The Practical Problems

Self steering systems are often criticised for lack of sensitivity in light airs. Attempts to improve the sensitivity by lightening the construction however lead to a fragile structure that may not be able to withstand heavy weather. The main enemies of sensitivity are friction and sloppy pivots, so the aim should be to build a robust device with close fitting low friction bearings. Naturally the less linkage and pivots in the system, the better. The ideal arrangement is to have the vane swinging on the same shaft as the trim tab. Then there is no linkage whatever so there is less friction and no loose joints. Such an arrangement would be possible where the rudder has a straight trailing edge. The H28, however, has its rudder hung at 45°to the vertical. To complicate the matter further, the rudder swings like a gate, so the trailing edge, (holding the trim tab), describes an arc centered on the rudder axis. I was therefore faced with the problem of, not only having to join the two shafts at 45°to each other, but one of them had to swing through an arc of 420mm radius. I had seen this problem lead to much heartache on another H28, where the linkage was prone to buckling at high rudder angles.

The solution came to me in a quiet little bay off Tory Channel when I meet Ben Gunn. We rafted together for the night and I noticed she had the two shafts connected by rigging lines and blocks. I took some sketches , went home and made up a mock-up in wood to determine the best location for the wind vane. Trial and error showed that the least variation in rigging line length occurred if the vane was mounted on the ships centre line, directly above the top rudder gudgeon. This very conveniently allowed me to mount the wind vane on the forward side of the pushpit, within easy reach of the helmsman, and it had the added advantage that a centreline mounted vane is less likely to suffer from interference to its airflow. A vane mounted on the lee quarter is subject both to turbulence from the cabin top when the boat is heeled, and to interference from the trailing edge vortex of the mainsail when close hauled.

To keep the whole structure light, and as friction free as possible, I decided to use 19mm stainless steel tubing throughout and to line all bearings with teflon. Teflon has a remarkably low coefficient of friction and will work equally well, above or below water without lubrication.

The next task was to get the power ratios right: which meant calculating the size of the trim tab needed to drive the rudder, and the size of wind vane needed to drive the trim tab. Most of the articles I had read cunningly avoided the questions of relative dimensions, or spoke glibly of "one fifth the rudder area…". There had to be an optimum size, beyond which working efficiency was defeated by increased drag. Then I came across an article by Mike Sanders in issue No. 125 of "Practical Boat Owner’. Here at last was a technical study of the relationship of the interrelationship between the size and position of the working surfaces.

Using Mike Sander’s techniques I found the underwater section of an H28 rudder has the characteristics shown in Fig 7, and the optimum area for the trim tab is derived from the formula:

A_{2 =} A₁ x H₁ / h₂ x (a₁k₁/a₂k₂)

Where

A₁ A₂ are the areas of the rudder and trim tab respectively,

H₁ h₂ are the perpendicular distances from the rudder axis to the respective C.P.s (centres of pressure),

a₁ a₂ are the stall angles of the rudder and trim tab, and

k₁ k₂ are constants based on aspect ratio.

The calculation then becomes:

A₂ = 618700 x 105/423 x 12/11 x .07/.09

= 13000mm²

I made up the trim tab by gluing together two layers of 12mm ply, tracing on it the curve of the trailing edge of the rudder, and cutting it out on a band saw to a width of 190mm. The trailing edge of the tab was faired to reduce drag and the whole thing was fiberglassed using light Dynel cloth. As will be seen from the diagram, the tab is located by making it neat fit in two stainless steel end caps welded up from 1 mm sheet.

Fortunately I had taken my rudder off for fibreglassing, so had a good chance to measure the underwater area accurately and install the four 100 x 9 mm threaded studs in the edge of the rudder to hold the trim tab gudgeon bearings. I found incidentally that the rudder, being buoyant, is not too hard to remove and refit without taking the yacht from the water. On the other hand, it can be quite sporty rowing a 3 metre long rudder ashore in a 2.5 metre dingy. Pick a calm day!