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Problem after repowering

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  • Problem after repowering

    I have recently repowered my boat with Merc F115EXLPT EFI with all the smartcraft instrumentation and so on. The height of the mounting has been done exactly how the installation guide says and how all the recommendations in the internet are suggesting. However, the problem is that I can only trim it up to number 4 mark according to the smartcraft trim gauge before it starts cavitating, but the trim range itself goes all the way up to 10. So there are actually several questions that bother me...
    - is the engine mounted too high?
    - am I missing like 60 % of the trim range that should be available?
    - how will it affect fuel consumption and speed if I Iower the engine but trim it up while on plane (am I back in the same exact point when I started before the repositioning?)

    so I'm confused and any help on the matter is appreciated!

  • #2
    First of all trim gauges are inaccurate, unreliable and therefore useless, IMHO. Your smartgauge is designed to work OK on all boats, but not specifically on your boat.

    Motors ventilate (likely your issue as cavitation is something else), at different points depending on motor height, type of prop, hull design and other factors. The point at which they ventilate is not as important as the performance they provide, before they ventilate.

    Trimming the motor by feel and sound is the best method. She should steer lightly, not chine walk and not ventilate.

    The way to determine if your motor is set up properly is to run speed/RPM tests. Run your boat the way it is and trim to max, before ventilation. Note speed and RPM. Now drop motor down a bit and run trial again. Now raise motor up a bit higher than original and run trial again.

    Remember to notice how driveable the boat is, at different heights. Must she always be trimmed down all the way to get on plane? Does she chine walk? See how she feels to drive.

    High performance boats will be affected by motor height and trim more than heavy runabouts.

    Hope it helps...


    • #3
      There are several things to consider on a situation like this.

      Does the boat porpoise (bow bounce up and down) when you trim up the motor? Or does it just ventilate the prop?

      Does the boat pull to one side or the other when you trim up? What about when down?

      Do you have pictures and measurements of the engine height relative to he bottom of the boat?

      What boat is the engine mounted on?

      It is set up with a 25" transom correct? You state it is the 115 EXLPT. XL means X Long - 25" shaft..

      What prop are you using?

      What engine was on the boat previously? How does the weight of the new engine compare to the old?

      Sorry to ask a lot of basic questions, but it may lead to a better understanding of the issue and to help you resolve it.

      As Chris says above, I wouldn't worry to much about the gauge if the boat is performing correrctly. If it is ventilating or porpoising with very little trim input, you may have an issue?

      We had an issue this past fall where a customer purchased a brand new boat from another dealer and had substantial performance issues. We found the engine was mounted too low right from the factory and the other dealer overlooked the issue. We raised the engine and the performance increased dramatically and all other issues were eliminated.
      2004 Four Winns 180 Freedom, 150 HP Evinrude Direct Injection
      1972 Chris Craft 31 Commander Sedan Bridge
      Various others at various times
      I've worked on and have access to pretty much all types of boats/motors.


      • #4
        What is the actual measurement of the transom.----That will determine what the issue is.-----And what model of motor was taken off ?----Model # of it was ?


        • #5
          Yes it is a XL shaft, meaning 25". The boat doesn't bounce when trmmed up, nor does it pull to either side, no matter trimmed up or down. Just when the smartcraft gauge shows 4 (out of 10) it starts to ventilate...you can hear and feel the engine starts to make humming sound and loses thrust. The prop is four blade Spitfire. The weight of the engine is couple of punds difference compared to old one (similarly sized Yama). Furthermore, the fuel consumtion is some 20% up in comparison what it used to be.


          • #6
            I would think the 4 is about right, the other 60% is from the top of the trim to full tilt

            The gauge measures the entire engine movement from full down to all the way up so I would say that is correct..

            Raise the engine all the way up and see if it hits the 10, if so that is the deal.
            GO IRISH!!!!


            • #7
              Originally posted by 5150abf View Post
              I would think the 4 is about right, the other 60% is from the top of the trim to full tilt

              The gauge measures the entire engine movement from full down to all the way up so I would say that is correct..

              Raise the engine all the way up and see if it hits the 10, if so that is the deal.
              That's not the case, unfortunately. The trim gauge range 0 to 10 is the true trim range only. After it has reached 10 it starts to go for the full tilt. Judging just by proportions the full tilt would be 25...30 or something like that but the gauge stops at 10.


              • #8
                Gee, numbers are cheep... who cares what the gauge says? The important part is whether the boat performs to it's capability.

                What kind of a boat are we talking about?

                Can you raise and lower the motor on the transom? As a starting point, you want the anti-ventilation plate of the motor on the surface at planing speed.

                i.e. On my 21 foot runabout, raising or lowering the motor made almost no difference.

                also i.e. On my speedboat, raising the motor on the transom makes the difference between a 55MPH boat and a 70MPH boat.

                What kind of prop do you have?


                • #9
                  I can't help you technically but can tell you trimming to get on plane in a boat is much like flying a small aircraft. You feel the right position dependant on water conditions just as flying is dependant on wind and weather conditions. Flying by the seat of your pants, same thing in a boat. I only look at the trim gauge when in shallow water. These guys will solve your problem, believe me they will.
                  Experience is a lifetime of mistakes, wisdom is not making them again.


                  • #10
                    I should clarify that I don't really care what are the numbers as such on the gage, but I'm interested whether "missing" a part of the potential range is an issue? A sign supporting the suspicion that something could be wrong is the fact that fuel consumption has gone up considerably, even though the engine is replacing a unit that was 10 years old, i.e generation or two older tech. If I remember correctly, the sort of most economical/comfortable cruising speed used to be 20-21 knots @3500rpm, now it is around 15-16 knots at the same rpm while using ~20% more fuel. Where is the logic in that?


                    • #11
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                      Are you missing anything? Nope. It's just where the boat wants to be trimmed in order to perform. Mine are typically down towards the bottom (trimmed in) as trimming up beyond that usually results in cavitation or porposing.
                      2017 Chaparral 19 H2O Ski & Fish w./4.3L 180HP Merc Alpha
                      2005 Four Winns 200Le x/5.0L Volvo SX-M (270HP - FI) - ordered new, traded in on Chaparral
                      1999 Bayliner Capri 1800LS w/2000 Honda 115HP - ordered new, traded in on Four Winns
                      1956 MFG 15' w/matching Evinrude Big Twin 30HP - presumed to be a sandbox somewhere

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