Comparing Inboard, Sterndrive and Outboard Power

There are three primary types of marine engine — outboard, sterndrive and inboard. So what’s the difference, and why so many options? Let’s take a look at the advantages, and most popular applications, for each propulsion system.

Inboard

Inboard power is the classic, original marine propulsion system. A gasoline or diesel engine is located within the boat, and power is transferred to the propeller by a shaft that passes through the bottom of the hull. A separate rudder steers the boat.

At one time, almost any pleasure boat bigger than a skiff had inboard power. If there are classic wood runabouts on your lake, they are almost certainly inboard-powered. In the early years of pleasure boating, only converted automobile or aircraft engines offered enough horsepower to move even midsize runabouts. That began to change in the mid-1950s as outboard motors became more powerful.

Today, inboard power remains most popular for specialized tow sports boats. It offers a number of advantages for tow sports. The engine makes a lot of torque, and the prop is always in undisturbed water under the boat, both of which help pull water skiers and boarders quickly up on plane, and hold well against the strain of the tow line. An inboard can also provide power without creating excessive turbulence behind the boat, which allows builders of specialized wake sports boats to shape the hull to create the most desired wake for skiing, boarding or surfing.

Perhaps the biggest advantage of inboard power is rarely talked about – the pros are well under the boat and there is little chance of a serious accident.  This important detail is what has kept sterndrives and outboards out of the wake surfing business until recently.

Both Volvo and Mercury have introduced forward-facing lower units that put the props about 26” (.66 cm) forward of where they would be otherwise. With the props no longer exposed, regular bow riders are now being used for wake surfing.

Perhaps the biggest downside to a single inboard is that they are challenging to dock for the beginner.  But, with experience and patience, most owners get onto it, and this issue did not keep over 13,000 people last year from buying inboard engine tow boats.

Throughout the 1960s to the 1990s, inboard gasoline engines were popular in express cruisers, cabin cruisers, express fish boats, and even some small motoryachts in the 3o” to 45” ( 9.10 to 13.71 m) range.  Unfortunately, global economic conditions, foreign wars, and misguided government tax policies spelled the doom for most of the builder that made this types of boats. Then the Great Recession of 2008 finished off the few that were left.

While some mid-range cruisers are available with inboard gas engines, they are quickly being supplanted by the large outboard engines, and they seem to be bound for extinction.

Sterndrive

The sterndrive engine (sometimes called an inboard/outboard, or I/O) was introduced by Volvo Penta in 1957 as a power system that combined the torque of an inboard engine with a steerable and trimmable drive capability of an outboard. This allows for easier, more confident maneuvering at low speed.

The trimmable drive improves performance and economy, and because the drive can be trimmed out, or raised, it allows the boat to be beached. Today, gasoline sterndrive engines range from around 200 hp to 430 hp.   

Single-prop sterndrives are for smaller boats and are matched up primarily with inline 4 , V-6 and V-8 engines. Dual-prop drives such as the MerCruiser Bravo Three and Volvo Penta Duoprop are typically paired with larger V-8s ranging from 350 to 430 hp and are used for pushing heavier cruisers and day boats.

The newest sterndrives are for wakesurfing and have forward-facing propellers, as mentioned above

Sterndrives once had a reputation for being prone to corrosion-related issues in saltwater, but manufacturers have significantly improved corrosion resistance on engines and outdrives. All come with sacrificial anodes to protect against galvanic corrosion. While the two major sterndrive makers have alloys and coatings that are less prone to corrosion, it remains a concern for many boats.

Indeed, 20 years ago about 75,000 sterndrives of all sizes were sold new each year, and that number has fallen to around 10,000 a year now.  There are many reasons for this, but a major one was that they required maintenance and TLC, characteristics that take time and forethought. With the advent of automobiles where owners almost never raise the hood, and new generation proved challenging for the industry.

Also, at the same time, 4-stroke outboard engines got rid of the 2-stroke smell, smoke, and temperamental behavior. And, they were much more fuel-efficient than the old 2-stroke and approached sterndrive engines in terms of fuel consumption.  Today, there is little difference between the two.

Outboard

An outboard is a dedicated marine engine that is attached directly to the stern of a boat, and today many family boat models are offered with a choice of outboard or sterndrive power. 

Originally intended only to power small fishing boats and runabouts, outboard engines have become more powerful and have often replaced inboard and sterndrive engines on larger cabin cruisers, day boats and high-performance boats. In fact today, with the advent of Mercury’s 600-hp V-12 outboard, these engines are being put on boats 50’ (15.24 m) and larger and are replacing diesel inboard engines.

Many new boats may be rigged with three or more outboards that, when combined, make more power than the biggest pair of sterndrive engines available, resulting in performance that was once unimaginable. Many new outboards are also more efficient and more powerful and feature-laden than the motors offered just a few years ago, all reasons outboards have become more popular on more types of boat.

Because the entire outboard engine is located outside the boat, more of the cockpit can be devoted to seating or gear storage, livewells and other features in the space a sterndrive or inboard engine would occupy — one reason outboard power has always been a popular choice for fishing boats.

The key trade-off for outboard power on family boats is that the motor intrudes on the swim platform. With sterndrive or inboard power, the boat can be designed with a flat, full-width platform that many boaters prefer for swimming and lounging. But lately, imaginative designers are creating a swim/staging area ahead of the outboards and behind the seating area that effectively gives occupants access to the water from either side of the boat.

An outboard is also self-draining, which makes it easy to prepare for winter storage, unlike sterndrives which had cooling systems and oil pumped out in cans which usually ended up making a mess.

Another drawback to outboards is that they are significantly more expensive than sterndrive engines. Most gas inboard engine blocks come from GM and were designed for large cars and trucks. Mercury makes its own blocks, but the design is virtually the same. Outboard engines are considerably more complex than truck engines, and were designed and built for a single purpose. (An exception being, Honda, which may use one of its engines, or a variation thereof, in its automobiles.)

Another weakness of outboards is the constraint on the diameter of their props, usually less than 16” in diameter. A single prop can have only so much blade area to transfer power to the water. This is why sterndrive builders introduced dual prop drives years ago – to have more blade area to transfer torque to the water, and they have other attributes as well. This is precisely why the Suzuki 350 outboard engine and the Mercury 600 both have dual props.

Which Type Engine to Buy?

The type of engine you buy will largely be dictated by the application of the boat you buy. There are simply not the choices that there once were – and that is largely because of the implosion of new boat sales from 2000 to the Coivd pandemic of 2020. Overall, sales by type of boat are 25% to 50% of what they were 20 years ago, and most of these losses have been in the larger boats.

Aluminum fishing boats and jon boats have finally recovered their numbers of the early 2000s, but they almost exclusively use outboard engines. And for the last 20 years, many boat owners that have stayed in the sport have moved from fiberglass sterndrive boats to aluminum pontoon boats. 

Today, consumers have a choice between sterndrive and outboard power mainly in the runabouts from 25’ to 40’ (7.62 to 12.19 m), while gas inboards are mainly the domain of tow boats.