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  • 25 hp blowout The

    I have a 1468 Lowe v Hull and a 1998 25 hp mercury 2 stroke. The transom on my boat is 20” the shaft on the Mercury is 19.5, which puts the cavitation plate about 1/2 inch above the bottom of the boat. The standard 13 pitch prop that came with the motor does not seem to like this as I experience blowout when I accelerate, just before it planes, also in the chop if the waves get very big. At full throttle with a full load I’m running 25 mph and 5600 rpm. Im wondering if getting a prop that’s happy a little closer to the surface wouldn’t be the best solution to this problem? What would you suggest?

  • #2
    Does motor sits perpendicular to water level (90deg) with deck weight well distributed and combo riding on flat calm water cond when experiencing this prop blow out and revs coming up ? If so means that motor sits too high on transom, those rebadged Tohatsu motors likes to ride while water flow skims right under upper small plate, if flow at plane passes near lower AV p'late will experience so. If trimming motor away from 90 deg, prop will lose its best prop thrust.

    To determine, plane the combo out, pull head out transom while sitting inside aft deck and check at which lower leg height is flow passing by, then will know what's really happening at back transom. Once you know can work from there...

    Happy Boating


    Sea Rider 320, 380 Sibs, 450 Rib, 2 Strokes Tohatsu 5,18 & 30 HP Proud Smokers

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    • #3
      The motor is trimmed 90 degrees to the boat with the cavitation plate in the froth with the boat fully loaded and balanced and on plane.

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      • #4
        If you have a local repair shop, you might ask them about having your prop cupped. That will bring the rpm's down a bit, and resist venting a bit at the same time.

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        • #5
          Pit up a pic. I would expect a Mercury 25 hp to measure around 23" for a longshaft. 1998 would not be a Tohatsu motor.

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          • #6
            Check your prop hub for slip.

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            • #7

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              • #8
                Pic of the motor on the boat.

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                • #9
                  Issues to have into account

                  -Doesn't matter if it's a Tohatsu, Merc, Yam or any other motor brand, all have upper and lower plates.

                  -On a dry installation (sitting a motor on transom) does't matter the height the plate sits with respect of rear hull, when hull at speed climbs out of the water it's a different story. There are only 3 possible situations : Back water splashes out/over transom, spot on lower leg/transom match, prop aeration achieved at different lower leg heights while motor sits on transom.

                  -If boat rides 25 MPH at 5600 wot rpm it's impossible to have a spun hub.

                  If wanting to solve the prop aeration fast, check at which lower leg is water flowing by as suggested. Max wot revs for that 25 HP must be 6000 rpm, if so, motor is 400 revs off when fully loaded. Do you always, usually go boating fully loaded ?

                  Happy Boating


                  Sea Rider 320, 380 Sibs, 450 Rib, 2 Strokes Tohatsu 5,18 & 30 HP Proud Smokers

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                  • #10
                    Originally posted by Sea Rider View Post
                    Issues to have into account

                    -Doesn't matter if it's a Tohatsu, Merc, Yam or any other motor brand, all have upper and lower plates.

                    -On a dry installation (sitting a motor on transom) does't matter the height the plate sits with respect of rear hull, when hull at speed climbs out of the water it's a different story. There are only 3 possible situations : Back water splashes out/over transom, spot on lower leg/transom match, prop aeration achieved at different lower leg heights while motor sits on transom.

                    -If boat rides 25 MPH at 5600 wot rpm it's impossible to have a spun hub.

                    If wanting to solve the prop aeration fast, check at which lower leg is water flowing by as suggested. Max wot revs for that 25 HP must be 6000 rpm, if so, motor is 400 revs off when fully loaded. Do you always, usually go boating fully loaded ?

                    Happy Boating
                    I’m taking the boat out tomorrow and will check where the water flows when the boat is up on plane as suggested. As to a full load— most of the time I fish with a friend pluss trolling motor and battery, so full loaded for a 14’er (500# not counting motor, gas, and hull weight)

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                    • #11

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                      • #12
                        Is that your engine sitting at its lower position ? If it is it looks a little highly trimmed out for your application perhaps.

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                        • #13
                          A photo of the motor, it’s a 25hp with 15 decals.

                          As as to the trim position, I moved it one pin closer to the transom and lost 2 mph and it pulled pretty hard to the side. It did improve the blowout problem slightly, as it buried the prop a little deeper, but didn’t eliminate it.

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                          • #14
                            Moving it in one hole may be part of your solution. 2 mph in top end not worth the issues caused by it blowing out too easily. That trim tab above the prop can be used to off set that torque you're talking about.

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                            • #15
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                              Even though the first photo shows the av plate in line with the bottom, I have a feeling it's not in line with the hull, just that the stick in touching the back of the hull. The second photo seems to support this. Move the engine tilit pin in one or two holes to allow a more 90 degree relationship between engine and hull bottom and test it. Engine mount height looks ok. As it is now, I would expect some porpoising at wot. aligning the thrust wiih the hull bottom will eliminate most of that.

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