planing the boat at around 12 knots under sail, or 18
knots under power

even well balanced that boat won't hardly do 4 knots under sail, let alone 3x
that, and under power not even 12 knots let alone 18.

300# of engine on the rear and 4 fat-assed people in the cockpit you can't sail
it at all and under power it won't plane.