Skin Friction Lines - Some Comparisons

Go read Hoerner's discussion on boundary layer tripping as a function of velocity fluctuations in Fluid Dynamic Drag Chapter 2, Section 6.

Nope, see the discussion above. Hoerner used data from plates, airships and spheres. Come on guys, if your laminar data comes from a wind tunnel with a 0.005% rms velocity fluctuations, real life performance will be different as you drive through seaweed, whale snot, and jetski wakes as Leo correctly points out below.

 

Keep in mind what you are trying to do here and what it
will be used for.
If you are interested in the fundamental physics of the
problem then differences of 1% are very important.
If you are only interested in the engineering aspects then
do what (I imagine) Petros did: take the stogie out of your
mouth, point at the 7%-10% scatter in predictions and get
back to work on the important aspects of the design.

 

To answer my own question, from the looks of the equation I would bet it comes from Hoerner. This is only the profile drag, you also have to allow for induced drag, a function of effective draft. Effective draft is difficult to estimate, probably the most difficult/important factor in a VPP after the effective sailing length.

As David Cockey elsewhere notes, you should remember that profile drag depends on CL - I was looking at the ORCi VPP, which appears to ignore that: see attached ORCi VPP appendage viscous drag. The canoe body viscous drag is still handled in the ORCi VPP by the ITTC 57-line... (see 2nd attach.)

ORCi could have included the CL-dependent part of appendage drag into the induced drag part, but I could not find evidence for that. The ORCi effective draft is quite a construction by itself.

 

That formula includes a CL^2 term so it tries to account for the increase in viscous profile drag due to lift. However the coefficient is not universal. It probably originated as a "best fit" to some experimental data.

 

All I know is that it becomes worse in a headwind