OB vs. IO Power

Many people considering the purchase of a new boat often ask about the differences between outboard and stern drive (or inboard/outboard) propulsion.  This article is not meant to recommend one propulsion system over another, but is designed to lay out the major differences between the two so that a buyer can make an informed decision about which is right for them based on their boating situation. 

The reason that we can be unbiased in our discussion is that we offer both outboard as well as stern drive boats.  You will find stern drive power in our family bowriders, deck boats and cruisers from Bayliner and Regal.  Bayliner also offers bowriders and deck boats with outboard power ranging from 16 - 21 feet.  Our Tidewater fishing boats, Zodiac RIBS and Sweetwater pontoon boats utilize outboard power as well.


Inboard/Outboard or stern drive propulsion systems basically have a marinized automotive engine inside the boat with an outdrive on the outside of the boat.  GM is the engine supplier of both major stern drive manufacturers, Mercruiser and Volvo Penta.  Each of these companies use a GM engine block and adds their own fuel delivery, ignition, exhaust and drive systems.  These types of drive systems are found mostly in family runabout boats and cruisers because of their user friendly layout.

Outboards are the oldest and most traditional form of gasoline boat power invented by Ollie Evinrude many years ago.  These engines are mounted on the transom.  Outboards are the primary choice for fishing boats.

Purchase Price Considerations

While outboard technologies have required much investment in R&D to meet the demands of the EPA,  the basic GM engines in stern drive applications have met these requirements for years with only minor enhancements required to bring them into compliance.  These engines have always utilized four stroke technologies. This translates into a significant price advantage at purchase for stern drives when compared to comparable four stroke outboards. 

However, the EPA has set new standards for emissions from stern drive engines which will require catalytic converters.  This adds considerably to the price of stern drive engines bringing the intial purchase price closer to that of outboard power.


Space and Layout Considerations

Because the engine portion is typically underneath an engine box or sunpad on the stern of the boat, stern drives have much better space utilization in the aft of the boat.  This affords much better access to the water and better space utilization throughout the aft of the boat.  There is no wasted space with splash wells as in outboard applications.  Further, there is no outboard on the stern to interfere with rear visibility.  As an added benefit, the extra padding around the engine reduces noise, providing a more comfortable ride in a stern drive boat.


Maintenance Considerations

Four stroke outboards and stern drives have similar basic needs when it comes to annual maintenance, both requiring oil changes.  However, stern drives require additional annual maintenance to the drive system.  One must compare the added annual expense to the initial extra cost of the outboard to determine how many years it will take to breakeven.  Further, one must consider that particularly in salt water markets, manifolds will require periodic changing.  Depending on flushing habits, one can expect manifolds to last 300-600 running hours.  Factor in this added expense when making your decision.

Noise Considerations

Due to the fact that stern drive engines are typically contained inside the boat under engine boxes or sunpads, the additional upholstery generally provides better noise absorption and therefore a quieter ride even compared to four stroke outboards.


Power Considerations

Power to get on plane quickly in many cases is an important factor, particularly if you plan on carrying a heavy load or if you plan on towing tubes, wakeboards or skiers.  Initial power to get on plane, or low-end torque, is really more of a question of four stroke vs. two stroke power. Four stroke outboards and stern drives have similar power bands and therefore will require a little more horsepower than their two stroke outboard companions.  For instance, an 18' bowrider with a two stroke such as a Mercury 90 Optimax can pull skiers with ease.  The same boat would require a 130 hp stern drive or at 115 to 130 hp four stroke outboard.


Shallow Water Considerations

Certainly one of the biggest advantages to outboard power is the fact that outboards can be tilted up to run in very shallow water.  This provides advantages for fishermen who need to get in shallow waters in search of baitfish, or to the homeowner who has to navigate very shallow waters to get into the neighborhood tidal creek.  That is not to say that stern drives can not be tilted up.  In fact, they can be tilted up, even with the keel of the boat in order to get off of a sandbar.  The difference is that since the outboard shaft is connected to the engine, and the entire engine tilts, there is no potential for drive damage.  When a stern drive is tilted, the engine remains in position.  The tilt is made possible due to a component known as a universal joint.  The engine should be turned off in order to get into the tilt position in order to avoid damage to the universal joint.

As you consider a new boat, take these points into consideration to be sure that you are getting the best boat to fit your needs.