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    • skiptabor

      The future of the JD Forum   07/31/18

      Thank you all for your participation on the JD Forums.  We have enjoyed answering your questions, providing project advice and technical assistance on this forum for a number of years. We believe that there are now better ways to provide this service for our customers and the community. With that in mind, we have made the difficult decision to discontinue the Jamestown Distributors Forum.   Our JD Tech Team is committed to being your trusted technical resource, and we encourage you to reach out via email, phone, social, product reviews, and to post questions using the Q & A 'Ask A Question' function on all product pages. Additional information can also be found on product page Technical Data Sheets (including detailed usage instructions and application data).   Thank You Jamestown Distributors 

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Showing results for tags '1:1 ratio trany'.



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  1. Hi, I'm building a 19' motorsailer with an inboard engine. It is the Atkin design "Surprise". http://www.atkinboatplans.com/Sail/Surprise.html The hull is made, as well as the decking, and it is time to mount the motor. I have a very nice Kohler, v-twin, 30 HP water cooled, fuel injected, non marine gas engine. I also bought a nice transmission, a PRM80. The problem is, I need a 1:1 ratio transmission, not the 2:1 of the PRM. I want to cruise the motor at 2400 RPM (3500 max rpm), and the 10" prop I have needs 2400 RPM to reach the design top speed of 20 MPH. I thought I could change the prop pitch to account for the reduction in RPM, but found it would require an extreme pitch and would have too much water slip. The only solution I see is to 1-find a transmission with a 1:1 ratio (which I have as yet not come across) or 2 - increase the RPM between the motor and transmission in some way. I have explored various belt designs, but at high max RPM 2 x 3500 = 7000, and the 20 to 30 hp, and the constrained space, I need either multiple V-belts, or synchronous belts, which become quite pricey and complicated to mount and couple to the motor and trany. Does anyone know of a 1:1 trany that will provide neutral and reverse? or have a suggested solution? Eric