I need a 1:1 transmission for an Atkins design

Discussion in 'Inboards' started by Eric770, Sep 5, 2014.

  1. Eric770
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    Eric770 Junior Member

    Hi, I'm building a 19' motorsailer with an inboard engine. It is the Atkin design "Surprise". [URL="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
     
  2. AndySGray
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    AndySGray Senior Member

  3. gonzo
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    gonzo Senior Member

    VelvetDrives are too big for a 30HP engine. Borg Warner made smaller transmissions. Yanmar manufactured transmissions under BW license that would fit also. However, you will need a bell housing and damper plate that fit.
     
  4. Eric770
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    Eric770 Junior Member

    The Borg Warner seemed like a good idea. The 71C series, 1017 model looked smaller than the others, but the BW site doesn't give clear specs. My motor only puts out about 50 ft/lbs. I called and went to a somewhat local marine junk yard but they only had large BW units. Can you suggest any key words for searching for the Yanmar trany?
     
  5. Eric770
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    Eric770 Junior Member

    I just don't understand why there aren't, or more likely, I can't find a small trany that would mate to a small sailboat engine. After all, diesels don't turn fast, so a 1:1 should be out there. A google search turned up a huge array of Chinese motors and tranys, but no 1:1 and nothing that wasn't scarey. I don't know....
     
  6. FAST FRED
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    FAST FRED Senior Member

    and the 10" prop I have needs 2400 RPM to reach the design top speed of 20 MPH.

    A bit of diameter , say 14 or 15 inches would be more efficient , and less costly to obtain than a 1-1.

    With a small boat why not a clutch , no tranny , and you are done.
     
  7. Eric770
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    Eric770 Junior Member

    Fred, I'm limited to the 10" by the hull design. The stern reaches out over the prop, and a rudder bracket runs underneath it. I'll be trailering the boat and on the Hudson River with tides and chops and cross winds, reverse is important to get it back on the trailer. My original thought was to make something with a thrust bearing, pulleys and either an electric clutch or idler pulley, but when I added up the cost, it seemed to make more sense to buy a trany and be done with it. I may have to fall back on Plan A.
     
  8. gonzo
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    gonzo Senior Member

    A 71 series will rob you of a lot of power. They are made to run 15 times the power of your engine. Call marinas or boat repair shops. They are usually plenty of used transmissions laying around. Have you tried ebay?
     
  9. Eric770
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    Eric770 Junior Member

    Gonzo, I agree, 71C will suck up too much power. I see that can support up to 400 HP.
    Yes, I saw a few on ebay. The shipping cost is very high, due to high weight I suppose.
    Still looking. Thanks for the input
     
  10. gonzo
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    gonzo Senior Member

    Have you looked at the small Hurth or Paragon transmissions? There should be some that came with Atomic 4 (Universal) gas engines around too. Those blocks tended to corrode and crack, so transmissions may be available.
     
  11. Eric770
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    Eric770 Junior Member

    I considered the Hurth but found quite a few negative posts and wondered about their reliability. Further more they are 2 or 3x the price new of a PRM, and I didn't see any 1:1 ratios. I'll give them another look.
    Do you know anything About PCM? The have a trany with 1.23:1. I looks pretty big.
    Thanks
     
  12. gonzo
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    gonzo Senior Member

    The PCM are the equivalent to a Velvet Drive.
     
  13. FAST FRED
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    FAST FRED Senior Member

    IF you can live with near 2-1 I have an excellent unit unit from a Volvo MD 7.

    It has a flat flange with 3 ears and with 3 ears on the engine output shaft a rubber biscuit does the driving.

    Fwd neut and reverse $ 500 , in Middletown CT , a drive from the Hudson , no shipping.

    The Volvo were 13 to 17 hp depending on the year so should be fine for you.

    https://post.craigslist.org/manage/4599487812
     
  14. sailmonkey
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    sailmonkey Junior Member

    Greetings everyone. I am considering an Atkins design as well and have a 20 hp engine. Can anyone direct me to a list of transmissions which may be on the used market and which may be suitable for application? I look on Craig's list and Ebay and national salvage sites almost all of the marine transmissions are for 100's of horsepower, which is frustrating.

    If I can not find one, I may try a torque converter from a snow mobile or go kart. They are inexpensive and light weight relative to marine transmissions. But have to have an engine shaft and a separate drive shaft, which complicates things in a small boat. Also, no reverse, which in a small boat isn't such a big deal.

    Thank you.

    John
     

  15. gonzo
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    gonzo Senior Member

    You will also need a separate thrust bearing with some kind of mount.
     
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