The Different Sections Of An Outboard & How They Work


When you are getting into the world of boating for the first time. There are many different terms that you will hear. Things like outboards, lower units, powerheads, and other sections that make up an outboard engine.

What Are The Different Sections Of An Outboard & How Do They Work? Outboard engines consist of three major sections. The top half is made up of the engine powerhead with all of it’s components. The mid-section which is in the middle of the engine. Then the lower unit that is located below the mid-section.

Here are all of the different parts that make up these main sections of the engine as well as the parts in them and how they work.

The Top Section

How A Combustion Engine Operates

Outboard Motor BasicsThe top section of an outboard engine is made up of many different components which makes the engine run. We will need to understand a little bit about a combustion engine in order to get a grasp of what these components do.

Without going into a huge lecture on how 2-Strokes and 4-Strokes operate. We’ll just give a quick, simple description.

A combustion engine has pistons that move up and down. This up and down motion spins a driveshaft that has something attached to it. In a car, it eventually turns the wheels on the car, moving it forward.

On a boat, with an outboard, it spins the propeller which moves the boat either forward or backward. These pistons use “combustion” to move up and down. Meaning they require an explosion for them to continue in motion.

The engine takes in an explosive substance, being fuel, and then sets the fuel on fire. When the fuel explodes, it pushes that piston down.

To keep this up and down motion. The engine will have different cylinders, each cylinder having it’s own piston.

Let’s say you have four cylinders with each cylinder having it’s own piston inside of it. The engine keeps this up and down motion by placing the pistons opposite from each other. One piston will be all the way up, then the next piston in the cylinder next to it will be in the fully down position.

So, as one piston is sent down by the explosion, it pushes the piston next to it up! Thus as each one is pushed down, they force the others up, and the motion will continue up and down, up and down until the fuel stops coming in!

The Different Systems of the Powerhead

Parts of an OutboardThe cylinders with their pistons are the mechanical parts that make up the powerhead. We won’t go into the powerhead break down here, but for more information on the powerhead. We recommend reading this article we wrote here on What Is An Outboard Powerhead!

The powerhead has three primary systems which operate the engine. These can get pretty complex and confusing to someone that does not have a mechanical background, or that has never worked as a mechanic. These three systems are the fuel, the electrical, and the air systems.

These systems all work together to provide the engine with the proper fuel and air mixture. Along with the spark which creates an explosion of that mixture to move those pistons up and down.

There are a lot of moving parts and a lot of action going on with an engine. With an outboard, most of that action is here on the powerhead, and down on the lower unit that we will cover in a little bit.

The Fuel System

The fuel system components are laid out around the engine.  They consist of a couple of different items in order to feed the fuel to the engine.

There is a low-pressure fuel pump that is used to suck fuel from the tank. Which is in the boat, and then brings it up to the engines fuel delivery system. The delivery system will vary depending on if you have a fuel injected engine or a carbureted engine.

There are more components outside of the engine that allows the fuel to get to the engine. We have already covered that pretty extensively here in this article about Why An Outboard Loses Prime & How To Fix It.

Once the fuel is delivered from the tank to the carburetors or either the VST or FSM. (Check this out for a break down of the components inside of an Outboard FSM or VST.)

The fuel is then mixed with the air in the carburetors and sent into the engine. To provide that mixture to the cylinders for that explosion to occur and move the pistons up and down.

If it is delivered to the FSM or VST, then it gets pressurized inside of the VST. After being pressurized, it is then sent to a fuel rail which then sends that pressurized fuel through tiny little holes in a fuel injector. Which creates the mixture due to the pressure and allows the cylinder to set the mixture on fire!

The Air System

Outboard Engine PartsWith fuel delivered to the engine, there is another element that is just as important as the fuel. Which is the air. If there is too much fuel, then it will not explode, if there is not enough fuel, it will also not explode.

There needs to be the correct mixture between the two components. There is a specific measurement that gives the perfect mixture of these two elements. That ratio is called the stoichiometric ratio which is 14.7 parts air to 1 part fuel, or 14.7:1.

The name comes from stoichiometry, which is two Greek words. Stoicheion means element and Metron means to measure. It is just to say that there is a chemical measurement and reaction of two chemicals! For a combustible engine, it is 14.7:1.

So the components that create this for the air side consist of an air intake. Then through some routings of tubes that funnel the air to a throttle body of some sort. This throttle body will open and close, changing the amount of air that is introduced into the system.

The Electrical Systems

Which leaves us with the last element, the spark! The spark is what starts the explosion of the fuel and air mixture. Giving the engine the up and down motion of the pistons, creating the power for the engine!

There are a lot of different sections and systems when it comes to the electronic portion of a combustion engine.

We could spend all day and 50,000 words to describe the depths of an electrical system on an outboard engine that they are now producing these days,

but instead, we will stick with two systems!

The ignition and the starting! The starting system is just the system that supplies power to the starter for it to start turning the engine’s components up and down inside of the powerhead. That allows the fuel, air, and ignition systems to kick off!

The ignition system consists of four major components; of course, there are a lot more to it, but for a simplified description. We will stick with the four! The Stator or Alternator, the trigger or pulser, the rectifier regulator, and the ignition coil.

The stator or alternator is what produces the AC electricity which is then sent through a rectifier regulator. This part then changes that AC electricity to DC voltage, that can be stored used for the spark as well as be stored in a battery for lights and other electronics to use on the boat.

That DC voltage is then sent to the ignition coil where it is amped up to a much higher amount of electricity. This higher amount will be able to “jump the gap” of the spark plug. Meaning that electricity will be passing from one side of the spark plug to the other side. Creating a “spark!”

The trigger or pulser is what tells that ignition coil to let that electricity go to the spark plug. Making sure that the fuel-air mixture in the cylinder is set on fire at exactly the right moment to push that piston back down!

Of course, these days all of these systems have been computerized and are all controlled by the computer. We don’t need to get into that confusing discussion here though!

The Mid-Section

Outboard Engine MidsectionThat wraps up the main components that can be found on the powerhead and the systems that make up the powerhead. The powerhead is then set down onto what is called the midsection. Which also has a lot of parts to it, but there is not a lot of moving parts like on the top section!

The mid-section of an outboard consists of a bracket that houses the power tilt and trim or trim unit, the driveshaft housing, oil pan on 4-strokes, the water tube, and the exhaust.

The bracket is what holds the entire outboard to the transom of the boat. Which also makes up the mechanism that allows the engine to turn left and right to give you the ability to steer the boat around.

Inside of the bracket, there is a trim unit. On smaller outboards most of the time there is not a power trim and tilt unit. There is just a tilt mechanism that allows you to lift the engine out of the water. On new models, there is usually a gas assisted trimming mechanism.

Then on the larger outboards, the bracket has the power tilt and trim unit. Which is an electric motor that runs a hydraulic pump that lifts the engine up and down by the press of a button. Making it super easy to get the engine out of the water!

Getting the engine out of the water for storage and other reasons is excellent, especially if you are keeping the boat in the water and in saltwater! Read this here to learn more about Leaving an Outboard Engine Up or Down and Some Storage Tips.

The Driveshaft Housing

Outboard Motor Break downInside of the driveshaft housing, there are a few essential parts held. The oil pan inside of a 4-stroke engine is located inside of the driveshaft housing.

This is also where the exhaust is routed to get the exhaust from the engine down to the lower unit which makes the engine run a lot quieter! Simply by putting the exhaust out under the water!

Also inside of the driveshaft housing, there is the water tube that comes up from the lower unit. The lower unit takes in water, which we will touch on next, and then is routed up to the powerhead through a water tube that allows the cooling water to get up into the powerhead!

The driveshaft housing also houses the driveshaft! Which is the component that allows the turning of the engine’s pistons to transfer that circular motion down to the lower unit.

Where the propeller is then turned to move the water and push the boat forward.

That basically sums up the mid-section in a short summary. It connects the powerhead to the lower unit and bolts the engine onto the transom!

The Lower Unit

Now the lower unit is just as complex as the power head as far as moving parts. It has a pretty simple design, but if it is not put together correctly, it can be problematic quickly!

Most lower units that come from the factory are good to go out of the box. They need to be topped off with gear lube and sometimes have a water pump put on them. Besides that, they do not need to be disassembled.

Now if you find that you need to disassemble a lower unit and rebuild it, this can become a pretty big job! You will know more about what is going on inside of the lower unit by the color of the gear lube. Here is more on What Color Should Lower Unit Gear Lube Be?

The inside of the lower unit has two main components that then have gears and bearings on them. You have the driveshaft and the prop shaft.

Lower Unit Driveshaft

The driveshaft is also what runs the impeller. It’s main purpose though is to transfer that circular motion of the engine down to what is called a pinion gear. This is a gear that is held onto the end of the driveshaft.

As the driveshaft spins, it spins that gear which is connected to the gears on the prop shaft.

There is a driveshaft bearing carrier that houses the bearings that hold the driveshaft in place and allow it to spin freely. The water pump housing is set on the top of the lower unit and has the driveshaft running through it.

This is where the water pump impeller is held and is stuck onto the driveshaft. As it spins, the impeller spins around and sucks water up from the water pickups on the gear case. It then forces the water up the water tube that is in the midsection and into the powerhead.

Cooling down the engine and allowing it to run at the correct operating temperature!

Lower Unit Prop shaft

The prop shaft is the more complex part in the gear case. This has a forward gear and a reverse gear that has what is called a clutch dog between the two gears.

This clutch dog is what moves when you shift the engine into forward or reverse. When you shift, it moves the shift shaft that moves the clutch dog into whichever gear that you shifted into. When the clutch dog moves, it engages the correct gear and then turns the prop shaft in that direction.

The gear is receiving it’s motion from the pinion gear that is on the bottom of the driveshaft housing.

Each gear whether it is forward or reverse has a bearing behind it that allows the gear to spin freely. Once the clutch dog is engaged into that gear, the prop shaft will spin!

On the end of the prop shaft, there are splines that the propeller fits into, and that is what allows the propeller to spin in whichever direction the prop shaft is spinning!

The Basics of An Outboard Engine

These are the basic components and sections of an outboard engine. Of course, this is not a super in-depth description of all of the components.

Hopefully, it does give you a better understanding of an outboard engine and the different main sections that make up the engine.

Things are so advanced these days with all of the computer controlled engines and the electrical parts. There are sensors for everything, and you almost have to have a laptop with the correct diagnostic software in order to work on them.

They have come along way though and have revolutionized the boating industry. There used to only mainly be inboards when it came to boating and when the outboard came out it was not the most reliable engine!

So deciding on what to buy was a real no brainer because the inboard was so much better. These days that is not the case. If you would like to know which one would be best for you, we wrote an in-depth article on Inboard Vs. Outboard Motor: What’s Right For You?

Check Us Out!

We’d like to invite you to come and check us out on our Youtube Channel: Born Again Boating! We cover all kinds of helpful boating tips and outboard service tricks!

We hope you will get subscribed to the channel and add us to your favorites so that we can become your go-to resource when it comes to all of your DIY boating projects!

Aaron Hilligoss

Aaron has been working in the Marine Industry for over a decade and holds certifications for Yamaha and Mercury Marine. It is not uncommon for him to own and be working on at least three different boats at any given point in time!

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