Would be rather prone to damage on canals, and like a ducted prop (also used at sea in low-speed vessels like tugs) more prone to fouling -- meaning not just weed, but the other stuff you get round the prop on canals... 😞
Incidentally the Schilling rudder works very well, light and very responsive to steer, and when hard over (75 degrees?) the wash comes out sideways and the boat moves laterally as if there was a stern thruster. Which is nice because that's what I was hoping for, which I'm sure will disappoint the sceptics... 😉
I didn't get a chance to make a direct comparison or measurements because there wasn't time, but Ricky had taken the boat out several times on trials, and his assessment agreed with mine -- and unlike me he's done the same with several otherwise identical boats with bigger conventional flat-plate rudders. The only downside he noted was poorer steering when not under power when there's no prop wash over the rudder, which isn't a surprise given that it's considerably shorter than a flat plate rudder, but then this isn't exactly a common use case.