propeller-gearbox match dilemma

Discussion in 'Props' started by tahrir, May 21, 2017.

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  1. tahrir
    Joined: Apr 2015
    Posts: 2
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    Location: Slovakia

    tahrir New Member

    So here's my situation. I'm building a dix 43 steel cutter. it's 15 tonnes, 11.87m at waterline, around 4m beam. The designer specifies an 18inch prop for a yanmar 4jhe which I believe is 44hp,3600rpm, with a 2.1:1 gearbox ratio. According to various online prop size calculators, 18 inch prop for that size of engine is too much.
    Regardless, I came across a good deal on a rebuilt nanni n4.60 (60hp, 2800rpm) and bought it thinking it would be nice to have a bit more power. The problem is that the transmission on it is 2.71:1 ratio, which means shaft rpms of about 1000. And the above mentioned calculators give 22inch propellers as most efficient.
    As I'm doing the build myself I can make modifications to fit a 22 inch prop but wouldn't like to deviate from the plans too much, especially because of added drag when sailing, unless absolutely necessary. And since I don't fully understand the implications of reducing the diameter and increasing the pitch of the prop, I'm hoping to get some pointers here.
    Or should I just get a lower ratio gearbox?
    Thanks in advance
     
  2. philSweet
    Joined: May 2008
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    Location: Beaufort, SC and H'ville, NC

    philSweet Senior Member

    It would help if you posted the data that you fed the prop calculators, and the entire results. I'm pretty confident Dudley can size a prop. For the Yanmar, Prop-it-now returns 18 X 10 given 41hp@3500rpm, 2.17 reduction, 3 bladed prop, 8.5 knots. Prop-It-Now // Michigan Wheel https://www.miwheel.com/inboard-propellers/prop-it-now/

    For the Nanni, if you underprop it a tad, 18x20 or 18x 21 would work fine in open water. This is for the same speed of 8.5 knots. Or 18x22 at 9 knots. The trouble with the Nanni will be with the idle speed. The boat will idle like a freight train through the marina with a 20 pitch prop. So if you can manage to modify the boat to take a larger diameter prop, you can pitch it down and get the idle speed lower. Or just accept that you will be slipping it in and out of gear a lot. On the other hand, a smaller, high pitch prop is nice for motorsailing.
     
  3. tahrir
    Joined: Apr 2015
    Posts: 2
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    Location: Slovakia

    tahrir New Member

    Thanks for the reply, it's very helpful. I've also heard back from the designer. He recommends a 22x14, three bladed prop... that should fit.
     
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