Damping

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One of the first lessons of helming a boat is to steer as little as possible. Vigorous use of the tiller or wheel to correct the course tends to be ineffective because the boat always turns too far, immediately necessitating another course correction in the opposite direction and leaving a snake wake trailing astern.

An experienced helmsperson, with greater awareness of the behaviour of the boat, keeps steering movements to a minimum, following one of two mental 'steering programs':

1. He or she tries to steer the optimum upwind course or, given another point of sail, precisely hold a desired compass course. A picture of concentration, our experienced helm studies the wind direction indicator, the sails or the compass closely, giving almost continuous small, occasionally larger, steering impulses to keep yawing and course deviation as small as possible;

2. he or she prefers a more relaxed attitude at the helm, correcting the course rarely and with small movements; the course varies over a greater range of angles.

How a boat responds to the helm is determined chiefly by design; a long keel boat will always be more sluggish than one with a fin keel and a balanced rudder.

Trim tab to main rudder proportions: this system type can make reversing under power awkward.

Experienced helms develop an internal 'damping program' which ensures that, almost without having to think, they are sparing in their use of the rudder. Rudder movements not only turn the boat, they also brake it, so minimising them preserves boat speed as well.

A windvane steering system lacks the wisdom of experience and, unless damped, will always turn the rudder too hard, too far and for too long, i.e. oversteer.

Damping must therefore be designed into system to replace its clumsiness with the gift of delicate steering and enable it to equal or even exceed the steering performance of our experienced helm. This can be done.

Principle 1: More damping equals better steering (although obviously not to the point where the system is so well damped that it does not move at all). Conceiving and building a system which properly balances damping and steering is the toughest challenge before any windvane steering designer. Systems must be powerful but must deliver their power in a controlled way.

Principle 2: The less damping there is built into the system, the more additional measures the helmsperson will have to take to offset this steering deficit and cajole the system up to a level where it can cope with a particular boat. This entails not only maintaining perfect sail trim but also reducing canvas early to cut the steering demands placed on the windvane gear. Poorly damped systems make particularly hard work of reaching and downwind courses and often surrender full control to the elements.

Principle 3: With no damping at all, self-steering is only possible if sail trim and sail area are so perfectly set that the boat steers straight ahead entirely of its own accord. Of course if your boat tracks along a straight line all on its own you might as well jettison the windvane gear altogether. Completely undamped systems can steer properly at just a few specific wind angles and are only really suitable as an aid to steering.

A well-balanced windvane gear will always put up the most satisfactory steering performance; it is best equipped to steer the boat under all sailing and weather conditions. Indeed, a good gear of this nature inevitably steers better than even an alert helmsperson because the continuous damping of all rudder movements keeps yawing angles permanently small and with a windvane, optimum heading with respect to the wind is guaranteed all the time. Such a gear can be rated as providing effective steering.

The term 'effective steering' is used to indicate the range of a particular windvane steering system. What use is a gear which can manage only 70% of given conditions or courses if it always retires precisely when manual steering appeals least, i.e. in heavy weather!

Squeezing satisfactory steering performance from a poorly equipped windvane gear means extra work for the crew. Eventually it makes more sense to steer by hand than keep running round the boat tweaking everything to prop up the gear.

Damping can be provided:

Damping at the windvane V vane:

A V vane rotating about a vertical axis (weathervane principle) is deflected very little by the wind, at most by the amount in degrees of the deviation from course, and there is almost always wind flowing along both sides of the vane. This gives a high level of damping.

H vane:

An H vane rotating about a horizontal axis can be subject to extreme deflection by the wind, in some cases through as much as 90 degrees, whereupon it hits its lateral end stops. The wind acts on one side of the vane only, and the amount of deflection is determined by wind strength rather than wind angle. The result is poor damping, since the windvane only begins to return to its centred position once the boat is back on course and the wind can reach the lee side of the vane and push it back upright. The vane therefore continues the steering impulse for too long, i.e. is damped too late. Inclining the horizontal axis, i.e. bringing it closer to the vertical axis, reduces the sensitivity of the system; the magnitude of the course correction signal declines because flow reaches the lee side earlier and more quickly, slowing the deflection of the vane.

The indispensable contribution of Marcel Gianoli, one of the pioneers already mentioned, to the development of windvane steering was to identify 20 degrees as the optimum angle of inclination for the horizontal axis.

Characteristics of the three types of windvane

Force Travel

Position in the wind

Space requirement/ turning radius Sensitivity Damping

H vane

Large Large Unsettled

Large

High Slight

V vane small small stable large low great

H vane, 20 degrees moderate moderate moderate moderate moderate moderate

Fig 4.1 An H vane rotating about a precisely horizontal axis can be deflected up to 90° to one side before wind reaches the other side of the vane to slow or damp the lateral movement. The steering impulse is too strong. A V vane rotating about a vertical axis can be deflected by no more than 10°, ie the amount in degrees of the deviation from course. The steering signal is too weak.

An H vane rotating about an axis inclined by 20° strikes the optimum balance between good steering and good damping.

Virtually all H vane gears make use of this feature. There are two principal categories: 1. H vanes which are fixed in this 20° position (Atoms, Fleming, Monitor, Mustafa, Navik, Cap Horn, Sailomat); and

2. H vanes which are freely adjustable, permitting them to be matched to different wind strengths in the interests of better steering, e.g. more upright in lighter winds, more steeply inclined in heavier winds (Aries, BWS, Hydrovane, Windpilot Pacific). Adjusting the angle of the windvane changes the leverage available to the wind, so an upright vane gives a stronger signal for light air on account of the longer lever, and an aft inclined vane gives a weaker signal on account of the shorter lever. The leverage exercised by the windvane declines as it is inclined aft.

Damping at the linkage

The steering impulse from the windvane is translated by a gear or rod linkage into a lateral rotation of the rudder.

Damping or manual adjustment:

1 Auxiliary rudder with V vane

No additional measures necessary since the inherent damping of a V vane is sufficient. Steering force can thus be transmitted via spur gears or toothed wheels in a 1:1 ratio (Windpilot Atlantik/C aribik).

2 Auxiliary rudder with H vane

Essential, as the rudder angle set by the windvane is a function of wind strength rather than wind angle and can thus become excessive, producing oversteering, in heavy air. The rudder angle can be changed and reduced manually at the gearbox to cut back the power of the H vane (Hydrovane).

3 Trim tab

Desirable, but complicated by the need to transmit the signals to an additional, remote shaft (trim tab shaft). The restoring forces produced by the auxiliary or main rudder to which the trim tab attaches usually furnish adequate damping. Manual adjustment of windvane to push rod signal transmission makes setting up the system easier (BWS).

4 Servo-pendulum gear

(see Yaw damping, Chapter 5)

Elaborate damping using a bevel gear in a step-down ratio of 2:1. This kind of damping is designated automatic damping because every steering impulse causes the pendulum arm to swing out in a precisely defined manner, the pendulum rudder simultaneously being brought back parallel to centreline (Aries, Monitor, Fleming, Windpilot Pacific). There are four main approaches to servo-pendulum gear design in this respect.

• Bevel gear - segment gear which only needs to cover a fairly limited range of operation between the two steering line guide tubes, mounted on both sides at the bottom of the gear, which restrict the lateral travel of the pendulum arm and make it impossible to raise (Aries, Monitor, Fleming).

• A 360° degree bevel gear - complete gear in which the gearwheels mesh over a pivoting range of 270 degrees, allowing the pendulum arm to be raised up laterally out of the water (Windpilot Pacific). 2:1 bevel gear linkages are now standard with all the major servo-pendulum system manufacturers (Aries, Monitor, Windpilot Pacific). The 2:1 transmission doubles the force of the steering impulse from the windvane while halving the lateral travel of the pendulum arm.

• Systems using other mechanical arrangements to control the movement of the pendulum rudder (Cap Horn, ATOMS).

• Systems in which the linkage performs no damping function.

5. Double rudder systems

These rely on the damping of the servo-pendulum system they incorporate. The categories are:

• Servo-pendulum systems with automatic bevel gear yaw damping and angling of the pendulum rudder shaft aft by 10°, in combination with the inherent damping of the auxiliary rudder (Windpilot Pacific Plus);

• Servo-pendulum systems damped by angling the pendulum rudder shaft aft 34° , combined with the natural controlling influence of the auxiliary rudder (Steger/Sailomat 3040).

Damping at the rudder

1 Auxiliary rudders steer directly and are reset/damped by the pressure of the water flowing past.

2 Pendulum rudder:

Angling the pendulum rudder shaft aft achieves a damping effect in the water similar to that of an H vane in air. Given this angled shaft, the pendulum rudder can only swing out a certain distance before the force of the water starts to push it back in. The alternatives are:

• vertical shaft and bevel gear linkage (Aries, Monitor, Fleming);

• shaft angled aft by 34° for damping, no bevel gear linkage. These systems require manual adjustment of windvane to push rod signal transmission characteristics to properly set the proportional relationship between the H vane steering impulse and the lateral pendulum rudder movement (Sailomat 601);

• bevel gear linkage, shaft angled aft by 10° (Windpilot Pacific).

3 Double rudder:

see previous section

A windvane gear with properly balanced damping properties will always turn the rudder by exactly the amount required, preventing oversteering. The feedback between the position of the rudder and that of the windvane ensures the steering pressure is only increased until the vane signals that the boat has begun to react and return to course. As the vane begins to move back towards its upright position the pendulum rudder reduces the steering force on the main rudder and returns to the centre.

This may seem rather complicated on paper, but fortunately it is not necessary to understand the science to appreciate the perfect steering a well-damped windvane gear will bring to your boat. Such a gear will also be very insistent in its criticism of your sail trim - if it never seems to be centred and is always working off to one side, you can be sure that something needs attention.

Every crew will realise sooner or later that it pays to act on these hints: correcting sail trim or adjusting the main rudder to relieve the pendulum rudder not only appeases the gear, it also improves boat speed. Systems with bevel gear linkages pull on the main rudder with gradually increasing force until feedback via the windvane returns the pendulum arm to its central position; oversteering is impossible.

A less well damped windvane steering system demands an attentive crew, particularly in changeable or worsening wind conditions. The steering system will have to be helped along by reefing early and reducing motion (staysail). Operating a system of this kind is taxing, especially for those with only limited knowledge of the processes at work in a servo-dynamic steering system. Such systems do not provide effective steering.

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