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O/B on jack plate, how high is too high???

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  • O/B on jack plate, how high is too high???

    19' fiberglass bass boat with 150 merc. What will be first thing I will notice when I get the eng. too high. I've been raising it a little at a time & nothing seems to change. Nose goes quite high on take off then plains off nicely, I'm thinking the higher the eng. the less the nose will lift. Right now cavitation plate is appx. 2" above bottom of boat with no adverse effects, no blow out, no porpoiseing, no big rooster tail, good water pressure. What will be the first thing I should notice when it starts to get too high?? Thanks

  • #2
    The prop will start to ventilate or the water pressure will drop.

    Comment


    • #3
      Originally posted by rphillips View Post
      What will be the first thing I should notice when it starts to get too high?? Thanks

      That depends on how observant you are, or how distracted you are at the time.

      Could be a slight drop in speed, or an increase in rpm.
      Or an overheat alarm

      Medford, WI


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      • #4
        You are correct that trim will have less effect on the bow lift, as you raise the motor up on the jackplate. You should use a GPS and accurate tachometer to find best speed and minimum slip. If your boat does not "carry" the bow easily, you will likely need to have the engine further down to allow more trim influence.

        The distance of setback also influences the amount of raising of the motor, she will tolerate.

        Trial and error, I am afraid....

        Also, drive-ability plays a part. If just you are going to exclusively drive the boat, she can be set up for higher performance, then if more inexperienced drivers will run her.

        Comment


        • #5
          A simple method : on calm water cond with boat running parallel to water level and trimmed to 90į, with well deck weight distribution, if OB sits way low will experience water back splashes or even over transom, if too high will have prop aeration on choppy, windy sea cond and at tight close turns even on calm water, only good for straight water courses...

          Other form, when ridding same pull head out transom and check at which lower leg is water flow passing at plane. Report if passing above or slight under upper splash plate or towards AV plate and if AV is exposed when at plane.

          Why are you using a jack plate, what if sitting OB straight on transom and testing that way..

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

          Comment


          • #6
            The boat was set up with the jack plate when I bought it. I think I would want to run with the motor as high as possible, without being too high, thus I'd like to know what is the first sign of too high, then back down just a touch. thanks

            Comment


            • #7
              What performance goal are you looking for? Jacking up the motor to the max may not give the highest speed, but it will certainly make the boat more difficult for the average Joe to drive.

              Comment


              • #8
                Don't care much about top speed, it rarely gets to WOT anyway. Right now the boat is as stable as any boat I've ever been in, at WOT, (appx. 50mph). My only complaint is how high the bow rises on take-off, & just logical thinking would make me think, the lower the prop in the water, the more bow lift, the higher the prop, the less bow lift.

                Comment


                • #9
                  Yes, the higher motor will generally mean less bow rise, but other things can affect it too, just changing props or moving stuff around in the boat.

                  Take it up and see what happens, just make sure it's getting water.

                  Comment


                  • #10
                    If don't want to lose precious boating time wild guessing wihich is the Tip Top Transom height for that OB post a pic shot sideways showing both lower leg plates to have a look..

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

                    Comment


                    • #11
                      What model boat? 4"5"6"8"10" jack plate? What year motor? Standard or Xr lower unit? Stainless or alum prop?
                      When in doubt,Trim it out!!!
                      Business Owner & Certifed Technician
                      Boats I own & use
                      20ft Javlin w/250 Junkrude
                      17ft Bullet w/200 Merc (SOLD!!)
                      13ft Allison w/150 I-6
                      17ft Hydra-Sport 175GT
                      15ft Allison w/75 Stinger(project)
                      14 x 48 Drifter with 1100SS Merc

                      Comment


                      • #12
                        Did yall notice, im pretty green. 1997 BMC mod. 1860. 1984 merc XR2 150. Lower unit uses the large hub prop. 4 blade stainless prop with no brand name visable, marked 24P. I didnt realize this would be sooo technical, thought I would hear the prop will lose grip in turns, /ou will start losing water pressure, handeling will start to get squirrelly. Guess ill just keep going up a half in. at a time till I feel, see, or hear something neg. Going on, then back it back down a little. Thanks for your help. Rick P

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                        • #13
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                          Visually checking what's going on at back transom to see at which lower leg height water flow passes at plane is faster and better than by ear method, once you know this height can raise/lower OB accordingly till Tip Top OB/Transom height is dialed and combo performs spot on. As you have checked, this is not a car....

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

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