Calulated propeller efficiency

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

  1. 7228sedan
    Joined: Nov 2009
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    7228sedan Senior Member

    Looking for recommendations on tuning my prop.
    Now that she has a healthy motor, the slip seems a bit excessive.

    340HP 454 1.5:1 Velvet V Drive

    Prop is a Michigan Wheel Dynajet 3 blade 17RH16 with no known modifications

    Performance readings:

    RPM Speed in knots measured by GPS and calculated slip% based on one of the various online slip calculators
    Idle 650 3.1 49%
    1000 4.2 52%
    1500 6.4 63%
    2000 7.6 56%
    2500 8.4 61%
    2750 11.6 52%
    3000 13.4 49%
    3500 1537 40%
    4000 17.4 50%
    WOT 4400 20.6 46%

    The boat is a 1972 Luhrs 280. Not a speedboat by any means... However, a 15 knot cruise at 3000 would be spectacular even if it took some off the top end.

    Any recommendations?
     
  2. Barry
    Joined: Mar 2002
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    Barry Senior Member

    I would call Michigan and get their input
    Their calculator shows 24 inch x 13 inch pitch efficiency of 50% but the calculator that was there did not include a hull type or weight? 13 inch pitch, maybe with the 24 but when I type in your 17 inch diameter it gives a 21 inch pitch

    Going from your 16 inch pitch to 21 inch pitch seems excessive


    Solas shows 14.8 by 18 p-----or -----14.8 x20------or------14.3 x 22

    Merc 7.4 shows 4200 - 4600 as an operating range so you have some room to increase the pitch. Is there not a Luhrs owners forum?
     
    Last edited: Nov 5, 2018
  3. DCockey
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    DCockey Senior Member

    Have you verified the reduction ratio?
     
  4. 7228sedan
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    7228sedan Senior Member


    The Luhrs owners forum unfortunately is lost on the older boats. Plus, most still have the original small block Chrysler and Paragon transmissions.
    The power should move the boat well if I can get it propped correctly.

    I don't have any room to increase diameter. I tried an 18/17 and it vibrated tremendously as the tip clearance was way too tight.

    I am going to take the prop to a shop to see what they recommend. I was thinking that a cup might help.
     
  5. 7228sedan
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    7228sedan Senior Member

    Yes sir, I installed this transmission myself.
     
  6. Barry
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    Barry Senior Member

    You are propped for max 4400 rpm, and the merc info shows a range of 4200 rpm at the lower end. So you can increase the pitch by 1 which will drop the rpm by about 200 rpm, rule of thumb,
    You can easily take this back down another 200 rpm so your max becomes 4000 rpm, so add another inch of pitch for a total of 2

    Find a prop shop that might lend you this bigger prop for a day.

    Consider a 4 bladed prop

    Run the boat at 3250 rpm, gets you almost to your goal of 15 knots. At 3500 rpm you have the least slip, but you should keep your eye on the fuel consumption. No use getting a prop that
    gives you your desired outputs if the fuel consumption is 10 - 15% higher, on a mpg basis

    Before doing anything, I would borrow/ buy a fuel flow meter *** and see what you are getting for mpg at 3000 rpm and 3500 rpm with the current prop. You may find that you are better to run the same prop at higher rpm for better fuel efficiency. efficiency in this case defined as higher mpg

    *** many of the older 454's had a return line which required a flow meter that deducted in essence the fuel flowing back to the tank. Many marinizers of marine engines were able to modify their return engines, ie remove them, so a single flow meter type set up was all that is needed.
     
    Last edited: Nov 5, 2018
  7. 7228sedan
    Joined: Nov 2009
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    7228sedan Senior Member

    Thanks Barry, I'd be fine with a 4000 max. I don't have a flowscan but I have done fairly accurate cruising to calculate burn. At 3000 it burns 10GPH. My goal it to try to keep the revs as close to 3000 as possible. However, cupping the prop to increase pitch and getting 15 knots at 3000 may very well increase the fuel consumption to where I am at 3200 now.

    I guess I was more surprised by the slip numbers then anything. Is 40-50+ "normal" for a boat such as this?
     
  8. Barry
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    Barry Senior Member

    I am not sure but the Michigan site and others will often show slip numbers with different input parameters
     
  9. gonzo
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    gonzo Senior Member

    If your max RPMs are 4000, the boat is overpropped. The engine should get up to at least the minimum range.
     
  10. Barry
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    Barry Senior Member

    200 rpm will not make much difference

    The miles per gallon will tell the story. Rarely would a person run the engine at max rpm anyway so you might as well find the spot were the mpg is optimum at the common running rpm

    We have had much of experience with measuring results with a given engine and different impellors and I don't have hard data in my hands right now but this is what we found
    In our formative jet boat building years we used Berkley 12j pumps and 330 hp naturally aspirated 454's

    At the time they only had an " A impellor" ( for the 330hp) rpm ran up to 4400 plus or minus at max, cruise at 30 mph was about 3700 rpm

    They developed the "AA" impellor, same hull same engine, rpm ran say to 4100 rpm at max, cruise rpm dropped to maybe 3400 rpm.
    This was below the engine manufacturers max limit of about 4200

    When Berkley brought out the "AAA" impellor, same hull same engine, rpm max ran to 3800 plus or minus, 400 rpm below the engine manufacturers lower limit, cruise at 30 mph was about 3000-3100 rpm

    What we noted that at cruise is that the mpg increased, which you would not have expected as the engine was "loaded up" as some might say. But we got as high as 4 miles per gallon with the set up at 32mph.

    All tests were ran on brand new engines, Floscan meters, an expensive test pitot tube/gauge set up and calm water.

    When we switched to Hamilton 212's we found the same situation, we picked the next highest hp rated turbo impellor and found while we would not reach the engine manufacturers
    rpm lower limit but got the best results as far as fuel consumption for gallons burned.

    I don't think that the max range is so tight as to impact the engine if you are a bit off.
     
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  11. 7228sedan
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    7228sedan Senior Member

    I'd be willing to let this mill max out at 4,000... That argument aside, what kind of modification to the current prop can be done to accomplish this?
     
  12. DCockey
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    DCockey Senior Member

    A guess, and just a guess, is you need a prop with much less pitch.
     
  13. baeckmo
    Joined: Jun 2009
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    baeckmo Hydrodynamics

    With a reduction ratio of 1.51:1 and 4400 engine rpm your propeller is cavitating quite heavily. In "clean water" it would require more power than you have got. This means that your prop is operating with huge losses at WOT and probably down into the cruising speed region. With a shaft speed of ~2900 rpms the 17" diameter is about right, but the pitch is way too high. If there were no disturbances to the inflow, you would need something like 12" plus a slight cup.
    Now the question is what is "un-loading" your 16" pitch prop to the extent that allows it to spin at 2900 rpms? What does the shaft support look like? Is there a square ending keel just in front of the prop or is there a ventilation path from the transom allowing air into the propeller from behind? A picture of the installation would be useful.
     
  14. Barry
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    Barry Senior Member


  15. 7228sedan
    Joined: Nov 2009
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    7228sedan Senior Member



    Other then the shaft angle being a little steep, there is nothing in front of the prop:
     

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