The nautical term "fetch" apparently has at least three different meanings. I take it that in this context it means "a distance over the water that the wind has to build up a wave pattern". Can I eyeball the average speed over that stretch at about 12 knots ?

i have the impression that many boat watchers are bow watchers. Until dolphins appear. The contrast between your video and the one earlier showing the praocargo cannot be more different. As an expert water flow watcher yourself is there some advice to amateurs to correct the frame of their intuitive interpretations of water moving about the bows? 

I am not particularly clear on the question but offer some comment.  The bow is interesting because it gives a good visual indication of speed.  For example I know when the attached flow on the bow of the 18m proa reaches deck level it is doing 8 knots.  The wake tells more about the wave drag of the boat.  

In the same vein estimating wave height seems to do little justice to the rather complex statistical parameters used in the physics of waves. What are the heuristics that you use as being the most practical?

I have fitted a curve to published data on average wave height versus windspeed for fully developed waves in deep water:

Height (m) = 10^[2*Log {Windspeed (m/s)} - 1.6]

For example 7m/s gives 1.2m height.   Obviously this formula is not useful for wind speeds below 6m/s where wave height is less than 1m.

Peak height is often stated as 50% higher than average.

The slope of a deep water wave, neglecting any windblown crest is around 1/7.  So trough depth to peak-to-peak distance is approximately 1/14. 

The well of the ww hull is essentially horizontal. So the the interaction with waves is boolean: they reach that height or not and until they do the boat profits from the slim hull with straight sides. A more classic hull with outward sloping sides balances out the relative disadvantage at low wave heigth with the lack of disruption at wave heights up to the height of the beams. With this hull form the waves are parted sideways while with the classic HP form the waves are slammed down. Intuitively the former seems less disruptive than the latter with a more comfortable ride. I also notice evolution on the design of the sides of the stabilizing parts of your V13 and the difficult choice of the height of the sole of the deck. Of course, this is a very different boat but I guess that some of the reasonings that went into the decisions taken there are also at work with HP planform boats.

A boat with two slender hulls needs to have clearance under bridging structure to limit the loss of speed caused by wave impact.  In my assessment longitudinal clearance needs to be no less than 1/14th of length, assuming reserve buoyancy in the ends, and beamwise 1/7th the beam to avoid continuous slapping of crests.  The 18m proa does not meet these criteria meaning waves over 1m will constantly hit the lowest part of the bridgedeck and slow it up.   

A hull with low buoyancy in the ends needs more clearance because it will ride a little lower in waves than in calm water.  A hull with flared sides and buoyant ends can be a little lower because it will ride a little higher in waves. 

I have not seen any cruising boats that meet these criteria.  I have read that one of the developments with the MOD70 over the ORMA 60 was increased beam clearance and reduced beam to reduce incidence of slamming.  I considered the ORMA 60 to have impressive beam clearance but apparently not enough for the beam.

The V13 design is intended for solar power and continues to evolve.  There are already a few variants of the design and some scale models.  I prefer to be on the water when the sun is shining and conditions are calm.  Like this ideal day:

I am aiming to have a solar boat that suits ideal conditions but still able to handle less than ideal.