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Motor set up for transom with a step

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  • Motor set up for transom with a step

    I have a 1990 Sea Pro 180 CC that has a step in the transom at the bottom. The step is approx 32 inches wide and 6 inches tall at the keel. It tapers to about 3 inches tall where the transom does not have a step on the outside edges. The step also goes in about 7 1/2 inches deep in toward the bow.

    I currently have a budget project going and I have a 1975 115 Johnson set up in the lowest hole, which puts the cavitation plate about one inch above the keel. With the motor trimmed all the way in it is still about 9 1/2 inches away from the step of the transom and about 5 1/4 inches below the top of the step. Sorry I don't have any pictures.

    I just got the motor running where I am confident that I am getting the power I can out of it and I am turning 4500 rpm with a 17 pitch prop. A couple of prop calculators have recommended a 17 pitch for speed. I also feel that this motor should spin it past 4500 rpm if it is set up right.

    My next course of action will be to raise the motor. I plan to go up at least two holes, maybe all three. How much of an rpm increase can I reasonable expect by raising the motor? I have heard on here before that you can raise motors a little higher than normal on boats with a steped transom. Is the true of just a myth that someone put out there. I did add a tell - tail to this motor so I will be able to verify that it is pumping water and that the pick up is still in the water. Looking at the motor in the water it does seem to be lower than it should be when running. Throws up more spray then other boats.

    The boat has also always had a prop torque problem. Will raising the motor help with this? I currently use smart tabs to help off set this, but they also kick up a lot of spray in the water. Hopefully raising the motor will also help with this. Does anyone have any other suggestions or warnings before I began to change the set up around? Now that the motor is running good I would like to get a little fishing in. Thanks in advance for any help.

  • #2
    Re: Motor set up for transom with a step

    The motor probably can go a bit higher. We're hoping you're not saying the top rpm is only 4500 but rather that's the cruise rpm. If that's the top rpm you're looking for at least another 1000 rpm and that's more than a prop change.

    Torque in the steering will be corected with the tab under the AV plate at the back, just above the prop. Once you get high enough that the tab is out of the water it won't do much good. You'll need to add a torque tab on the skeg and work with it and skeg shape. Trim tabs (of any design) probably won't do much to counter steering torque.

    How high you can actually go largely depends on your prop. the more technology built into it, the higher you'll be able to go.

    Get youself a water pressure gauge. Don't count on being able to see aerated water.


    • #3
      Re: Motor set up for transom with a step

      Thanks for the reply Dhadley,

      Unfortuantely the WOT is 4500rpm with the motor at the height now which does seem to low by how the motor looks like it plows through the water at the AV plate instead of skinning on top of it.

      The prop torque is where the boat tilts to port not in the steering. There is not flotation in the bottom of the boat it is up on the sides. The boat does not list when sitting in the water only under power.

      I have been keeping my eyes out for a 13 x 15 pitch prop which is what is recommended for my boat for hole shot and water sports. I need a spare anyway. I figure that will only bring my rpm up to the 47-4900 range though. I was hoping that raising the motor would also free up a few rpm's. I wanted to do the things that only involved time first before switching parts and spending more money.

      I'll add the water pressure guage to the list.


      • #4
        Re: Motor set up for transom with a step

        What is your speed?Boat weight?
        Is the steering on the starboard?Of course trim is important as well.Does the bow seem to come up when you trim?Can you trim all the way without blowout?Any ventilation in turns.If you have no venting problems your probably too low.If you can gain an inch height you could get a couple mph and some rpm.Be sure throttle opens all the way.


        • #5
          Re: Motor set up for transom with a step

          Boat speed is about 32 mph haven't really concentrated too much on speed just yet. Total weight is probably just over 2000 lbs, 1500 for the boat about 30 gallons of gas me and a little equipment.

          It is a center console

          I can trim almost all of the way up before I get any blowout. It also doesn't really vent in the curves. I am sure that I can come up some. I really haven't had this boat out much, still in the setting up process. The motor is now running really well and the butterflys are straight across at WOT. The boat is not what I would condsider a handling or speed boat as it doesn't really make sharp turns very well, but it is stable and remains fairly flat. I am going to have to trim the tilt and trim bracket to get it to clear the splashwell drain/scupper so I can raise the motor. Its never as simple as you think it should be.


          • #6
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            Re: Motor set up for transom with a step

            i got a 1990 20f Lowe pontoon boat the motor bolts on to a peice of ply wood and it broke on me in the water what do i need to get to replac it it holds the motor on