Servopendulum systems

Since this is the most popular system today we shall devote the next few pages to a detailed look at its various features.

How they work

The windvane rotates the rudder blade via a linkage. The rudder blade is mounted on a shaft which is able to swing from side to side like a pendulum (hence the name). When the rudder blade is rotated, the force of the water flowing past pushes against it and swings it out to one side. The shaft on which the pendulum rudder swings is connected to the tiller (or wheel) via lines, so the lateral movement of the rudder blade is translated into a pulling force on the tiller (or turning force on the wheel) which effects the course correction. Once the boat is back on course, the windvane returns the pendulum rudder blade to the centre.

 Steering impulse = wind Steering force = water Steering element = main rudder Power leverage (PL) = up to 200 cm / 80 in

The enormous power leverage of the servo-pendulum design compared with other gears clearly reflects the considerable steering and servo forces it is able to generate.

This Windpilot Pacific V vane servo-pendulum system MKI (1969), is made of sstainless steel.

The traditional Monitor H vane servo-pendulum system.

The servo principle

Imagine you are standing at the stern of your boat doing 6 knots holding a 2 m / 6 ft long wooden plank in the water. Align the plank directly on centreline and you can just about hold it with 2 fingers. Rotate it slightly, however, and it swings powerfully to one side (your shoulder joint represents the pendulum axis).

Using this principle the hydrodynamic force of the water flowing past can be harnessed to generate a tensile force of up to 300 kg / 660 lb. This explains how servo-pendulum systems are able properly to steer large, heavy boats: not only does a bigger boat require more steering force, it also inherently produces greater hydrodynamic force for the steering gear to exploit.

Fig 5.14 This figure shows how the torque at the pendulum arm of a servo-pendulum gear on a displacement boat reaches a natural limit defined by the maximum speed of the boat. ULDBs have no such limit because the boat speed can rise rapidly during surfing. The formula is applied here to a Windpilot Pacific gear with a 12x90 cm/ 4.8x36 in rudder blade and a standard power leverage (PL) of 190cm/ 76 in.

Yaw damping

A sailing yacht with nobody at the helm is an inherently unstable system, as it will immediately turn towards the wind until the sails flog. A sailing yacht with a helmsperson/autopilot/windvane gear in control is a stable system. The difference between these two states, which amounts to nothing more than the load on the rudder, may be very small or very large depending on sail trim, weather conditions and the characteristics of the boat. Sometimes a couple of fingers on the wheel are enough to keep the boat on course, but on other occasions the helm can be strenuous work.

Fig 5.15 Windpilot Pacific: Rudder angle and pendulum arm angle as a function of deviation from course for a windvane rotating about a 20° inclined axis.

A deviation from course of 10° causes the pendulum arm to swing out 27°, which turns the main rudder by a maximum of 13°. The servo-pendulum gear oversteers only slightly ( 10° deviation, 13° main rudder movement ). This explains the good steering performance of such systems.

The enormous potential force of servo-pendulum gears is also their main problem: unless the force transmitted to the main rudder can be regulated in some way, there is a danger that rudder movements will be too large or too prolonged, leading to oversteering.

Consider how an experienced helm steers. He or she knows that delicate rudder movements are sufficient for course corrections and would never steer with big or vigorous tiller movements. Abrupt, sweeping movements of the helm make it very difficult to judge the boat's exact course and consequently cause oversteering. Not only that, but unnecessarily sharp rudder movements are also bad for boat speed.

Alternatively, consider the blade of a folding propeller (eg Max Prop), which, in its sailing position, is stationary in the wake of the keel. If it receives a mechanical impulse, it begins to spin endlessly around the shaft, stopping only once it has been reset. In this analogy, the propeller blade represents the pendulum rudder, the prop-shaft is the pendulum rudder axis and the windvane provides the mechanical impulse.

If the steering impulse from the windvane was transmitted directly to the pendulum rudder without any kind of braking at all, the pendulum arm would swing out too far to one side, possibly even lifting out of the water, until the wind delivered a steering impulse in the opposite direction. This wide a range of movement would require excessively long lines to transmit the correction to the main rudder and would turn the rudder too far, resulting in oversteering.

Yaw damping in a servo-pendulum gear basically amounts to limiting the lateral travel of the pendulum arm. A balanced system combining a damped windvane and a bevel gear with a step-down ratio of 2:1 is able to do this. There is also a second, more critical reason for limiting the lateral swinging range of the pendulum arm: the maximum heeling angle of a sailing boat is about 30°, so the maximum possible working range of the rudder has to be no more than about 28° to ensure that the combined effect of heel and a large rudder movement does not lift it out of the water to windward. Mounting a servo-pendulum gear offset to one side would obviously accentuate this problem, further reducing the useful working range, and is therefore not at all advisable (see Fig 5.16). Most course corrections involve the pendulum rudder swinging out to windward; this is the movement which directs the main rudder to bear away, and bearing away is by far the most commonly required course correction.

The steering impulse from the windvane swings the pendulum arm out no more than 28°. Every time the windvane rotates the pendulum rudder blade, the pendulum arm is pushed out to one side, simultaneously rotating the rudder back parallel to centreline (but so as to remain offset laterally from centreline). This arrangement keeps the maximum steering line travel down to about 25 cm / 10 in and effectively prevents oversteering.

The state of the art in servo-pendulum gears today is a horizontal vane, inclined by 20° (see Chapter 4), acting through a bevel gear with a step-down ratio of 2:1. Aries, Monitor, Pacific and Fleming use identical configurations in this respect.

A servo-pendulum system with a bevel gear provides perfect steering and always delivers precisely the force required to bring the boat back to the desired course. If attitudes to sail

trim become more relaxed, the gear automatically generates greater pressure on the main rudder which is then adjusted more forcefully.

Servo-pendulum systems which do not have yaw damping are demanding of the crew, placing undue emphasis on balance, sail trim and the idiosyncrasies of the boat. Wind and sea conditions can also provoke an unacceptable deterioration in steering performance.

Windpilot Pacific 360 degree bevel gear linkage ratio 2:1

The push rod

The steering impulse from the windvane is transmitted via a vertical push rod to the linkage where, after being amplified by the bevel gear, it effects the lateral adjustment of the pendulum rudder. The forces encountered here are generally quite moderate, and the key is to ensure that the impulse is sensitive, prompt and reliable even in light air. Manufacturers have tended in the past to overestimate considerably the loads on the push rod and consequently to overbuild it. Aries use a chunky cast push rod which weighs in at over 1 kg / 2 % lb and Monitors component is 450 g / 1 lb; the more modern Windpilot Pacific manages with an 8 x 1 mm stainless steel tube which weighs only 143 g / 5 oz and has proved its strength on thousand of boats over the last 12 years. Not surprisingly, two such different designs give very different steering performances.

Remember: The push rod is one of the factors which determine light air steering characteristics; it must be as light as possible and no stronger than absolutely necessary.

The Windpilot Pacific (left) and Monitor are indentical in design, but have substantially different dimensions of pushrod, pendulum axle diameter and pendulum carriage.

Steering force transmission

Lines are used to transmit the force generated by the pendulum rudder to the boat. The steering lines in a conventional system (Aries, Monitor) are attached to the pendulum arm itself and start at the bottom of the gear. From there the two lines (one on each side) are each guided through 3 blocks up to deck level, where 2 more blocks each lead them to the tiller or wheel. These systems therefore require 10 blocks and correspondingly long steering lines. Modern designs, in contrast, have the pendulum arm extended upwards, enabling the steering lines to start at deck level. The number of blocks required is reduced from 10 to just 4, and the steering lines are also correspondingly shorter. Care must be taken with modern systems to ensure that the steering lines from the pendulum arm are initially led parallel to the vessel's transom. Although a certain deviation from parallel is tolerable, the effective line travel will be reduced if the transmission angle is too unfavourable. Larger boats in particular need the full line travel.

Steering line routing on a double-ender

To facilitate the use of this type of system on double-enders and extreme 'sugar-scoop' sterns, the Windpilot Pacific design includes a cross-member with fastening points at each end for the steering line blocks. This option is not available with the Sailomat 601.

A servo-pendulum system will only work well if the force from the gear is transmitted smoothly to the main rudder. Arrangements with shorter transmission paths (line lengths) and fewer turning blocks accordingly deliver better steering results. Put another way, the longer the transmission paths and the longer the lines, the greater the transmission losses will be. Slack or stretchy lines and stiffness in the main rudder reduce the efficiency of the system. A servo-pendulum system is only as good as the quality of the force transmission.

Short transmission path to the tiller with the Windpilot Pacific. The steering lines start at deck level.

Steering line travel

The amount by which the steering lines move as the gear goes from lock to lock, the maximum line travel, is only about 25 cm / 10 in for a servo-pendulum system with bevel gear yaw damping. Ineffective force transmission, slack, stretchy lines or excessively long transmission paths can quickly eat into this figure. Given a combination of factors, it is not inconceivable that the maximum line travel could fall to as low as 10 cm / 4 in. A gear with this restricted a range will demonstrate its impotence at the slightest opportunity; sooner or later the rudder will lose control. A good servo-pendulum gear can generate up to 150 kg of steering force - enough to keep anything up to a 60 footer happily on course. The key to actually achieving good steering performance from a servo-pendulum system is, quite simply, the quality of the force transmission arrangement.

The 25 cm/10 in steering line travel demonstrated here on an Aries, but indentical on both the Monitor and Windpilot Pacific.

Tip: The effective line travel, and hence the magnitude of corrective rudder movements the gear can make, can be increased by moving the centred position of the windvane more to leeward (which is easy to do by adjusting the tiller mounting or wheel adaptor). This method relies on the fact that almost all windvane gear course corrections are bear-away movements. It may be the only way in extreme conditions of achieving sufficiently large rudder angles.

Transmission to a tiller

Boats with tiller steering provide ideal circumstances for the transmission of the steering forces. Aft cockpits allow the transmission paths to be kept short and the attachment point of the steering lines on the tiller can be moved around or even, as is found on some lighter or faster boats, be mounted on an adjustable track. The steering lines are preferably attached to the tiller by a short length of chain, one link of which latches between the two teeth of the tiller fitting. Some systems use an arrangement in which the lines are fastened to the tiller with comb cleats, but this is not a particularly practical arrangement.

The tiller fitting is mounted at a point about six-tenths of the way along the tiller, i.e. just aft of the area normally held for steering. The path of the steering lines out to the blocks at the side of the cockpit is angled aft slightly so as to follow the turning radius of the tiller. One advantage of this is that the lines always assume the correct tension when the chain is latched into the tiller fitting and the gear engaged.

Tiller fitting and chain link, Windpilot Pacific

The lines used to transmit the steering force must be prestretched rope. They should not be set too tight, however, as this causes excessive bearing friction in the turning blocks and reduces the efficiency of steering force transmission to the tiller. Bearing friction can be countered by using ball-bearing blocks on the steering lines. Other factors which adversely affect the efficiency of transmission include too many turning blocks, stretchy or excessively long steering lines and stiffness in the main rudder.

Braided, pre-stretched 8 mm diameter rope makes very good steering lines. The breaking strain is well above the loads the lines will actually be exposed to, so stretch is minimal. It is a good idea to end-for-end the lines every so often on longer voyages so that the wear from turning blocks etc. is not always concentrated in the same spots.

Fine-trimming with tiller steering

The chain latch arrangement makes it easy to fine-trim the boat's course when the steering gear is in use, and also enables the crew to disconnect it instantaneously if necessary (e.g. for emergency manoeuvring). Once the chain has been disconnected from the tiller the gear has no influence on steering, and just follows along behind the boat like an obedient dog. Since the gear does not disturb the helm at all, there is no need to remove the windvane.

Transmission to mechanical wheel steering

The transmission of steering force to a wheel is somewhat less efficient because the transmission path - from the gear to the wheel and from there via the wheel-steering mechanism to the rudder quadrant and the rudder itself - is longer. Obviously transmission losses are higher, so the effective line travel (25 cm / 10 in) is reduced. Practically every boat over 11 m / 35 feet now has wheel steering. The reason for this is that the main rudder is too large to be managed comfortably by the helmsperson without some kind of step-down gear. That said, many boats have wheel steering simply because it has become fashionable. Wheels take up more of the cockpit, and many boats which have them would be better steered with a tiller.

Modern wheel-steering systems transmit the steering force mechanically to the rudder quadrant through sheathed cables. The average wheel has a diameter of about 60 cm / 2 ft and goes from lock to lock in approximately 2.5 turns. Almost all servo-pendulum system manufacturers design their wheel adaptors around this standard. Wheel adaptors are, with few exceptions, around 16 cm / 6 V2 in in diameter (i.e. around 53 cm/ 21 in in circumference). Given these dimensions it should be clear that even under perfect transmission conditions with no loss at all, a steering line travel of 25 cm / 10 in amounts to slightly less than half a revolution of the wheel.

with no loss at all, a steering line travel of 25 cm / 10 in amounts to slightly less than half a revolution of the wheel.

All wheels are designed assuming the same amount of steering force input by the helmsperson. This means that larger diameter wheels generally go from lock to lock in fewer revolutions, so a wheel adaptor on a large diameter wheel has to exert more force with less line travel.

There are three main alternative arrangements for transmitting force from a servo-pendulum gear to a wheel steering system. The steering lines may be reeved as follows:

• doubled up, i.e. double the line travel for half the force,

• running blocks, i.e. half the line travel for double the force.

These systems all provide adequate steering performance if installed appropriately. Nevertheless none can touch the quality of transmission possible using a tiller with its adjustable attachment point and transmission ratio and much shorter transmission path. Windpilot introduced an infinite transmission force adjustment facility on their Pacific model in 1997, giving it the same range of adjustment as a tiller.

Tip: Running both the steering lines to the wheel adaptor along the same side of the cockpit keeps access free on the other side. The four lines in the cockpit (two leading from the gear and two for connecting these to the wheel adaptor) can easily be mixed up in this configuration, so the pairs which belong together should be clearly marked. It may also be useful to fit 4 snap-shackles as connectors for joining the lines.

Slack and stretch are most easily cured in this arrangement by positioning an additional turning block on one of the lines between two existing blocks and moving it up, down or to one side to tension the steering lines. When the additional block is released, the slack returns and the connectors (snap-shackles) joining the gear to the wheel adaptor can be opened easily.

Steering lines led on one side, connection point with snap-shackles

Steering lines can be tensioned easily using an additional block.

Steering lines led on one side, connection point with snap-shackles

Steering lines can be tensioned easily using an additional block.

The factors highlighted as being detrimental to transmission efficiency with tiller steering also apply to wheel steering. Deficiencies in the boat's own steering system (e.g. stiff rudder, slack, poor transmission) can further reduce efficiency, so not all of the original 25 cm / 10 in steering line travel (see Steering line travel) is ultimately available for turning the main rudder and steering the boat.

The Windpilot Pacific (1998 model) has an infinite transmission force facility.

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How To Have A Perfect Boating Experience

Lets start by identifying what exactly certain boats are. Sometimes the terminology can get lost on beginners, so well look at some of the most common boats and what theyre called. These boats are exactly what the name implies. They are meant to be used for fishing. Most fishing boats are powered by outboard motors, and many also have a trolling motor mounted on the bow. Bass boats can be made of aluminium or fibreglass.

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• renee
How servo pendulum gear works?
12 months ago