What is propellor slip?

Discussion in 'DIY Marinizing' started by Rangerspeedboat, Apr 18, 2009.

  1. Rangerspeedboat
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    Rangerspeedboat Senior Member

    Well, what is propellor slip?
    I dont kave a clue besides what my dad told me about a rubber or neoprene bushing that allows the prop to slip if it hits something. How do you calculate for it?

    Like in this propellor calculator http://www.rbbi.com/folders/prop/propcalc.htm

    My real question should be How do I figure out propellor slip for props that I dont even have.

    For example: What is the propellor slip for a mercury 9.9 4-stroke?

    Is there a database that shows this stuff? I could not find one.

    Thanks, Ranger
     
  2. Guest625101138

    Guest625101138 Previous Member

    The neoprene bushing is a protective device and has nothing to do with prop "slip" in the normally used context.

    I expect the best way to understand slip is to use an example. Say a prop has a geometric pitch of 1m. That means every revolution the prop will push through a column of water 1m long.

    To develop thrust, the prop must push more water past it than passes the hull of the boat. To continue the example lets assume the water past the hull each revolution of the prop is 0.8m. Hence the prop has pushed back 0.2m more water than has passed the hull. This is the slip. It is usually given as a percentage of the geometric pitch so in this case 20%.

    Taking the extreme example of a tug undergoing a bollard pull test with it tied to a solid pier the prop here would have 100% slip as the tug is not moving.

    The slip of a prop changes with operating conditions. You might find a simple calculator that will give you a rough guide but if you really want to understand how propellers work and do realistic calculations then I recommend you learn about JavaProp:
    http://www.mh-aerotools.de/airfoils/javaprop.htm
    There is a Java applet on the site that you can run on your computer to do propeller design or use it to determine the operation of an existing propeller.

    If you want to have a go at this I can guide you through its use.
     
  3. Rangerspeedboat
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    Rangerspeedboat Senior Member

    Thanks for the response. I see what you mean, slip is like the efficiency of a propellor.

    So its like saying what the prop will do on paper and what it actually do. Right?

    I would like to think that I understand, but I'm not sure.

    I dont really need to understand more about props than if I turn them they will push or pull me, so I dont need the java prop thing. It is interesting though.
     
  4. Fanie
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    Fanie Fanie

    If water was a solid then you would have no slip, similar like a nut that screws on a bolt. The efficiency will be the proportion of energy that is required to get a perfect propulsion to what is actually achieved.

    If you swim in water the water flows around your hand so you slip.

    In the case of a prop it slips a lot at low speed but as soon as the boat speeds up slippage reduce some and you get better bite in the water, so it becomes more efficient.

    He he... ever heard of Alice in Wonderland ? :D
     
  5. Guest625101138

    Guest625101138 Previous Member

    Slip is one component of efficiency - will be the most significant with small diameter heavily loaded props. The blades also have drag as they are forced through the water. Even if the boat had zero drag and the prop operated with zero slip it would still take power to turn the prop. This is the viscous friction of water over the blades. There are also other components of drag associated with generating the thrust.

    On paper you would like to take into account all the loss components so you can determine power. If you know the slip of the prop you can determine the speed though and get close to the required power. The slip can be determined from the diameter of the prop and the thrust required at any give boat speed.

    Thrust = water density * Vp * pi * D^2/4 * (Vp - Vb)

    Where:
    Vp is the velocity of flow over the prop (rps * pitch)
    Vb is the boat speed
    D in diameter
    Slip = (Vp - Vb)/Vp

    An example using metric units. Boat doing 10kts (say 5m/s) with 12 X 12 (0.3 x 0.3m) prop at 1500rpm (25rps).

    Thrust = 1025 * 7.5 * pi * 0.3^2/4 * (7.5 - 5)
    Thrust = 1358N (304lbf)

    Slip of course = (7.5-5)/7.5 = 33%

    The power on the hull is 1358 * 5 = 6792W or 6.8kW

    Power from thrust in the prop water stream is 1358 * 7.5 = 10188W or 10.2kW.

    I have attached the JavaProp design page for the same conditions. You will see that it determines 11.4kW so the other losses in addition to slip are 1.2kW or roughly an additional 10%. Overall efficiency is 60%.

    This thrust would be close to the condition for an outboard on say a 14 to 16ft (loaded weight about 1t) runabout just getting on the plane although the diameter of the prop is larger than a typical outboard on a dinghy so the efficiency of 60% will be better than you could expect under these conditions. The efficiency improves as the boat weight is reduced to lower drag.

    Rick W
     

    Attached Files:

  6. Rangerspeedboat
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    Rangerspeedboat Senior Member

    I understand now that I cant answer that question. LOL


    Dont be throwing no math at me Rick :D Get me all confused :rolleyes:

    I understand the equation. Problem is, I dont have any of the information needed for my situation. Just getting ahead of myself I guess. However its good info to know.

    Thanks for the Help, Ranger
     
  7. gonzo
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    gonzo Senior Member

    Without slip a propeller would have no thrust; it would stall. A propeller is like a wing , it needs flow on its surface.
     
  8. Ad Hoc
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    Ad Hoc Naval Architect

    Rangerspeeboat

    Ok, so in words, the technical definition is thus:

    Slip Ratio (its correct terminology) = analysis pitch - advance revolution.

    analysis pitch = the distance moved by a propeller during one revolution when delivering no thrust, ie just imagine turning it by hand.

    advance revolution = when loaded (that is when actually delivering thrust) this 'distance moved' is less than the analysis pitch.

    You then get into:

    Apparent Slip
    True Slip
    and
    Effective Slip

    Just to confuse matters!
     
  9. PAR
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    PAR Yacht Designer/Builder

    Propeller slip is when a prop says something it probably shouldn't have . . .
     
  10. Ad Hoc
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    Ad Hoc Naval Architect

    ..is that the same as the disc in your back?? ;)
     

  11. gonzo
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    gonzo Senior Member

    It's a Froudian thing
     
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