Outboard vs inboard vs inboard/outboard – the engine guide!


Choosing an inboard or outboard motor for your boat is simple once you are familiar with how your needs affect the decision. Inboard motors offer greater stability and are well suited to certain water skiing applications, outboards are general purpose and offer easy control, maintenance and replacement. There are no hard-and-fast rules, however, and personal preference will affect your decision.

Read on to find out how all the different engine designs stack up against each other and what fits your best.


If you spend a lot of time around waterfronts, you might notice that the outboard engine is the default choice for fishermen and other light commercial inshore boats. Outboards are popular because their mounting position high up on the transom makes them easy to access and they can be tilted completely out of the water when not in use. There is the ease of swapping to a bigger motor. When it comes to reliable service, many seasoned boaters find the outboard simply tough to beat. A draw back is that to get the power that the I/O has may take multiple outboards. With a set up like this the boat will need a thicker transom which will make the boat heavier. The outboards also leave less room at the rear of the boat for reenter.

Inboard/Outboard and Inboard Motors

That cute fiberglass runabout with the stern drive tucked under the swim platform is actually an inboard/outboard (I/O). The term inboard is reserved for motors mounted at mid ship that drive a propeller shaft that passes through the bottom of the hull. A true inboard system will rely on a separate rudder to enable steering. Inboards are popular both with slalom skiers because they produce little wake and with smaller fishing boats that work in heavy seas because of their low center of gravity. An inboard system is required on heavy vessels requiring larger motors where the size and weight are not appropriate for mounting at the aft end of the hull. Inboards are less common in smaller recreational boats because they require a large box right in the middle of the boat to house the engine, are more costly to produce, and are more difficult to load on a trailer. These engines were crafted after car engines and have more horse power and torque then the outboards. They also get better gas mileage and are quieter. Inboard boats that do not have stern drive units can have one of two types of transmissions or gears, a straight drive transmission or a V drive transmission. Both of these transmissions are inside the vessel. More stress is placed on the V drive transmission since the drive shaft forms a 45 degree angle with the engine. The benefits of a V drive are interior cabin space. A V drive allows the engines to be set back further aft midship giving more interior cabin space. One heavily talked about draw back is the chance of a fire. Yes, there is a chance of a fire but if you run your bilge blower you should be fine.

Power Systems

The power system you choose will have a direct effect on how you control your boat at low speeds. An outboard with its integral skeg and directional thrust enables effective maneuvering with or without power. Where an inboard has a drawback is that it does not steer effectively unless thrust is applied, and docking can be more challenging as a result. If the addition of joystick control to an I/O system can simplify docking, but it is not available on every model and at every price. Joystick control is also available for inboard some systems (Volvo Marine’s IPS system). The I/O has close to the same steering as an outboard when not under power. This is due to the fact that an outdrive is modeled after the lower unit of an outboard.


Outboard engines are perhaps the easiest to maintain. From inside the boat you can readily access the motor. On a trailer, the entire system is within easy reach. The factory engine housing is another benefit as it provides a fully integrated seal and protected environment for the engine electronics and mechanicals. An I/O has the motor located in the bilge of the boat where it is susceptible to water damage and moisture vapor. Access will sometimes be limited to a hatch in the floor. Inboard systems are similarly located in the bilge of the boat. There is a draw back to I/O and outboards in that since their drive gears are always in the water it will cause other repairs that you will not see with an inboard boat were the whole drive components are inside the boat. The inboard and I/O motors are like servicing a car engine. It is a little harder to change oil, but if you purchase an oil pump it is easy. Outboard engine also do not have to drain for winter because all the water drains out when removed from the water. I/O are cheaper to replace than outboards and require less maintance than outboards. Most outboards last 750 hours before needing a major overhaul like new power head. Inboard and I/O gasoline engine last about 1500 hours and diesels last 3000+ hours. The outdrive of the I/O general last 750 hours and the transmission of the inboard last 3000 hours before needing major overhauls.

I Prefer I/O

The inboard/outboard configuration is very popular with recreational boaters and builders. With the motor tucked away at the stern, the rest of the boat is left open for seating. Designers like the I/O layout because it makes the transom easy to style, and it leaves room for a full width swim platform and additional amenities like showers and storage lockers. Small and midsized cruisers similarly benefit in that they will have more room below deck for accommodations. If you are in boating for the sheer fun of it and your needs are not too demanding, an I/O may be the best choice. But don’t over look the inboard boats or v drive boats.

To sum it all up for everyday boating with the family I will go with an inboard or I/O. But if I am looking at a fishing boat I will take and outboard just because the motor can be lifted out of the water to fish shallower without getting stuck.

Thanks for reading!
Nathan B


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Posted on: August 16th, 2010 at 7:34 am