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Thread: Convert Jet lower unit back to prop lower unit.

  1. #1

    Default Convert Jet lower unit back to prop lower unit.

    Here's the question. When I switch out the units will I need to lower the entire motor deeper in the water. Common sense tells me yes. The motor is a 2007 115HP Mercury Optimax. The hull is a 1996 18" Roughneck tunnel. Any suggestions?roughneck.jpg

  2. #2
    Member broncoformudv's Avatar
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    Yes the prop will need to be mounted lower so it does not cavitate. In your picture it looks like you have some room left to come down with your mounting bolts so might be enough. Most jetboats don't have enough room to play with and require adding a extension on the lower unit or a lift plate.

  3. #3

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    Quote Originally Posted by broncoformudv View Post
    Yes the prop will need to be mounted lower so it does not cavitate. In your picture it looks like you have some room left to come down with your mounting bolts so might be enough. Most jetboats don't have enough room to play with and require adding a extension on the lower unit or a lift plate.
    Thanks. Yes I have lots of room to drop it. The question is how far?

  4. #4
    Member broncoformudv's Avatar
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    You need to drop it till the cavitation plate is level with or 1-2 inches below your hull. Every boat is a little different on where they like the prop to be but that should get you in the ball park. The cavitation plate is the flat area that runs around the top of the lower unit. At least I think that is called a cavitation plate, heck I might have my terminology all screwed up.

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    Generaly, there is a difference of 5" when switching from prop to jet or the reverse. If that is a 15" transom at the tunnel and you have a 15" shaft outboard, then installing the prop lower and then setting the outbaord all the way down against the transom, should put it right where it needs to be.

  6. #6

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    Quote Originally Posted by broncoformudv View Post
    You need to drop it till the cavitation plate is level with or 1-2 inches below your hull. Every boat is a little different on where they like the prop to be but that should get you in the ball park. The cavitation plate is the flat area that runs around the top of the lower unit. At least I think that is called a cavitation plate, heck I might have my terminology all screwed up.


    No your terminology is correct. Thanks for the reply. I kinda thought that might be a starting point but did not know for sure. Thanks again

  7. #7

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    Quote Originally Posted by .338WM View Post
    Generally, there is a difference of 5" when switching from prop to jet or the reverse. If that is a 15" transom at the tunnel and you have a 15" shaft outboard, then installing the prop lower and then setting the outboard all the way down against the transom, should put it right where it needs to be.
    I just went out and measured from the top of the tunnel to the top of transom and came up with 25 inches. I understand about the 5 inch difference between the two units, it could go either way. Just going to have to do it and see what comes about. Thanks for the input .338.

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