Propeller size

Discussion in 'Props' started by Duco84, May 26, 2015.

  1. daiquiri
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    daiquiri Engineering and Design

    I am glad to hear this. Well-done, Duco! :)
     
  2. daiquiri
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    daiquiri Engineering and Design

    I had said that you wouldn't manage to arrive to more than 9.5 kts. My guess is that part of the speed increase is very probably due to the transom extension, the effect of which is very hard (read: almost impossible) to evaluate on paper.

    Anyways, your goal was reached - that's what matters. :)
     
  3. Duco84
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    Duco84 Junior Member

    And now new Challenge :) - Higher speed with new engine, gear, propeller..
    What are your opinions? Engine approx. 240HP.
     
  4. Jamie Kennedy
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    Jamie Kennedy Senior Member

    How much will the new engine add to the total displacement of the boat?
    How much can you lighten the boat in other areas?

    In theory you might get another knot of speed by tripling the power. Before you do that it might be fun to see how much speed you can get through weight reduction. A good test is to progressively add weight and plot speed against weight over several points, and then extrapolate back non-linearly the other way. As long as you don't extrapolate too far it should be close. I also wouldn't go beyond a 2nd or 3rd order polynomial.
     
  5. Duco84
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    Duco84 Junior Member

    "daiquiri"

    Please, do You have time to calculate possibility for higher speed with engine cca.250hp? Of course with new propeller.
     
  6. daiquiri
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    daiquiri Engineering and Design

    Hi Duco,
    I can try but it will necessarily be a guesstimate. The big unknown here is the effect of the new aft platform on the hydrodynamic performance of your boat. It is no longer a displacement-speed double-ender boat, and is not a planing prismatic hull either. It is something in between, with a very vast range of possible resistance curves.
    I'll see to give you some reasonable numbers tomorrow.
    Cheers
     
  7. Duco84
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    Duco84 Junior Member

    Thanks a lot. Just to inform You that you was in right, when I checked again top speed it was 9.5.
     
  8. daiquiri
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    daiquiri Engineering and Design

    Duco, do you perhaps have any photo of the underwater hull shape (or a drawing of the hull lines) with the transom extension mounted?
     
  9. Duco84
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    Duco84 Junior Member

    Please find some photos
     

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  10. Çemberci
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    Çemberci Senior Member

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  11. daiquiri
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    daiquiri Engineering and Design

    Duco,
    I am sorry to have made you wait, but I had a lots of work to finish before I could dedicate myself to your request.
    Anyways, here it comes.

    Without too much theory, we now have two measured operating points:
    1) HPsh=68 HP, Vmax=8.5 kt
    2) HPsh=100 HP, Vmax=9.5 kt​
    The 68 HP is a figure which comes from the prop calcs in my post #28 (which turned out to give the correct resistance curve), and 100 HP is a likely max. power your old engine is currently delivering to the prop shaft.

    Now, you are looking for an operation in the high-displacement mode (commonly called semi-displacement), but with a very small contribution of dynamic lift. Your transom extension works mostly as an extension of the LWL.

    In this speed range, the resistance curve can be conveniently approximated by the following expression:
    HPsh = k Vmax^n​
    where HPsh is the shaft horsepower, V is the max speed in kts.
    By substituting the two known pairs of HPsh and Vmax, we can find the two constants k and n:
    k = 0.0405
    N = 3.47​
    So this is the current power curve for speeds close to your actual Vmax:
    HPsh = 0.0405 Vmax^3.47​
    With a new prop shaft power of 250 HP, we can solve the above equation for the maximum speed which can be obtained. The result is:
    Vmax = 12.4 kt​

    Considering that it is a bit stretched extrapolation (3 kt above your current max speed), you should stay on the safe side and read the above number as:
    - "approximately 12 kt, if I manage to find a propeller which can give me at least 50-55% efficiency in these conditions".

    Cheers
     
    Last edited: Aug 2, 2015
  12. philSweet
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    philSweet Senior Member

    Daiquiri, typo, k=0.0405. Predicted speed is correct, though.

    Duco, this question is a very different one than your previous one. The prop will now be highly loaded, and things are getting fiddly. It will be much harder to calculate a prop spec for this case. You will probably need to try a couple different ones. The prediction is quite sensitive to the accuracy of the data, and the power model is suspect in this speed range. It would be worth trying to find a way to increase prop size some if you do the swap.
     
  13. daiquiri
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    daiquiri Engineering and Design

    Right, thanks. I'll correct it.
     
  14. Duco84
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    Duco84 Junior Member

    Only few data regarding the transom extension. Without subject spoiler boat has very big trim. After instalation of same, trim is almost zero, even negativ, until speed of 7kn, after that trim is very small.
     

  15. daiquiri
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    daiquiri Engineering and Design

    So, let's try to see what prop could suit the new engine and speed. As Philsweet correctly noted, it will be a highly loaded prop, and I expect that the main issue will be the cavitation. I have seen it happen before.

    Following pretty much the same steps seen in my post #28:
    1) Engine: 250 HP (186 kW) @ 2400 RPM
    2) Boat: LWL = 13 m , BWL =3.5 m , Displ. = 13 t
    3) Propeller: Diam = 25.6" (0.65 m - max. allowed by the available space), Pitch = unknown, Z = 5, B.A.R.=0.75 (to minimize cavitation)
    4) Max. speed: V = 12.0 kt = 6.173 m/s
    5) Motor power: P = 250 HP = 186.5 kW
    6) Motor rpm: Nm = 2400 rpm
    7) Gear ratio: 1:1.962
    8) Shaft rpm: Nsh = 2400/1.962 = 1223 rpm = 20.38 rps
    9) Assumed prop efficiency: 0.5
    10) Transmission efficiency: 0.95
    11) Overall mechanical efficiency: eff = 0.5*0.95 = 0.475
    12) Effective power: Peff = eff*P = 88.59 kW
    13) Hull resistance: Rt = P/V = 14.35 kN
    14) Thrust deduction factor: t = 0.10
    15) Prop thrust: T = Rt/(1-t) =15.95 kN
    16) Wake factor: w = 0.15
    17) Water speed at prop: Vp = (1-w)V = 10.2 kt = 5.247 m/s
    18) Advance ratio: J = Vp/(Nsh D) = 0.396
    19) Prop thrust coefficient: Kt = T / (rho Nsh^2 D^4) = 0.210​
    From the propeller charts:
    20) P/D ratio: 0.78
    21) Pitch: 0.78*0.65m = 0.507 m (20.0")
    22) Prop efficiency: 0.495 (very close to the desired value of 0.50)
    23) Kq = 0.026
    24) Torque: Q = Kq rho Nsh^2 D^5 = 1284 Nm
    25) Shaft power: Psh = 2pi Q Nsh = 164.4 kW
    26) Motor power (check): Pm' = Psh / 0.95 = 173.1 kW (232 HP)​
    The difference between the assumed motor power and the absorbed shaft power is 7%. Considering all the imprecisions of the calculation and in the initial data, the accuracy is sufficient and no further iterations are necessary.

    Conclusion:

    Assuming that the input data (both measured and estimated ones) are correct, by installing a 5-blade 0.75 B.A.R. prop with 20.0"pitch and a 1:1.962 gear, while keeping the current diameter (25.6" = 0.65 m) should get you very close to the point in which the 250 HP (@2400 rpm) engine gives the maximum power. It should increase the max speed of the boat to approximately 12.0 kt.

    A couple of important notes:
    1) The assumed weight of the boat is 13 t. Duco, you should check the weight of the 250 HP motor + gearbox which you are considering, and see that they are compatible with this assumption. If you manage to keep the added weight less than 500 kg, you can take the above numbers as valid.
    2) The analysis was done for an engine which gives 250 HP @ 2400 RPM (example: Perkins M250C: https://www.frenchmarine.com/Datasheets/M250CInfoPack.pdf). If the max-power RPM changes, the gearbox has to change in order to give approximately 1200-1250 RPM at the prop. I can help you with that.
    3) It is possible that at 12 kts the transom extension might give a contribution which is more important than what was considered in the resistance estimate in the post #56. In that case you might (just might, but I doubt it) end up with a slightly higher max speed than the estimated one, which means that the new prop will have to be re-pitched in order to optimize it for the real conditions. It is something you won't be able to know until you have made a few test rides with the new engine and prop (if you ever decide to re-power the boat). No theoretical analysis will give you that answer before a test run of the re-powered boat is done.

    Good luck. :)
     

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    Last edited: Aug 4, 2015
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