I have decided to build a semi-balanced NACA foil section rudder with Bolger specified end plate. Bolger has specified a semi-balanced rudder on the 20' Chebacco RD (see Ben Ho link in the favourite links section) but all of his Chebacco rudders are a much simpler shape.
Because the rudder is mounted behind a full keel (there is going to be a 3" gap) I figure there is increased chance of turbulence and stalling of the rudder. NACA foils are less prone to stalling so I hope the extra effort of carving a mathematically determined foil will improve rudder performance. The shape was determined using a spreadsheet I developed (see below if you want a copy) then I cut lots of parallel sawcuts in the cedar rudder cheeks to guide the chisel and plane.
Here you see the post that will be the trailing end of the keel. There is a 3" gap to the rudder (hopefully this will be enough) and the rudder pivot will be 3" further behind that.
The advantage of a rudder mounted behind a full keel becomes apparent here - you don't ever have to lift the rudder when you come into shore and it will be hard to damage as the keel will hit the bottom first. It is also unlikely to pick up seaweed or floating nets or rope like spade rudders might.
This is a work in progress - I don't know how this is going to work yet.
The post is solid 32mm 316 stainless. I am not too keen on welding stainless in a spot that cannot be inspected so I have opted for a more mechanical solution for attaching the wooden rudder to the rudder post. Holes are drilled in the post and 12mm SS rods inserted into the holes. The rods and post shape is then routered out of the two halves of the rudder and the whole assembly is sandwiched with epoxy between the two halves. Now, when the post is turned the torque is transferred to the rudder via the rods. A hole at the top of the post is for bolting on the tiller.
I thought I had better check to see if this rudder will float or sink. If it floats it will rub against the underside of the boat. I did the calculations and it looks like it will be about neutral - despite 6.5kg of steel rudder shaft. (lucky I didn't use Stainless Tube - I was considering it, but tube wasn't available in the size I needed) I will ponder if I should add some lead to the rudder to keep it sitting on the bottom pintle bearing. (I didn't in the end)
The bearing was made by greasing the shaft, inserting it into the tube in the hull and filling the space in between with epoxy filled with 10% graphite. The result is a perfect bearing, smooth, water tight and slop free. It shouldn't wear out in a hurry either.
It was a messy job - but the result was worth it.
And now it is coming together.