Calulated propeller efficiency

Discussion in 'Props' started by 7228sedan, Nov 5, 2018.

  1. 7228sedan
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    7228sedan Senior Member

    Thanks Barry, I had read this post before as I was considering a diesel re-power at one time. Seemed that there was nothing conclusive.
     
  2. Barry
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    Barry Senior Member

  3. 7228sedan
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    7228sedan Senior Member

    Thanks Barry, I'll have to bring the tape over to the yard when I have her hauled. Based on my best guess however, I cannot get the numbers to line up.
    This calculator is saying that I shouldn't see more then 17 knots at WOT based on the 330 HP. I see a solid 20 into the wind. in order to get the 20 knots I need to enter 450 HP.
     
  4. Barry
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    Barry Senior Member

    [​IMG]
    Waterline length in feet: 26 feet Beam at the waterline in feet: 10.5 feet Hull draft in feet (excluding keel): 2 feet Vessel weight in pounds: 10000 lbs Engine Horsepower: 330 HP
    Number of engines: 1
    Total Engine Horsepower: 330 HP

    Engine R.P.M. (max): 4200 RPM
    Gear Ratio: 1.5:1
    Shaft R.P.M. (max): 2800 RPM

    Number of shaft bearings (per shaft): 1
    Hull Constant: 150
    Desired speed in Knots: 15 knots
    Horsepower Calculations
    This will calculate the maximum horsepower and torque available at the prop(s).




    Speed & Power Calculations
    Calculations based on desired speed and available HP
    HP required at propeller(s) for desired 15 knots speed: 100.0 HP
    Estimated speed with existing 330 horsepower:
    This is the speed we will use for the propeller size. 26.63 Knots

    At this point it is important to note that all of the calculations above are based on full RPM and HP. Most engines are rated to run at a percentage of their full RPM. This is what will determine your maximum cruising speed. The propeller sizing calculations below are based on 90% of full RPM, which allows the engine to develop it's maximum power without overloading. The chart below shows typical engine ratings, you can find this information in your engine specifications.



    Propeller Size
    Number of blades Diameter (inches) Pitch (inches)
    2 Blade 17.9 X 15.0
    3 Blade 17.1 X 14.9
    4 Blade 16.1 X 14.6
    xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
    The table info is above the xxx line



    I had to add in some guesses as to waterline length etc
    You have to use a planing hull as that is type of hull that you have. So when you import weight, beam, and your horsepower, the table calculates a top speed with the 330 hp that you have even though I entered 15 knots as a desired speed. It shows you needing 100 horsepower and as a rule of thumb before EFI engines, 1 gallon per hour produced about 10 hp, so your fuel usage at 10 gph producing the 100 hp as per the table is pretty close.
    You are at 17 x 16 and the table shows a 17.1 x 14.9 in a three blade. Your current prop is not far off the mark. I would have imagined that Luhrs would have originally selected a good working prop.


    Just another observation. Baekmo suggested a lower pitch prop so that you do not have as much cavitation at your 15 knots. I was under the impression that lowering the pitch number allowed the engine to spin faster. Baekmo can you elaborate. If you only goal is to cruise at 15 knots, then the lower pitched prop might be more efficient, slip efficient at this speed though you MIGHT get an increase in max rpm that you don't really want to run at for sustained periods.








    [​IMG]
     
    Last edited: Nov 8, 2018
  5. 7228sedan
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    7228sedan Senior Member

    I had used the semi displacement calculator... Hence my difference in the calculations.

    Your water line length and beam are what I had used.

    She's a little heavier then 10K based on research Ive done.
    The Power Boat Guide lists the following:
    Length 28'0"
    Beam 11'2"
    Draft 2'8"
    Weight 11,500

    That likely measures up to why I only get 20 knots compared to your calculated 26.63

    I wonder if a 4 blade is more appropriate for my application?
     
  6. Barry
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    Barry Senior Member

    As Baeckmo is usually right, lots of cavitation, then a lower pitched prop might be the way to go or more blades but there is a chance that you might increase the rpm that the engine might
    attain. But then you rarely will run it faster anyway. So a lower slip number at the 3000 rpm target that you are trying to achieve
     
  7. baeckmo
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    baeckmo Hydrodynamics

    Ok Barry, it may seem contradictive, but since the 16" pitch prop is spinning faster than it should if working in "green water"; thus it is either cavitating or ventilating (i.e. operating in a "lighter fluid" than water). In this operating range, the gas/vapour volume on the suction side of the blade increases with increasing angle of attack, often resulting in a non-linear behaviour of the torque and thrust curves (in fact a "dip" in performance).

    In cavitating/ventilating mode, all work (i.e. deflection of the flow) is done by the pressure side of the blade. The normal, almost flat pressure side of ordinary propeller blades, have low lift/drag ratios in this case, because they produce a pressure "spike" in the front third of the chord. The best l/d ratios in cavitating flow is produced by a profile that is producing the dominating lift close to the trailing edge, and with zero angle of attack at the leading edge.

    By reducing the overall pitch, the angle of attack to the leading edge is reduced, and thereby also the detrimental cavity volume. Of course the flow deflection (ie work done) is also reduced. But by adding a cup to the trailing edge, the required total loading is restored and distributed where it results in optimum l/d ratio in cavitating flow.

    This non-linear behaviour is typical of semisubmerged and fully cavitating propellers; in the mid speed range they overrev and are known to have problems "getting over the hump", because at the same speed range, the hull drag is higher than at cruise speed.

    Now, that said about the propeller as such, the important question is still: cavitation or ventilation? Having seen the picture of the installation, I suggest that there is actually a ventilation track from the transom. Four reasons with additive effects:

    • The propeller loading (as expressed by the thrust constant ct) is very high, causing the propeller inflow to have a high degree of radial direction (inflow diameter much larger than propeller disc), and on top of that a very high outflow velocity (i.e. a small, concentrated jet).
    • The high shaft angle is directing the outflow jet downwards, not following the hull bottom. This jet acts like a jet pump, lowering the static pressure along the hull past the rudder stock and to the transom.
    • The shaft bracket is attached to the hull with no fairing at the rear end. The step height seems to be in the same size as shaft diameter, leading to a wake or a "deadwater zone" just where the outflow is causing a pressure reduction.
    • High shaft angle creates high variation of angle of attack between local positions around the propeller disc.
    Added together, I see a clear risk for a ventilating case, allowing the basically overpitched propeller to spin at 2900 rpm, instead of something like 2300-2400 that would suit the power available (if the gear ratio was matching). Unless there is some other hull openings allowing air into the prop, I'd suggest the following in order (and in the suggested sequence) to get it right (and matching the gear ratio):

    • If possible, trim down the bracket-to-hull attachement plate's rear edge and fill the wake space so that the angle between hull and filling is less than 8 degrees.
    • Make a smooth fairing aft of the rudder stock.
    • Attach a small vertical rail to the transom. Width ~450 mm and protruding ~5 mm below the bottom line with a sharp front edge. This is called an interceptor and will create a local pocket of increased pressure in the rear part of the bottom and reduce the risk for backflow.
    • With the above done, test with your present propeller and report the result to us; then we can decide on the right propeller.
     
    Last edited: Nov 9, 2018
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  8. 7228sedan
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    7228sedan Senior Member

    Wow, this is some excellent information and advise. Thank you!

    Regarding the recommended "trim down of the bracket to hull plate's rear edge..." I want to make sure I am following. Are you referring to essentially removing the material in between the strut and the hull to decrease the vertical edge at the rear of the strut? If so, would another option be to fare that edge to remove that vertical blunt edge? There is a wedge of sorts between the strut and the hull. Should I look to possibly removing that wedge so that the strut is mounted flat to the hull? This would obviously require possible tube modifications as well as a complete realignment. However I have always felt that the angle on the shaft was too steep. With that added to the hydrodynamic effects of the blunt trailing edge, could there be multiple problems? The boat gets the best performance numbers with significant if not full trim tab. The trim tabs are almost always fully deployed regardless of the wind/wave action and results in the best efficiency (RPM and Speed)

    The second bullet: "Make a smooth fairing aft of the rudder stock" Are you referring to modifying the round stock to more simulate a vertical stabilizer aft?

    And lastly, the Interceptor could be just flat stock of the recommended dimensions fixed to the transom, correct?
     
  9. baeckmo
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    baeckmo Hydrodynamics

    I'll be back with a sketch later.
     
  10. 7228sedan
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    7228sedan Senior Member

    You are a Gentleman, as well as a Scholar sir. Regards
     
  11. baeckmo
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    baeckmo Hydrodynamics

    Thanks, nice words are always sinking in, but since my speculations are based on "second hand info", there is a big IF involved, so let's wait with the celebrations for a while.....

    You note that your trim tabs are practically always deployed close to full extension, meaning that you (well the boat...) need a considerable lift at the rear. The interceptors create lift with a higher l/d ratio than most trim tabs, so I suggest that you fit the interceptor rail all the way between the insides of the trim tabs and let it take over the main lifting duty. In doing so, it will be even more efficient in stopping back-flow aeration, meaning you may well do without the fairing aft of the rudder shaft, see sketch enclosed. It could be wise to use a slightly wider bar, say 60 mm, and make the mounting holes oval to allow some height adjustment.

    Note that the horizontal force on the interceptor is quite high, attachment must be solid, and the importance of smooth radius on the bracket edges!

    Btw, check the tacho calibration before doing anything else!
     

    Attached Files:

    Last edited: Nov 10, 2018
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  12. 7228sedan
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    7228sedan Senior Member

    This is great information Baeckmo, thank you. I have dual helm stations, both tachs are reading consistent.

    Should I be concerned with reducing the prop to hull clearance with the recommended fill & faring as per the drawing?

    I was considering a composite board epoxied to the hull to fill, most likely King Starboard or similar. Then I can transition the edges with the grinder or epoxy filler.

    Does that sound reasonable?
     
  13. baeckmo
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    baeckmo Hydrodynamics

    Your tachos most likely get their frequency signal from the same source; i'd guess the alternator. Both tacho's will be off calibration if the alternator rpms have been altered, or the alternator has been swapped or... or...., so to be sure, check shaft rpms directly with a light-spot tachometer.

    Can't see that there should be any issues with prop clearance, and the filling suggestion seems fine with me, but since I'm a "metal man", let's hope someone with plastic experience will chime in on that one.
     
  14. DCockey
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    DCockey Senior Member

    Luhrs was a longtime, large production builder. Presumably they knew how to select the props for their boats. Is the boat which is the subject of this thread in the original configuration? Does anyone know the original propeller size. If so did all the Luhrs 280 have high propeller slip?
     

  15. philSweet
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    philSweet Senior Member

    You are probably not going to like this answer because it is kind of pricey, but I don't think you are going to fix this problem without throwing the strut away and fitting a new one. It is quite hopeless for the arrangement you show. It needs to have about four times the meat on it that it has, and a longer cutless bearing would help (say, twice as long). Now you can go back and look at that slightly larger prop, with 4 blades, and a more sensible pitch.

    [​IMG]
     
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