Trimtabonmainrudder systems How they work

The trim tab is mounted on the trailing edge of the main rudder, moving it directly.

Steering impulse



Steering force



Steering element


main rudder

Power leverage (PL)


30 - 50 cm /12 - 20 in

Popular in the early days of windvane steering, this configuration did well on boats with long keels and an outboard rudder and was suitable for DIY construction. Bernard Moitessier used the simplest of all trim tab gears on Joshua. The trim tab was connected to the trailing edge of the main rudder and sat on an extension of the vertical windvane shaft.

These systems are prone to oversteering and generally have no yaw damping, so they will only work well on an optimally balanced boat. Trim must be close to perfect so that the boat can be steered with very small movements. Under some circumstances this can mean drastically reducing sail area to enable the steering system to stay roughly on course.

The absence of yaw damping on most gears of this type makes them hard work. As a result even France, once a stronghold of trim tab systems, has seen a move towards modern servo-pendulum gears.

Trim-tab-on-main-rudder designs have many drawbacks: providing them with yaw damping is difficult, the trim tab causes problems with manoeuvring under engine and any kind of mass production is all but impossible because the key parameters vary so much from boat to boat. Virtually every yacht design has a unique main rudder with its own rudder post angle and balance proportions and each one consequently requires a unique trim tab. This system has all but disappeared now.

Trim-tab-on-main-rudder system manufacturers: Atlas, Auto-Steer, Hasler, Saye's Rig, Windpilot

The Saye's Rig is a hybrid pendulum/trim tab system in which the power leverage (PL) is increased by a bracket attached directly to the main rudder.

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How To Have A Perfect Boating Experience

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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