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cavitation due to oversized prop

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  • cavitation due to oversized prop

    I have an 07 X-45 with the 8.1L. GSA tabs and lots of ballast in the boat for surfing. When I set all this up about 5 years ago, I spoke live with Eric at OJ Props who suggested the OJ 829 (15 x 16.5). I've had that prop on for years - it's great for surfing.

    But, at the end of every year there are chips in the fiberglass right by the prop. I used to think it was a rock getting kicked up by the prop and chipping the fiberglass. But damage was particularly bad this past season. See pics here: https://photos.app.goo.gl/q6uFvhELTB2JK3x66

    I now think the damage is from the prop having so little clearance to the boat and is is causing cavitation.

    I've been googling and found a few sites saying you want at least 1" between the prop blade and the bottom of the boat. I would guess I have about 1/4" inch of space - hard to measure exactly.

    I tried a new approach of not only filling the cracks but also putting in a very thin steel plate to withstand the pressure from the prop much better. But of course now I'm looking at about 1/8" of space between the plate and the prop.

    I called Eric at OJ and he said they could reshape the prop to give it a little more pitch but less diameter. I might lose a little power but would gain back the space which could improve overall water flow. I'm curious if anyone has heard of any of this or has put oversized props on their boats for more surfing power? Any issues with a tight fitting? Should I just send back to OJ and have them reshape to a smaller diameter?

    Curious what folks would do.

    Thx

  • #2
    Originally posted by joelevy View Post
    I have an 07 X-45 with the 8.1L. GSA tabs and lots of ballast in the boat for surfing. When I set all this up about 5 years ago, I spoke live with Eric at OJ Props who suggested the OJ 829 (15 x 16.5). I've had that prop on for years - it's great for surfing.

    But, at the end of every year there are chips in the fiberglass right by the prop. I used to think it was a rock getting kicked up by the prop and chipping the fiberglass. But damage was particularly bad this past season. See pics here: https://photos.app.goo.gl/q6uFvhELTB2JK3x66

    I now think the damage is from the prop having so little clearance to the boat and is is causing cavitation.

    I've been googling and found a few sites saying you want at least 1" between the prop blade and the bottom of the boat. I would guess I have about 1/4" inch of space - hard to measure exactly.

    I tried a new approach of not only filling the cracks but also putting in a very thin steel plate to withstand the pressure from the prop much better. But of course now I'm looking at about 1/8" of space between the plate and the prop.

    I called Eric at OJ and he said they could reshape the prop to give it a little more pitch but less diameter. I might lose a little power but would gain back the space which could improve overall water flow. I'm curious if anyone has heard of any of this or has put oversized props on their boats for more surfing power? Any issues with a tight fitting? Should I just send back to OJ and have them reshape to a smaller diameter?

    Curious what folks would do.

    Thx
    That's not cavitation, if it was cavitation you'd see damage to your prop (pitting) as well. My guess would be erosion damage due to the force of water flowing across your hull.

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    • #3
      gotcha. That helps and I'm sure you're right from the force. Same questions apply though - I can fix with a metal plate but not sure the downside of little clearance.

      Comment


      • #4
        Originally posted by joelevy View Post
        gotcha. That helps and I'm sure you're right from the force. Same questions apply though - I can fix with a metal plate but not sure the downside of little clearance.
        Not sure how much but with an overhung rotor like a prop shaft, it's got to deflect some under power. Any idea ? I doubt it's 1/8" though. More like something in the thousands of an inch.

        Comment


        • #5
          I am not an hydraulics engineer, but I would think, that you need to have a certain about of ancillary space around the prop to efficiently funnel the water through and around the prop. Otherwise, I think you would have a low pressure area that would cavitate really easily. the damage is probably a combination of junk in the water and also the water itself.

          Michigan Wheel, who manufacturers props had this to say on their website:

          What is the minimum allowed clearance between the propeller blade tips and hull bottom? How much space should exist between the propeller and strut or keel?

          The clearance between the propeller blade tips and the hull bottom should be at least 15% of the propeller diameter and ideally, 20% or more. So for example, a 20” diameter propeller would have 3” minimum clearance and better still 4” or more. A common guideline for recommended propeller to strut or keel clearance is 20% of propeller diameter measured between the propeller blade edge and strut leg or keel. This is often measured at a point on the propeller blade edge about 70% of the distance from the shaft centerline to the blade tip.


          Would a smaller diameter propeller with more pitch provide the same performance?

          A propeller with smaller diameter and larger pitch might provide the same load on the boat’s engines but may not have the same efficiency as the suggested propeller. Propeller diameter is chosen to be optimal for the boat’s particular combination of horsepower, RPM and speed. Deviating significantly from the best diameter may result in slower acceleration, reduced cruise speeds and efficiencies, and more difficulty in slow speed maneuvering.

          Comment


          • #6
            I'm assuming you haven't tried it in the water yet. Given that you've done the work, that's what I would do next. I wonder if it'll be noisy. But other than that, I think the suggested clearance are primarily to prevent gel coat and fiberglass damage. I doubt that'll be an issue with your steel plate.

            The only other consequence I can think of, is if you ever run through a soft mud bottom that has rocks mixed it, you're more likely to damage the prop if a rock is kicked up and gets between the prop and that plate. I wouldn't worry about that myself on my wake boat because I'm never in shallow water. I would worry about it with my ski boat because one end of our course is very shallow, but with a very soft, muddy bottom. I end up running the prop through the soft mud a couple of times a season. Typically, without any real damage. I only mention this just in case you happen to be in shallow water regularly.
            -----------------------------------
            Mastercraft ProStar 2019 5.7L - Current
            Mastercraft X25 2014 6.2L - Current
            Nautique 200 OB 2012 5.7L - Current

            Comment


            • #7
              Originally posted by lakedrum03 View Post
              I am not an hydraulics engineer, but I would think, that you need to have a certain about of ancillary space around the prop to efficiently funnel the water through and around the prop. Otherwise, I think you would have a low pressure area that would cavitate really easily. the damage is probably a combination of junk in the water and also the water itself.

              Michigan Wheel, who manufacturers props had this to say on their website:

              What is the minimum allowed clearance between the propeller blade tips and hull bottom? How much space should exist between the propeller and strut or keel?

              The clearance between the propeller blade tips and the hull bottom should be at least 15% of the propeller diameter and ideally, 20% or more. So for example, a 20” diameter propeller would have 3” minimum clearance and better still 4” or more. A common guideline for recommended propeller to strut or keel clearance is 20% of propeller diameter measured between the propeller blade edge and strut leg or keel. This is often measured at a point on the propeller blade edge about 70% of the distance from the shaft centerline to the blade tip.


              Would a smaller diameter propeller with more pitch provide the same performance?

              A propeller with smaller diameter and larger pitch might provide the same load on the boat’s engines but may not have the same efficiency as the suggested propeller. Propeller diameter is chosen to be optimal for the boat’s particular combination of horsepower, RPM and speed. Deviating significantly from the best diameter may result in slower acceleration, reduced cruise speeds and efficiencies, and more difficulty in slow speed maneuvering.
              Good info.

              Comment


              • #8
                Thanks folks - as always this forum is great. I think I'm going to get it modified and try that. Never a perfect solution but does seem better to have 1" or so of space b/w prop and boat.

                Comment


                • #9
                  Originally posted by dpb185 View Post

                  That's not cavitation, if it was cavitation you'd see damage to your prop (pitting) as well. My guess would be erosion damage due to the force of water flowing across your hull.
                  ^ this

                  Originally posted by joelevy View Post
                  Thanks folks - as always this forum is great. I think I'm going to get it modified and try that. Never a perfect solution but does seem better to have 1" or so of space b/w prop and boat.
                  and this ^

                  .
                  93 190
                  John 14:6
                  .
                  2 Peter 1:3–8
                  .
                  Matthew 6:1-4

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