Prop advice for a Roth Bilt 15

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  • Prop advice for a Roth Bilt 15

    Hello, first time posting. I came across this site while trying to figure out if I need a new prop. Site has tons of good info, so hoping I can get my situation figured out. I have a 15ft Roth Bilt - fiberglass, center console (in foreground of picture) that I bought a year ago. It has a Johnson 35hp 2-stroke on it. I currently have two props that I have tried for the boat. Each gives good performance, but each has drawbacks. First is a 10"x13" and the other is a 10.5"x11". Both are aluminum. The 13" pitch prop is great with light loads on the boat. It tops out at 5800rpm and 27 mph WOT. Mid-range is also good, 20mph @ 4700rpm. However, if the boat is loaded to go to the beach, it struggles to get on plane. With the 11", I have the opposite problem. With a light load the motor hits 6300rpm and 22mph WOT. But gets up on plane with a heavy load.

    Hoping there is something in the middle? I don't mind dropping top end speed for better mid range performance with heavier loads. First thought was to switch to stainless to get the benefits of a stiffer prop over aluminum. But then what pitch? 12"? Or stay at 11" with the added performance from stainless? Maybe a 4-blade prop?

    Admittedly, the boat is probably a little under powered. But it's what I've got at the moment. Any thoughts, advice, or ideas would be appreciated.


  • #2
    Propeller calculator ('Prop Wizard') by Turning Point Props


    I like this "prop wizard" by Turning Point propellers as you can use your specific make/model boat or one as close to yours that you can find. For example, I couldn't load a ProCraft tin boat from their database (years ago), but found a Smoker Craft of similar/same beam, length and type, for example. You might need to find a similar flat-bottomed hull to match - or call them, as their Customer Service is excellent!

    Then you can add weight for how much gear and fuel the boat typically carries on every trip, then even add for # of people carried aboard. Even better than the Prop Wizard ... I love their props! They will also exchange your prop for another one if you try one out and don't like the results!

    FWIW I have successfully rigged 6 boats 16' to 21' in the past few years using this calculator and their props, with awesome results! Their new 'pressed aluminum' blades offer thin blades and what they call "SS performance at an aluminum prop price". I rigged my old 16' tin skiff with 60hp 2-stroke Johnson OB with one of the props and the #s were dead on PERFECT!

    For larger HP and/or offshore boats, I call Ken at, as he too will swap out props until YOU are happy.


    • #3
      Ok, so I believe that motor has a 2.15
      13 pitch, 5800 RPM at 27 MPH is 19% slip (not great)
      11 pitch at 6200 an 22 MPH is 39% slip (really bad)

      the prop I'd suggest would be a Powertech SRA3
      12 pitch, you might actually pick up a couple MPH over that 13 you have, and still have good acceleration.
      you could actually go 11 pitch, and if it hooks up like it should, you would still get 26, maybe 27 MPH out of it at 5800 RPM and acceleration would be great.

      They cost $249
      if you get one and decide you want a different pitch or style, you can swap for 35 plus shipping.

      Ken Reeves
      [email protected]


      • #4

        Appreciate the information and the SRA3 looks like a good choice. If you had to pick one, would you recommend the 12" or the 11"? I'd love to go with the 12" if you don't think it will lag under heavy loads.



        • #5
          go with the 12, if you need to swap out its $35 plus shipping to go to an 11 if you think its too much.

          Ken Reeves
          [email protected]