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  1. #1
    Petty Officer 3rd Class barthel's Avatar
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    Default Prop Size and Pitch Effect

    Hey All,

    I'm not trying to fix or cure any situation with my boat, I just have a dumb prop question. I have been considering getting a spare but now have to replace it as I bent it today.

    My boat is a '98 Stingray 180RS, 3.0L, Alpha One Gen II outdrive. It came stock with a 13.75" by 21 degree pitch prop. I thought that was what was on there but when I serviced the outdrive last week found out that the prop that's currently on the boat is a 14.5 by 19 degree pitch.

    My question is, what would have been the effect on the boats performance changing from the stock prop to the current prop? It seems to holeshoot OK at the moment, but the speed seems down at WOT (4400 RPM's), I'm only indicating mid to high 30's on the speedo.

    Again, not really complaining, just "curious"

    Thanks.
    "Every normal man must be tempted at times to spit on his hands, hoist the black flag, and begin to slit throats." - H.L. Mencken

  2. #2
    Rear Admiral Maclin's Avatar
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    Default Re: Prop Size and Pitch Effect

    The lower number 19 pitch, when compared to a 21 pitch, will give better holeshot at the expense of top speed. If the boat was used mostly for skiing by the previous owner they probably changed to the 19 to pop skiers out of the water easier.

    A 21 pitch, assuming the engine can still hit 4400 rpm, would gain 2-3 or maybe even 4 mph but would not be quite as good at pulling skiers up.

  3. #3
    Petty Officer 3rd Class barthel's Avatar
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    Default Re: Prop Size and Pitch Effect

    Would the diameter difference have had the same effect?

    I sort of got the "pitch" explanation by reading it here. I guess I was wondering if the difference between the 2 was negligible as the higher pitch prop would have been 3/4 of an in smaller in diameter.
    "Every normal man must be tempted at times to spit on his hands, hoist the black flag, and begin to slit throats." - H.L. Mencken

  4. #4
    Rear Admiral Maclin's Avatar
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    Default Re: Prop Size and Pitch Effect

    The pitch of the prop determines the speed of the boat at any given rpm, diameter will not change that. A larger diameter could keep RPM's down if the engine does not have enough torque to spin the larger diameter prop, as it will need more power to spin the same due to more drag caused by the extra weight of the prop and the larger blade area. Most prop diameter changes are not that extreme though, but could still make a slight drop in top RPM/speed.

  5. #5
    Petty Officer 1st Class
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    Default Re: Prop Size and Pitch Effect

    not to be a nit picker but although pitch can be measured in degrees it is actualy 21" per revolution of your prop. The 21 will probably give you more speed if you go out by yourself, but when you load it up with people and gear you are likely to loose speed and go below your current 30mph.

  6. #6
    Captain Philster's Avatar
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    Default Re: Prop Size and Pitch Effect

    You will often see that a drop down in diameter means the engine can handle more pitch.

    When playing with pitch, stick w/ the same diameter prop.
    Checkmate 2400BRX/Optimax 300xs
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    Alumacraft 17 w/ Merc 40 4S

    Date an I/O, but marry an outboard!


  7. #7
    Petty Officer 3rd Class barthel's Avatar
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    Default Re: Prop Size and Pitch Effect

    So if I kept the same pitch (19) but dropped diameter (say to 14 or 13.75) would that theoretically give me a few more rpm's on the high end?

    My initial sense was to replace like for like (what's on there now) but just wanted to make sure I make the right choice. I doubt I'll be going to a 21 pitch as I am never in the boat alone. We don't do major watersports, just tubing and kneeboarding so I could see a 21 pitch prop not being conducive to that.
    "Every normal man must be tempted at times to spit on his hands, hoist the black flag, and begin to slit throats." - H.L. Mencken

  8. #8
    Captain Philster's Avatar
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    Default Re: Prop Size and Pitch Effect

    I'm guessing the diameter and pitch change offset eachother somewhat.

    You're at the bottom of the recommended RPM range @EOT? You're 19p on there is barely conducive to any water sports. Lightly loaded, you should be turning closer to 4800 rpm. 4400 RPM can be be acceptable if you were carrying a full load.

    You now have to answer all the questions posted in the 'sticky' at the top of the thread list in this (prop) forum.
    Checkmate 2400BRX/Optimax 300xs
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    Alumacraft 17 w/ Merc 40 4S

    Date an I/O, but marry an outboard!


  9. #9
    Petty Officer 3rd Class barthel's Avatar
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    Default Re: Prop Size and Pitch Effect

    1) Performance issue you are trying to correct.
    Not really trying to correct anything, just trying to make sure I get the right prop for the job since I have to replace what's on there anyway. We basically cruise and do minor watersports, tubing and kneeboarding. Haven't tried to pull a skier yet but I did just buy a new pair of skis.

    2) Current prop manufacturer, model, aluminum or stainless as a minimum.
    Merc Black Max aluminum 3 blade

    3) Current prop diameter and pitch (required).
    14.5" X 19 pitch

    4) Wide open throttle RPM and speed with an average load (very helpful)
    4400 with 5 people in the boat. Not sure about speed as the speedo went Tango Uniform when we were out, it was bouncing all over the place. We will always have 3 to 6 people in the boat.The last time it worked it registered 38 to 40 MPH on the top end, but am not sure of the accuracy.

    5) Engine/drive make, model, year, and HP
    Merc 3.0L, Alpha One Gen II out drive, 1998 listed at 130HP

    6) Boat make model, year, length and weight
    1998 Stingray 180RS, 18' open bow, weight listed from factory was 2100#
    "Every normal man must be tempted at times to spit on his hands, hoist the black flag, and begin to slit throats." - H.L. Mencken

  10. #10
    Captain Philster's Avatar
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    Default Re: Prop Size and Pitch Effect

    Sounds like you might be better than I initially thought. I thought you were scraping the bottom of the max RPM range with a light load.

    Might not have to do anything. I am going to say your speedo was reading 2-3 MPH high. If so, that puts you at 9% slip, as I believe your gear ratio is 2:1.
    Checkmate 2400BRX/Optimax 300xs
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    Alumacraft 17 w/ Merc 40 4S

    Date an I/O, but marry an outboard!


  11. #11
    Petty Officer 3rd Class barthel's Avatar
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    Default Re: Prop Size and Pitch Effect

    Correct, the outdrive is a 2:1

    So, based on what we know, no harm in sticking with what's on there? (14.5" X 19 pitch)

    BTW, thanks for all the input, I'm still sort on new at this shi-at.
    "Every normal man must be tempted at times to spit on his hands, hoist the black flag, and begin to slit throats." - H.L. Mencken

  12. #12
    Captain Philster's Avatar
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    Default Re: Prop Size and Pitch Effect

    Opinions might differ here. I know -- right?

    The slam dunk might be a 18p prop. 19p = acceptable. 18p might be closer to perfect.

    Only customizing a prop will ever make it perfect provided you have the exact load on the boat at all times. Props ain't nothin' but compromises.
    Checkmate 2400BRX/Optimax 300xs
    .
    Alumacraft 17 w/ Merc 40 4S

    Date an I/O, but marry an outboard!


  13. #13
    Petty Officer 3rd Class barthel's Avatar
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    Default Re: Prop Size and Pitch Effect

    OK, so one last dumb question. What's the effect (top end wise) going to an 18 pitch? Would I still keep the same diameter, move up to the 15 1/2, or go to a 4 blade 18 in 14"?
    "Every normal man must be tempted at times to spit on his hands, hoist the black flag, and begin to slit throats." - H.L. Mencken

  14. #14
    Rear Admiral Maclin's Avatar
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    Default Re: Prop Size and Pitch Effect

    An 18 inch pitch prop will give slower top speed than the 19 but could deliver slightly better holeshot.

    Again, diameter will not affect the distance moved thru the water for each revolution but will affect rpm and could improve (lower) slip. That means the RPM's may drop some with larger diameter but the same speed achieved with same pitch due to less slip. Realize that less slip = more load on the engine. Since you are working with only the 3.0 engine the increased load could be more noticeable.

    If it was my experiment I would try the "stock" diameter 21 inch pitch prop and see what performance you get.

    Here is a link to a prop calculator, it does not have diameter as a factor though, but will give you the speed at any given combination of gear ratio, pitch, slip and rpm.

    http://www.marksmarineinc.com/prop_calc.aspx


    If you have exact ratio and pitch and rpm and speed then it will calculate your prop slip. To compare pitches of props use 12 to 15 as a prop slip constant.

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