For any hull with no rocker to plane it needs a positive angle of attack meaning trimmed bow up.  Under sail the 18m proa inevitably trims bow down.  Under motor there is a little buoyant lift from the bow wave that trims it slightly up but speed is too low to generate any dynamic lift.

In light displacement as launched and spotless bottoms the 18m proa hulls exhibit a drag curve of:

Drag = 19.7 X  (Speed in Knots) ^ 1.78.

Hence power will be a function of speed raised to the power 2.78 - not a lot different to cubic relationship.

What you are observing in the videos is characteristic of a slender hull in displacement mode.   Water crowds at the bow providing some buoyant lift and there is a wave trough at the stern causing the stern to sink a little.  That has nothing to do with dynamic lift generated when planing.  

Phase velocity of waves (your 1.34 * Feet ^ 0.5) has little relevance to slender hulls.  By your reasoning rowing shells are planing at their racing speeds around 9kts.  I can assure you they are not.  My flat bottom pedal boats generate dynamic lift that is noticeable over 7kts and becomes significant in drag reduction at 10knots but still not planing.  This clip gives an example of some dynamic lift:

At rest the foot of the stem is 30mm below the surface with my weight.  At 8kts it is clearly above the water level due to some dynamic lift - or semi-planing.  The trim change is due to bow lifting not stern squatting as the stern is near its static level. Towards the end I slow and the foot of stem drops below surface albeit somewhat out of view.

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