Rudders, let's start here

Icon new rudder.jpgThis 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.


Thanks Paul. You made a  good boat even better.



I started with this rudder because I think it represents the epitome of where rudder evolution has taken us. I'll work back wards now and go into some of the older rudder shapes, i.e. the "elephant ear" rudder as my pal Thorvald likes to call them and explain why they were not the best way to steer a boat.


I met with Thorvald this weekend. He is rebuilding an old Bill Garden yawl with a long keel and an attached rudder that is kind of shaped like an elephant's ear, hence the term. We spread out the drawings to Thorvald's boat and studied the old rudder shape. My conclusion was that he should leave his rudder as is. The underwater profile of his boat is so pretty that I just couldn't;t see enough benefits from altering the rudder planform to justify modifying it from the original. It was a case of respecting the original work of Bill Garden.


So understand. The best rudder for steering a boat is an independent spade rudder. Or if your boat has a really fat fanny twin high aspect ratio spade rudders. If your boat has moderate proportions without exaggerated beam at the stern one rudder will work fine. But it's waste of time to try to justify other styles of rudders. All sorts of weird rudder shapes will work but the clean foil of the spade rudder works the best. Carbon fiber construction can make the rudder tremendously strong and free from all the problems that come with grp skins over foam attached to a metal pipe stock.


I will look at some of the older rudder shapes next.

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This page contains a single entry by Robert Perry published on October 10, 2011 12:25 PM.

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