Prop pitch (again...)

Discussion in 'Boat Design' started by Deering, Apr 26, 2019.

  1. Deering
    Joined: Feb 2005
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    Location: Juneau, Alaska

    Deering Senior Member

    Hi all,
    I know this topic has been beat to death, and I’ve read back through a lot of the discussions, but I’m sort of thick so I need to ask again. Apologies.

    During sea trials of my powercat redesign I noted that it couldn’t quite achieve full rated RPM. Rated for the diesel Volvo D4 is 3,500 RPM (225 hp). These are new engines, gear box, props. Reduction is 2.5:1. Prop is 22x20 inches. I could reach about 3,400 RPM at 17 kts. That will no doubt drop a bit more at full load.

    My understanding is that prop sizing should allow the engine to achieve full or a bit above rated RPM or risk overloading the engine. But I will rarely if ever run this boat at full power. Peak torque (based on manufacturer’s curves) is at 2,500 RPM, which happens to correspond with my target cruise speed (12 kts). Because the engines are new I’m trusting that the published hp and torque curves are accurate - maybe a bad assumption?

    So my questions:
    - Will I damage my engines running at cruise speed for this summer?
    - If I never run near top speed, is there a reason to adjust pitch ever?
    - Is there a way to estimate the RPM range where the engine would not be overloaded based on the information I already have (engine curves and max measured RPM)?
    - Are there efficiency advantages to the current configuration?

    One other note - the 225 hp version of the D4 series is the lowest output flavor they sell. They can wring up to 300 hp out of the same block. Does that ‘de-rating’ of my engine yield an advantage in this matter?

  2. DCockey
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    DCockey Senior Member

    philSweet likes this.
  3. philSweet
    Joined: May 2008
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    philSweet Senior Member

    You are fine as is. The 3500 rpm rating is only for use in light planing craft going 25 knots or so, and limited to 1 hour out of a 12 hour jaunt. Going slower is tougher on the engine. So you should be running less than 3500 with only a 17 knot top end. The same engine is available as 180 hp @ 2800 rpm with a heavier rating and sounds like it is closer to where you should be anyway. I'd just fit a stop at 3300 and call it good.

    Engine rating key - Engine Ratings

    Volvo Penta series - Diesel Inboard

    D4 - 180 pdf -

    D4 - 225 pdf -
  4. Deering
    Joined: Feb 2005
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    Location: Juneau, Alaska

    Deering Senior Member

    Thanks guys! David, that is exactly the type of info I was looking for. But one note Calder made caught my eye. ‘As props get bigger they impose higher torque loads at every speed, increasing cylinder pressures, temperature, and other stresses’ (p. 58). Am I safe in assuming that as long as I’m not running the engine at its upper bounds this won’t become an issue?

    Phil, thanks. I had forgotten about the 180 hp D4. I’d have to go back to my earlier calcs, but at that time a 225 hp engine was indicated as I recall. Comparing the D4-225 with the 300, peak rated torque is 540 vs almost 700 Nm respectively. Is it fair to conclude that engine stresses will always be lower on the 225?

    Based on what you guys are sharing with me, I’m not sweating the prop size too much, and in fact it might be beneficial. If other folks can confirm my logic, or have a counter argument, I’d be interested to learn more.

    Thanks again.
  5. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    As Phil notes.
    Simple relationship:
    rpm (new) = cube root [RPMo x (Power(new)/Power0)]

    0 = original
    new = target
  6. Deering
    Joined: Feb 2005
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    Location: Juneau, Alaska

    Deering Senior Member

    That’s not making sense to me. If Power(new) = 135 kW (at 3,300 RPM from manufacturer) and Power(old) = 160 at 3,500, your formula yields 14.3 for the RPM(new). Huh? For a given power I already know the RPM from the power curves.

    If I follow Phil’s advice and limit higher RPM to 3,300, is there a lower RPM that I should stay above?
  7. baeckmo
    Joined: Jun 2009
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    baeckmo Hydrodynamics

    The D4/225 hp engine has a very flat power curve on top. It delivers ~218 hp @ 3000 rpm, and 223 @ 3400. You are quite on spot, even with a slighly higher displacement. If you arrive at an operating point (extreme load or towing) with maximum rpms of 3000 at full throttle, then you should back off slightly.

    If you find the working point (223 hp/3400 rpm), the propeller power curve will approximately follow the polynome P=const*n^2.5; where P is power and n is rpm. Starting with the working point, you get the constant value c=223/3400^2.5; . So, if you throttle down to 3000 rpm, the propeller will absorb ~163 hp at whatever speed this will propel the hull. Remember at all operating points, except at max throttle, it's the propeller that decides what power is developed, not the engine.

    The exponent 2.5 used here differs from the exponent of 3 used by Ad Hoc, which is the theoretical value for equal inflow factors at all speeds. But with a real hull, the inflow is varying since the hull resistance in reality follows a "non-linear" characteristic, thus the adjusted exponent.

    ...and don't bother with limiting the rpms with this prop setup.

  8. Deering
    Joined: Feb 2005
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    Location: Juneau, Alaska

    Deering Senior Member

    I’ve heard that statement several times in the past but never really grasped it. Your explanation, along with the other replies above, finally turned on the lightbulb for me. At say 2,500 RPM, the propeller is going to create a certain torque, depending on the hull, loading, and sea state. The engine can not produce any more power because the propeller is not going to provide any more torque unless the RPM is increased. If the torque of the propeller at 2,500 RPM exceeds what the engine can deliver, the RPM will decrease until it can.

    And now I have a better understanding of the 2.5 exponent referred to in the manufacturer’s propeller power curve. Two lightbulbs.

    Thank you for your input. It was very helpful.
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