavitation, while the 13" has a cyclic cavitation pattern. If non-cavitating, the 12" prop would use some 160+ hp instead of the ~100 it is using today.

    So Sindel, I think that Michigan Propeller Co has a prop series called "DynaJet" that is designed for this kind of operation. Its blade profile is cambered to work better in a cavitating environment. The optimum pitch for you is probably slightly less than 14", say 13,5 but with a cup.
     
  5. Guest625101138

    Guest625101138 Previous Member

    beackmo
    The attached might help you understand how p-factor forces are determined for a propeller. While still simplistic it may get you to the next level of understanding.

    Your notion of prop wash causing the steering moment is hogwash.

    Rick W
     

    Attached Files:

  6. baeckmo
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    baeckmo Hydrodynamics

    But holy smoke, that is what I have tried to make you understand! What is not shown in your thumbnail is the important factor that the angle of attack is varying around the perimeter as well, and with that, the lift or better the flow deflection. The "thumbnail propeller" is rotating the same way as Sindels prop. The velocities and angles I gave earlier are the result of exactly the same considerations, with shaft angle 15 degrees.

    This is a well known phenomenon in industrial pump installations and also a great problem in most jet propulsion arrangements; reducing efficiency and causing high radial forces on impellers. Any textbook on pumps will cover the issue.

    So, go back to basics and try to understand that, then we can discuss Sindels problem (or is it your ego that is most important here?)!
     
  7. Guest625101138

    Guest625101138 Previous Member

    beackmo
    It is good to have agreement on the cause of the problem. So you now accept that the downgoing side (starboard side) is creating the high thrust, as explained in the above attachment, and increasing vertical force and the upgoing side has lower load and less downforce. Net result being steering moment to port (or left as noted in the diagram) and overall uplift. (I believe this is opposite to what you originally stated)

    If Sindel does not want the skeg then he needs to stick with lower diameter props that operate at higher slip. The negative pressure on the front of the the blade on the starboard side is limited by partial cavitation. The larger diameter prop has less of the blade in cavitation hence it is able to produce higher differential forces across the prop.

    Rick W
     
  8. baeckmo
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    baeckmo Hydrodynamics

    Nope, the blade force here is roughly a product of velocity x angle of attack. If the aoa > zero (as for the downgoing 13" blade; 0,55 degree), there is no lift no matter the relative inflow velocity, just drag. The change in momentum is small. In fact, if induced inflow velocity is taken into consideration, the angle of attack at the 90 degree position is negative. A flat face profile has often a zero-lift aoa of ~ -1 to -2 degrees, so the downmoving blade on the 13"x13" prop is only producing drag.

    On the port side however, with blade moving up at an angle of attack of 4,7 deg (close to max L/D for these profiles), the product of W*(angle of attack) will be higher than on the stbd side in spite of the lower inflow velocity. The result is a flow leaving the blade with a higher peripheral composant (swirl), on that side. Most of this propeller's momentum change is produced in the sector 120 degr>60 degr, the rest is more or less "idling" due to low angle of attack.
     
  9. Yellowjacket
    Joined: May 2009
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    Yellowjacket Senior Member

    Interesting analysis and results.

    One point not made is that the higher trim angle (created by the prop down wash) actually makes things worse (and contributes to an even higher trim angle) in that the shaft angle will go up from 15 degrees to maybe one or two degrees higher, so the angles will all go up on the thrust side and down more on the "non'thrust side. It isn't "running away", but a couple of degrees more is clearly making a difference.

    So, if Sindel wants a nibral prop, what specs should he be using to get closer to where he wants to be? That is, assuming that he can go to a prop supplier, what rake and/or cup and pitch should he be trying to spec that will get him what he wants?
     
  10. baeckmo
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    baeckmo Hydrodynamics

    The resulting forces, vertical and longitudinal, are totally depending on the lift and drag coefficients in the respective position of the blade. These coefficients change dramatically as the local cavitation number is reduced; I suspect that Mr W is not including this effect in his analysis, so let's have a look at the 90 degree and 270 degree positions with the 13" propeller.

    I use profile data from a crescent shape profile (no S710) that has been used for both industrial pumps and waterjets. As we are only interested in the relative values for comparison, we may lump the water density and the infinitesimal area dA into a common factor K. With a NPSH of 10.8 m we get a local cavitation number on stbd blade of 0.134, compared to 0.193 on the rising blade, all referring to the 0.7*radius.

    The rising blade, with angle of attack 4.7 deg will then have: Cl=0.21, Cd=0.03 and a total force coefficient Cf=0.212. the descending blade will have Cl=-0.05, Cd=0.03 and Cf=-0.058.

    With these figures, the force on the descending blade is -0.058*39.72^2*K. That equals -91.99*K, acting rearwards and directed down 3.8 degrees; the vertical force just a fraction.

    The force on the rising blade on the port side has the value 0.212*33.13^2*K = 232.7*K, acting forwards and down with the angle 17.03 degrees. The vertical composant is here 232,7*K*sin(17.03) =68.2*K, directed downwards. This equals 31% of the horizontal thrust from this area strip!

    With profiles, less suitable for a cavitating environment, the degradation of profile performance is even worse than in this example, resulting in erratic propeller behaviour, often occurring suddenly and without warning.

    Without cavitation, there may well be a lifting force from this propeller, depending on the balance of lift and drag, even if the main thrust is still generated by the port side blade.

    Yellowjacket: The streamlines into the prop are more or less directed by, and following the hull bottom, so the influence of trim is not significant. As for the original question about rake: it cannot change things in this case. It is the blade profile that is important, it should be designed with cavitation in mind!
     
  11. Guest625101138

    Guest625101138 Previous Member

    baeckmo and others following
    It may help you if you read through this whole link. It explains p-factor and some of the misunderstandings around it.

    http://www.qmfc.org/school/asym.htm

    You have reached the right conclusion regarding cavitation but do not understand how the blade forces are generated.

    Pushing propellers are self-stabilising. There are strong forces that are trying to align the prop with the flow. When the prop is angled downward the forces are pushing it upwards to get it aligned. It is possible to operate a pushing prop without a shaft strut. It locks itself in the flow as if supported by a rigid support. Holding a prop at an angle to flow induces large forces in the shaft and strut.

    Rick W
     
  12. Yellowjacket
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    Yellowjacket Senior Member

    Beckmo..

    Thanks for the clairfication on relative prop flows...

    How far away does the prop have to be from the hull for the flow field not to be considered to be flowing along the lines of the hull? For instance, cleary up where the tip is in close proximity to the hull, the flow field will be parallel to the hull surface. As you move down toward the bottom of the prop the flow field will slowly straighten out. There are also probably induction effects where the flow is being pulled toward the prop at work here too. Lots of things going on down there...
     
  13. Guest625101138

    Guest625101138 Previous Member

    YellowJacket
    The flow field is a complex picture. It will be slower relative to the prop near the hull. This means that the bottom of the prop will be producing more thrust than the top.

    Depending on the blade shape and how much of the blades are in cavitation through their rotation, the thrust and moment produced by the prop could be resolved by a single force acting lower than the bottom of the prop, further to starboard than the blade tip and angle upwards by as much as 40 degrees.

    The smaller prop has more blade in cavitation and this limits the variation in force on the prop disc due to the complex flow field. There is also less variation in velocity over the prop disc from proximity to the hull because clearance is greater. The smaller diameter also results in smaller moment arms.

    A larger diameter prop should work OK if the blade area was reduced. This will increase the required operating pressure and more blade will cavitate.

    With this boat and set up the prop cavitation is helping with boat stability.

    Rick W
     
  14. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    From RickW
    "...If you do the analysis correctly.."

    Hmmmmm, as always, you seem so 100% convinced of this, but when asked to show your numbers, in the same way you ask everyone else ad nauseam your reply is:
    "I have not shown you my analysis..."

    Why is this, why don't you show, so we can all see...

    It is very easy to follow beackmo's very well explained reasoning and analysis, as he does this more times in one day in his real job than i suspect you ever do in reality. So, where is your analysis then, other than just words copied from websites?

    And since you love to "show and tell"...once you have posted your calculations and analysis, for the record, just so everyone is clear, how many high-speed monohulls with single prop propulsions have you designed?
     

  15. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    yellowjecket

    Sorry you're getting the usual flip-flopping from Rick...on one post the flow is simple and easy to understand, in a patronising manner and very dismissive, but suddenly now it is all very complex. Well, that is the MO of a non-naval architect. Anything to twist away from answering the real question....

    In most normal cases anything that is 20% diameter from tip to hull is considered acceptable. Some Class societies allow down to 17%, but anything below 15% is considered unacceptable, period. However i do know of a patrol boat that has 12% clearance, it performs ok..but is not exceptional! It has the associated vibration problems etc...
     
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