1. chasintail

    chasintail Senior Member

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    I have a 24 x21 prop on a 13/4 shaft if I went to a 24x24 what effect should that have ...... THANKS
     
  2. mudhake

    mudhake Captain

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    at least a 300rpm loss, depending where you are now on the correct rpm scale.
     
  3. ulf75

    ulf75 Senior Member

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    I have a 24x25 or 26 4 blade nibral, Left hand, 1"3/4 shaft NIB

    Bought for a project, wound up going w different motor

    If your interested

    -Scott
     
  4. chasintail

    chasintail Senior Member

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    iveco 270 hp 35 Bruno extended keel looking for more speed ........... so going to a24x 24 will bring 2900 rpm wide open down to 2600 it will make me slower ?
     
  5. chasintail

    chasintail Senior Member

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    what should I go to for more speed ?
     
  6. Genius

    Genius Captain

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    You should be thinking about engine health, more than speed. If you are at the upper end of of your engine HP at WOT, getting a prop with longer legs will not do anything for you but cost you money in engine repairs/life.

    Best to run your numbers through a prop calculator or find someone who has a similar set up to compare (even then, its not the same). If you play around with props you may figure out where you are with engine loading.
     
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  7. plowin

    plowin Captain

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    What should be wide open throttle rpms be?
     
  8. chasintail

    chasintail Senior Member

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    wide open in neutral is 2900 ....2950
     
  9. Genius

    Genius Captain

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    I have no idea about isuzu engines, not familiar with them, need a model number. I believe it was meant as isuzu not iveco.
     
  10. chasintail

    chasintail Senior Member

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    iveco 270 hp I get 20 knots wide open clean bottom run at about 2200 15, 16 knots with pulpit and crows nest on
     
    Last edited: Feb 6, 2017
  11. Genius

    Genius Captain

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    my bad. What's your WOT RPM, in gear.
     
  12. chasintail

    chasintail Senior Member

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    2900 ..........2950
     
  13. Brooksie

    Brooksie Captain

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    If you are running at the engine makers specified RPM at WOT with existing prop, and if existing prop has been checked & tuned; going for more pitch or diameter will just overload your engine. Going from 4 to 3 blades often results in slightly more speed IF the 3 blade has proper blade area to prevent cavitation. If not, you will loose speed. Occasionally a different brand or style propeller will produce slightly more speed.
    Maybe shed some weight first as Bruno's are not the best weight carriers also have your tach checked to be sure what you are saying is actually true.
    Finally, an unmodified 35 can really only handle a 22" prop as a 24 does not have enough clearance either above or below. I have always run a 22" on mine and picked up a full knot by dropping my skeg bar and shaft years ago, giving it proper room to work, fairing the deadwood didn't hurt either but I can't remember how much...
     
    Last edited: Apr 14, 2017
  14. chasintail

    chasintail Senior Member

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    I modified it extended keel by about 6 or7 " shaft tube, dropped shaft,, longer rutter ,fairing some that's how I got to The 24 ....I could have a little capitation now looking at prop but not sure
     
    Last edited: Feb 6, 2017
  15. Genius

    Genius Captain

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    so....like I said, I don't know the engine (or model #). But, if you are 2900-2950 in neutral (governor) and also hitting governor set point in gear and the specs for the engine support that, you may have some room to increase pitch. If you have the gear ratio, water line length and displacement I can run a prop calculator?
     
  16. chasintail

    chasintail Senior Member

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    thank you , I don't know the displacement though , I had it figured when I put that motor in 4 yrs ago , and that's how I ended up with that 24 x21 ......
     
  17. hardcore1

    hardcore1 Captain

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    The most important factors are what is the engine supposed to reach at wot? and what is your gear ratio? Then what are you achieving with the existing prop which appears to be 2950rpm. Given all of that info you can be strafed in the right direction
     
  18. Numasahake

    Numasahake Captain

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    Why don't you install a pyrometer in the exhaust manifold. Then you can monitor EGT's. As long as EGT's don't go too high at wot, you'll be fine with the bigger prop. Heck, I run a pyrometer in my 17 hp Lombardic Diesel powered Arctic Cat 700 ATV. I'm sure I'm the only one in the universe to have done that...
     
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  19. Wharf Rat

    Wharf Rat Captain

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    Hey Guys,

    Really sorry I missed this thread, I have it set up so I get an email for the electronics posts, but not this one apparently. That said, it looks like you guys have already taken it to another level, and I'll be totally honest that this sht is WAY over my head, and my boss aka 'Greg Diesel' on the forum (although I don't think he knows how to post or has the time) is still in Islamorada fishing as we get the whole month of Feb. off for VaCa. We're back in the office on Monday, he's most likely going to have a pile of work to do but I'll fill him in and see if he as anything worth contributing. I will say I have actually learned a bit from this thread in particular, but I know that under/over propping is beyond any kind of exact science, but it's good to know once again the DEBF crew has it covered. Talk to you guys soon, I'm actually itching to get back to work, although by Monday afternoon I will regret even thinking that.
     
  20. chasintail

    chasintail Senior Member

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    IF I GO FROM A 24 X 21 TO A 24 1/2 X 26 WHAT EFFECT WILL IT HAVE ? MORE SPEED OR MORE TORQUE ? THANK YOU
     

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