Trimtabonauxiliaryrudder systems How they work

The steering impulse from the windvane is passed to a trim tab attached to the trailing edge of the auxiliary rudder. As the trim tab is pivoted out to one side it pushes the trailing edge of the auxiliary rudder to the opposite side. The movement of the auxiliary rudder effects the course correction. The main rudder is fixed in place and used for fine trim in the same way as with a simple auxiliary rudder gear.

Steering impulse

=

wind

Steering force

=

water

Steering element

=

auxiliary rudder

Power leverage (PL)

approx. 20 cm / 8 in

Trim tabs are very small, normally no more than 20% of the area of the auxiliary rudder blade.

There are two advantages to diverting the steering impulse from the windvane via a trim tab on its way to the auxiliary rudder:

a. Since the trim tab it has to turn is very small, the windvane can also be small b. The distance between the trim tab axis and the auxiliary rudder axis generates a servo effect which gives this type of system more steering power than a simple auxiliary rudder gear. This is analogous to the way a small trim tab on the trailing edge of an aeroplane wing is able to turn the whole flap and steer the plane.

Length of lever = servo force:

The separation between the axis of the auxiliary rudder and the axis of the trim tab accounts for the leverage which creates the servo effect. The distance between the two axes is usually about 20 cm / 8 in, so the maximum achievable servo effect with this type of system is relatively small. The servo effect can be enhanced to a certain extent by prebalancing the rudder, but the achievable steering force will never be very large because the trim tab is unable to turn the auxiliary rudder any more than about 10%.

Trim tab operated steering gears represent an important development in the evolution of the windvane steering system. Using the trim tab to amplify the force generated by the vane was the first step towards smaller windvanes and higher steering forces. Today this type of gear is outdated and, as we shall see, windvane steering technology has moved on.

Advantages and disadvantages Advantages:

Smaller windvane but somewhat greater steering force, functions independently of the main rudder; can be used as an emergency rudder. These systems also have all the advantages of simple auxiliary rudder gears.

Disadvantages:

Even larger, bulkier and heavier than simple auxiliary rudder systems. A particular disadvantage of these systems is that they make manoeuvring under engine even more difficult; an auxiliary rudder with a trim tab is more or less impossible to fix in place, so motoring in reverse is no fun at all. It is not easy to fit a yaw-damping device to systems of this type, so most sailors manage without.

Fig 5.5 V vane auxiliary rudder with trim tab system.

An RVG V vane auxiliary rudder with trim tab system, fitted on the 10m/33ft glassfibre Sy moored in Palma de Mallorca

A Mustafa H vane auxiliary rudder with trim tab system - the dinsosaur among the vanes.
V vane trim-tab-on-main-rudder H vane trim-tab-on-main-rudder system, custom-built for a 10m/32 system, Windpilot Pacific custom-ft Olle Enderlein design. built for a Danish Kaskelot.

Installation

Auxiliary-rudder-with-trim-tab gears need to be mounted on the centre of the transom. Some sea conditions can place substantial loads on both the system and the transom and the mounting must be very robust to support the considerable weight of the gear. V vane systems have a relatively large turning radius so H vanes, which are more easily kept clear of mizzen masts, are more suitable for ketch- and yawl-rigged boats.

Trim-tab-on-auxiliary-rudder system manufacturers: V vane: RVG,

H vane: Autohelm, BWS Taurus, Mustafa.

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