Bob Perry has a new blog and one of his first posts was on ICON’s new rudder designed by Paul Bieker.

ICON's new rudder designed to prevent stalling.

This is ICON’s revised rudder. As you probably can guess the bumps on the leading edge are what was added to my original rudder shape. These bumps are called “Tubercles” and can be found in nature on the leading edge of the fins Humpback whales. They can be seen as vortex generators. So, why were they added and did they work?

ICON’s original owner and skipper were Happy with the boat’s performance and I never heard any complaints about the rudder. But ICON sold to a new owner and he was campaigning the boat heavily and quite successfully. The new owner, Kevin, felt the rudder was stalling too quickly when the boat was being hard pressed. He asked Paul Bieker to take a look at the rudder and suggest a modification. I’ve known Paul for years. He was an intern in my office years ago and lived in my house while interning. Paul is one of the most clever and technically agile designers I know. Paul pulled out a technical paper,” HOW BUMPS ON WHALE FLIPPERS DELAY STALL, An Aerodynamic model.” by Ernst A. van Nierop, Silas Alben and Michael P. Brenner. This highly technical paper goes into the theory behind how the bumps can delay the stall angle of a fin. It’s full of complex formulas and to be honest a bit over my head. OK, way over my head but I can look at the pictures, the graphs and read the text and get a good understanding of what and why they were doing it.

Paul applied the theory in the paper and suggested three bumps on the leading edge of ICON’s rudder. The idea being that when ICON was heeled over and rudder was applied at high angles of attack the rudder would suck air down the leading edge leading to a stall and loss of rudder control. The bumps direct this downwash back across the rudder foil and delay stalling. Cool huh. And yes, according to Kevin, the bumps worked great. There is very little additional drag from the bumps but the effect on stall angle was dramatic.

On many modern high performance boats with broad transoms twin rudders are used so that the leeward rudder is well below the water’s surface. This prevents the leeward rudder from “ventilating” and stalling. But on ICON we had one rudder and changing to twin rudders would have been very expensive. Paul’s solution was a simple, quick and economical fix. Plus, it looks sexy.

You can follow Perry’s blog and read the entire post at Yacht Design According to Perry.