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Thread: Prop position = prop transom lift leverage?

  1. #1
    Join Date
    Dec 2018
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    TX
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    Default Prop position = prop transom lift leverage?

    Hi all,

    I am still a beginner with boating and looking to learn from those more experienced.
    After playing with a 25 degree rake prop I am getting a feel for what transom lift is and general adjustments to various setups. My new build is a 1886 Aeromarine cat (42.75").

    I have seen my Aeromarine cat setup with the prop under the transom and others with the prop extended several inches beyond the transom.
    My assumption is that you want the spray off the prop to not be blocked in any way... so beyond the transom might be ideal?

    I am trying to understand the impact of placing the prop further back, I believe doing so gives the "lift" more arm leverage? Am I thinking about this the right way?

    Attached is a simple drawing to illustrate the 2 setups.
    Attached Images Attached Images

  2. #2
    Join Date
    Apr 2007
    Location
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    As water leaves the mono’s transom it begins to rise back up towards the original water surface. Eventually it actually rises above that surface before dropping back. As you move the prop further and further behind the transom it gets buried deeper into this rising water surface, lifting the prop and sometimes the entire hull.
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  3. #3
    Join Date
    Dec 2018
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    TX
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    Quote Originally Posted by Fluid View Post
    As water leaves the mono’s transom it begins to rise back up towards the original water surface. Eventually it actually rises above that surface before dropping back. As you move the prop further and further behind the transom it gets buried deeper into this rising water surface, lifting the prop and sometimes the entire hull.
    That is an interesting point I had not considered. I do have a mono which I will keep this in mind for.

    What about for a Catamaran where it is single prop in the center where the water surface has not been touched? I am thinking the length of the strut placement and ultimately the prop acts like a lever against the center of gravity and hull area touching the water. If so.... a medium or high "lifting prop" would behave more strongly due to that length beyond the transom.

  4. #4
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    I set my big cats up with the prop 3-4” behind the transom. Lifting props are usually not the best choice for cats, the lift isn’t needed and low lift props are more efficient. Those who do use high lift props usually heavily cup them to reduce the lift. Placing the prop far behind the transoms has been tried, but most who did returned to standard setback. In theory high setback could help reduce bow lift and blowovers, but I haven’t seen that happen irl.



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  5. #5
    Join Date
    Dec 2018
    Location
    TX
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    Quote Originally Posted by Fluid View Post
    I set my big cats up with the prop 3-4” behind the transom. Lifting props are usually not the best choice for cats, the lift isn’t needed and low lift props are more efficient. Those who do use high lift props usually heavily cup them to reduce the lift. Placing the prop far behind the transoms has been tried, but most who did returned to standard setback. In theory high setback could help reduce bow lift and blowovers, but I haven’t seen that happen irl.



    .
    Thanks I really appreciate the help!

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