The big misconception

Servo-pendulum systems operate on the basis of servo-dynamic force. Essentially, the mounting at the stern only has to withstand the force transmitted through the steering lines to the main rudder and support the gear itself. High loads, such as pounding waves, do not as a rule reach the gear, and breaking waves are more likely to knock the whole boat to leeward than force the pendulum rudder out of its position in the wake. A swell which catches the boat side-on acts on not only the pendulum rudder but also the main rudder, causing both to rotate slightly and absorb some of the force of the wave. The connecting lines from the gear to the main rudder thus act as a kind of sliding clutch, allowing the steering system as a whole to damp every movement.

Notice how the Pacific gear is fastened (4 bolts) to the heavy gaff-cutter shown in the cover photograph. Despite its fragile appearance the mounting already has eleven trouble-free years of service behind it including plenty of blue-water mileage. This should not really seem so surprising: the pendulum rudder follows the boat as effortlessly as a gull behind a trawler when the steering lines are disconnected, so the load on the mounting amounts to nothing more than the weight of the gear. Reconnecting the steering lines adds only the force generated by the pendulum rudder to turn the main rudder and effect the course correction.

Release one bolt, remove the system and the swim ladder is

An offset mounted servo-pendulum system will not function effectively

Experience, of course, is the real test. If wave action really could bring damagingly large forces to bear on a trailing pendulum rudder blade and its mounting, we would expect to find at least a few instances amongst the thousands of Aries and Monitor systems in use of the pendulum arm being bent against the steering line guide tubes which extend at the bottom of both these systems. This type of damage turns out to be all but unheard of. The configuration of the bevel gear linkage in both systems ensures that the pendulum arm is always brought back into parallel with the keel, i.e. is damped, before it can travel so far sideways. This remains true regardless of wave action or even capsizes.

An offset mounted servo-pendulum system will not function effectively

Wooden, steel, aluminium and solid laminated GRP hulls need no extra reinforcement on the inside of the transom. Only on sandwich construction hulls is it recommended to fit additional wooden blocks or aluminium plates rather than sandwich material at load bearing points.

The apparently greater load distribution provided by the larger number of bolts (up to 16) on conventional servo-pendulum systems (Aries, Monitor) is not technically necessary and the mass of bolts contributes to the visual pollution of the stern. The loads may simply have been overestimated by the designers at the time these traditional servo-pendulum systems were conceived.

This mounting for a Windpilot Pacific on a 25 tonne gaff cutter has worked well for 12 years.

Mounting on a 20 tonne Colin Mounting on a HELMSMAN 49 Archer

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