prop hel[

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  • prop hel[

    new to boating, especially pontoons. i have a 2012 south bay 525cr pontoon with 2012 mercury optimax 90. top speed is 21.3 GPS at 5400 rpm. using mercury online slip calculator, this comes out to be 35% slip. is this normal? what could I do to reduce slip and improve efficiency? current prop is a three blade Mercury 13.75 X 15. Top speed is fine for me where it is at, just looking to make it more efficient and save some fuel. Also, when turning sharp, prop really loses grip and blows out, speed goes way down (around 13mph).

  • #2
    Prop slip is not always what the term implies. What happens is, the column of water the prop is pushing to create thrust, is actually moving through the water. This effect is greatest during static conditions and diminishes as the boat accelerates through the water. There will always be "slip" during propulsion through a media that is fluid. Not so with car tires and the road...unless you're peeling out and spinning the tires.
    When using pitch to calculate how much a prop should move through water for each rotation, remember this number is ideal and without factoring in "slip". This slip is not so much the water slipping across the blade face, but the entire thrust column moving rearward through the water around it.
    Larger diameter props with less blade angle (pitch), which produce a larger thrust column and mass, reduce this effect. Smaller diameter props with higher blade angles increase this effect. There are other factors that affect how a foil affects the medium, such as blade design, blade area, blade cross-section, trailing edge cupping and rake (sweep angle).

    Bottom line...a bigger prop with big blades will help, but you'll have to reduce pitch.


    • #3
      Thanks for the response

      I was thinking of prop slip as a measure of efficiency. Is this a correct way of looking at it?

      the initial reason I thought that I had an issue with prop slip is that when I tried to use the mercury prop finder, the lowest speed it gave me to pick from with a 15in pitch at 5400 rpm with 2.33 gearing was 25mph which would be 25% slip. So my slip is too high to enter my data into the prop finder which led me to believe that it must be abnormally high.

      would going to a 4 blade improve efficiency and if so what pitch would be recommended to stay below the 5700 rpm max?

      i cannot run a larger diameter than what I have now due to the 90hp being the smaller gear case. Max diameter would be 14”


      • #4
        You're probably fine. How does it cruise? It's a pontoon boat. Not a speed demon. Enjoy the ride.


        • #5
          I am looking for better fuel economy more than anything, and to me if something is only operating at 65% efficiency, then it certainly has room to improve. Just looking for some thoughts from people with more experience in this area than myself so I don’t end up throwing a bunch of money away on things that do not improve the performance or efficiency.