Years ago the debate about the most effective rudder for steering was settled, and the cantilevered spade configuration was the winner. But what about hitting debris, running aground, and catching nets you might be thinking.
First: structure. We can tell you from our own experience, and the feedback from owners who have tested our theory, that a properly engineered spade rudder is stronger and more damage tolerant than skeg or keel hung fins.
There are three keys to this strength. The first is a substantial strong rudder shaft. Our norm is to use twice ABS and/or Lloyds Special Service rule as the baseline stiffness specification. Next comes sufficient hull strength and gusseting of rudder bearing carrier(s) where they connect to the hull for long life and abuse. The last item is a frangible rudder tip, so that it goes away before the rudder shaft itself is damaged, leaving the upper two thirds of the blade with which to continue on.
The photo above is the rudder on the FPB 64 Iron Lady, which spent several hours pounding on a reef. It will give you a feel for how tough these blades are.
Next comes catching things on the rudder. Here we have a swept leading edge to shed, and almost total protection from the full depth prop skeg.
The alternative is a skeg hung rudder, or a spade with a beam from the trailing edge of the keel to the bottom of the rudder. The problem with both of these approaches is damage tolerance. If either is bent from pounding, then the rudder is jammed and you have no steering.
We’ve cruised well over 250,000 miles now with spade rudders, and done our share of running aground, and we are happy to report we’ve yet to have a problem.