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Thread: 28 pitch thru 32 pitch, same speed

  1. #1
    ghittner
    Well, here's where I am as far as prop selection goes and I've run into this with most all my power plants over the years... On GPS, my boat runs 73 MPH Period!
    If I run a 32 pitch, I run 73 @ 4500 RPM. If I run a 30 pitch, I run 73 @ 5200-5300 rpm. If I run a 28... Well, you know the rest....
    The application is a 1989 Arriva 24" open bow boat V bottom with a Bravo 1, 1.5:1 ratio. at what I guess is about 3500-4500 pds. The props are all bravo one 4 blades. The engine is a blown 498 w/ 10 pds boost, good heads and large cam. So, anyway is this what I should expect? It seems to be the same for all the plants I've run over the years. The first was 50 MPHw/a 330 HP 454. The second, 57 MPH the third 63 MPH and now 73 MPH@ approx 900 HP. All with the same results regardless of any prop changes.
    Here's my question. Is this what you guys see too? Am I missing something? I guess I should run the 32 pitch as it delivers the same top end @ 4500 RPM rather than spinning 5500 or 6000 for the same top speed. The boat planes instantly no matter what prop. What to do? What to do??

  2. #2
    DMOORE
    That's pretty damn weird. 28-32 pitch props and they all run 73. Not sure what to think of it. I would say though, do not run the 32. If your only able to turn 4500 rpm's, your lugging the motor too much. This causes a greater chance of developing detonation. Also it's harder on the drive. Run the 30 pitch and let the motor rev out a bit more. Easier on motor, easier on drive and gives a better mid range punch.
    Darrell.

  3. #3
    MACHINEHEAD
    Basically your engine has the same HP at 4500 as it does at 5300. This is typical of a large CID blower motor. Run the 30 for reasons above. I might start looking at X-dimension. 3 inches below bottom is where Id like to be.

  4. #4
    SnoC653
    Why not try a half inch smaller prop with the 32 pitch or even a 1/4 inch smaller. Pitch dictates the forward speed/rpm but the smaller circumfrence should allow more rpms with the same pitch. You might loose just a little out of the hole. But, I think you'll have enough HP to still launch hard.

  5. #5
    cfm
    Try using the trim button, and if you have tabs raise those too. LOL. Just joking btw.
    Sounds like your boat doesn't want to move. Got any pictures , especially the bottom along as mentioned above how deep your drive is ?
    You sure the Bravo style prop is what your boat wants ?

  6. #6
    ghittner
    Try using the trim button, and if you have tabs raise those too. LOL. Just joking btw.
    Sounds like your boat doesn't want to move. Got any pictures , especially the bottom along as mentioned above how deep your drive is ?
    You sure the Bravo style prop is what your boat wants ?
    There's a thing called a trim button?

    The trim button is my happy place, I'm all about it. Anyone who wants max speed has his finger on the trim button constantly. I like it so much that I moved one onto my throttle, right in my hand!!!!
    All speeds taken by GPS

  7. #7
    ghittner
    Basically your engine has the same HP at 4500 as it does at 5300. This is typical of a large CID blower motor. Run the 30 for reasons above. I might start looking at X-dimension. 3 inches below bottom is where Id like to be.
    I've always been very suspicious that my drive was way way way too low.
    How do I take my measurements? Thanks.

  8. #8
    TIMINATOR
    It takes X amount of HP to run X speed. The top speed will stay the same as long as your engine makes comparable HP at the different RPMs, and most blower motors and many milder motors do. The exception is a high strung motor with big heads,and large cam, giving a peaky power band, those motors tend to react quite a bit to RPM changes, and the speed will react accordingly. If you are happy with the holeshot, run the biggest prop and keep the revs down. TIMINATOR

  9. #9
    Jerrys59
    Well, here's where I am as far as prop selection goes and I've run into this with most all my power plants over the years... On GPS, my boat runs 73 MPH Period!
    If I run a 32 pitch, I run 73 @ 4500 RPM. If I run a 30 pitch, I run 73 @ 5200-5300 rpm. If I run a 28... Well, you know the rest....
    The application is a 1989 Arriva open bow boat V bottom with a Bravo 1, 1.5:1 ratio. at what I guess is about 3500-4500 pds. The props are all bravo one 4 blades. The engine is a blown 498 w/ 10 pds boost, good heads and large cam. So, anyway is this what I should expect? It seems to be the same for all the plants I've run over the years. The first was 50 MPHw/a 330 HP 454. The second, 57 MPH the third 63 MPH and now 73 MPH@ approx 900 HP. All with the same results regardless of any prop changes.
    Here's my question. Is this what you guys see too? Am I missing something? I guess I should run the 32 pitch as it delivers the same top end @ 4500 RPM rather than spinning 5500 or 6000 for the same top speed. The boat planes instantly no matter what prop. What to do? What to do??
    What kind of speeds are you looking for from that hull?

  10. #10
    ghittner
    What kind of speeds are you looking for from that hull?
    Simple, as much as possible. But I realize I may be at the limit or close. I just don't know.

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