I cannot say much about outboards, but here is the theory on hull speed:
the speed v for a displacement hull is
v x approx 1.34 x square root of{LWL}
where v is in knots and LWL is in feet
that’s a bit over 5 knots for a Wayfarer
So to exceed that speed the boat needs to plane. Which I guess needs a lot more power.
More info here;
http://en.wikipedia.org/wiki/Hull_speed
hope that helps.