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Building a prop for the 1/60th Scale Walrus

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  • Building a prop for the 1/60th Scale Walrus

    I will be making the prop for the Walrus.
    I am running into a problem of figuring out the prop. It is a 5 blade prop. From plans it is for a 1/60th scale model making the prop 60mm or 2.3622
    Click image for larger version

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    Now I need to make the blades. But because the blades on the plan are curved, how do you figure the proper pre-shaped contour?

    I tried seeing if there were other sources and I found a paper model, but its blades looked thin (black line) then I used the real prop (in red) and overlaid it, but the angle of the photo is off skewed. I have tried to combine and average, but I am not so confident of my abilities.
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    Any suggestions?
    If you can cut, drill, saw, hit things and swear a lot, you're well on the way to building a working model sub.

  • #2
    As a rule of thumb, the pitch of a marine propeller for submarine use is about unity, i.e. the same. Both are linear measurements, not expressions of angle. Pitch is the non-slip distance, along the shaft axis, a propeller makes through the fluid in one complete rotation. That distance dependent on the angle of the blades. As the tip of the blade traverses more medium than the root, it's angle must be less than that of the root, and all radius points along the span of that blade have to be so angled to all travel the same distance in one rotation -- this propeller is said to be a constant pitch type. This is why the helical twist to the blades.

    Another accepted rule for submarine propellers: the total developed blade area, less the hub, is around 60% of the disc. Graph-paper is your friend as you work out blade profile and blade area.



    Take that nice picture of the ass-end of that sub and enlarge it so the propeller diameter is equal to the scale model propeller you want to make. The slight off-set is small here and can be discounted. Your view is a projected one, you can't detect from observation in this plane the actual cord of the blade root because of the helical twist. Once you make your blade chart, and can observe there the actual cord at that radius point, will you be able to establish the blades true surface outline -- the developed projection. And that's what you'll need to do before preparing the blade blank and cutting it to form. You want to lay down the profile of the developed blade, not the projected (what you see) blade.

    A developed blade profile looks like this:



    Note the foil sections at selected radius points along the span of the blade -- useful stuff, if you can secure contractors drawings. If not, wing it! The suction side (forward) of the blade has camber, the pressure side (aft) of the blade has little camber or is flat. No under-camber, this ain't an airplane!

    Once you have your developed blade profile worked out, make a template and transfer that outline to a block of pattern makers medium like sugar-pine or dense RenShape.



    The root squared off to capture the angle at that radius. The tip squared off to capture the angle at that radius.

    Noth'n to it!

    (the first smart-ass who says, "screw it, I'll just poop one out on the 3D printer", gets my foot up there ass!)

    David
    "... well, that takes care of Jorgenson's theory!"

    Comment


    • #3
      Originally posted by He Who Shall Not Be Named View Post
      (the first smart-ass who says, "screw it, I'll just poop one out on the 3D printer", gets my foot up there ass!)
      Some pay good money for that!

      DIVE IN! Go on, go on, go on, go on, GO ON! http://www.diveintomodelsubmarines.co.uk

      Comment


      • #4
        Using a bit or Pythagoras (a2+b2=c2), and interpolating the drawing with the picture, I came to a figure of about 13.2mm width for the root of the blade.

        What I would do, is make some simple templates from scrap sheet metal, which you can twist, then take some face on shots until it looks authentic.
        DIVE IN! Go on, go on, go on, go on, GO ON! http://www.diveintomodelsubmarines.co.uk

        Comment


        • #5
          Originally posted by Subculture View Post

          Some pay good money for that!
          (you're not helping, Andy)
          "... well, that takes care of Jorgenson's theory!"

          Comment


          • #6
            Originally posted by He Who Shall Not Be Named View Post




            David
            On the graph, I notice you mark up 2.25" on the y coordinate (each block is 1/4") equalling the pitch, so on mine it would be 90mm (60mm x 1.5) to be easy I am keeping metric system. Yet your r1 - r5 is marked out how on the x coordinate? Is it spaced 1.5" (original diameter of prop) or what?
            If you can cut, drill, saw, hit things and swear a lot, you're well on the way to building a working model sub.

            Comment


            • #7
              Originally posted by trout View Post

              On the graph, I notice you mark up 2.25" on the y coordinate (each block is 1/4") equalling the pitch, so on mine it would be 90mm (60mm x 1.5) to be easy I am keeping metric system. Yet your r1 - r5 is marked out how on the x coordinate? Is it spaced 1.5" (original diameter of prop) or what?
              The distance of the R-point along the X-line is the circumference of the disc at that radius point. 2(pie)R

              The line from the R-point to the pitch-point represents the angle of the blade at that point along the span.

              David
              "... well, that takes care of Jorgenson's theory!"

              Comment


              • #8
                Tell me if this looks like I am on the right path.

                Click image for larger version

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                If you can cut, drill, saw, hit things and swear a lot, you're well on the way to building a working model sub.

                Comment


                • #9
                  Too much pitch. You have 1.5 times diameter, which will lower required shaft speed, but make the boat far more prone to torque roll. Aim for no more than 60mm pitch (1:1 or square), and preferably less e.g 0.8x diameter.

                  Boats with contra-rotating props, or single screw inside a propulsor/pumpjet with fixed stators can run more pitch, as you have a counter force.
                  Last edited by Subculture; 02-17-2017, 06:02 AM.
                  DIVE IN! Go on, go on, go on, go on, GO ON! http://www.diveintomodelsubmarines.co.uk

                  Comment


                  • #10
                    Originally posted by Subculture View Post
                    Too much pitch. You have 1.5 times diameter, which will lower required shaft speed, but make the boat far more prone to torque roll. Aim for no more than 60mm pitch (1:1 or square), and preferably less e.g 0.8x diameter.

                    Boats with contra-rotating props, or single screw inside a propulsor/pumpjet with fixed stators can run more pitch, as you have a counter force.
                    Andy's right, Tom. Plug in a 60mm pitch to the chart, connect the dots, and you have a reasonable pitch chart for that wheel. (you were likely queuing off that VICTOR-3 propeller(s) chart -- special circumstance, and not typical).

                    David
                    "... well, that takes care of Jorgenson's theory!"

                    Comment


                    • #11
                      This is good stuff, right here!

                      Comment


                      • #12
                        If the prop diameter is small in relation to the sub ( e.g. Russian Kilo class) then you can get away with more pitch. Most modern subs tend to have large props. Also the heavier a boat is, the larger the torque moment it can handle.
                        DIVE IN! Go on, go on, go on, go on, GO ON! http://www.diveintomodelsubmarines.co.uk

                        Comment


                        • #13
                          Originally posted by SubHuman View Post
                          This is good stuff, right here!
                          I blame Tom. He's such a pain-in-the-ass!

                          David
                          "... well, that takes care of Jorgenson's theory!"

                          Comment


                          • #14
                            Click image for larger version

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                            Here is the updated drawing. Does the Walrus have a small prop?

                            Click image for larger version

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                            If you can cut, drill, saw, hit things and swear a lot, you're well on the way to building a working model sub.

                            Comment


                            • #15
                              Okay, can see it's shallower despite still saying 90mm pitch. If you want you can make the outer pitch (R5) a bit shallower in pitch, and the inner (R1) a bit steeper. In reality you have a lot of allowance with model props- we're not governed by ultimate efficiency or worried by vibration irritating the vessels occupants. You'll just give a few fish the bums rush!

                              Walrus prop is pretty large, compare it with say your Gato.
                              DIVE IN! Go on, go on, go on, go on, GO ON! http://www.diveintomodelsubmarines.co.uk

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