Proper Prop pitch? Help please

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  • Proper Prop pitch? Help please

    Hi,

    So I'm prop tuning a new tohatsu 90 fron Marita, currently on the 3rd attempt to get it right. Think I'm close now. Engine specs max RPM range as 5150 - 5850. I now can get 51-52 WOT and trimmed. Engine is pushing a 98 Hewes 18' Searunner.

    Should I step down one more time to get closer to top of the max rpm range? What are the trade off's between RPM vs a flatter pitch? Does it really matter?

    Thanks in advance,

    dc

  • #2
    Tune it like you will normally run the boat. I'd say are probably propped about right. When you drop down to the next prop you'll typically pick up 500 rpm, which puts you over your rev limit. If you will normally go out more heavily loaded, you might want to drop a couple of inches in pitch.

    As it's a really good idea to carry a spare prop, you could just get the next lower pitched prop as a spare.
    Those that are successful in Alaska are those who are flexible, and allow the reality of life in Alaska to shape their dreams, vs. trying to force their dreams on the reality of Alaska.

    If you have a tenuous grasp of reality, Alaska is not for you.

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    • #3
      Thanks Paul,

      Actually, this sea trial was a party of two, a more typical load would be four folks, so meebe I'll drop it to 13.25/14. Part of the deal w/ Marita includes a SS prop, after I determine the right pitch. Will SS prop perform any differently, RPM wise? Also, would you comment on the advice quoted below concerning motor setup for SS vs aluminum. IS that correct?

      A standard aluminum setup is when the cavitation plate (that fin thing that is just above the propeller) is in line with the bottom of the boat. A stainless steel setup is when you raise that cavitation plate about 4 cm above the bottom of the boat (you normally raise the motor by about 2 holes on the transom but on certain setups it can vary). The reason that a stainless steel propeller can be higher out the water is because of its non flex ability in the water.

      http://ezinearticles.com/?Ultimate-Outboard-Propeller-Information-Guide&id=654730

      Thanks,

      dc

      Comment


      • #4
        I think you are on the right track too- but for others reading this thread here is a lesson I just learned while setting up my new Alumaweld 20ft Intruder.

        The boat is "lightly" powered with a Merc 4stroke 115 ( my choice because of escalting fuel prices -- especially here in Canada) The motor came with a 14DX17P aluminum prop -- but because of a screw-up -- no tach. When I got the boat back home from Oregon and had a tach installed, I found that the 17"P did not bring the WOT up to specifications (5-6K rpm) I went to a propeller shop to see if they could lower the pitch on the orginal Mercury 17" P prop. Nope-- not to the 15 " pitch I wanted to try..... But did they have a deal for me!!! A new aftermarket 15" prop that they assured me was as good as the Merc and almost as good as a stainless.

        Signed the credit card slip and I am on my way. Tried it out and sure enough-- the WOT is right where it should be. But I was still unhappy as the prop cavitated if I pushed the throttle a bit quickly when climbing out of the hole.

        Called the service folks where the boat was purchased.. suggested going to a stainless ($600+ ouch!)

        But my local Mercury service tech, said that for a low reving engine like a 115, the improvement would be marginal. He asked if the original 17P Merc prop cavitated as well--- I couldnt remember, but I agreed to put it back on as see. Well now-- NO cavitation with the Merc prop. He then said its your prop---"Did you buy it here?" Ummmm- no "Well try this Merc 15"P and see how you do." Man-- I cant believe the difference-- I DONT need a stainless prop.. the Merc prop functions just fine. Lesson learned-- All props are NOT created equal..... Hope my experience helps someone else.

        BatBreath
        Last edited by BatBreath; 06-19-2008, 16:51. Reason: Spelling

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        • #5
          Originally posted by Dogcliff View Post
          Hi,

          So I'm prop tuning a new tohatsu 90 fron Marita, currently on the 3rd attempt to get it right. Think I'm close now. Engine specs max RPM range as 5150 - 5850. I now can get 51-52 WOT and trimmed. Engine is pushing a 98 Hewes 18' Searunner.

          Should I step down one more time to get closer to top of the max rpm range? What are the trade off's between RPM vs a flatter pitch? Does it really matter?

          Thanks in advance,

          dc
          Per your post you are at the bottom end of WOT and moving to the next lower pitch will move you to about the 56 to 57 hundred range. Trimmed it should be closer to the top of the range. The stainless prop may drop that back about 100 to 200 rpms as there is not as much flex in the SS.

          As far as moving the motor so the cavitation plate is above the hull I do not recommend unless you are also going to carry an SS spare prop. There also is little to be gained by doing so. Boat also could get load sensitive if moved up.

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          • #6
            Thanks all... 'preciate the advice. I think I will drop one more pitch increment and ler 'er rip.

            ca

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            • #7
              Originally posted by Dogcliff View Post
              Thanks all... 'preciate the advice. I think I will drop one more pitch increment and ler 'er rip.

              ca
              Well what's the verdict????

              Comment


              • #8
                A SS prop will run a couple hundred rpm higher than an aluminum one will. Run the same pitch as the aluminum one. Ebay has the best prices on "used" props anywhere, you can't tell the difference from new most of the time.
                The emphasis is on accuracy, not power!

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