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  • Boat Speed

    I have 21' pontoon with a 90 HP Mercury. At 5200 RPMs I was at 23 MPH using the GPS. Is this close to what I should be getting for speed? Thank you.

  • #2
    Not sure what year your 90 is or if its a 2 stroke or 4 stroke...

    Outboard and stern drive engines are not supposed to do any particular speed.. although sylvan advertises a specific setup and a resulting speed.

    Your engine's maximum rpm is controlled by the propeller size ( dia and pitch) max rpm will be indicated in your owners manual or on the engine itself.. pull off the cowl and look for the maxium rpm or WOT rpm.

    If your not making max rpm when the boat has only a driver... it will certainly not make that number when loaded with passengers and gear.

    The problem that can arise from under propping ( too small of dia and or pitch) is a over revving condition and that can lead to internal damage.

    If your over propped (too much pitch and or dia) the engine will be lugging and create a condition called detonation... which in short is a vibration caused by irratic combustion .
    And this can and will lead to too much combustion chamber heat and eventually severe mechanical damage to pistons, cylinders, valve's if a four stroke.

    Speed is whatever your engine is capable of producing at WOT (wide open throttle) the most important factor is does your engine hit the maximum WOT rpm.

    Check your engine specifications and then you can confirm if that speed is at its max.

    Best of luck

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