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i have an old gasoline fueled skiff that only makes 3-4 knots. I find this to be strange since the engine is rated at 10-12 horsepower. It isn't that heavy and i figure it should be able to do more. Could this be because the flywheel is TOO heavy? And i'm talking MASSIVE. it's like 50kg. Maybe even more

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Flywheel weight will not impact the power output of your engine. The mass of flywheel will change the rate of change of speed of the engine:

  • A heavy flywheel will mean the engine is slow to speed up and slow down. This is good for smoothing out a rough engine.
  • A light flywheel makes the engine more responsive and also less stable, it can change gears quickly and will possibly stall more easily and feel less smooth.

A heavy flywheel is suited well to a boat where engine speed transients are not desirable.

I suspect you have another issue impacting the output and consequentially the top speed of your boat.

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  • An important parameter for the maximum speed of a boat is the "hull speed" which depends on the length of the waterline: $V = 1.34 \sqrt(L)$ where V is in knots and L is in feet. To pass that speed, you need enough engine power to get the boat to "plane" by climbing on top of its own bow wave. Also, your rated engine horsepower is only relevant if it is properly matched to the propeller, otherwise you can't use the available power.
    – alephzero
    Commented Sep 25, 2018 at 18:27
  • Also see this answer as well as this answer, and this answer for more flywheel related details.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Commented Sep 25, 2018 at 19:47
  • I am by no means a boat expert but I know they design props blade pitch for speed or pulling . Could you have the wrong pitch prop installed?
    – mikes
    Commented Sep 25, 2018 at 20:06

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