I purchased an Onno P-41 rudder in early 2009 to replace the nearly useless flat-plate over-stern rudder the previous owner had installed on my WSBS Thunderbolt. The boat once had an under-stern rudder, but in river racing the prior owner had the rudder break off and punch a hole in the boat, so he installed the over-stern rudder. As a river racer, I wanted to stay with an over-stern rudder, but I needed one that was truly effective. At the same time I purchased the Onno foot pedal system to activate the rudder - another excellent product, but that's for another review.
The first thing that struck me about the Onno P-41 rudder is the beauty of it - it's practically a work of art. Superbly crafted, the rudder has an airfoil cross-section and an elliptical platform, the combination of which is designed to provide maximum response with minimal drag. The symmetrically airfoiled (hydrofoiled?) section in theory should reduce turbulence around the blade and keep the flow attached longer to provide greater rudder effectiveness at higher angles of incidence in relation to the line of the hull. The elliptical platform in theory should reduce tip vortices (and the resultant drag) from the end of the rudder. I don't know how much analysis, if any, Pat put into this rudder, but I will say it works superbly. My Thunderbolt went from being a challenging boat to paddle just trying to keep it on course, to being a superbly responsive boat with excellent tracking - and excellent speed. Turning and tracking with a 21 ft boat can be somewhat critical in a race that involves turn around points and obstacles.
One of the really cool features of this rudder is the ability to change the rudder length just by inverting the rudder in relation to the housing. The rudder can be left fairly long, which is great for deep water and waves, or shortened to a much more reasonable length for river running (and it's still very effective).
There's no provision to pull the rudder up, but it does have an adjustable kick-up feature to keep the rudder and boat from being damaged by underwater obstacles. I have found that I can flip the rudder up vertically to keep it out of harms way when out of the water, if I so desire -the tension strap will give enough to flip the rudder 180 degrees and then hold it there.
I've been amazed at the toughness of this rudder. In river racing, it's not unusual to hit areas shallow enough to drag the rudder, and sometimes I cringe as I hear the rudder drag, expecting the rudder to be chewed up when I get done, but so far the rudder looks practically new.
Pat thoughtfully includes a bracket for mounting the rudder to the stern of your boat. I've had some challenges getting the stern of my Thunderbolt to have enough beef to keep the screws secure, but it works well enough otherwise.
This is an excellent rudder system, and well worth the money.