Auxiliary Rudder vs Servo Pendulum
Windvanes 101—Crash Course in Selfsteering Systems
Auxiliary Rudder vs. Servo-Pendulum
Scanmar International can be objective about comparing the two major types of windvane selfsteering systems because we manufacture both - the Monitor (servo-pendulum) and the Auto-helm (auxiliary rudder). The price is approximately the same for these two windvanes so we have no problem recommending the best system for your boat. Scanmar bases our recommendations on over 30 years of business and the feedback from thousands of installations as well as the personal experience of a five and a half year circumnavigation by Scanmar's founder, Hans Bernwall.
HOW THEY WORK
A servopendulum windvane will turn the wheel or move the tiller that controls the boat's own rudder, just like a helmsman or an autopilot. Generally, the best way to steer a boat is by using the boat's own rudder, which was designed to steer the boat in the first place.
An auxiliary rudder windvane adds a second smaller rudder to the boat. It works independently of the boat's own rudder, which is locked on boat centerline and used only for trimming the boat. There are three different principles used for auxiliary rudder systems:
'B' and 'C' are considered the better designs since the small trim-tab or servo oar generates additional power to turn the auxiliary rudder. 'A' relies on the airvane alone being able to accurately control a semi-balanced rudder.
WINDVANE RACING HISTORY
The hardest tests for windvanes were probably the single-handed circumnavigating races BOC and Around Alone. In four races between 1982 and 1999 many windvanes were used. The boats in those races were rather like the fast cruisers of today. Today's single-handed racers are a different breed. They have major sponsorship, and their boats are custom-designed for speed alone with enormous beams and are capable of speeds up to 30 knots. Most now use sophisticated electronic autopilots and elaborate charging systems to keep them operating. Windvane steering systems are just not appropriate for this new breed of 'racing sled'. The early boats, though, were similar to today's cruisers and were the perfect platform for improving windvane selfsteering design.
BETWEEN 1982 AND 1999 ONLY ONE AUXILIARY RUDDER WINDVANE WAS USED (1982-83). DURING THE SAME PERIOD, TWENTY MONITOR SERVO-PENDULUM SYSTEMS WERE SUCCESSFULLY USED. IN THE LAST TWO RACES, TWELVE BOATS HAD MONITORS - NO OTHER WINDVANES WERE PRESENT.
WHY IS THE SERVO-PENDULUM SYSTEM PREFERRED?
SERVO-PENDULUM: It produces stonger and faster steering power as the wind increases, and it works directly on the boat's own rudder, which was designed to steer the boat in ALL conditions, as long as there is wind.
AUXILIARY RUDDER: The auxiliary rudder might not be large enough to work in rough conditions, especially on a larger boat. In light winds auxiliary rudder systems work well because the trim-tab is easy to turn.
SERVO-PENDULUM: It's protected by its own ability to swing to the side. Most servo-pendulum systems also have overload protection in the form of a breakaway tube or latch release in case of an overload. The Monitor sero-pendulum system has a safety tube - a mechanical fuse - built-in.
AUXILIARY RUDDER: It's a rigid spade rudder with no designed overload protection. Breakage will occur if something like a log or a lobster pot gets stuck between the two rudders. In a violent broach the boat will be pushed sideways and the load on the auxiliary rudder can be very high, putting major strain on the rudder, the rudder shaft, the mounting system and the boat's transom.
FLIP UP/DOWN WATER PADDLE
SERVO-PENDULUM: The paddle can be raised or lowered. The servo paddle is only in the water when the windvane is steering. The Monitor's paddle is easily remotely lowered and raised.
AUXILIARY RUDDER: It's always in the water no matter who or what is steering - windvane, auto-pilot or helmsman. It remains down when anchored or docked. An auxiliary rudder can only be removed with difficulty.
SERVO-PENDULUM: The paddle is easily raised before motoring, and propwash is not a problem.
AUXILIARY RUDDER: Vibration because of propwash turbulence can be a real problem limiting motoring to low RPM's. This problem varies from boat to boat, and depends on the size and location of the prop as well as the design of the boat, and is difficult to predict.
SERVO-PENDULUM: Hand steering is not affected at all since the paddle is out of the water and the operating lines are disengaged.
AUXILIARY RUDDER: Tight turns will be adversely affected, and extreme care has to be taken to lock the auxiliary rudder before attempting to go in reverse.
POSITIVE YAW DAMPING
SERVO-PENDULUM: This is an inherent feature in servo-pendulum systems. If a boat begins to broach the paddle will be pushed in the opposite direction by the water. This movement will make the boat's own rudder correct the course and stop the broach.
AUXILIARY RUDDER: In a broach, an auxiliary rudder will be pushed in the wrong direction and will make it worse. An auxiliary rudder system with a servo-oar might work at correcting the broach but the smaller rudder area might not be sufficient to correct it completely.
SERVO-PENDULUM: The loads on the transom are relativey light because the paddle is small in area and can swing to one side in an overload situation like a broach. In most cases the standard supplied mounting bolts and fender washers are sufficient, and additional backing plates are not needed. Most servo-pendulum systems also have a built-in weak link or mechanical fuse which will protect both the boat and the windvane.
AUXILIARY RUDDER: A large spade rudder puts a lot of load on the transom in rough conditions. If the boat broaches or the auxiliary rudder gets jammed there is a good chance that damage will occur since there is no overload feature. Both the windvane and the transom can be damaged. Scanmar always recommends backing plates for our Auto-helm auxiliary rudder windvane - they must be fashioned to suit by the installer.
CLAIMED ADVANTAGES OF THE AUXILIARY RUDDER WINDVANE
IT CAN DOUBLE AS AN EMERGENCY RUDDER
The assumption is that if the boat's rudder is lost, the auxiliary rudder can steer the boat to the next landfall. However, if conditions are severe enough to break the boat's main rudder it's quite possible that the auxiliary rudder would also be affected - if the main rudder is carried away it can hit and damage the auxiliary rudder. If floating debris like a log got between the two rudders both could get jammed and damaged (this happened to Scanmar's owner during his circumnavigation using a home-made windvane).
To make sure that your emergency rudder is in good condition when you need it we believe that it should be kept safely stored until needed, just like your life raft. The Monitor servo-pendulum windvane has a conversion kit - the M-Rud - that converts it into an emergency steering system. The M-Rud is stored in a four-foot bag, ready for fitting if you ever have to use it. If you do, you know that it is in the same condition as when you bought it.
THERE ARE NO LINES IN THE COCKPIT
This is true - an auxiliary rudder system has no operating lines running to the boat's wheel. However, this supposed advantage should be examined more closely. When the windvane is steering the boat there's no need to be sitting at the wheel. To gain access to the wheel, a servopendulum system's operating lines are normally set up to one side of the wheel leaving the other side free, and the lines can be quickly unshipped when desired.
THE AUXILIARY RUDDER WINDVANE AND ELECTRIC AUTOPILOT CAN BE USED IN TANDEM
One auxiliary rudder manufacturer claims this is an excellent setup for heavy weather sailing but we feel that this is proof that their windvane is unable to cope with rough conditions, because the rudder is simply not large enough to control the boat in that situation. The electric autopilot would be attempting to steer a compass course, and the windvane would be attempting to follow a bearing off the wind - any wind shift will cause the two systems to fight each other with great strain on both of them. We feel that this can be a recipe for disaster. A servopendulum system uses the boat's own rudder, which is designed to handle the boat in any weather. The harder the wind is blowing and the faster the boat is going, the greater steering power is generated.
We feel that Scanmar is in the best position to recommend the correct selfsteering system for any boat because we manufacture three different windvanes - the Monitor servo-pendulum, the Auto-helm auxiliary rudder, and the Saye's Rig hybrid servo-pendulum/trim-tab.
The Monitor servo-pendulum system is the best windvane for most boats - it uses the boat's own rudder, can be installed by the purchaser, and can be easily shifted to a new boat by changing only the mount and possibly the length of the safety tube.
The Auto-helm auxiliary rudder system is recommended for boats with hydraulic steering and no access to an emergency tiller deck fitting, boats with a stiff and unfixable steering system, and boats with davits. (The Auto-helm's separate airvane mast can be mounted on a davit crossbar or anywhere convenient on deck.)
The Saye's Rig is often the only windvane that can work on very large boats with hydraulic steering and high freeboard.
Engineering, construction, and ease of use are all important. We manufacture the best windvanes available, all supported by our three-year warranty, and our company's reputation is second to none. Contact us for a recommendation for your boat.