I just made a permanent mount for my Humminbird portable sounder. The installation instructions are similar to those posted. It seems to work fine from dead stop to my top speed which is about 21 knots, but omagawd for spray. I have to make some kind of deflector to get that under control. I'm sure it also produces some drag, but I don't know if it is measurable in speed loss.
1993 Bayliner 3288, 351 Fords, converted to tuned port programmable EFI. 2012 AB 9.5AL RIB, Yamaha 15MSH (tender)