Propeller size

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

  1. daiquiri
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    daiquiri Engineering and Design

    The prop works behind a full-length keel. The boat is a typical Mediterranean double-ender, which seldom have streamlined trailing edge of the keel. Hence, I have used the value 0.15 for the wake fraction.

    I don't think it is safe to consider 10 kts as maximum design speed. The engine is 20 yrs old and IMO we should assume that it will not be able to deliver the declared 110 HP due to wear. Then you have a driventrain which will absorb some 5-6% of the power, leaving you with some 100 HP at the shaft, maybe even less.

    Considering the current boat performance, it is IMO unlikely that it will be able to arrive to 10 kts with this available power.

    Cheers
     
  2. Joakim
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    Joakim Senior Member

    Wake fraction of 0.1 was just a guess. It may be 0.15 or even higher, but it has a rather small effect. With 0.15, 29" and 8 kN at 10 kn:
    708 rpm
    Eff: 0.507

    I agree that 10 knots is very optimistic. It may even be that 8.5 knots is all he gets. I just showed the calculations at the same speed you used.

    8 kN, 0.15 at 8.5 knots:

    29"
    676 rpm (clearly too heavy!)
    Eff 0.456

    25"
    747 rpm
    Eff 0.469

    Current propeller with another gear ratio
    796 rpm
    Eff 0.474

    1:2 gear ratio and 16" pitch
    1036 rpm
    Eff 0.456

    The truth is somewhere between the 8.5 knots 0.15 and 10 knots 0.1 calculation. I don't know where. In both cases it seems changing the gear ratio a bit with the current propeller gives better efficiency than changing the pitch.
     
  3. daiquiri
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    daiquiri Engineering and Design

    Let's see what will I get for the 1:2 transmission, with the use of charts.

    Carrying on from this point of the post #28:

    With the 1:2 ratio, the prop rotation speed becomes:
    Nsh" = Nm / 2 = 1100 RPM = 18.3 RPS
    J" = Vp' / (Nsh" D) = 0.349​
    The new Kt is
    Kt" = T' / (rho Nsh"^2 D^4) = 0.14​
    and from the charts we get:
    P"/D = 0.62
    P" = 0.40 m (16")
    Eff" = 0.495
    Kq" = 0.016
    Q' = Kq" rho Nsh"^2 D^5 = 637 Nm​
    The shaft power with this new prop is
    Wsh" = 2 Pi Nsh Q' = 73 kW (98 HP)​
    and it is also very close to the max. available.

    I am including the updated charts, with all three solutions.

    Conclusion:

    There are two possible solutions which would allow the motor to work at full power:
    1. Keep the existing gear and change the prop pitch to approximately 28"
    2. change the gear to 2:1 ratio and change the prop pitch to approximately 16".
    Both will result in a similar propeller efficiency of approxim. 0.50.
    The solution n.1 is clearly less expensive.

    Changing the gear only, without changing the propeller pitch, would decrease the propeller efficiency to 0.42 and is IMO out of consideration.

    Cheers
     

    Attached Files:

  4. Joakim
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    Joakim Senior Member

    The other results are very similar to mine, but I really can't get that low efficiency for changing the gear only. Or are you refering to changing the gear to 2:1 and still using 22.8" pitch? That is out of the question, but using the current propeller with 2.6-2.7:1 (depending on the actual speed, target rpm and wake fraction) would be slightly better than the other ones.

    If I let the software optimise for 8 kN, 0.15 and 25.6" diameter at 9.5 knots, it suggests 19" pitch, 0.5 expanded area ratio (10% Burrill cavitation limit), 933 rpm and 0.522 efficiency. So about 2.4:1 gear ratio should be close to optimal and the current propeller with about 2.6:1 wouldn't be that bad (0.507).

    The optimiser doesn't always work that well, thus there may be a even better solution.

    So according to these, the best efficiency would be to change both the gear and the propeller, but that is obviously the most expensive option. Changing just the gear is slightly better than changing just the propeller. The OP probably knows best what is the local cost and feasibility of these options.
     
  5. daiquiri
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    daiquiri Engineering and Design

    Look, I have shown all the calcs and charts involved in the process of finding the correct propeller, so it can be reviewed and checked by anyone if necessary.

    Hitting the "Run" button in the POP software (or whichever you use) cannot give a similar insight and control of the parameters. I prefer the manual method.

    It is absolutely necessary to include the hull resistance estimation in the process, otherwise the results will be detached from the physical reality, which is: hull + prop working together and mutually influencing each other. The POP optimizer will find a result based on your input and will give a result which you have no choice but to trust, since you have no control over the process and no knowledge of the algorithm used. Most importantly, you will never know if the interpolating curve has gone out of the boundaries at some point during the iterative process. That might give false yet apparently correct results.

    With the manual method, it is all immediately visible in the chart. If you get out of boundaries, you literally go out of the paper sheet.

    Cheers
     
  6. Joakim
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    Joakim Senior Member

    Please show the calculation resulting to only 0.42 efficiency.
     
  7. Duco84
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    Duco84 Junior Member

    Thans a lot. I prefer the manual metod, and according to this and costs, I will try with pitch 29". Once again thank you.
     
  8. Duco84
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    Duco84 Junior Member

    Sorry 28"
     
  9. daiquiri
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    daiquiri Engineering and Design

    Actually, it is 0.40. My mistake. The dotted line extended to the P/D=0.9 (current prop) gives the Kt, Kq with 1:2 ratio gear, max. engine RPM. In reality it is just an approximate and not attainable value since it would result in a massive required power, so it would be necessary to perform a couple of iterations in order to match the speed and resistance for both the hull and the prop. I don't have time to do it now, job is waiting.

    The calculations for other configurations are shown in my previous posts.

    Check the attached charts, where I have extended the dotted lines for you.

    Cheers
     

    Attached Files:

  10. Joakim
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    Joakim Senior Member

    Yes, I just opened your files before you answered. Didn't realise, that you had the lines there, not just the basic charts.

    2:1 with the current propeller is surely out of the question. Way too heavy. Yes you get 0.40 efficiency, if you force 1100 rpm, but then you need almost 300 hp and propeller is heavily cavitating, which makes the result totally irrelevant. In reality you would get over 0.5 efficiency and less than 800 rpm thus quite close to what the OP now has but engine rpm less than 1600 instead of 2200 and overloading the engine.

    As I have said a couple of time, about 2.6:1 would work with the current propeller and have a good efficiency, if changing gear is an option (OP asked about that) and that gear is available.

    I don't see much difference using the same charts via software or directly. You have all the same parameters. Using the automatic optimiser is a different thing. I don't usually use that at all.

    Yes it would be important to know the resistance as well. We both have calculated it using propeller thrust. Then you can try to calculate the resistance at higher speed, but with the data we have that is not going to be accurate at all. So we don't really know what the speed will be.
     
  11. daiquiri
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    daiquiri Engineering and Design

    At 1600 RPM the engine gives far less than 110 HP - perhaps some 70-80 HP, like what it gives now. Hence the 10 kts you have used to obtain 53% efficiency is simply not realistic. IMO, more realistic speed for this configuration and available power would be more like the current 8.5 kt, and you'd discover that the efficiency falls down to much lower values in this condition.
     
  12. daiquiri
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    daiquiri Engineering and Design

    Good luck and let us know how did the new prop behave. :)

    Cheers
     
  13. Joakim
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    Joakim Senior Member

    The engine seems to give about 85 hp at 1600 rpm assuming it is the same as the current model: http://www.imr-rakovica.com/englishbre/imr-motormS46TGrafbig.htm

    So it would be slightly better than the current situation, but overloading the engine. No good! The efficiency would be around 0.5.

    I have not said that you could reach 53% with the current propeller and 2:1 gear. It may be possible with 2.6:1 gear, if the speed increases to about 10 knots, which is not likely to happen, but not impossible.
     
  14. Duco84
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    Duco84 Junior Member

    Once again thank You. In next two days I will visit workshop and change pitch of current propeller on 28". I will try, but I worry that it will be to heavy for this engine.
     

  15. Duco84
    Joined: Feb 2014
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    Duco84 Junior Member

    Pitch 26.8"
    Result- Speed 10kn :)

    Thanks a lot and best regards.
     
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