Oversized prop compromise

Discussion in 'Props' started by DogCavalry, Sep 24, 2020.

  1. DogCavalry
    Joined: Sep 2019
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    DogCavalry Senior Member

    I'm wondering about the tradeoff between prop size requirements at low speed vs high speed for the same boat.

    Props for planing boats are sized for the upper end of the speed range, because the inflow rate allows an appropriate mass flow for the boat to efficiently use the engine's power. If the prop is sized for the lower end of the boat's speed range, it can use full available power, but once it gets up to top speed, the prop is much larger than it needs to be. So I'm wondering how much extra fuel is burned at the top end for the oversized prop.
     
  2. gonzo
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    gonzo Senior Member

    You won't get to the top end with an oversized prop.
     
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  3. DogCavalry
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    DogCavalry Senior Member

    Good start. Extra blade area makes extra drag, costs efficiency. How much? How close?
     
  4. gonzo
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    gonzo Senior Member

    It is not only more blade area, but more pitch too. Look at any propeller power absorption and an engine power output curves. They are curved differently.
     
  5. DogCavalry
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    DogCavalry Senior Member

    Of course. But pitch and blade area are separate values. I just poked some numbers into a prop calculator and got a recommendation of a 10 by 16 prop. How much top end speed is likely to be lost with a 16 by 16 prop.
     
  6. DogCavalry
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    DogCavalry Senior Member

    Maybe if I rephrase the question with a concrete example. I'm building a boat that will carry loads greater that its empty but fuelled and crewed weight. Prop calculations for different conditions give different numbers, obviously. If I use the prop for light condition, high speed, there's a good chance I won't be able to get on the plane at twice that weight. And if I prop for fat and slow, there's too much prop area once I've unloaded onto the dock.
     
  7. gonzo
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    gonzo Senior Member

    If you keep the same pitch, the propeller would have less diameter to be optimized for less speed. The blade loading will be less at lower speeds.
     
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  8. DogCavalry
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    DogCavalry Senior Member

    Certainly. But that's easy. It's the other end that's tough. Running light, but with the correct pitched prop, and too much blade area.
     
  9. DogCavalry
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    DogCavalry Senior Member

    Although the blade advance rate isn't going to be that much greater than hull surface velocity (ignoring laminar flow issues) with a low slip setup, and the wetted area of the hull is so much greater than that of the prop that its surface drag is a small part of the total. Bigger prop is better, given available torque to spin it at rest.

    Right, thanks Gonzo for helping me clarify my thinking.
     
  10. BlueBell
    Joined: May 2017
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    BlueBell Ahhhhh...

    What if you could get a gearbox in there so you could change gears according to sea conditions, loading, time schedule, etc.

    I rode once in a speedboat, two seater, big V-8, high pitch prop.
    I knew the guy but had never seen his boat.
    I hop in, with a PFD on, he hits the gas and off we go.
    Up on plane in no time and then he reaches down and,
    you guessed it, shifts gears!!
    Then we really started to fly!

    Then there's adjustable pitch props to consider if an inboard.

    The whole thing has little to do with prop efficiency and more to do with prop compromise, matching performance curves and sea-trials.
     
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  11. DogCavalry
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    DogCavalry Senior Member

    That's amazing Bluebell. I might have to do something like that myself. Light and heavy are such different conditions.

    ZF makes a 2 speed transmission, maybe it was one of those. And cajunpockettunnel recently sent me to look at his interest: top fuel drag boats. Those things have 6 speed transmissions.

    Maybe we need a thread to discuss multispeed transmissions, because I don't think I can follow Gonzos advice with a 1 speed. Too much stress on the engine power curve. Need a 2 humped curve, like a bactrian camel.
     
  12. missinginaction
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    missinginaction Senior Member

    With an oversized prop, your engine won't reach it's rated RPM. This will put excessive stress on the engine and drive line, shortening it's life and causing it to run hot. You'll burn more fuel since you'll be loading the engine beyond its designed limits. You'll pull more manifold vacuum which will richen the mixture.

    I spent all of last winter studying propellers. There are so many variables. Diameter and pitch are two. The expanded area ratio (EAR) is also important. What's often overlooked is the way that gear ratios effect propeller performance. With a given amount of horsepower you can spin a smaller diameter prop faster or a larger diameter propeller at a slower speed. For instance a ski boat might work well with a 15 inch diameter prop and a 1.5:1 gear box. The three bladed prop typically used has an EAR of about 0.55. The ski boat is light and fast. A larger cruiser might require something like an 18" to 20" diameter prop. If it's a small cruiser it might be possible to run it with the same horsepower as the ski boat and the larger propeller. It won't be as fast but depending on the horsepower it will plane off. There is a big but though. Swinging that larger prop is going to require more leverage. All things being equal power wise it's possible to gear down the cruiser to 2:1 or maybe 2.5:1. At 1.5:1, the shaft RPM on the ski boat will be about 2,900 RPM at 4,400 engine RPM. The cruiser, swinging the larger prop at say 2:1 will see a prop shaft RPM of only 2,200. That 700+ RPM difference in propeller shaft speed at full power is the trade off necessary to swing the bigger prop with equivalent horsepower. You can spin it, just not as fast. Since you have more blade area (the bigger prop, especially if a four bladed propeller could have an EAR of as much as 0.86) you can develop a lot more thrust. You'll need that thrust to push the heavier boat, you'll need the additional blade area (EAR) and slower prop shaft speed to absorb the power as the propeller has to work harder to push the heavier boat.

    If you're really interested spend some time with Dave Gerr's book "The Propeller Handbook". If you can use a scientific calculator (necessary to solve the engineering equations) you'll gain a healthy understanding of what makes a propeller work.

    As with many things a propeller is a compromise.
     
  13. DogCavalry
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    DogCavalry Senior Member

    Well, interesting, and if the damn library would open I'd already have Dave's book. Phoned him yesterday, btw. He pronounces it like Gear.
    It would take a huge prop to reduce rpm to the point that the engine suffered, I think. Since loading or unloading a bunch of friends would have a much bigger effect on speed, and therefore loading, speed, slip rate. If the pitch is correct for the power/weight ratio to achieve desired speed.
     
  14. missinginaction
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    missinginaction Senior Member

    You're right about loading up your boat. Consider though, if your boat were already over propped and then you loaded it up with a bunch of friends, what would happen to your performance? You'd likely be hard pressed to even get the boat planed off. That's one reason why modern boats have a loading placard installed.

    I've never seen a shiftable boat, not to say that they don't exist because I don't know. What makes no sense to me is that the prop must be matched to the engine and the gear ratio. If you shift, you change the gear ratio, right? If that is the case you'd need to change the prop specs to match the new gear ratio. When you shift, you'd change the shaft RPM relative to the engine RPM. Perhaps anything's possible but for practicality with a boat or runabout built for mass consumption......I just can't see it.

    Just order Mr. Gerr's book. It's easy to get. I refer to my copy often and like the idea of owning it so I can refer to it at any time.
     

  15. jehardiman
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    jehardiman Senior Member

    It is possible to have a shifted gearbox with a fixed pitch propeller with the proper selection of engine, pitch and gear ratios. It is a function of how a boat draws power, a prop delivers it, and an engine provides it.
    (NAs out there, let us keep it simple ...OK)
    Boats need horsepower as thrust (T) at speed (V). This is effective horsepower required (ehp =(T*V)/550 with T in lbs, V in ft/sec)
    Propellers produce T as a function of blade area (A), diameter (D) and pitch (P) times propeller rps (n) when there is slip (s), i.e. slip ratio s=1-(V/n*P). Think of a large s as a large angle of attack of the propeller blade.
    Propeller absorb horsepower as a function of Torque (Q) and n, where Q is a function of A, D, and s. This is the delivered horsepower (dhp = (Q*n*2*pi)/550)
    Engines provide horsepower as a function of engine torque (q) and rpm. This is shaft horsepower. (shp = (q*rpm)/5252)
    Because n and rpm are locked by gearing, the torque (q/Q) relationship is the inverse of the gear ratio (60*n/rpm) = (q/Q).
    And typically shp>dhp>ehp where dhp = 0.95*shp and ehp = 0.75 dhp.

    So to produce an exact T at an exact V, there are many combinations of D, P, A, and n.
    However at any P, the n interacts with the V to generate a specific s, which sets Q....larger s, larger Q
    Given Q for that specific A, D, P, s and n then the engine q output for that combination of n and rpm needs to exactly match, or exceed the required Q, this is called the operating point.
    1) Too little engine torque, and the engine slows down (i.e. "lug" or overload the engine) until a) the Q is supportable, or b) you stall the engine.
    2) Too much engine torque and the propeller speeds up (i.e. the engine over speeds) until either a) the engine throttling limits n, b) s and Q increase to match engine output, or c) the prop tears the water apart and the prop fully cavitates in which case Q falls to effectively zero and the engine runs away.
    3) Increase the A, D, or P, you increase the Q at a given n, see 1)
    4) Decrease the A, D, or P, you decrease the Q at a given n, see 2)

    If Pitch is high enough, it is possible that at low speed s and Q were too great and the engine could not produce sufficient q at low rpm to turn the wheel, so the engine was geared down. However, once a certain speed was reached, the engine could be up shifted to increase rpm because s and Q actually decrease as speed increases relative to n (see any propeller Kt,Kq diagram). I am very familiar with this because we did run a gear change set on the high pitched propellers on SUBHUMAN II and III.
     
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