Self Steering

Jul 14, 2018 | Technical

The H28 with her long keel, is a particularly stable vessel and easy on the Helmsman. She will even steer herself for short periods while the solo helmsman attends to sail trim. But, as any long distance sailor will tell you, the pointless drudgery of long hours at the tiller takes much of the pleasure out of passage making. The answer is a self steering arrangement.

When I took Jambelot to Picton in 1979, I borrowed a homemade “QMF” type vane, which drives the tiller lines (see fig1) for the journey; but, in spite of much fiddling, I was unable to get satisfactory performance on any point of sailing. Obviously more power was required. The more complex servo powered systems like “Navik” and “Aries”, are rather expensive, So I decided to build a trim tab system (Fig 2) based on a collection of ideas from books, magazines and existing systems.

The Theory

The principle is simple. Fig 3 shows a boat on close reach, sails trimmed and self steering set. At Fig 4 she has wandered off the wind a few degrees. The windvane weathercocks and in doing so, turns the trim tab in the same direction. The water flow over the trim tab then causes the rudder to drive the other way (Fig 5), thus bringing the boat back onto the desired heading (fig 6).

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.

The Structure

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.

The Trim Tab

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:

A2 = A1 x H1 / h2 x (a1k1/a2k2)


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!

The Wind Vane.

Wind vane size is obtained from the formula:

A3 = A2 x H2/H3 x G x 100

Where G is the gear ratio between the vane and the tab, and 100 is an empirical figure based on experimentation.

Much has been written about the merits of wedge shaped vanes, high aspect profiles, trailing edge flaps and fabric covering but I could find no authoritative conclusions on these factors. I wanted a small, strong, light sensitive vane that could be quickly unshipped in heavy weather conditions. I noticed that the vane size could be reduced by decreasing H2 or increasing H1. H1 is limited by the fact that the vane may strike the backstay in downwind conditions, but my research showed there is a definite mechanical advantage in moving the vane’s leading edge downwind from the axis of rotation. For this reason my vane is narrow (350mm) and its edges parallel the slope of the backstay. I was able to reduce H2 by moving the leading edge of the trim tab forward of the axis of rotation. I did this by simply moving the lower tab bearing away from the trailing edge of the rudder. With H2 down to a mere 10mm the vane calculation became:

A3 = 13000 x 10/550 x 1 x 100

= 238000mm2

Strength and lightness were obtained by making the vane from 8 mm polystrene sheet sandwiched between two layers of chopped strand matting. It is held in the frame by two sets of lugs and secured by a single bolt so I can get it off fast in bad weather.

The Control System

The vane shaft rotates in two Teflon bearings mounted on a simple wooden board, clamped on the pushpit. Vertical thrust is carried by a tufnol clamp, which rides on the top face of the upper bearing. A similar Tufnol block, secured to the control disc, is clamped to the lower end of the vane shaft. The shaft is therefore free of welded or through bolted sheaves, (I know of two vanes that blew away in gales when the shafts fractured at the bolt holes!), and the whole vane assembly can be lifted off by merely undoing the bottom Tufnol clamp.

The control latch (Fig 8), is welded to a “T” piece made from 5 mm strap, which is free to rotate on top of the 6mm toothed control disc. The spring-loaded latch plunger can be disengaged, for tacking, by pulling the knob and giving it a quarter turn either way. The teeth are at 5°spacing, which I find is quite fine enough for accurate steering.

The two braided polyester control lines run from the transverse bar on the trim tab shaft (again secured by a tufnol clamp), through small cheek blocks on the arms of the “T”piece, to a double jam cleat where the tension can be easily adjusted.

The most difficult job was alignment of the bearings, which must be perfectly true to avoid friction or jamming. All five bearings (see fig 9), are made up of 31mm tube, welded to pieces of 25 x 6 mm strap and lined with interchangeable teflon sleeves.

Performance Testing

The first test was carried out on a very gusty day in Queen Charlotte Sound (not a good area for constant winds). Nevertheless the structure proved its ruggedness when we were twice knocked down by a vicious squalls, and some promising runs were achieved hard on the wind. Proper testing, however, came when we departed for Auckland a month later. The only adjustment needed was to the height of the steering bar on the trim tab shaft to obtain the most effective “lead” for the steering lines. Note also that the gear ratio can be altered by moving the anchorage of these lines along the holes in the bar.

I was delighted with the steadiness with which the heading could be maintained and the ease with which both large and small alterations of heading could be made. On several occasions the crew forgot to unlatch the gear when tacking, (twice the boat described 360° turns with the gear engaged!); but the remarkable “dual rein” linkage simply crossed over and waited for the helmsman to get himself organised. Wind speed variations seemed to have little effect on steering ability, even under gale conditions off Castlepoint when we had some of the most unusual sailing conditions. I came to the conclusion the gear would do its job provided the boat was balanced and there was enough breeze to move her through the water. It was not until we passed East Cape that we encountered a following wind and were able to test the down wind performance. Vane steering gear is notoriously inefficient in light winds from behind. Our run across Bay of Plenty was pleasing enough, but the real achievement came when we were almost becalmed off Mercury Island. We set the spinnaker in about a 5 knot breeze and engaged the latch without any real confidence. The breeze picked up to about a 8 knots and as I sat there, beer in hand, watching the vane sniffing at the wind, I knew this was the moment that made it all worthwhile.

Brian Greer