OMG!

Back working on the boat.
The rudder shape is known as a "barn door" and is only as deep as the keel. As such it means slow ponderous changes of direction and in roughish water when the boat is heeled there's very little in the water - which makes steering a tad difficult.
So I'm converting it to a deeper modern shape with a lifting blade that sticks out below the keel. The area of the blade is the same as the old one, it is just a different shape.
I spent a long time measuring, checking the drawing, measuring again before I cut the old blade off - it's done now so it can't go back on. (Well it can actually, I was careful to keep the old bit so it can be replaced if the new one's rubbish).