The pendulum rudder material buoyancy shape and balance

A pendulum rudder should react as sensitively as possible to every steering signal from the windvane. A balanced shaft design and buoyancy in the rudder blade improve the sensitivity of the gear, and the shaft and rudder blade should ideally be no heavier than absolutely necessary. The loads on a pendulum rudder and its shaft are for the most part only moderate; even pounding through waves is unlikely to damage the gear in its protected position at the stern. Nevertheless, the force generated by the pendulum rudder does make significant demands on the axle on which the pendulum arm swings. This is reflected in the visibly stronger construction employed in modern systems (Sailomat 601, Windpilot Pacific). The common servo-pendulum gears have the following pendulum arm axle dimensions:

Aries

25 mm,

Monitor

22 mm,

Sailomat

40 mm,

Windpilot Pacific

40 mm;

erial thickness for the pendulum rudder shaft:

and the following ma erial thickness for the pendulum rudder shaft:

and the following ma

Aries STD

aluminium tube 38 x 6.5 mm,

Monitor

stainless steel tube 41.3 x 1.25 mm,

Sailomat 601

aluminium tube 60 x 6 mm,

Windpilot Pacific

aluminium tube 50 x 5 mm.

The pendulum rudder does not need to have a profiled section because its maximum angle of attack is inherently very small. Every time the windvane rotates the rudder blade and increases the angle of attack, the blade swings out to one side and immediately reduces the angle back to approximately zero. The angle of attack, which is a function of the rudder pressure required (to bring the boat back on course), never exceeds 3 - 5°. Such small figures effectively preclude flow separation, and the truth of the matter is that in an emergency even a simple wooden slat can function as a pendulum rudder blade, assuming it can be fastened to the shaft (no problem with the forked shaft of the Sailomat 601 or Windpilot Pacific).

The balance proportions of the pendulum rudder directly influence the sensitivity of the whole system. If the gear is to deliver good steering performance in light air, for instance, even a relatively weak steering signal should suffice to effect a sustained deflection or rotation of the pendulum rudder. A pendulum rudder with a large or variable balance proportion is inherently easier to rotate than one which is completely unbalanced.

Ultimately, steering performance is determined by the combined effect of all the various parameters involved in the functioning of the windvane steering system. Refining each parameter in turn takes considerable practical experience and testing, so it is unsurprising that the major manufacturers all go their own way in this respect.

The rudder profiles (width and depth of section) and balance proportions of the most common servo-pendulum systems:_

Aries STD

170 x 50 mm

balance proportion: 19.4 %

buoyant - rigid foam

Monitor

170 x 48 mm

balance proportion: 20.0 %

buoyant - rigid foam

Sailomat 601

170 x 25 mm

balance proportion: 20.6 %

not buoyant -aluminium profile

Windpilot Pacific

120 x 19 mm

balance proportion: 22.5 %

buoyant - wood

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