What is a Lower Unit on a Boat?

At its core, boat propulsion is simple. Whether a boat is propelled by a water jet or a propeller, a pressure difference is created, providing the required thrust to move the boat through the water. 

There are a wide range of propulsion systems on modern watercraft, incorporating different technologies and layouts to achieve optimal performance. Inboard, outboard, and sterndrive are the most commonly seen designs.

The sterndrive, also known as an inboard/outboard, incorporates elements of both inboard drives and outboard drives, and relies on a component called the lower unit to transfer the engine’s power into the thrust needed for the boat to move. 

Like any component, the lower unit requires upkeep and maintenance to avoid issues. In this article, we will dive into what a lower unit is, how it works, and some maintenance tips to keep your boat issue-free all summer. 

What is a Lower Unit?

The lower unit (sometimes called a gear case) is not limited to boats with a sterndrive engine setup. Outboard engines have a lower unit as well, which is part of the entire outboard engine assembly. When looking at an outboard engine, the long extension with the propeller on the end is considered the lower unit. 

Graphic displaying the engine and lower unit layout on a sterndrive boat

Sterndrive boats, which utilize a larger engine onboard, rely on a separate piece of machinery called an outdrive to power the propeller. If you look at a stern drive boat out of the water, you will see something similar to an outboard engine hanging off the back. the outdrive connects to the engine, which is located on the boat. In a sterndrive application, the lower unit is at the bottom of the outdrive, just behind the propeller.

What Does a Lower Unit Do On a Boat?

The lower unit allows for power transfer from the engine to the propeller, similar to a transmission on a car. A major difference here though is the lack of differing gear ratios. Typically, boats have a single forward gear, single reverse gear, and neutral. The outdrive also pivots in response to steering inputs, and the lower unit usually has a rudder below the propeller, allowing the boat to steer.

Close up of dual boat propellers

The outdrive receives power from the engine via an input shaft where the unit connects to the boat. Power is sent through the top of the outdrive and down to the lower unit through gears and drive shafts, eventually turning the propeller.

On some boats, the lower unit supplies the engine with cold water to keep it at a safe operating temperature. An impeller draws in water from the lake or river, pumping it through the engine (in the same way coolant is cycled through the engine of a car).

Lower Unit Common Problems

Keeping up on regular maintenance and following proper winterization steps will help prevent lower unit damage, but issues can arise for even the most diligent boat owners.

Water in the Gear Case

If the oil in your lower unit appears milky, water has made it into the gear case. Usually, a bad seal is to blame. Water can wreak havoc on the internals of a lower unit if it’s present when the temperature gets below freezing since it will expand as it freezes.

Impact Damage

The lower unit typically sits below the bottom (keel) of a boat, increasing the chance of hitting hazards in shallow water. A low-speed impact can result in damage to the propeller, while a more severe collision can damage or destroy the lower unit

Worn Out Impeller

On some sterndrive boats, the impeller is housed in the lower unit. The impeller pumps cold water through the engine, keeping it cool while running. Since the impeller is made out of rubber it will eventually wear out and fail, potentially leading to severe engine damage. You can read our article on impeller replacements here.

Lower Unit Maintenance

The lower unit, like any mechanical component, requires regular maintenance to work right. With only one drive gear, the rotating mechanism inside the lower unit is subjected to lots of stress, and the lubricating oil inside needs to be changed regularly. 

At the end of every boating season, you have the lower unit gear oil changed and add new grease to all grease fittings. 

If you dock your boat on the water during the summer, you may have accumulated buildup on the lower unit. Winterization is the perfect time to remove buildup and clean the lower unit. 

Lower Unit Services at Matson Point S

Whether you are experiencing problems with the lower unit on your boat or need help with winterization, the expert marine technicians at Matson Point S in Riverton Utah are here to help. Give us a call or schedule an appointment online today!

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