[omc-boats] Undersatnding steering mechanics

Hi,

OMC electric stringers use some innovative ideas that permit a full 75 degrees of tilt, while permitting a full 90-degrees of movement around the vertical axis. Try to match those specs in any MerCruiser ever built!

When trying to talk about maintenance on these systems it is probably a good idea to separate the boat-mounted steering components from the actual sterndrive steering systems. Chasing down phantom problems in one area without checking the other area could lead to frustration.

All Evinrude/Johnson/OMC factory-built boats used the aircraft-inspired rope-cable-drum system. A carry over from the low-power outboard days. It was possible to use such a system on the stringer sterndrive because of the relatively large vertical "spread" between the upper gearbox steering pivot bearing and the lower steering pivot bearing mounted below the water pump. The upper bearing is lubricated by the upper gearbox oil, but the lower bearing needs proper lubrication (early models were self lubricating and protected by o-ring seals). You also want to check the steering shaft gears and worm gear drive to determine that there's no binding in the stern drive itself before looking at the boat-mounted steering mechanics.

The question that was brought up concerned a third-party OEM boat that used TruCourse steering (never used on E/J/O boats). TruCourse used a worm gear and a sealed push-pull cable system, and eliminated the exposed rope/pulley/drum system. Both ends of the TruCourse cable require proper tension adjustment and originally had plastic covers to keep pout the water, dirt and silt, but often these covers get discarded by mechanics in a hurry to finish the job. Again, before trouble shooting anything make certain the stern drive bearings and worm-gear system are working properly.

Another mechanical system often seen on electric stringers in OEM boats is the mechanical rack system which operates an exposed, thru-the transom tiller arm, which physically bolts to the side of the sterndrive cover. The rack takes up considerable amount of behind-the-dash real estate. But this system has its built-in mechanical advantage and does not require the sterndrive's internal steering worm and worm wheel gear reduction to steer the leg.

The final steering system item that often gets overlooked is the adjustable trim tab-exhaust port on the lower case, directly aft of the prop. The tab is designed counteract the engine torque while underway. The tab angle is user adjustable to achieve a neutral "on-center" steering effort, where the boat should track straight. If mis-adjusted the steering would be noticeable more difficult due to increased drag in one direction.

Lee Shuster
Salt Lake City

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Received on Wednesday, 25 October 2006