thanks for participating in the discussion

my crude look at your thought that a surfboard does not go fast enough for turbulent flow:

according to Wikipedia the Reynolds number formula for a foil is Vc/v where V is velocity, c is chord length and little 'v' kinematic viscosity.

Take a surfboard fin chord length at base 0.12m travelling at 10 m/s (36 km/h) kinematic viscosity 0.0000010533 water

Reynolds number = 1,139,276 at the base of the fin.

Cessna light aircraft speed 27.7 m/s (100 km/h) chord about 2.2m / kinematic viscosity of air 0.0000146 = 4,068,493

So such a plane does have 4 times as much turbulence than the surfboard fin base.

However if considering diagonal flow across the hull of a surfboard, we could be looking at about 0.5m of "chord" which would the produce a Reynolds number of 4,746,985 - more turbulent than the Cessna.

Regarding the speeds I chose - I once taped a GPS device to the nose of a CI tufflite New Flyer at Bells beach and got close to 40km/hr - was consistently holding over 30km/hr on the wave so I don't think it was some erroneous velocity spike from hitting a lump of chop or something. 100 km/h for the light aircraft is an approximate take off speed for that aircraft according to the internet.

Water does not always, stick, in extreme circumstances there is a phenomena known as "cavitation" where the low pressure side of a foil will turn the water to steam, however it takes something like an internal combustion engine driving a propeller to do that I am told and definitely no cavitation for our surfboard fins, let alone vacuum. However, what npsp and myself are suggesting is that the flow can stop following the contours of the foil nicely and instead become turbulent - things like vortexes start happening.

However I am intuitively with you on your other comments - surfboards don't travel with fins and hull at constant AoA and hold steady speeds, plus there is all sorts of other stuff happening like foamy water you mentioned. So we cannot simply model our boards on simple formula's such as the Reynolds number and Bernoulli's equation - so shapers will never get replaced by hydrodynamics engineers.