Difference Between Inboard and Outboard Boat Motors


I never knew there was a difference between outboard and inboard motors until I began to familiarize myself with boats. I took an interest and decided to share what I’ve learned.

What is the difference between the inboard and outboard motors? An inboard motor is placed on the inside of the boat, generally at the hull. If it’s a gas (or petrol) inboard, it has combustible engines with cylinders that will fill with fuel and air. Outboard motors are placed outside the boat, generally at the stern, or back, of the boat.

They both function differently with the different purposes they are meant for. There are pros and cons to each and the pricing is vastly different. Each one has its better use options.

Functional/Technical Difference

Just as each motor is placed in different positions on the boat, they are also different in their function. Each one is made to function for certain boats and those boats are all used for different types of activities.

Inboard Motors

Inboard motors sit inside a compartment for the engine under the deck, so they don’t stick out like outboard motors. They actually work in a similar way to car engines. If it’s a gas or petrol inboard, it has combustible engines with cylinders that will fill with fuel and air.

This mixture ignites with a spark and creates an explosion of sorts, or what is called combustion. There is a piston connected to a drive shaft, or crankshaft, and gets pushed down from the combustion.

This turns the shaft part of the way. Multiple pistons that are pushed down at intervals create the spinning motion in the drive shaft.

Propellers are typically placed under the hull of the boat and are connected to the shaft. When the drive shaft starts to spin, the propeller will also start to spin.

“An inboard engine works just like a car engine in terms of how it creates power and delivers drive to the boat.”

Steve Smith

The faster the drive shaft goes, the faster the propeller goes as well, so the faster an engine explodes or combusts, the faster a propeller will move.

An inboard motor cools by using the water around it. The water is pumped into the inboard from the outside by means of a hose, a hole in the stern, or another method, depending on the exact type of inboard boat you have.

This water cools the engine and is then pushed out of the system with the exhaust from the engine.

All of this gives an inboard motor a lower center of gravity, which can help in rough seas. The inboard motor has less of a noise issue because it is tucked inside the boat rather than loudly working on the outside.

They also typically have more horsepower because they can be larger and are more evenly distributed in weight than outboard motors.

Outboard Motors

There is a fuel tank which sends fuel to burn in the cylinder to make power. Because of the fuel, the piston is activated and moves back and forth according to the type of outboard motor.

Fun Fact: The largest outboard boat motor seen has been a Seven Marine motor. This motor contains 627- horsepower and was first shown on the Miami International Boat Show in 2015.

The piston rod turns the crankshaft, which turns the driveshaft (a piece that runs down the spine of the motor). At the bottom of the driveshaft is a gear which changes the vertical spinning motion into horizontal spinning.

“Powered by the burning and expanding fuel gases, a piston moves back and forth in the cylinder. This is just like the piston in a car-engine cylinder and often works through the same four-step process.”

Chris Woodford

This spinning motion turns the propeller which makes the boat move forward. Outboard boat motors work on a cycle. There are different types of outboard cycle types which are geared towards different things like speed and wake size, etc.

It all depends on what you are looking for.

What are the Pros and Cons of Each Motor?

Inboard and outboard motors are two different types of motors that can be built into a boat. Both motors are named for their difference in placement on the boat.

An inboard motor is placed inside the hull of the boat, where an outboard motor is placed outside on the stern. Both have their pros and cons and their technical differences.

Pros of an Outboard Motor:

  • An outboard motor is easily accessible when it has a problem and breaks down because it’s on the outside of the boat.
  • The outboard motor has a better drive and can handle lower speeds, unlike the inboard/outboard motor* design.
  • It is lighter than other motors and is specifically designed for boating.
  • They are extremely reliable according to many boaters.
  • They can be easily stored in the wintertime and you won’t be getting unwanted water critters caught inside.

*Note: Lower down in this post is a description of an inboard/outboard motor. (or I/O)

Cons of an Outboard Motor:

  • While the outboard is easier to steer and is simple maintenance, it doesn’t have enough power for bigger boats.
  • Outboard motors are also closer to the water and more exposed than an inboard motor. It is more vulnerable to the elements.

Pros of an Inboard Motor:

  • Inboard motors put forth a lot more power and have great speed.
  • The motor is more heavy duty and can handle hauling more weight than an outboard.
  • Because the inboard is placed inside the boat, there is more space in the interior of the boat for more people.

“There are a few major drawbacks with inboards, but many boat owners overlook key features. Though inboard motors require a large box in the middle of the boat, which will keep the engine, they are much quieter than outboards, which is great for entertaining.”

Samantha Pudney

Cons of an Inboard Motor:

  • Inboards are harder to control at lower speeds. Steering is less effortless than in the outboard because you don’t have your hand on the motor.
  • It is harder to do maintenance when something goes wrong because it’s on the inside of the boat.
  • These motors produce a smaller wake (waves) and are popular with fishermen because they don’t disrupt the water as much as an outboard motor.

Which One Should I Get?

What you choose to purchase and get for yourself all depends on what you are going to be doing with your boat. If you are planning on entertaining a large group of people, you’ll need a larger boat.

The inboard motor option is the best for a larger group. It leaves more space on the boat because of the fact of it being placed inside the upper part of the hull.

Even though the outboard is placed on the stern, it cannot carry as much horsepower as an inboard motor. It may be the easiest to control, but not to entertain.

Different Types of Inboards and Outboards

There are a few different types of both inboard and outboard motors that can be used for a boat. Each one has a different purpose and goal. Because there are different types of activities you can do in the water, there are different motors to choose from.

Direct-Drive Inboard

A direct-drive or D-drive inboard motor is typically used for ski boats. It leaves little wake and usually goes about 30 mph and above. With these motors, an automotive V8 is mounted longitudinally, with the flywheel at the rear and angled downward at the back.

The prop and rudder are built into the hull underneath the boat, and the engine is mounted in the center of the boat. A D-drive motor has about the same power path as a rear-drive car.

V-Drive Inboard

V-drive motors also have a prop and rudder built into the hull underneath the boat. They are stowed under the transom seating, and manufacturers use unique hull designs, as well as surf and ballast systems to manipulate the wake.

These motors also use an automotive V8, but it’s mounted with the flywheel or rear of the engine, facing forward. Boats with this type of engine are usually watersport boats because they are good with wakeboarding, tubing, wake surfing, skiing, kneeboarding, and almost any other watersport you could do behind the boat.

This is possible because the weight of the transmission is farther aft, and the stern rides lower in the water and therefore throws a larger wake. Who doesn’t want a huge wake to do all those watersports?

The wake makes the whole day!

Two-Stroke Outboard

A two-stroke outboard motor features a compression stroke followed by an explosion stroke, which uses previously compressed fuel. They don’t include valves, which can make construction simpler and the mechanics of it easier to work on.

The compression stroke in an engine is where the air or air/fuel mixture is compressed before ignition.

The functions of suction, compression, expansion, and exhaust occur in two strokes of the piston, which is where this type gets its name. These motors are also fairly light and cost-effective.

Many are air-cooled as well, eliminating the need for a water-pump and allowing the boat to operate in shallower water.  

There are some drawbacks to this type, however. A two-stroke motor requires a mixture of oil and gas to lubricate its moving parts. It’s also less fuel efficient, and these motors produce a lot more emissions and pollution, with a stronger gasoline smell, then it’s a four-stroke counterpart.

Four-Stroke Outboard

A four-stroke motor runs one compression stroke, followed by one exhaust stroke, and then quickly followed by one return stroke. These motors don’t need a mixture of oil and gas, as they use plain unleaded gasoline in the fuel tank and motor oil in the engine case.

The exhaust stroke is a movement of an engine piston that forces the used gas or vapors out through the exhaust ports.

Merriam-Webster Dictionary

These engines produce fewer emissions, which is better for the environment. They tend to be quieter than two-stroke motors as well.

Many of these also feature high-tech computer management systems, which keeps the motor running smoothly giving the boat a greater performance.

The drawbacks of this type are that they are bigger and heavier. They are also more expensive to build and require more regular oil changes. They may be more difficult to repair as well. These are a bit slower than their two-stroke counterparts.

Pricing

There are many different types of horsepower for our different motors. Each brand has a different price for these various ranges of horsepower as well. Here are some tables to give you an idea of what you’d be getting.

Outboard Motors Pricing Based on Horsepower

Low Horsepower

BrandHorsepowerPrice
Suzuki2.5 hp$780-805
Honda2.3 hp$950-972
Tohatsu3.5 hp$1,000-1,030
Mercury3.5 hp$1,050- 1,068

Medium Horsepower

BrandHorsepowerPrice
Suzuki15 hp$2,995
Honda15 hp$3,117
Tohatsu15 hp$2,615-2,619
Mercury15 hp$3,050- 3,070

High Horsepower

BrandHorsepowerPrice
Suzuki30 hp$4,675- 4,775
Suzuki25 hp$4,000-4,100
Suzuki20 hp$3,550
Honda20 hp$4,106
Tohatsu20 hp$3,850
Mercury20 hp$3,550- 3,571

Inboard Motors Pricing Based on Horsepower

BrandHorsepowerPrice
Volvo200 hp$5,995
Rotax250 hp$4,495
MerCruiser300 hp$7,495- 10,399
MerCruiser350 hp$10,499
MerCruiser375 hp$19,900

The pricing of inboard and outboard motors is variable on many elements. The main element that drives up prices is horsepower. However, there are also other factors such as weight. The heavier the motor is, the more it could drag down the boat to a slower speed.

Typically, the heavier the motor is the more expensive the motor will be. However, the more defining feature that changes the price of a motor is the horsepower. The higher the horsepower, the more desirable the motor is and, therefore, the more expensive it is.

Typically, the inboard motors are more expensive because they have higher horsepower. However, they are much heavier than outboard motors. Depending on which aspect is more important to you, horsepower or weight, your decision will be affected.

What’s an Inboard/Outboard Motor?

There’s also another type of motor that combines both inboard and outboard motors together in one design. This is called an inboard/outboard, I/O, or sterndrive motor. The unit lies below the swim deck and it has the motor mounted under the transom.

These typically have more horsepower, longer life, and are more attractive as the engine is hidden below the boat. They can also be a bit heavier, which can help with stability, and make it a good boat for fishing.

These also have no need for a prop shaft or rudder system, which can make it easier to operate in shallow water.

“Inboard/Outboard (I/O) engines, otherwise known as Sterndrive, combines inboard power with outboard driveability.”

Used Boat Warehouse

Essentially, this type of motor combines the best of both inboard and outboard motors, with its car-like engine that has good weight distribution and enhanced handling, the ability to tilt up and to trim the drive.

These can be nice to maneuver and often allow for a full-width swim platform as well.

These do, however, have more potential failure points than inboard and outboard motors, and they can be expensive to repair. They also typically need to be replaced about every three to five years, which can be expensive as well.

Maintenance for Inboard/Outboard Motors

Because both motors are very different, their methods of maintenance are different as well. Each one is placed on the boat in a different place. While the Outboard motor can be removed and cleaned, the inboard cannot.

Inboard Motor Maintenance

An inboard engine functions similar to a car engine. It is a combustible engine that has cylinders. When combustion occurs, it pushes a piston down. The piston is connected to a crankshaft, so it turns the shaft away. From the pistons being pushed down, it causes the drive shaft to spin.

All of these parts need to be checked on regularly to make sure they are functioning well and kept up properly. Before using the boat when you first take it out for the season, it’s good to do the following:

  • Check and possibly change the engine oil and filters. It’s also a good idea to have a spare filter and extra oil on board in case there’s ever an emergency. Use a cleaner in the engine to remove the debris that it may have accumulated over the season. The water will come out clear when it is clean.

During the season, also check the following:

  • The Prop Shaft- make sure it turns without issues.
  • Water Pump Impeller- make sure the rubber isn’t broken off, because that can lead to problems.
  • Stuffing Box- check while the boat is docked but still in the water. Check for signs of leakage.
  • Engine Hoses- make sure there are no cracks because that can cause water to fill up in the engine room.
  • The Smell of the Engine Compartment- that can be indicative of a leak in the engine.

Before the winter, you should check the antifreeze. You should also warm up the engine to loosen up the oil, and then let the oil drain, and finally replacing the oil filter with a new one.

Fill the crankcase with oil and inspect the level once it gets warm. It’s best to store the boat in a place with climate controls, but if that’s not possible, using a drop light, placing it under the engine, and running it overnight can be a good option as well to prevent it from freezing.   

Outboard Motor Maintenance

After every trip you take, there are certain maintenance requirements that will help you to keep you outboard motor for a long period of time. There are also just routine checkups to do on your outboard motor.

After every trip you should:

  • Flush out the engine. Although this is important on saltwater trips, you should also do this when you have been in freshwater.
  • Start up the engine and let the water pump run. The water pump will do its job while the engine is running.
  • Check the water flow of the water pump. By putting your finger into the stream of water, you can feel the temperature. Make sure it is not hot. Warm is perfectly acceptable. Also, if there is not a strong current, there may be debris stuck in there. Insert wire and work it back and forth.
  • Allow the engine to burn the fuel in the carburetor. After the engine is flushed, disconnect the fuel line and run the engine.
  • Check for fuel or water leaks. Take the engine cowling off and check. If there are leaks, contact your boating mechanic.
  • Replace the cowling. Wipe down the cowling. Keep a cover such as a canvas or plastic cover on your engine in between trips.

For routine checkups:

  • Check the fuel line for cracks and worn spots.
  • Check the primer bulb. Make sure it is pliable and does not have any cracks.
  • Check the fuel-line fittings. Make sure they are seated properly and not leaking.
  • Check the clamps on the fuel line. There may be rust or corrosion.
  • Check fuel tanks. There could be damage or corrosion that would need to be fixed.
  • Check the tank vent. Make sure it is aspirating properly.
  • Check for water in the fuel.

These lists of objectives will help you to keep you outboard boat motor for a long time. With all the money spent on a motor, it is significant that we do these checkups in order to maintain our motor.

It is better to have to fix a small part of the motor than to buy a new one altogether. There are boat mechanics that can also do these checkups for you if you are not confident in these steps.

Related Questions

Which boat motor, inboard or outboard, offers better fuel economy?The outboard boat motor, specifically the four-stroke, offers better fuel economy. The outboard motor uses fewer gallons per hour and has more miles per gallon compared to the inboard boat motor.

What things do I need to do to maintain my outboard boat motor?Here are a few things to remember when doing your outboard motor maintenance:

  • Flushing (flush through with fresh water when the boat motor has been in salt water)
  • Oil Change
  • Changing gear lube
  • Fuel filter
  • Checking engine oil

What do I do if my motor is overheating? Locate the source. It is likely from obstruction in raw water intake. Find the intake and clean it out. This overheating is caused by a lack of water flow in the cooling loop. A loose hose clamp or a split hose can slow down the water flow to cause the heating.

Geoff Southworth

I am a California native and I enjoy all the outdoors has to offer. My latest adventures have been taking the family camping, hiking and surfing.

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