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40337Re: [bolger] Re: design ideas

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  • Jeff
    Nov 2, 2004
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      >Something I've never understood about paddlewheels is how to work out
      >the gearing. Obviously the paddles, at the bottom of the paddlewheel,
      >have to move backwards faster than the speed of the water moving past
      >the hull; but how much faster? Does anyone know? And what is the rule
      >of thumb that determines the under-water area of the paddle blades?
      >
      >Whatever the required speed is, the paddleshaft(s) would have to turn
      >a lot slower than the half-shafts of a road-vehicle.


      I've done a lot of research on this recently and here's what I've found out on paddlewheels.

      There is no set rule as to RPM but most stern wheels rotate anywhere from 40 to 100 RPM with 60 - 65 being the most common RPM. Unless you have a specially built wheel like Bolger designs on his Fast Sternwheeler you get a lot of foam and air injected going faster than 100 RPM causing very low efficiency.

      You should figure at least 30% slip and probably closer to 50%. In a rough estimate you can figure the required RPM by the wheel diameter and intended speed. So if you intend to build your boat to hit 10 MPH using a 6 foot diameter wheel you first get the circumference of the wheel which is 18.84 feet. To go ten miles per hour you need to cover (5280 feet X 10 miles ) divided by 60 minutes or 880 feet per minute. Since the paddle is 18.84 feet around you need about 47 RPM then allow for 30% slip, you'd need about 60 RPM on the wheel. Of course the thrust is based on power and paddles, this is just the speed you need on the wheel to get to 10 MPH.

      The rule of thumb is one paddle for every 1 foot of diameter plus 2. On a 6 foot tall wheel, that's 8 paddles.

      The paddle size is somewhat guess work based on length, width, depth, and HP. One rule that is floating around the WWW is to divide your HP by the feet in width of the paddle wheel to get the inches of paddle width PLUS 2 inches. If the wheel is 5 feet wide using a 30HP motor you get an 8" paddle. But, 1 or 2 inches make a big difference in how well the engine can handle the wheel and how efficient the wheel is to the power, oh, and changing RPM will affect things as well.

      Here's a good site to start with http://gemort.wirefire.com/contents.htm

      Jeff


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