Gear ratio vs prop pitch

Discussion in 'Inboards' started by 202_ski, Jul 10, 2009.

  1. 202_ski
    Joined: Mar 2008
    Posts: 12
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Australia

    202_ski Junior Member

    I have a V drive boat that is 32% downdriven. I'm putting in a new motor (that only revs to 4000) with almost double the power of the old. I would like to change gears to atleast 1:1 but it is much easier and cheaper to change props. I am looking for max speed. Will going up in pitch make up for lack of revs due to the downdriven v drive?
  2. Guest625101138

    Guest625101138 Previous Member

    The range of pitch for a given diameter is limited. Common pitch to diameter ratios are around 1. Very aggressive ratios as high as 2 are possible but very unusual. Common ratios are say 0.6 to 1.2.

    If your current prop has a low pitch for its diameter then there is a good chance you will be able to get a standard prop with roughly 30% higher pitch required to absorb twice the current power.

    If you can also increase the diameter as well as the pitch then you are more likely to find a suitable prop. Larger diameter will also improve the efficiency so you will get a slightly better top speed.

    It would be helpful if you provide more detail on the existing prop - like diameter and pitch as well as the current top speed, rated engine power and rated engine rpm. Most props have their diameter and pitch stamped on the hub. Given this information it is possible to make quite a good estimate of the most suitable prop for the new motor.

    Also helpful if you determine the maximum diameter you can swing. Need 'some' clearance from the hull but it is a matter of the compromises you want to make. This will enable performance estimate for the biggest prop you can swing.

    One very important consideration is the survival of the box. Does it have the capacity to withstand the extra torque and thrust? Is it adequately rated for the new motor?

    Rick W
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.