Naval architect calls them ‘fishtail’ rudders,
explains what makes them work better
By Lou Codega, NA
Rudders with flat plate or conventional airfoil shapes in cross section tend to stall when deflected to high angles, particularly so at low speeds. Once stall occurs, water is no longer flowing smoothly over the low-pressure side of the rudder; that is, the side toward which the stern is swinging. Instead, the rudder face is more or less covered with eddies and vortices. Lift drops dramatically and the turning moment remains essentially constant from that
rudder angle onward. Its pretty obvious that this is exactly what the captain wants not to have happen, and, in my experience at least, the richness of captain’s vocabulary does very little improve the hydrodynamics.
The new rudder design for the Great Harbour and N37 boats is really an old one, and is derived from historical designs that sought to both extend the stall free range of rudder angle and develop high lift at all rudder angles. The end plates top and bottom increase the rudder’s effective aspect ratio. In layman’s terms, they force the water into behaving as though the rudder is far deeper than it actually is by preventing water from flowing over the ends of the rudder. This would rob lift producing pressure differential and create turbulence to increase drag. The fish shaped cross section, though, is really the key to success.
The rounded leading edge delays the stall angle, as does the wedge shaped trailing edge on the low-pressure side. The wedge on the high-pressure face greatly increases lift from the back end of the airfoil, which is typically quite ineffective. And it shifts the center of pressure of the rudder aft a bit, allowing the rudder stock to located closer to the center of the rudder, which in turn forces more of the rudder blade into the propeller’s slip stream.
The first trial results have been outstanding. On side-by-side tests of identical boats fitted with the old and new rudders, the new rudders turned in one half the turning diameter of the old at low RPMs and about 60 percent of the diameter at high speed. The turning diameter at full RPM is now about three boat lengths
Disadvantages? For these boats, I don’t think there are any. I was a bit concerned about astern performance, but there does not seem to be any difference. Rudders like this are not more widely used because they are terrifically difficult to build in ship sizes, and result in a lot of drag at high speeds in general and always if they cannot be perfectly aligned with the prop wash.
All in all, the rudders fit perfectly into the GH/N37 philosophy of efficient, shallow draft, low-power cruising.
(Lou Codega is the naval architect who designed the Great Harbour series hull. Read more about Lou Codega by clicking here.)