Lower Unit

The Lower Unit is the lower half of an outdrive or outboard, containing the drive gears, propeller shaft, and propeller. When all of the parts are working correctly, all motor power is sent to the lower unit. The lower unit has to be big enough to hold the gears, and small enough to present the least resistance possible. If the gears are too small, they break easily, yet if they are too big, pushing the oversized lower unit through the water takes energy away from the forward motion of the boat.

Clutch Dog

A clutch dog is a type of clutch that couples two rotating shafts or other rotating components not by friction but by interference. The two parts of the clutch are designed in such a way that one will push the other, causing both to rotate at the same speed and will never slip. Clutch dogs are used where slip is undesirable and/or the clutch is not used to control torque. Due to the fact that slippage is not an issue, clutch dogs are not affected by wear in the same way that friction clutches are. The clutch dog is a stout, hollow, sliding cylinder attached to the propeller shaft, with teeth cut into each face for engaging the adjacent gears. The sterndrive is in neutral when the clutch dog is between the nearby gears. The drive engages forward or reverse by sliding the clutch dog into the appropriate gear that turns with the engine.

Gears

A gear is a toothed wheel that engages another toothed mechanism in order to change the speed or direction of transmitted motion. A wheelwork is a connected set of rotating gears by which force is transmitted or motion or torque is changed. Gears that spin freely on the shaft are adjacent to the clutch dog on the propeller shaft, which coincide with a third gear that is driven by the engine. These gears are always turning in opposite directions while the engine is running.

The engine turns the drive shaft, which spins in the opposite direction as the prop shaft. The pinion gear on the end of the drive shaft drives both the forward and reverse gears at the same time. Inside the forward and reverse gears is the prop shaft which has four large “dogs”. When the shift lever is moved, the dogs connect with either the forward or reverse gears. Only one of them can engage the drive shaft at a time. The pinion gear is smaller, with fewer teeth. It turns 2.5 times for every revolution of the forward and reverse gears. The prop and prop shaft are therefore turning slower, but with more power (torque) than the engine.

Shafts

There are two shafts in a lower unit, the drive shaft and the prop shaft, with the drive shaft running from the bottom of the crankshaft to the lower unit. The prop shaft will bend on impact, and, once bent, the shaft and the prop will vibrate due to the prop not turning in a perfect circle. If this continues, seals wear out from the vibration, water enters the lower unit, and the bearings excessively wear. If vibration is allowed to continue, replacing the bent shaft will still allow some vibration because the bearings will be worn.

Propellers

Propellers come in different sizes and pitches, with the size of the motor determining the diameter of the prop, and the size and load on the boat determining the pitch. The load determines the engine’s speed -- if the load is too large, the rpm is too low and there is great stress on the motor; if the load is too light, the rpm is too high, and the motor will self-destruct from the inertia of the piston as it goes up and down. If the propeller doesn’t have enough pitch, the engine will turn too fast. Balancing the propeller to the load and controlling the engine’s speed is critical for the life of the engine.

Trim & Tilt

Many outboards and most inboard/outboards (I/Os) come equipped with power tilt and trim which raises or lowers the drive unit. The "trim" part of the system is used to adjust the angle of the engine, and is usually accomplished using two hydraulic pistons set into the engine bracket. The trimming movement raises and lowers the bow, and it can also effect steering and performance. There is also a single hydraulic rod connected higher on the engine, which is the "tilt" part of the system that is used to tilt the engine above the water. Most outboard engines 30 hp and below do not have power tilt and trim, rather, they usually have 5 holes into which a pin can be inserted, varying the angle of the engine relative to the transom of the boat.

Trim

Trimming the engine will adjust the angle at which the boat rides through the water. In smooth water, a GPS makes finding the best angle easy. Just trim the engine all the way down, set the power to your normal cruise setting, and start trimming up slowly in small increments. The speed will increase as the hull finds the most efficient trim angle, and when you have gone too far, it will start to decrease again. The three positions of trim are trimming in (down), neutral trimming and trimming out (up). Trimming in lowers the bow and results in quicker planing, neutral trimming results in greater efficiency, and trimming out lifts the bow and increases the top speed, clearance in shallow waters and the steering torque. The proper trim angle for outboard engines depends on the speed and the conditions.

Tilt

If the engine is tilted too high, the bow of the boat will ride too high and the stern will "squat" in the water. When riding with large following seas, tilt the engine higher than you ordinarily would, and run the boat just barely on plane -- the bow will stay safely above the waves. If the engine is not tilted high enough, it will push the bow of the boat too far down, making it plow along. For cruising in smooth water, it is slow and inefficient, and the engine should be tilted further out for proper trim. Most boats will ride more comfortably if they are cutting through waves with the sharp part of the hull facing forward, rather than bouncing along the top, hitting the waves with the middle of the hull.