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Advice required for motor mounting height on transom

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  • #1

    Advice required for motor mounting height on transom

    Hi folks,

    Got a boat and motor that is new to me and I have been reading about the fact that boats are often supplied from dealers or previous owners with the motors mounted too low on the transom which results in more drag and less than optimal prop performance/overall performance. I have a Norwegian built 16ft daycruiser with a deep V hull and my 90 hp is mounted on the lowest position possible out of the 4 mounting holes on the bracket. Before buying the boat I was reading about the performance of the 90hp setup and people have quoted 40-42 knots top speed. I am only getting 37-38 knots top speed and on normal trim settings (down) my motor tops out around 4700 rpm and 35 knots. If I trim up to max before ventilation I can increase rpms to around 5000 and gain 2 knots up top. Then the other day I saw an ad online for the same setup as I have (albeit with a newer model Yam 90) and the motor mounting this guy has is like something I've never seen before. He claims 45 knots top speed. I've attached a photo for you to see the motor mount position. It is about 4 inches higher than my mounting and about 4 inches set back. He's using a stainless prop with a pitch of 21". I have no markings on my prop but it looks original Yam and I'm guessing 17" pitch.

    Can anyone give some explanation of how moving towards a setup like this could bring me some benefits?

    My motor ventilation plate (or cavitation plate?) is pretty much level with the bottom of the V at the rear of my boat. I should take it for a run tomorrow to check what level its at relative to the water line when on a fast plane.

    Any info appreciated!
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  • #2
    Originally posted by ollyjscott View Post
    Hi folks,

    Got a boat and motor that is new to me and I have been reading about the fact that boats are often supplied from dealers or previous owners with the motors mounted too low on the transom which results in more drag and less than optimal prop performance/overall performance. I have a Norwegian built 16ft daycruiser with a deep V hull and my 90 hp is mounted on the lowest position possible out of the 4 mounting holes on the bracket. Before buying the boat I was reading about the performance of the 90hp setup and people have quoted 40-42 knots top speed. I am only getting 37-38 knots top speed and on normal trim settings (down) my motor tops out around 4700 rpm and 35 knots. If I trim up to max before ventilation I can increase rpms to around 5000 and gain 2 knots up top. Then the other day I saw an ad online for the same setup as I have (albeit with a newer model Yam 90) and the motor mounting this guy has is like something I've never seen before. He claims 45 knots top speed. I've attached a photo for you to see the motor mount position. It is about 4 inches higher than my mounting and about 4 inches set back. He's using a stainless prop with a pitch of 21". I have no markings on my prop but it looks original Yam and I'm guessing 17" pitch.

    Can anyone give some explanation of how moving towards a setup like this could bring me some benefits?

    My motor ventilation plate (or cavitation plate?) is pretty much level with the bottom of the V at the rear of my boat. I should take it for a run tomorrow to check what level its at relative to the water line when on a fast plane.

    Any info appreciated!
    I am surprised no one has replied.
    Yes to all your questions are plausible.
    First raising your motor will help(judging from the picture)
    - just a hole at a time
    Offsetting rear wards allows raising of the engine because the water is actually going upwards after leaving transom (but usually about 1 inch up for 2 to 4 rearward a)

    - this is running below sea level so to speak
    The moving out also allows the prop to run in "cleaner" water
    - there would be considerable turbulence from what appears to be a "W" hull
    Any change needs to be assessed with what happens during sharp turns

    With that hull experimenting is feasible by looking at that one picture

    Comment

    • #3
      When underway at your cruise RPM and cruise trim angle look over the transom and see if the anti-ventilation plate can be seen. If it is buried in water the motor is lower than its optimal position.

      If the motor presently has the bottom of the skeg even with the bottom of the V of the boat my guess is that is on the low side. If a boat builder/dealer follows Yamaha recommendations more times than not the motor will be mounted lower than for the optimum position. Yam does not want problems, they are not into maximum performance.

      Depending upon what is seen you might need to raise the motor two or three bolt holes.

      Keep in mind that if the motor is mounted high enough for optimal speed/efficiency performance there is a possibility that the propeller may ventilate in turns. In this case, either lower the motor down a hole or two or be prepared to trim the motor down during turns and then back up coming out of the turns.

      Here is a link to a photo where the plates can clearly be seen.
      https://www.google.com/search?q=phot...6g74qjfh0KM%3A
      Last edited by boscoe99; 07-19-2015, 08:59 AM.

      Comment

      • #4
        Some props can be run higher than others without ventilating. Generally speaking SS can be run higher and provide better bite and less ventilation than aluminum.

        Comment

        • #5
          Great link Boscoe.
          Solves some of (all) my other "venting" problems and then some!
          Even gives a picture of Steve McQueen!:

          Comment

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