Years ago a Factory rep for one of the largest windsurfer custom fin designers/manufactures gave a talk on the importance of foil shape and repairing any nicks, scrapes or gouge in a fin. He said they found a definitive test by placing the fin in a fast flowing stream of a Artesian well (solid water...no aeration)...He said when a properly foiled fin with no surface imperfections was introduced in the flow, the fin would offer very little resistance to holding it in the flow...however if a fin was introduced into the flow with surface imperfections the fin would be ripped right out of their hands. It became a standard practice for them to check their personal race fins for resistance using this method...I would think this same method would work for cat rudders and dagger boards as well.

Mathematical Calculations and scientific research are the accepted ways to define and quantify resistance, but this simple method can show whether the theory lines up with real world results.

Best Regards,
Bob