is my engine is mounted too high?

Discussion in 'Powerboats' started by the brain, Oct 6, 2016.

  1. gtflash
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    gtflash Senior Member

    The age old rule of plate 1" above is a rule of thumb for bog standard boring boats

    Another rule of thumb is for every 6" of rear offset, you can go up 1/2" so in theory 1.5" up in this scenario, but all boats are individual and there is no exact science. I am with ondarv. Plate doesn't need to touch water, but some people say it needs to be splashed with water

    Highly unlikely you are going to run surface piercing, so with what ever prop you have, does it plane, does it feel stable, does it not over heat, does it corner. If all are ok, you may not be too high, if it gets hot, lower it. If it won't plane, lower it. If your WOT is too low, get a smaller prop If it's too high get a bigger one. Add a blade and you will loose some rpm. Switch to stainless from Ali and you will gain a tiny bit of rpm It's an absolute black art, friends with props are best friends
     
  2. the brain
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    the brain Senior Member

    I've decided on replaceing the bass boat prop 14 1/2" X 24 w/ a 14 3/4 x 21
    thanks for surport
     
  3. the brain
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    the brain Senior Member

  4. gtflash
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    gtflash Senior Member

    I am not familiar with the prop you have or sent link too. It looks like you have a 3 blade stainless. As others said it looks like your 24 is too big Dropping to a 21 or 19 will give you more revs. If the one you have has a nice attitude and it grips sufficiently then look for something similar. If you are after more speed I personally think you could go up in height but my boat runs several inches above standard so it looks low in comparison. Might be better to set height before buying props as the height may change prop choices
     
  5. the brain
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    the brain Senior Member

  6. Mr Efficiency
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    Mr Efficiency Senior Member

    Something not right here, you have gone down in both pitch and propellor diameter, and lost rpm ? What about the engine height, did you go down ?
     
  7. the brain
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    the brain Senior Member

    actually I was planning on going up one set of holes I feel the engine could be dragging creating more drag.

    did you see the video
     
  8. Mr Efficiency
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    Mr Efficiency Senior Member

    The video in #1 post ? yes, it certainly was not too high in that. Bear in mind when going up, that whilst you might be able to raise it substantially to good effect in calm water, rough conditions might see it needing to come down. Maybe a jacking plate is in order.
     
  9. the brain
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    the brain Senior Member

    you make some good points.

    I actually have a hydrolic jack plate however I don't want anymore setback.

    from what I've read when boat is on plane the anti cav plate should be skimming ontop of water so in either situation smooth or ruff water not should how this applies to my situation?
     
  10. kapnD
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    kapnD Senior Member

    I see in your other thread you are in the process of enclosing the cabin, adding a genset and AC, fuel tank, livewell, deep fryer, etc.
    I would leave the prop selection until you are finished adding weight.
    Having an unrealistically high target speed will not help you in selecting an appropriate prop that will have acceptable performance at all speeds.
    The weight of the rig, the shape of the hull, and the horsepower all impose limitations on what speed can actually be attained.
    David Gerr's book titled "Propeller Handbook" explains the intricacies of propeller selection in easy to understand terminology.
     
  11. the brain
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    the brain Senior Member

    I had another chance to take the boat out I was actually incorrect about my previous findings.

    w/ the new 14 1/4 X 21 aluimin prop I am now able to achieve higher RPMs closer to the 5500desired RPM (haven't completely document exact RPMs)

    the higher than 5K RPM w/ new prop was achieved w/ trimming up seamed like the more I trimmed up the higher the RPMs I believe it I trim up to much then the prop. loses it's traction correct.

    edit this last shakedown was in saltwater if that has anything to do w/ results.

    edit I estimate the addition of the rear inclosesure to be around 100LBs.
     
  12. Steve H
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    Steve H Senior Member

    Keep in mind that anytime you go from a stainless prop to an aluminum prop, performance will be lost.
     
  13. ondarvr
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    ondarvr Senior Member

    Not if you go from the wrong SS prop to the correct aluminum prop.
     
  14. Steve H
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    Steve H Senior Member

    Then why did he lose RPMs with a lower pitch prop until he overtrimed it?

    Stainless will always out perform aluminum. I know they are more expensive, but on this particular application a SS prop is the way to go.
     

  15. Mr Efficiency
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    Mr Efficiency Senior Member

    Stainless is certainly the way to go, in this instance, or any other where there is a greater chance of ventilation/cavitation issues.,
     
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