Increase of pitch rule of thumb ?

Discussion in 'Prop Shop' started by fredslann, Oct 18, 2009.

  1. fredslann

    fredslann I Love microskiff.com!

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    I had read that its a rule of thumb to allow 200RPM for each inch of pitch...good info or bogus info?

    Got the tiny tach and with the 9.9"X9 prop its turning about 6300 today...and the cavitation plate is 1" below the hull bottom...which I'd like to raise to level. I'd like to go with a ss wheel but I do tend to hit stuff like oyster rocks on occasion...so....I don't have a option to try a wheel before buying one here so I gona have to wing it...

    Does the data support going to a 9.9 X 12 3 blade?

    thanks for any suggestions
     
  2. Brett

    Brett > PRO STAFF <

    http://www.propmd.com/t-propellerbasics.aspx#pit

    covers most of what's needed to make an informed decision.
     

  3. Frank_Sebastian

    Frank_Sebastian Well-Known Member

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    If that is the 2 stroke Merc 25 HP that you mentioned in an earlier post and the boat is an aluminum skiff with a light load I would think a 3 X 10 and 3/8" X 13 would be about right for the boat with a light load?

    A 9 7/8" X 11 prop would likely be hitting a rev limiter and doesn't sound right for the engine mentioned earlier.

    Best regards,
    Frank_S
     
  4. fredslann

    fredslann I Love microskiff.com!

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    thanks for the info...
    It was a yami and I ordered a 10 X 12 solas...at least for now. That's takin it up 3 inches...and sometimes I have a passenger that may go more than 220lbs...but your most likly correct that this prop may also hit the limiter with just me and a light load.

    When I get the new wheel on, Ill post what it did to the topend and RPM's

    Thanks
     
  5. Frank_Sebastian

    Frank_Sebastian Well-Known Member

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    There is nothing wrong with under propping a little. Almost every commercial fisherman does it in hopes of a big catch. Then when (and if) you do find yourself with a heavier than expected load you will be able to get it up on a slide (plane) and get home before daylight.

    Best regards,
    Frank_S
     
  6. fredslann

    fredslann I Love microskiff.com!

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    just an update...
    added the Solas Alum 9.9X12 and got and avg. speed of 29.5MPH by GPS while taching out @ 6000...1/2 load 4"chop on the water. I had raised the motor up one inch on the transome..it didn't blow out until I put it hard over. I have the moter at the most angle the yoke will allow so the next thing is to see if I can make a wedge so I can get the nose up a tad more...it seems to be running with most of the water coming off at midway of the hull....which is not a flat bottom alum skiff...it does have some V but it does appear to have some hook in the hull bottom at the Transome...that may explain not getting the bow to rise.

    But otherwise the prop works nicly!!! I'm pretty happy with that speed!
    Thanks foe the help fellas
     
  7. Flyline

    Flyline Won &quot;Do More With Less&quot; Award!

    wow! that 's fast for 9.9hp so can u please please post a pics so we all see the pics! :) :) :)
     
  8. Lil_Tate

    Lil_Tate Well-Known Member

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    it says 25 merc on this thread, does it not?
    i dont see where it says 9.9hp anywhere on here?
     
  9. Flyline

    Flyline Won &quot;Do More With Less&quot; Award!

    Ohh... :-[ nope....I guess I read too fast... ;D
     
  10. fredslann

    fredslann I Love microskiff.com!

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    Just one more update....
    The boat is a 87 Alumacraft...same as the Express boats now...16fter. motor is a 2strk 25hp yammi.
    I made a 10 deg wedge to see if that'd help get the nose up and it did...but was squirlley. MPH only increased by 2.. and blowouts took place in the slightest turns. So I took the wedge out and I'll take the gain in speed I first got.