What Is an Outboard’s Lower Unit & How It Works

As the name implies, the outboard motor’s lower unit is the bottom part of an outboard motor. It transforms the power from the engine to the rotation of the propellers.

Without an outboard lower unit, you can’t go anywhere.

So it’s important to understand the parts of an outboard lower unit and how they work.

Typically, an outboard lower unit includes the driveshaft, water pump, propeller, shift shaft, gearcase, and skeg.

If the outboard is intended for saltwater applications, you will also see anode on its lower unit, which is used to protect the engine from corrosion.

Here is a diagram that shows the different parts of an outboard lower unit:

Editor’s Note: The above-mentioned parts of an outboard’s lower unit are commonly found on combustion engines regardless of the brands (including Honda outboards, Yamaha, Johnson, Mercury, etc). The modern electric outboard motors feature a much more simplified lower unit.

Basically, the outboard lower unit is like a gearbox with different components, responsible for the power transmission. All the power of the outboard motors is sent to the lower unit and transferred to the prop shaft and the propeller.

All parts of the outboard lower unit work together to make your boat move. Here is how each component works:

• The drive shaft transmits power from the engine (or powerhead) to the gearcase. It’s also important in engine cooling through its synergy with the water pump.
• The water pump is attached to the drive shaft and gets power from the rotation of the drive shaft. It circulates water through the internal cooling system to keep the engine cool.
• The shift shaft is responsible for transmitting motions to put the outboard into or out of gear, enabling you to select different gears (forward, neutral, reverse) and control speed. For example, if you twist the shaft counterclockwise, it puts our engine in forward gear.
• The gearcase is the core part of the lower unit which contains pinion gear, dog clutch, forward gear, and reverse gear. It transfers vertical rotational power from the engine to horizontal rotational power to turn the propeller or rotor.
• The skeg helps to keep your boat moving straight and turn your boat accurately. It also protects the propeller from underwater obstructions.

Editor’s Note: Mercury, Yamaha, and other combustion outboard lower units all work in a similar way as illustrated above. However, the electric outboard lower unit works differently.

The outboard lower unit is one of the most easily damaged parts of a boat motor, especially if you are using a gasoline/petrol outboard and travel shallow rocky rivers a lot.

So it’s important to maintain the outboard’s lower unit regularly to prevent damage and increase its life span.

Here is how:

#1. Service Your Outboard Regularly

You need to follow the maintenance guidelines of your outboard motor (provided by the manufacturer). For example, change the gear oil in your outboard motor lower unit regularly (every 100 hours or once a year) and lubricate the gears and bearings properly.

#2. Check for Signs of Wear & Damage

It’s frustrating to see damages on any part of your boat, and when it comes to the outboard lower unit, it’s a real pain.

There can be all kinds of boat lower unit problems, such as water in the gear lube, damaged water pump impeller, leaking seals, and bent propeller. If you want to get the very best of your outboard, you need to deal with them right away!

So how do you know if your lower unit is bad on your outboard?

Here are some typical signs and symptoms that indicate your outboard’s lower unit is damaged:

Signal #1. Discoloration of Gear Lube

If you see a foamy or cloudy fluid, metallic particles in the tank, a foul odor, or milky liquid on the dipstick, it’s highly possible that water enters the gear lube (perhaps due to a faulty water seal, a leaking drain plug, or a stuck water drain valve).

Signal #2. Strange Noise from the Outboard Lower Unit

If you hear whining, whirring, or grinding noise coming from the lower unit, it’s very likely that your water pump impeller is worn out or damaged.

Signal #3. Leaking Gear Oil or Other Fluids

If you notice an oil slick in the water or wet spots near the lower unit (or an unusual increase in fuel consumption), it signals that the lower unit’s seals or gaskets are leaking and need to be repaired or replaced immediately.

Signal #4. Bent or Damaged Propeller

Another sure sign that your outboard’s lower unit is bad is the bent or damaged propeller, which can be caused by common wear and tear, vibrations from the lower unit or hitting underwater obstacles like rocks and reefs.

Signal #5. Reduced Power or Performance

If you find your motor isn’t as powerful as before or even locks up, there can be problems with your outboard’s lower unit (such as a damaged shift shaft or broken propeller).

All the failure symptoms above indicate that you need to take your outboard to a professional and fix the problems as soon as possible.

Here we also collected some hottest questions about outboard lower units. Continue reading to find quick answers.

1. How much does it cost to replace a lower unit on a boat?

The outboard lower unit price varies between different models, brands, and labor required. Generally, a used outboard lower unit goes for $800-1000, while a new replacement can cost around $1100 or higher (taking a 150hp Yamaha outboard as an example).

If you are looking for an outboard lower unit replacement, you’d better have a mechanic look at it for professional advice. If you are mechanically inclined, you can also DIY an outboard lower unit and rebuild it on your own to save money.

2. How to change lower unit oil?