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prop pitch........

Discussion in 'Boats and Motors' started by Alwsfishin, Feb 13, 2005.

  1. Alwsfishin

    Alwsfishin '73 24' Stamas

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    Which prop will give you better preformance.....10 3/4 x 11.......or a 10 1/2 x 13........'61 40 hp .......if it matters.....I'm looking for higher rpm.....theres a 2" difference in pitch in these 2 props .....thats 300 to 400 rpm difference......which is the higher rpm 11 or 13?...thanks
     
  2. ncraft150

    ncraft150 Buckeye-Basser


  3. freyedknot

    freyedknot useless poster

    an 11inch prop will move you 11inches per revolution and a 13 will move you 13 in. per revolution. just make sure of your rpm,s are right for the motor.
     
  4. UFM82

    UFM82 The one others want to be

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    Pitch is how far a prop will move ahead through a SOLID medium- that is, no slip. All props will slip in the water, some more than others and boat design has a major influence. Light easy planing hulls slip the least while heavy displacement hulls slip more.

    It is true that pitch influences RPM- adding an inch of pitch will usually result in a 150-200 rpm change. Thus, going from the 11 pitch prop to the 13 pitch prop will drop your rpm by 300-400 rpm. BUT, changing the diameter of the prop has an effect as well. Larger blades tend to hold the water better and move larger hulls better. Smaller diameter blades offer less drag and are "faster", if that makes sense. Diameter can have serious influences on RPM but I don't think the 1/4" difference in these two props will have much effect at all.

    Do you know now what your WOT RPM is? If you are already at top RPM, the bigger prop may lug your engine too much. If you are low, the 13" prop will be too much for the boat. If you are overrevving, the 13" prop is needed to bring the RPMs down.

    If you don't have a tach and don't really care where the RPMs are, (some guys just run 'em by ear, you know?) the 13" will probably give you better speed but may lack power- it may slow down a lot in turns. Take off may be sluggish as well.

    That 40hp should be able to do the 13" prop though. My 50 hp ran a 17" SS prop and it did very well with it.

    UFM82
     
  5. Alwsfishin

    Alwsfishin '73 24' Stamas

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    Thanks for the replies.........thats the problem I was having.....the larger blade prop was definitly lugging the motor .......then I took a prop off another motor I had and it doubled the rpm as well as the mph....I'm running the 40 hp on a 16 1/2 Lyman with new paint........not much drag on this hull......
     

    Attached Files:

  6. UFM82

    UFM82 The one others want to be

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    Those two props look VERY different. That first one looks like a big trolling motor prop- high thrust/low rpm blade. That second one looks like a typical small outboard prop. If you want more pitch in that prop, take it to a shop and have them add pitch. Up to 2" of pitch can usually be added to most props without messing them up. That would give you some speed and drop the rpm's down a bit and maybe work out for you.

    UFM82

    Edit: Oops, I saw that you wanted to increase the rpms- sorry. Have the shop take some pitch out. Just the opposite of what I said. :eek:
     
  7. Alwsfishin

    Alwsfishin '73 24' Stamas

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    I ran a prop similar to the white one and the motor seem to be running at 1/2 the normal high rpm....then I put one on similar to the black one and she went from 15 mph to 25/27 mph, result from a serious jump in rpm's ....now I was going to tinker with different pitches in the bl prop style.....thanks again !!
     
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