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New Props ACME vs MICH WHEEL

Discussion in 'Props, Shafts & Seals' started by MBevins, Aug 22, 2019.

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  1. MBevins

    MBevins Senior Member

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    Okay title might be just a bit misleading.
    I've already purchased the ACME'S.
    I was running 24x26 Mich Wheels for ever. One of them was inadvertently redesigned beyond recovery.
    So new ACME'S were the order of the day. They are 24x25 based on their improved design criteria.
    Took them out for a spin today. They're very nice at cruise picked up 3/4of a knt.
    However my WOT would not go over 2250, J&T spec's them at 2450.

    So before I get concerned about over propping, I've got a full load of fuel, a full load of water, plus all the cruising gear. ( We're cruising right now)

    I'm thinking that is why I can't get any higher RPM. Once I dump fuel and water I'll try again and see if it'll hit a higher number.

    Does this make sense?
  2. PacBlue

    PacBlue Senior Member

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    Yes, once you dump some weight your max rpm’s will pick up. Which J&T engines do you have? If you stay at 80% off 2250 you will be ok. What rpm are you cruising at?

    I like the Acme’s, they are CNC milled to a nice tolerance and should be relatively vibration free.

    What model Michigan Wheel’s did you replace?
  3. MBevins

    MBevins Senior Member

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    I typically run about 1900-1950.
    That's the 80% sweet spot based on my spec'd 2450. The engines are 6-71 TIs.
    I'm afraid running at 80% of 2250 wouldn't be in the cards. That drops me down to 1800 which is about 16.5 knts, that's lugging for that boat. She'll do it but not efficiently. I'd be better off to take some pitch out of ACME's.
    As to the MW s , I don't have a clue, they were on the boat when I purchased her 20 years ago.
  4. PacBlue

    PacBlue Senior Member

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    So those J&T 6-71TI’s are rated at 2500 rpm and should turn 2550 rpm to make everything “nice”.
    You probably went up in DAR and are pulling more blade area than the MW props, that’s why I wanted to know the model.
    While we are at it, which model of ACME props are you running?
    I agree, like to run these at 1950 rpm.
    Other thing - need to check full throttle at neutral idle at the dock to make sure you have full travel on the throttle sticks and the engine rack. The fun begins!
  5. MBevins

    MBevins Senior Member

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    Haven't checked this in a couple of years.
    The invoice says this....
    24x25x2 ACME 4 blade Nibral .045 cup.
    I'm pretty sure they're 2450 FL with. But I'll check in the morning when I do my prerun check.
  6. PacBlue

    PacBlue Senior Member

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    I like the cup as it gives you some extra lift in the stern, but the pitch is about 2” over if you can only pull 2250 rpm.

    You may also want to verify the engine speed matches your gauges by checking with a photo tach.
  7. MBevins

    MBevins Senior Member

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    I'm sure gauges are correct, I've had them a while and I've gotten pretty personal with them.
    ACME took an inch off the pitch when they spec'd them so I'm hoping they got it right.
    As to the WOT situation , I believe those numbers are determined using low water and about a half tank of fuel are they not?
    I can't see dropping 3 inches of pitch from one MFG to another, but I'm not a prop expert, if I was I guess I wouldn't be inquiring.
  8. chesapeake46

    chesapeake46 Senior Member

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    HA ! Well put !
  9. Capt Ralph

    Capt Ralph Senior Member

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    It's the cup kicking in way to much.
    Think of it as a two speed prop, the lower pitch and slipage gets the boat up, as wheel rpm comes up, that last edge, the cup, kicks in, less slippage and high gear comes on. Send them back and remove the cup spec.

    Who ever ordered the wheels screwed up. Depending on your deal, you may want to argue a lil before putting any more bux to this.
    If they scanned the Mich Wheels, they may not have found any cup.
    I hate modifying new props to make it work when the correct (new) ones could be in tomorrow.
  10. Capt J

    Capt J Senior Member

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    I would have ACME take care of it right away. They should cover this on their end if you gave them all of the Michigan specs.
  11. Pascal

    Pascal Senior Member

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    You ve got to turn rated RPM at full or near full load especially if you cruise at 80%.

    Make sure the bottom is clean. Obviously the new wheels are clean but if they were swapped in water make sure the struts, rudders and hull are clean

    What’s the HP rating? JTs are usually tweaked and close to the limit, not the kind of engines you want to overload. Also make sure you trim tabs are working, that helps creating lift at the stern especially if the boat is underpowered
  12. PacBlue

    PacBlue Senior Member

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    The most conservative route is to prop your wheels for full load - full fuel, full water , no waste, all gear onboard. Prop it for 2550 rpm and you will get max life out of the DD’s.

    I agree this seems to be a ACME problem if you gave them the right data. If you gave them low water and half fuel data, then you may have to circle back to that condition and see where you are at on rpm’s on these props.

    I don’t like taking 3” of pitch out of new wheels, that doesn’t sit well with a correct propeller sizing. You have a medium cup, which is good for double cabin motoryacht that tend to squat. If the removal of 2” of pitch is too much and you push past 2575 rpm’s, you can go to a heavy cup and it will dial right in. We just need to know what loading condition (fuel and water) you started out at?
  13. PacBlue

    PacBlue Senior Member

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    More on cup - it is a second variable that makes it a bit more difficult as you have to adjust pitch and potentially cup. Two variables now instead of one. You have a few choices:

    1, get back to the right loading condition when you gave performance data to the prop shop.
    2. remove cup and retest, assuming they used proper cup gauges, 1 thru 12, what number are you on, a 6 or?
    3. Make decision with the new data - more pitch, more cup, what combination of the two?
    4. Depending on the age, condition and model of the MW, it is not necessarily surprising to see a 3” swing in pitch, as blade shape and blade area are sure to be different.
    5, losing pitch can be a bit of a bummer as you lose speed potential, but cruise is what matters. If it squats too much without cup and needs a little extra load at your desired operating condition, you will have to re-introduce some level of cup, which is not necessarily a bad thing. Focus on your desired cruise speed.

    Lot’s to think about.
  14. Capt J

    Capt J Senior Member

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    Do you know what RPM's you made at WOT with the pre-reengineered original wheels? Did they make proper RPM?
  15. MBevins

    MBevins Senior Member

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    Yes they made wot within about 30 rpm.
    But I'm sure that was with 1/2 tank of fuel. .
  16. PacBlue

    PacBlue Senior Member

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    Then you should have seen 2530 on the original set?

    WOT on 6-71TI's is 2500 rpm, and the No Load maximum rpm is 2700 on a healthy engine. You need to verify those data points.

    Another question - how deep is the water where you are collecting your performance data? I prefer 3 to 4 x my draft as an absolute minimum, but really like to see 20' plus under my props to eliminate any shallow water effects.
  17. Pascal

    Pascal Senior Member

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    Have you checked whether your trim tabs are working?

    Fuel and water load isn’t going to cost you 200 rpm. How big are the tanks?

    Not sure water depth negatively affect performance on plane. At hull speed yes, you will loose as much as 1.5 kts in shallow water. On plane though if anything I ve noticed slightly better speed and fuel burn than in deep water.
  18. PacBlue

    PacBlue Senior Member

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    I mentioned it to further eliminate other technical variables, there is a lot of technical documents available to Naval Architects on the topic, you can google and read as you like. True high speed planning hulls may see a resistance decrease in certain water depths but the propeller may also see a decrease in efficiency. Squatting, getting on plane (over the hump) are all affected negatively by shallow water depths.

    In order to optimize propeller selection, get some decent water below you, at a minimum use half the length of your boat as a guideline. It makes the technical side of propeller selection that much simpler to not have to factor in reduced water depth.
  19. Capt J

    Capt J Senior Member

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    I agree with this. On faster planning boats (25+ knots I see increased speed of about 2 knots in depths less than 2-3x the draft, slower boats like the OP's they tend to stay the same or slow down a bit a lot of times.)
  20. Pascal

    Pascal Senior Member

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    I agree about the squatting making it harder to get in plane. I never try to get on plane under 10’ But have noticed better fuel and speed in 10 to 13’ than in deeper water