flyingcab said..
For laminar flow on a fin you need a reynolds number to be less than 5×10^5
This number is a ratio of the inertial forces to viscous forces and is used in everything in fluid dynamics.
For a fin of length 10cm, travelling at 15 knots, your reynolds number is going to be 767000 which means that you will have turbulent flow. In fact, you will have turbulent flow for the last 3.5cm of your fin.
20knots, more than half your fin will experience turbulence.
30 knots over 70% of your fin will be in turbulence.
The faster you go, the more turbulent the flow will be.
That may suggest at first that our fin should be long but narrow ? Is it right? In order to avoid turbulence.
Lets perform mental experiment and take that sharpest katana blade. What will be resistance if you run it trough the water at 40-50 knots
a) perpendicular ( vertical)
b) parallel to our direction ( horizontally)
Obviously total surface area remains the same.
What will be difference in lateral resistance of vertical and horizontal blade?
and another consideration regarding surface directly
(((((Without going into details I could speculate that regardless how smooth your surface becomes , eventually the gross of the interaction between boat/ fin surface and water depend on quantum effects known as Kashmir effect. The mentioned distance about 0.1mm may further firm the suggestion. Our goal could be then eliminating this Kashmir effect if possible. In order to do so I imagine that we need to separate this two surfaces by the distance that quantum effect becomes neglected. I imagine that we should be able to create precise grooving along the flow. This way water may have direct quantum contact only with this apexes of the grooving, space between filled with gas/air trapped between that doesn’t provoke Kashmir effect. Ideal material allow then create ultra sharp apex , then groove/ valley between. I am not sure if continues groove is needed or just hill , sharp needles like.
Translating this to our simplified technology I imagine that surface of our fin / or bottom of the boat should be ideally coated with hard material grooved precisely along. Instead of mechanically grooving ideal could be nonomaterial of specified surface structure already.
Practical simplified method could be hard coating our fin then sanding along the flow.
Waring is required when playing with Kashmir effect. Wrong size of structure may cause opposite effect magnify the force to the point that surfaces becomes almost "glued" together ! look gecko climbing your ceiling )))))
that means they sometimes kinda half let go when accelerating under pressure) for the first few runs! I wonder if that was linked to the info in the above statement?
and those funny little brief 'squirrels' would disappear.
. The hail drop's the water temp at the surface and provides a ball bearing like surface to reduce friction
I'm pretty sure the top speeds at Luderitz have been done with both. Most have more chance of going faster than before by spending more time on the water trying to be the fastest at their local spot honing their skills. 
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