Windvanes 101—Crash Course in Selfsteering Systems
Auxiliary Rudder vs. Servo-Pendulum
Let us try to fuse the discussion of self-steering principles with the practical engineering aspects. From the classification of different types self-steering designs it should be clear that the ones that work best use a servo device (trim-tab or servo-oar). The better performing and most common gears are trim-tab-on-auxiliary-rudder and the servo-oar-to-main-rudder. So, which should you choose?
PerformanceFrom the standpoint of performance, the servo-oar-to-main-rudder does a better job on a reach and in hard weather, when it takes a lot of power to steady the yacht. A STANDARD SERVO-PENDULUM GEAR CONTROLS THE BOAT'S OWN RUDDER, WHICH IS ALREADY DESIGNED TO STEER THE BOAT IN ALL CONDITIONS. It will generally handle a larger yacht better than a standard auxiliary rudder gear.
A servo-pendulum usually has a bit more weight and surface area than the trim-tab mounted on an auxiliary rudder. Although the servo-oar is much more powerful than the trim-tab, there is also much more resistance to its turning the main rudder. Because of this the trim-tab-on-auxiliary-rudder gear has an edge in very light air when the slow speed of the boat does not provide much leverage to the servo-oar.
The yaw damping feature is unique to the servo-pendulum and is extremely valuable in controlling the course of the yacht, especially downwind.
In summation, it seems that the servo-pendulum provides better all-around performance on the majority of yachts. This statement, of course, assumes that the actual engineering of a gear is not preventing it from operating to its potential, which carries us over to the practical aspects in comparing the two self-steering principles.
An advantage often mentioned of the trim-tab-on-auxiliary-rudder is that these devices provide a back up steering system if something happens to the main steering of the yacht. However, in years of involvement with self-steering we have seen again and again that if anything breaks it is the auxiliary rudder of the vane gear rather than the main rudder. The self-steering rudder is a spade rudder, which is normally much weaker and much more vulnerable than the boat's own rudder. If conditions get bad enough to damage the boat's main steering you can be almost certain that the auxiliary rudder will break also. An emergency steering add-on to a servopendulum selfsteering system, such as the M-Rud add-on to the Monitor windvane, is free of that possibility since it is stored below until needed.
Several negative features of the auxiliary rudder-trim-tab principle are related to the much greater size, weight and underwater surface area of this system. The bulk and the rigid shaft of the auxiliary rudder result in much greater loads than in a pendulum gear. To begin with, this makes the installation crucial and sometimes more time consuming, since special transom reinforcement and larger back-up plates are always necessary.
Another aspect of the considerably larger underwater areas is much greater vulnerability. A servo-pendulum is protected by its small size and its freedom to swing to the side on impact. It is fairly easy to equip a servo-oar with some kind of overload protection, but that feature is not found on any of the auxiliary rudder systems currently in production. An auxiliary rudder requires a strong, rigid attachment, making it extremely difficult and expensive to provide an overload fuse.
For the same reasons an auxiliary rudder cannot be mounted and dismounted or have the underwater parts removed in the easy manner of a servo-pendulum vane gear. The auxiliary rudder remains in the water when it is not in use, creating extra drag on passages and inviting growth damage in port and in anchorages. On some boats, turbulence from the propwash causes excessive vibration which can limit motoring at full speed because of strain on the rudder bearings and mounting system. Because of its greater surface area and its inability to be removed the auxiliary rudder will interfere with maneuverability in tight situations, especially when reversing.
Due to its much smaller surface under water and the swinging shaft of the pendulum, the servo-oar vane gear is much less vulnerable than the auxiliary rudder type. Weight is much lower, installation is much easier, there is much less drag and interference with maneuvering. In a well designed pendulum gear an overload protection is provided that works on impact from any direction. Also, the servo-oar can be removed, with ease even at sea in large seas and in darkness.
The main negative aspect of the servo-oar principle is that it does not provide a good emergency steering system, even though it can sometimes be used for this purpose on a smaller yacht. The pendulum lines are taken off the wheel adapter or the tiller and cleated. The servo paddle is now in a locked position and by moving the counter weight by hand or temporary rigged lines the servo paddle can be converted to a rigid, small auxiliary rudder that will serve as an emergency rudder. Expanding on this idea, Scanmar offers the add-on M-Rud option for the Monitor windvane - an oversized paddle and servo-pendulum locking bracket that converts the Monitor windvane into a proven true emergency steering system.
Clearly, the need for lines into the cockpit constitutes an added obstruction, which is not necessary with the auxiliary rudder device. The argument against the lines in the cockpit seem to disappear with use of a good windvane. If the installation is well thought out the lines should only be a slight inconvenience. Once the vane gear is set up the crew will find out that there is no need to sit at the wheel, which is in an exposed location. Time will be spent under the dodger, at the nav station, in the bunk, in the galley or relaxing on deck. If the pendulum lines have a quick release system they are very easy to remove in port when more time is spent in the cockpit. Additional information about operating line setup can be found in the Monitor section on the "Lines in the Cockpit" page, and in the FAQ article "Operating Line Setup".
In weight of these arguments the servo-pendulum vane seems to be the clear winner both by performance and practical criteria. This has made the servo-oar gear the most commonly available gear and the vane system most often seen on cruising boats today. It is the only type of windvane used in the single-handed around-the-world races like the BOC and Around Alone - with one exception. One auxiliary rudder system was used - once in the early 1980's. It had breakage problems in its overload system, and has been withdrawn from the market.