Scott,

Boats with a spade rudder and no skeg are inherently course unstable. Your boat is, and so is Weaverbird. 

A self steering gear for such boats must add the stability that is missing.

The OGT does not do this. A trim tab does not do this.

Both might give adequate steering with the wind forward of the beam, because then the combination of sail, hull and and rudder tends to have stable characteristics (where the hull and rudder alone do not). Neither would work off the wind sufficiently well off the wind to be worth the effort of making them. 

An auxiliary rudder gear (the Hydrovane is a commercial example) adds stability to a certain degree, because the main rudder is fixed and acts as a skeg.

Only a servo pendulum gear does a fully reliable job of adding the yaw damping feature that such boats need to make them course stable. And even then, I have found that unless the servo pendulum has an inclined axis so that it tends to trail and self-centre, small boats with dinghy-type spade rudders will not hold a steady course on a run. I have known this since the early 1970s, when I was working with Blondie Hasler, trying to get a Hasler SP servo pendulum gear (with vertical axis vane and horizontal axis pendulum) to work on Pilmer, his Kingfisher 20 with a balanced spade rudder.

That is why I developed the two pendulum gears for which there are drawings in  https://junkrigassociation.org/members_files > drawings > David's vane gear drawings.

I have to recommend that either you build one of these, or you take the easier option of installing an electric autopilot. For short passages in good weather, the latter should serve reasonably well.

Arne:

Scott,

Test 1
you could for instance drive your boat for motor in a straight line. Then you put the engine in free and leave the tiller. If the boat starts swinging into a steeper and steeper turn, to this or that side, then it is unstable.

Test 2 (..if the boat went straight in test 1):
Motor into a medium steep turn, set the engine in free and let the tiller go. If the boat straightens out the course, then it is definitely course stable.

The quicker the boat straightens up, the better is the course stability.

Arne

I went out to my boat right around sunset yesterday. There was almost no wind and Lake Macatawa was very calm. I think there was just one power boat that briefly threw a wake across the water.

Test 1: The boat goes straight after I put the engine in neutral and let go of the tiller.

Test 2: The boat never straightens up. The tiller stays more or less where it was when I let it go, until the boat has turned around and slowed down. After slowing down, the tiller moves and puts the boat into a steeper and steeper turn.

So ... it is stable until something pushes the rudder over, I guess.

A man with good taste (he sails a Contessa 26) made his own version of the trimtab SS and proceeded to do an Atlantic circle.  Details per the video

https://youtu.be/Wcd9Y5AS6qE

Scott,

Test 1
you could for instance drive your boat for motor in a straight line. Then you put the engine in free and leave the tiller. If the boat starts swinging into a steeper and steeper turn, to this or that side, then it is unstable.

Test 2 (..if the boat went straight in test 1):
Motor into a medium steep turn, set the engine in free and let the tiller go. If the boat straightens out the course, then it is definitely course stable.

The quicker the boat straightens up, the better is the course stability.

Arne

Scott, there is also the OGT MkI model. It is a little bigger than Mk II, and Belcher reckoned it is twice as strong as the Mk II.

It is not easy to sit on one's armchair and say "this will work" or "this will not work", in particular with windvanes. 

It depends on the tiller forces on your boat. My 1.4ton Malena had an unbalanced rudder so tiller forces were noticeable.  If your tiller forces are high, then I would suggest the MkI vane directly on the tiller.

In case the boat is directionally unstable, I suggest you fit a rudder-like thing on the transom and lock it in mid-position. That will act as a skeg, and can also work as a spare rudder if needed. You may well make this before you build a wind-vane.

Arne