Propeller size

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

  1. Joakim
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    Joakim Senior Member

    BAR=0.75 seems quite risky regarding cavitation. I would recommend BAR~1.
     
  2. daiquiri
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    daiquiri Engineering and Design

    From the cavitation point of view, things would be a bit better with BAR=1, but IMO not so much better to justify the loss of efficiency due to higher friction drag of blades. A lower efficiency means lower attainable speed and higher fuel consumption for the same speed. For this reason too a certain small degree of cavitation has to be accepted (and it usually is) if the prop can give a better efficiency.
     
  3. Joakim
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    Joakim Senior Member

    Propeller selection is often a compromise between efficiency and cavitation. I don't know how well Burrill's and Keller's methods work for this boat, but both suggest that BAR=0.75 is too small. Burrill's back cavitation is clearly >10% for BAR=0.75 and Keller's formula suggests BAR=1.2 (with K=0.2).

    Will the efficiency even be better with lower BAR when it is cavitating more? The curves you use are for non-cavitating conditions.
     
  4. daiquiri
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    daiquiri Engineering and Design

    That's right - the curves are for non-cavitating props, hence are not strictly usable in the high-load conditions. Things will bee even worse if we look at that big cut-off keel in front of the prop, which will almost certainly act as a promoter of cavitation.

    Nevertheless, I have done another quick calc, to see if we can get any other useful info on the two props. Just out of curiosity, I have put the numbers from the post #60 into the chart for BAR=1.05 5-blades prop. See the PDF attachment. I get the following values:
    Kt = 0.210
    Kq = 0.028
    Eff = 0.475

    For a BAR=0.75 prop the efficiency figure was 0.495 (attachment in the post #60), which is a 4% higher value.

    Since I don't have time to calculate cavitation properties and dig through the cavitation charts, I will let POP software do the calcs for these two props. See the JPG attachment.

    What you can notice is that, according to POP, both BAR 0.75 and BAR 1.05 props have nearly identical cavitation number when working at the same J. How realistic it is, I don't know, but that is the result. Perhaps we should question the output of the POP software, that is yet another possibility.

    So, let's assume that the cavitation number is indeed the same in both cases. Then we have two possibilities:
    1) We can assume that the output numbers are correct and hence the BAR 1.05 gives no advantage over the BAR 0.75 prop. But it has a disadvantage of 4% lower efficiency.
    2) We can intuitively decide that the BAR 1.05 prop must have at least a slightly better cavitation performance than a BAR 0.75 prop. But it also has a 4% lower nominal efficiency. So, in that case, do these pros and cons of the BAR 1.05 cancel out each other and give the same overall performance as the BAR 0.75 prop?

    That's how we realize that we have finished in the grey zone of propeller design, in which very little can be said beforehand, based on available data.
    So I have chosen to express an opinion based on the data and charts I have at hand. Only after the first test run was made with the new propulsion setup, we shall know the actual state of things and will be able to evaluate further optimization steps - if they show to be necessary.

    I am attaching here the POP files too, so you can check the output if you have that software.

    Cheers
     

    Attached Files:

  5. Joakim
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    Joakim Senior Member

    Cavitation number is just the ratio of available pressure head to cavitation and dynamic pressure of the total (apparent) velocity at 0.7 radius. So it does not include BAR nor propeller loading at all, just ship speed, rpm and propeller diameter. So it is NOT usefull for propeller selection by itself.

    Burrill's back cavitation chart includes also propeller loading (based on total velocity at 0.7 radius) and it gets lower with higher BAR (Burrill's chart is based on projected area). POP also gives a warning, if 5% or 10% Burrill's back cavitation is exceeded depending which one you have selected.

    You can use e.g. this to calculate Burrill's and Keller's cavitations: http://14.63.170.163/v1/AUTH_078229b7-f0f5-49f9-9a0b-08fef890f216/KOCW/document/2014/mmu/baikshinyoung/9.pdf
     
  6. daiquiri
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    daiquiri Engineering and Design

    Ok, but what is the conclusion regarding the selection of the prop, in particular regarding the points 1 and 2 in my previous post?
     
  7. Joakim
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    Joakim Senior Member

    1) According to both Burrill's and Keller's method BAR=1.05 should be clearly superior to BAR=0.75 regarding cavitation.

    2) In non-cavitating conditions BAR=0.75 has 4% better efficiency, but I don't know which one has better efficiency in this case, in which BAR=0.75 has more than 10% back cavitation (= higher pressure on >10% of the back side compared to measurement = lost thrust) and BAR=1.05 clearly less.

    3) The propeller's available for this type of boat are most likely designed for non-cavitating or very little cavitating conditions (unlike e.g. high power OB propellers) and thus are likely to suffer badly in performance from cavitation. But I don't know how much 10% affects them.
     
  8. daiquiri
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    daiquiri Engineering and Design

    Ok, I would say that Duco has enough info for taking a rational decision.
    Cheers
     
  9. Duco84
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    Duco84 Junior Member

    Thanks a lot. First I must choose engine, and of course make decision regarding the gearbox and propeller.
     
  10. philSweet
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    philSweet Senior Member

    For what it's worth, when I looked at it rather briefly, I chose a lower reduction to get the blade speed up. I also used .45 as an expected efficiency. I didn't do a formal cavitation calc, but I did decide that I wouldn't go with 250hp using that prop diameter.

    I think you should consider two options -

    1. Leave the shafting and prop diameter as is and use a more conservative engine, say 210 hp.

    2. Bight the bullet and do what is necessary to carry a bigger prop. This would involve modifying the deadwood and redesigning the rudder as a minimum.

    By the way, what is the shaft size and length material? And have you checked to see if the existing shaft is able to handle the bigger torque? (This is one reason I used a lower gear ratio - hedging my bet a wee bit)

    Also, regarding the Wake Factor, taken as 0.15 so far; can we not argue that it ought to be smaller for the higher horsepower motor than for the lower hp motor driving a prop of the same diameter? Does anyone have any opinions on that? It's probably a small change, but if we know the trend, we should use it.
     

  11. Duco84
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    Duco84 Junior Member

    Unfortunate it is impossible to increase space for propeller. Even now distance between top side of propeller and hull is only 3cm.
    My plan is to change engine in order to achive max speed 12kn, and service speed 9-10kn.
     
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