Prop Thrust Calculation

Discussion in 'Props' started by takoateli, May 31, 2011.

  1. takoateli
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    takoateli Junior Member

    I'm making a long tail outboard motor using a diesel engine and I'm trying to determine roughly the thrust the prop will be generating. Any help would be greatly appreciated.

    The prop is a 7.75" diameter, 6" pitch prop (cupped).

    The engine is turns at 3600rpm max. The boat it will be pushing will be a metal boat shaped like a dugout canoe and I'm guessing it will travel at 8-10mph.

    The reason this is important is I'm considering connecting the long drive shaft (which will have the prop on the far end) directly to the crankshaft of the
    motor which will impart the thrust generated by the propeller to the crankshaft of the motor. The motor is rated to take 287.7 LBS of axial thrust. That's the magic number. If my prop is going to generator less than 287.7 LBS of thrust I can skip the thrust bearing.

    I've seen photos of motors that are the same general design as what I'm building, they take any old motor and slap a drive shaft and prop on it, and it appears without any additional external thrust bearing. It appears they let the engine's crank shaft take the thrust.

    Any input you could give me would be greatly appreciated. I'd like to know how much thrust the prop would develop as well as anyone's thoughts about applying the thrust directly the engine's crankshaft is a good or bad idea.

    Thanks!
    You can see the motor at http://gregihnen.me, then under about me and my motor.
    Greg Ihnen
     
  2. gonzo
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    gonzo Senior Member

    The thrust developed against the shaft depends on the resistance of the hull.
     
  3. takoateli
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    takoateli Junior Member

    Yeah, I've read that too and have seen it in action.

    I was hoping someone might have a ball park idea about how much thrust that motor might make.

    Greg
     
  4. daiquiri
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    daiquiri Engineering and Design

    Not always so, Gonzo. ;)
    Consider these two cases, for example:
    - an accelerating boat, when the prop thrust is higher than boats' resistance, the difference being necessary to overcome the inertial force
    - the bollard pull, when the boat is standing still yet the prop is pushing hard.
    Cheers
     
  5. daiquiri
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    daiquiri Engineering and Design

    If it's just for this time, than the very approximate calculation goes like this:
    1. Take the engine power in Watts. If it is given in HP, then multiply by 746 to obtain the power in Watts. Call this power BSP (brake shaft power).
    2. Multiply BSP by transmission and propeller efficiencies. You can assume around 0.5 as a maximum total efficiency of such a small prop. Call this power PE (effective power).
    3. Convert the speed to meters per second (m/s). To do that, multiply the speed in knots by 0.514 . Then multiply again by 0.95, to approximately take into account the eventual effect of the hull wake. Call this speed Vp.
      There might be no wake effects if this is a planing hull with props clear of obstructions ahead. But still I'd keep that 0.95 factor above, because it will give a safety margin to the calculation of the thrust.
    4. To obtain thrust given by the prop, divide PE by Vp: T = PE / Vp .
      This will be the available thrust in Newtons, for that speed and engine power. I'm using all SI units, to avoid errors related to inconsistant units. To convert this thrust to lbs, multiply by 0.225 .
    That's all.

    If you think you'll need to do more such calculations in future, and/or have them more precise, I suggest you to buy the excellent Dave Gerr's book "The Propeller Handbook". You can find it at Amazon for $12, a real bargain for the knowledge it contains: http://www.amazon.com/gp/offer-listing/087742988X/ref=tmm_hrd_used_olp_0?ie=UTF8&condition=used

    Cheers!
     
  6. gonzo
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    gonzo Senior Member

    Accelerating fast (hole shot) is a function of the mass of the boat more than anything else. However, that would be of short duration.
     
  7. takoateli
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    takoateli Junior Member

    Thanks! I wish I could buy you a beer or a few to have you do the math. But it does sound straight forward. I'm just curious about the prop efficiency you mentioned above. It sounds like it's only 50% efficient, that half of the power just goes for drag on the prop itself? Wow!

    Knowing that boats I will run this motor on with a 15 hp will do around 14 mph I'm assuming I'll be shy of that, maybe around 10 mph. so I'm going to use that in my calculations as a starting point.

    Now as I understand it that formula only good for when the boat is underway otherwise you get a divide-by-zero error. There's a separate calculation for "bollard pull" thrust. Would you recommend I calculate that too so I know the thrust from a hole shot? Or should I just refrain from pulling bollards. : - )

    Greg
     
  8. takoateli
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    takoateli Junior Member

    Daiquiri,

    I ran the calcs at 8 mph and 10 mph and the thrust was fine. Well below what the motor will tolerate. But, I'm betting the farm on that .5 prop efficiency. If that's not in the ballpark, that's to say if the efficiency is much higher, then my motor could be toast. Are you pretty sure of that one?

    I just have to ask, strawberry, pineapple or banana?

    Greg
     
  9. daiquiri
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    daiquiri Engineering and Design

    For a prop of that size, 50% is even a good efficiency (it contains the drivetrain efficiency too) if you're lucky to have a well-choosen prop and if you're diligent enough to keep it clean of marine growth. :)

    Yep, the above math is valid for non-zero velocities. For the bollard pull, you can use the approximate method from Dave Gerr's "The Propeller Handbook". It says that the pull is given by:
    T = 11.87 * ( SHP * D )^0.67
    where:
    - T is the bollard pull, in pounds
    - SHP is the engine shaft horse power (calculate it as brake horse power, BHP, times 0.96)
    - D is the propeller's diameter in inches.
    - ^0.67 means "elevated to 0.67th power".

    And yes, it would be wise indeed not to waste your fuel for pulling stupid bollards... ;)

    Cheers!
     
  10. takoateli
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    takoateli Junior Member

    I really like math in situations like this, like I like a hammer or a saw, because it's doing something cool for me. In school I hated math. Now it's my friend!

    Thanks so much! I really appreciate the help!

    I wish I could buy you a daiquiri!

    Greg
     
  11. daiquiri
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    daiquiri Engineering and Design

    See the previous comment. Even if it should be higher, it will hardly go over 55% for the prop of that size.

    One more thing. Don't make a confusion between mph and knots. Mph means "statute miles per hour", while a knot is "nautical mile per hour".
    1 knot = 1.15 mph.

    I knew it would happen, sooner or later... :eek:
     
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  12. takoateli
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    takoateli Junior Member

    Cool. Then I'll do a worst case scenario and see what happens with a .55 efficiency and maybe even just a tad slower in the water. I could end up using this thing to push our fuel barge.

    Yeah, I caught that but thanks for pointing it out. I did the mph to knots conversion on my Mac's dashboard widget.

    Thanks so much! I greatly appreciate everyone's comments, thoughts and time.

    Greg
     
  13. gonzo
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    gonzo Senior Member

    You are getting great advice from daiquiri
     
  14. quantumgremlin
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    quantumgremlin New Member

    Propeller thrust

    I strongly recommend getting a copy of "The propeller handbook", it is a suprisingly interesting read even if you don't wan to do the maths.
     

  15. FAST FRED
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    FAST FRED Senior Member

    A simple rule of thumb is a good prop will make 20 lbs of thrust per hp.

    Sure in theory it can be higher , but 20 works.

    Use your engine rated power , 10HP x 20 would be 200 lbs thrust.

    That work for you?

    FF
     
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