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Thread: true transom measurement question

  1. #1

    Default true transom measurement question

    I was wondering if the categories of 15 inch transoms for short shaft motors and 20 inch transoms for long shaft motors are exactly what the transom heigth should be or if these are more guideline numbers and transoms will vary either side an inch or two?

    The reason I am asking is I am having problems with my jet motor starting to cavitate when I get my 16 ft alum. boat to start to plane out and I have to back off on the throttle to keep the foot in the water enough to keep it running smooth. I measured from the top of transom to bottom of boat in the middle where the foot is and it is 21 1/2 inches. Is this an odd ball size or are there fluctuations in the exact measurement of a 20 inch transom? As you can see in the picture the transom has a built up raise to it, I did not do this I bought it this way with no motor attached but am wondering if it should be there of if I need to make the transom exactly 20 inches to solve my cavitation problem.

    Also from the pictures can you tell me if the foot looks like it's in the right position, It seems close to the hull to me and if I have the motor in the straight up and down position the foot will hit the hull when I turn too sharp. I would guess this problem would be solved if the foot stcuk down a little below the hull but shouldn't it be even with the botom of the boat?

    Any thoughts or opinions would be great, thanks.
    Attached Images Attached Images

  2. #2

    Default Transom measurements are different for jets.

    It looks like you have a custom riser on that transom adding some height to it. Outboard jets need to be adjusted vertically on their individual applications for the best performance. If you try and run a short shaft jet on a factory 20" transom it will not perform well. A good place to start is to run a straight edge down the bottom of your boat and make sure it lines up with the top of the front pin holding in your grates. A lot of boats setup to run real shallow will cavitate in hard turns. Check your grates to make sure they are not bent or obstructed. They also make fins that will bolt to the side of your foot to help funnel more water in which may help with your cavitation. Switching to the new style liner in the jet foot is also an easy way to drop the foot about .25". They make it much easier to shim your impeller as well.

  3. #3


    from the pic i would say that you need to drop it down about 1/2 inch or so i have setup lots of them if you are in the valley i could help ya out Trevor

  4. #4
    Join Date
    Jun 2006

    Default jet foot location

    This should give you a starting point.

    (Just a quick sketch...sorry it's kinda messy!)

    One other point, if you don't have good laminar (smooth, undisruptive) flow over the hull of your boat, air bubbles will form causing cavitation, ESPECIALLY on a flat bottom boat.

  5. #5
    Join Date
    Apr 2006
    Fairbanks Alaska

    Cool Smooth Bottom

    One of the most common problems is having dents at the back
    underside edge of the bottom. A hooked bottom will tend to
    deflect water down and away from the intake. A mallet and a couple of blocks can correct a lot of problems. Are you hitting the bottom or the bottom extension when you turn? You may need to remove the extension.


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