Overload protection

A. In the transmission components

The steering lines of a servo-pendulum gear should always be pre-stressed rope and should have a diameter of at least 6 mm % in or better yet 8 mm 1/3 in. Lines of this nature have breaking strains well above the maximum steering forces (300 kg / 660 lb) they are likely to encounter so they will stretch little in use.

If the rudder suddenly loses control or the boat is hit by a gust, a servo-pendulum gear will exert its full force on the steering lines and the main rudder. The force in the steering lines can be enough to bend the stanchions or pushpit to which the steering line turning blocks are bolted. A good safety measure in this respect is to make sure that one block on each side of the boat is fastened to the rail with just a cord lanyard which will break if overloaded and protect everything else.

B. In the rudder shaft

The rudder blade of a servo-pendulum gear trailing along in the water could hardly be better for catching kelp, fishing nets and general flotsam, and overload protection should therefore be regarded as a priority. Here are the options:

1. A designated emergency breaking point between the rudder shaft and the pendulum arm is incorporated in the form of a notched tube (ARIES). The breaking parameters are difficult to determine: the total leverage exerted by the rudder can be quite large, so it is hard to know at what stage the breaking point will give way and at what stage the whole gear will rip out of its mountings. The pendulum rudder should be tied to the mounting with a safety line so that it is not lost when the shaft breaks.

2. The connection between the rudder blade and the shaft is protected by a spring-loaded catch which releases if the blade strikes anything (MONITOR). This design effectively protects both the rudder blade and the mounting against collision damage.

3. The rudder blade is held in the forked end of the shaft by a rubber band (CAP HORN) or splint (ATOMS) which gives when the rudder blade is overloaded.

4. The rudder blade is kept from pivoting up by lateral M8 bolts (SAILOMAT 601). The force required to shear an M8 bolt is often too large in practice to prevent damage to the gear mounting.

5. The rudder blade is retained in a large area shaft fork by friction. As long as the bolt which closes the fork is not overtightened, the rudder can pivot up fore or aft in a collision (WINDPILOT PACIFIC). The rudder blade in this design must be positioned carefully to ensure it is balanced. Subtle changes in the balance increase or decrease the sensitivity of the gear.

Diy Small Boat Steering System Plans
Overload protection (left to right): Aries, Sailomat, Monitor, Windpilot Pacific (old), Windpilot (new)

Tip: The rudder blade of a servo-pendulum system should not be fully immersed until the boat speed is approaching hull speed, i.e. until the system needs to generate maximum force. This means that when the boat is stationary the top of the rudder blade will be a certain distance out of the water. The stern wave on some boats, especially those with more traditional sterns, can rise surprisingly high above the level of the surrounding water, so the exact distance will depend on the type of boat. If the rudder blade is too deeply immersed, the shaft will drag through the water, causing unnecessary turbulence and slowing the boat. This is easily avoided by mounting the system higher on the transom, which also improves the operation of the windvane.

The Windpilot Pacific cuts smoothly through the water causing little drag.

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