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  1. #1

    Prop question 28 x 28 vs 28 x 31

    My understanding is that for a 53MY the stock prop is 28 x 30

    As I have posted in a previous post recently I had a bent up prop, and bought some from SKY that are 28 x 28, I got them on and all vibration is gone, and they work great!

    I just dropped off the bent set to a local prop repair place and they found the stamp on the old ones to say 28 x 31......

    So, on our trip bringing her home we got 1.25 MPG, running about 1200 rpm and 9.2 knots.

    New props at 1200 rpm seem to run a noticeably amount slower I think I'm getting 8 or 9 mph (yeah not knots) but yes slower.

    So I am wondering if it really matters for fuel economy?
    If my thinking is correct, I might need to bump the rpm up to achieve the same 9.2 knots, but that should take the same HP and therefore the same GPH correct?
    If I am looking at this correctly it should theoretically be a slight bit less torque on the motors and thus be a little easier on the motors?

    I'm not planning on running on plane, so let's remove that from the equation.
    Just wondering about mileage and wear and tear on the motors.

    Your thoughts?
    1980 53' Hatteras MY, Hull # 592

    "Moon River" <-- Finally picked a name

  2. #2

    Re: Prop question 28 x 28 vs 28 x 31

    The factory spec props are 28x31 for 8v71ti’s. 28x28 for naturals. 28x32 for 6v92ta’s. None of which matters as much as your boat reaching 2300rpm loaded. Yours probably does because you just bought it, give it a year and it won’t anymore because of all the crap you don’t realize you’re bringing aboard. It happens. For what it’s worth I tried a second set of props at 28x29 and didn’t like them for the same reasons as you, and I put the 28x31’s back on. I am trying to keep the boat on a diet.

  3. #3

    Re: Prop question 28 x 28 vs 28 x 31

    Yes, bump up the rpm a little to get hull speed back up. Shaft horse power is low now because you in a lower pitch (gear ratio). By raising rpm to hull speed, shaft HP will come up and consume the same amount of fuel as before.
    What I'm trying to say, same hull speed - same shaft HP required, just a higher RPM to get the same results.

    Go back to the 31 pitch soon, As now you will over rev at WOT.

    It's like driving your car in a lower gear, never the final drive ratio.
    Last edited by Captain Ralph; 08-24-2022 at 10:14 AM.

  4. #4

    Re: Prop question 28 x 28 vs 28 x 31

    Bob, I have the 28 X 31 and was doing around 10.5 mph SOG at 1,100 RPM last weekend. 11 - 11.5 at 1,250 if this information helps.
    Dragonten
    1981 53' MY Hull #603
    Woodstock, Illinois

  5. #5

    Re: Prop question 28 x 28 vs 28 x 31

    Quote Originally Posted by Captain Ralph View Post
    Yes, bump up the rpm a little to get hull speed back up. Shaft horse power is low now because you in a lower pitch (gear ratio). By raising rpm to hull speed, shaft HP will come up and consume the same amount of fuel as before.
    What I'm trying to say, same hull speed - same shaft HP required, just a higher RPM to get the same results.

    Go back to the 31 pitch soon, As now you will over rev at WOT.

    It's like driving your car in a lower gear, never the final drive ratio.
    That is what I thought. I will leave them on for now. I don't mind it turning a few extra RPM's.
    In fact it might be better if it helps the turbo's kick in a little.
    I don't know what rpm that happens. I built a turbo bike a few years back and the boost didn't kick in until 4,000 rpm or so LOL. Whole different world.
    1980 53' Hatteras MY, Hull # 592

    "Moon River" <-- Finally picked a name

  6. #6

    Re: Prop question 28 x 28 vs 28 x 31

    Quote Originally Posted by Dragonten View Post
    Bob, I have the 28 X 31 and was doing around 10.5 mph SOG at 1,100 RPM last weekend. 11 - 11.5 at 1,250 if this information helps.
    We were getting 9.2 knots at 1200 with the 28 x 31's (with one a bit bent up I think) so that sounds right.
    Thank you
    1980 53' Hatteras MY, Hull # 592

    "Moon River" <-- Finally picked a name

  7. #7

    Re: Prop question 28 x 28 vs 28 x 31

    Quote Originally Posted by cww View Post
    The factory spec props are 28x31 for 8v71ti’s. 28x28 for naturals. 28x32 for 6v92ta’s. None of which matters as much as your boat reaching 2300rpm loaded. Yours probably does because you just bought it, give it a year and it won’t anymore because of all the crap you don’t realize you’re bringing aboard. It happens. For what it’s worth I tried a second set of props at 28x29 and didn’t like them for the same reasons as you, and I put the 28x31’s back on. I am trying to keep the boat on a diet.
    Thank you for the info, I was mistaken thinking the spec was 28 x 30.
    Good info!
    1980 53' Hatteras MY, Hull # 592

    "Moon River" <-- Finally picked a name

  8. #8

    Re: Prop question 28 x 28 vs 28 x 31

    If you don't know what WOT rpm is before or after the prop change, you won't know where you need to be. Push it up to WOT now and see what rpm you can achieve. With my 6v92's, I was over loaded, so I took pitch out to get to 2350rpm (those props are 28 x 28.5 BTW). You may be the same. You just don't know until you run them up. (which is good for them and you should get it on plane from time to time to keep from wet stacking)
    Sky Cheney
    1985 53EDMY, Hull #CN759, "Rebecca"
    ELYC on White Lake--Montague, MI

  9. #9

    Re: Prop question 28 x 28 vs 28 x 31

    You know your under propped now. At WOT you will be hitting the governors speed stop.
    What was your high idle vs wot specs when you purchased the boat? Pull out that survey.
    I would aim for the original 31" pitch and diameter.

    Also keep the in-laws stuff off of the boat.

    We lived onboard for 18 years. I can tell you when we took on family stuff (weight).
    Still have the numbers for our artificial reefs donations.

  10. #10

    Re: Prop question 28 x 28 vs 28 x 31

    Quote Originally Posted by Captain Ralph View Post
    You know your under propped now. At WOT you will be hitting the governors speed stop.
    What was your high idle vs wot specs when you purchased the boat? Pull out that survey.
    I would aim for the original 31" pitch and diameter.

    Also keep the in-laws stuff off of the boat.

    We lived onboard for 18 years. I can tell you when we took on family stuff (weight).
    Still have the numbers for our artificial reefs donations.

    How does he know he's under propped now? He's never had it to WOT before or after changing props. You're assuming things not stated. Did they run it up during survey? What RPM did it make? I don't think we know that.
    Sky Cheney
    1985 53EDMY, Hull #CN759, "Rebecca"
    ELYC on White Lake--Montague, MI

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