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Thread: Bravo Ratio

  1. #1
    Posey
    The drive ratio on my bravo one is 1.36 I'm currently looking at some different boats and most have 1.5 ratio. My Baja cruiuses at about 40mph@3200rpm. Seems as though the other Ratio revs the engine higher to go the same mph, which is better? any feed back on this subject.
    Thanks, Posey

  2. #2
    MrHavasuCat
    I am currently having my 1.50's changed to 1.35's because of the very problem you mentioned. The 1.50's kill top end speed.

  3. #3
    not guilty
    All depends on the hull design,weight & h.p.Most boats with 1.50 are made to run well with that ratio & change to higher with substantial engine upgrades.Hope 3200 rpm isn't WOT on your present boat.Sounds like a V-8 was traded for 4-popper.
    [This message has been edited by not guilty (edited July 31, 2001).]

  4. #4
    shockwaveharry
    Whats the difference? eg: A boat spins a 28 with 1.5 gears at 5000 RPMs , changes to 1.36s and drops down in prop size to a 24. Engine RPMs remain the same, prop RPMs go up but its should still moving the same amount of water, same boat speed. Is there a speed advantage to spin a small prop faster? My thinking is that on a car, if if you change differential gears to a lower ratio to increase acceleration, you lose top speed. Put a set of taller tires (changing the prop), your back where you were and have gained nothing. I'm running 1.5s now and as I increase HP I plan on just changing prop pitch. Am I missing something?

  5. #5
    Havasu Hangin'
    It's my understanding, that you would switch to the 1.36s if you are getting close to running a 30" prop, as there are less choices in the 30"-34" range.
    I think it also has something to do with the prop slippage, efficiency, and high-torque engines utilizing the extra revs.
    http://www.goldenplasma.f2s.com/foru...es/happy25.gif

  6. #6
    shockwaveharry
    OK, I get it. Just because the monster under my hatch could spin a 42 dosen't mean I'll be able to find one. Thanks for the info.

  7. #7
    MrHavasuCat
    Once you start running props bigger than 30" they become less efficient. I am running a 32 with 1.50's, and have friends running 34 and 36's. The slip increases with these bigger props. You are better off spinning a smaller prop faster. with my new gears I can drop to a 30" and maybe even a 28". Iwill have to do some testing and see. the problem is alot of people are running the new xr drive which only comes in the 1.50 ratio. I am going to teauge platinum foot gears in the 1.35 ratio, and I hope they hold up.

  8. #8
    Posey
    Thanks for the info,my Baja is 27ft 502mag (carbed)probably around 400hp. WFO is 5000rpm @ 68-70mph 1.36ratio swingin a 21p Mirage 3 blade. I tried a 25p and it couldn't pull it. I guess more pitch you need more HP. With a 1.5 ratio you can swing more pitch with less HP moving the boat faster at the same RPM? Right?
    Posey

  9. #9
    MrHavasuCat
    In theory spinning a larger prop faster should produce more speed. Like I stated above the bigger props seem to be less efficient from experience. Jumping from a 21" to a 25" prop is substantial. I would try to dial in a smaller prop with your current ratio. You can have your prop reworked by a reputable prop shop and maybe able to get your max rpm out of your current prop. I don't know what max rpm on your motor is, i would guess it would be no higher than 5200 on a stock motor. Call a prop shop and ask them about different prop options. I don't think changing to 1.50's would help you. my.02.

  10. #10
    Posey
    Hey Thanks MrHavasuCat I would like to try reworking my Prop. Merc. says max. rpm is 4800-5000 on my '92 502mag sounds like I'm there, might push to 5200 but I'd rather not. RPM's a killer. Do you know a prop shop thats good and has a good turn around time?
    Posey

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