engine-propeller matching

Discussion in 'Props' started by Walid, Oct 21, 2020.

  1. Walid
    Joined: Aug 2020
    Posts: 20
    Likes: 2, Points: 3
    Location: algeria

    Walid Junior Member

    it's my first threard in this side and I hope things will going alright

    so,I will begin by the keller criterion for the"CAVITATION"
    the K factor depend on the number of popeller,type of the vessel,they mention:
    k=0 for high speed twin screw
    k=0.1 for Twin screw
    k=0.2 for single screw

    to be in the safe side regarding the cavitation issue,I will use k=0.2 for a single screw Xboat.

    in order to select the optimum propulsive system for Xboat I found the folowing path is the most suitable when it comes to the optimum shaft-RPM and the required DHP for a given diametre and thrust,therefore we can select the right engine and gear-ratio which can suit our propeller.
    firstley,we have:
    Z=4"blade number"
    t=0.2"by assuming the resistance augmentation factor"
    w=0.2"by assuming wake fraction"
    D=0.6m"with the adequat clearnce"
    Rtot=...KN"bare hull resistance"

    output data;
    the required thrust to reach the desiride speed is:

    we can apply the cavitation criterion;
    EAR=AE/A0=0.55+0.2=0.75 "just an example"
    now,we must plot the curve of the polynomial Kt=cont.J^2 on the B4-75 chart.
    the required shaft-RPM:
    the higest efficiency=...
    the required delivred power:
    and by assuming the mechanical losses:
    shaft efficiency=0.98
    gear efficiency=0.98
    we can find the BHP requied and the suitable gear-ration

    as we can see above,there is a clear influence of the K,t,w factors on the final results.

    Any suggestions regarding this factors!!!
  2. baeckmo
    Joined: Jun 2009
    Posts: 1,694
    Likes: 700, Points: 113, Legacy Rep: 1165
    Location: Sweden

    baeckmo Hydrodynamics

    Personally, I prefer the Burrill method for the basic estimate, because it gives an indication of the amount of blade area that is cavitating. You may have to live with a certain amount, but then a basic knowledge of the pattern gives possibilities to adjust blade shape locally to improve performance/operating life. When you have the results (rpms, propeller dia, pitch, blade number, blade area ratio aso) from the first turn of the optimizing spiral, you check the thrust coefficient vs cavitation number (both according to Burrill). Then you have a preliminary idea about the cavitational state, and what measures to take to avoid cavitation to a reasonable degree (or promote/stabilize in the case of trans- or supercavitating propellers).

    In reality, given the huge variations of cavitation qualities in the water, the relevant question is not "why does it cavitate", but rather "how come this one does not cavitate". The answer must come from an understanding of the first principles, not by picking a more or less random constant. The better your estimate of the local pressure/velocity field is, the better the final outcome. Remember that too much blade area reduces the efficiency!

    Now when satisfied with the performance at the design operating point, you do it all over again for at least two off-design points. You then have to check possible engine overload situations due to high resistance or wave/hull interaction in particular speed range(s). Again, cavitation performance may be critical due to changes in inflow velocity and leading edge angle of attack. It is not uncommon to find that the full speed design condition is conflicting with low speed/high resistance conditions.
    Last edited: Oct 22, 2020
    Walid likes this.
  3. Walid
    Joined: Aug 2020
    Posts: 20
    Likes: 2, Points: 3
    Location: algeria

    Walid Junior Member

    baeckmo thanks for your contribution,
    I found another answers for anther questions here,but it's okay
    so,based on the "cavitation occurs at high prop speed"and for a prilminary design phas I will make sure to pass through the fully loaded condition as a first step with their speed limit,in order to find the proper shaft-rps which can reach the desired torque to produce the required thrust and of course to minimize the cavitation damage as possible in term of the right EAR"keller or Burrill",once we satisfait this I guess it will be no barriers for the engine to reach his top rpm unless we have an improper gear-ratio "in cas of selecting an engin for a given prop diameter"
    Last edited: Oct 22, 2020
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