Skip to main content

Pros & Cons: Our propulsion special

A comparative look at inboard, outboard and sterndrive power, with a discussion of Volvo’s new Inboard Propulsion System and a roundup of new outboard engines

A comparative look at inboard, outboard and sterndrive power, with a discussion of Volvo’s new Inboard Propulsion System and a roundup of new outboard engines

In looking toward the purchase of a new or used boat, among the considerations to address is which propulsion system would be best.

Automobile manufacturers make engine choices relatively simple, if they allow you to choose at all. The marine world is very different in that respect, as not only is horsepower and torque variable, but there are different methods of transmitting that power to the prop.

The three most common marine propulsion systems — inboard straight shaft, sterndrive and outboard — are being used successfully in a wide range of vessels varying in size, displacement and price. And the new Volvo Inboard Propulsion System, or IPS, adds a different dimension to the inboard power options.

In an attempt to determine which propulsion systems are most favored for certain applications, I consulted with six authorities on powerboat design: naval architect and professional engineer Lou Codega of Smithfield, Va.; Mark Ellis of Mark Ellis Design Ltd. in Oakville, Ontario; naval architect David Gerr, director of Westlawn Institute of Marine Technology; Michael Peters of Michael Peters Yacht Design in Sarasota, Fla.; powerboat expert and author Eric Sorensen (“Sorensen’s Guide to Powerboats”); and Doug Zurn of Zurn Yacht Design in Marblehead, Mass. I also spoke to both novice and experienced recreational boaters.

The goal here is to inform you of specific features and potential issues with each system, allowing you to make an informed decision. The perfect boat may not be perfect for you if the propulsion system doesn’t meet your needs.

“The drivetrain, regardless of type, really has to be treated as a system, not as individual parts,” says Codega. “Decisions regarding engines, transmissions, appendages and propellers must be taken with the understanding that all the individual components are interconnected, and all relate to the performance and expectations of the boat. There are many tradeoffs, and very seldom is there a single answer, regardless of how simple the question might seem.”

Most of the designers agree that speed and draft requirements are the key factors in selecting a propulsion system for a vessel. Zurn adds that design should allow for good access to all components, a sentiment echoed by anyone who’s ever had to work in an engine compartment.

For novice boaters, sterndrive and outboard propulsion are considered better entry-level systems, based on maneuverability.

In discussing vessel lengths for the practical application of outboard and sterndrive propulsion, there is a key issue not commonly addressed, but one that boaters should keep in mind. “As the vessel size and displacement increases, the blade area of the prop must also increase to absorb the extra horsepower at lower speed,” says Gerr, noting that outboards and certain sterndrives are limited to a 14-inch-diameter prop.

There are a few options that will enable an increase in prop size while still utilizing a sterndrive. The MerCruiser Bravo 2 outdrive will accept a 21-inch-diameter prop, and the Bravo 3, being a dual-prop drive, will provide greater total blade than a single large wheel. Volvo’s “Duoprop” is another alternative for powering heavier vessels with sterndrives that wouldn’t be practical with outboards. The sterndrives that incorporate two props in the same axis also will run smoother, due to the higher frequency produced by the increased number of blades.

Inboard, straight shaft

Let’s start with the basics. The oldest of propulsion systems, the inboard drive system consists of a gas or diesel engine mounted inside the vessel — normally below the deck — that transfers the engine’s power through a transmission to an output flange and then to a straight shaft, commonly referred to as the prop shaft. The prop shaft exits the hull through an adjustable fitting or gland called a stuffing box, which maintains compression around the shaft to keep sea water from entering the boat while allowing just enough water around the shaft for lubrication and cooling.

The shaft runs through and is supported by one or more struts mounted to the bottom of the hull. Depending on the hull design, the prop shaft may pass through the keel first. The shaft extends through the cutless bearing in the strut, and the propeller is attached at the end.

The ability to steer an inboard boat comes from a separate rudder assembly that looks like a vertical blade suspended from the vessel undersides and mounted abaft the propeller. Water flow from the prop passing over the rudder blade creates a pressure differential on both sides of the blade, effectively steering the vessel. Simply put, when the rudder is operated to port, the force of the water hitting the left side of the rudder will swing the stern in the opposite direction, hopefully swinging the bow to port.


• In general, inboard propulsion is a relatively inexpensive method of powering a boat.

• General service and maintenance are readily available almost anywhere.

• The engine is positioned in the boat to provide good balance fore and aft, and creates a low center of gravity, further improving handling characteristics.

• It is generally considered the simplest, most efficient method of transferring torque from the engine to the propeller.

• Many commercial users still rely on this system due to its relative simplicity and repairability.

• There are fewer critical corrosion issues than with either outboard or sterndrive propulsion.

• Inboard propulsion can be used effectively in a wide range of vessel sizes, from a small launch to a megayacht, keeping within the design parameters of the boat.


• Inboard propulsion occupies a lot of space inside the hull and limits the interior floor plan.

• As the propeller is in a fixed position below the hull, maneuvering at slow speed and in making sternway can prove more challenging for novice or entry-level boaters.

• Trailering an inboard boat is more complicated, as the underwater running gear extends below the hull and is fixed in position, requiring specially designed trailering equipment.

• Prop fouling and damage cannot normally be attended to while the vessel is in open water, whereas outboards and sterndrive systems provide relatively convenient accessibility to free entanglements or for emergency prop replacement.

• Inboard propulsion requires a more extensive installation due to the requirements of the cooling system, steering and exhaust through-hull fittings that aren’t required with sterndrive or outboard power.


Although an inboard boat is considered by many to be at a disadvantage in close-quarters maneuvering, I have witnessed seasoned skippers piloting inboards calmly execute intricate docking maneuvers that would have most of us breaking out in a cold sweat. Twin engine installations, in my opinion, should be considered only for additional speed, not from a maneuverability standpoint. There are alternatives such as bow thrusters or split transmissions that will aid in maneuvering, are less complicated, and are more cost effective.

In the past most inboard marine engines were converted automotive power plants. Manufacturers are now producing both gas and diesel engines designed from the ground up as marine units. Although there have been no major mechanical changes in recent years, overall engine efficiency has increased, due to sophisticated electronic engine management packages.

The relative ease of maintenance when compared with sterndrive and outboards makes an inboard system more desirable when cruising in remote areas, where sophisticated replacement parts may not be readily available. General mechanical skills usually will be adequate to cover most emergency repairs.

Starting with boats around 30 to 35 feet, Gerr prefers an inboard and straight shaft setup unless shallow draft is a concern. He feels it is the most reliable package, with fewer corrosion problems than sterndrives. And it’s much less expensive to replace a shaft or prop than a lower unit.

As a marine surveyor and technician, I view inboard propulsion as several independent systems, each with specific service and maintenance issues. It isn’t uncommon to find that the engine has been well maintained, while the rudder packing has been ignored. One of the least understood and most overlooked issues with inboard systems is engine mounting and shaft alignment. Alignment issues can create drive system vibration, stuffing box leaks, transmission failure and loosening of struts, to name just a few areas of concern. Proper selection, installation, inspection and maintenance of engine mounts and shaft alignment are critical for proper operation.

Whether we like to admit it or not, most of us at some point will experience either a hard or soft grounding. The fact that the inboard drive is fixed below the hull can create more opportunity for the grounding to occur, as well as complicating ungrounding since the running gear cannot be tilted up out of the way. I don’t consider this to be a weak point of inboard systems, but rather something to remember when venturing off known and charted waterways.


An outboard engine is a complete, purpose-built and designed marine package comprising a vertical-shaft engine on top — the powerhead — that drives a vertical output shaft. The shaft runs down through the intermediate housing to a set of bevel gears in the lower unit. The bevel gears effectively redirect the power 90 degrees to the horizontal plane, where a propeller is attached. This output redirection can reduce the power available at the prop by 4 to 5 percent. The entire outboard swivels on its mount for steering control and is commonly fitted with hydraulically adjustable tilt and trim.

All outboard engines are raw water cooled, meaning they are cooled by the water they are floating in, as opposed to a closed, recirculating freshwater system.


• Outboards are considered light in weight for the horsepower they produce, and free up precious cockpit space.

• They are available in a wide array of horsepower ratings, especially in the lower range, and are relatively simple to install without special equipment.

• They are easy to maintain from an accessibility standpoint. If stored on a trailer, the entire unit can be serviced and maintained while out of the water.

• Outboards are easily removed from the boat, should they require work on a bench.

• They are easily replaced in the event of catastrophic engine failure.

• Trailerability and the ability to run in shallow water are assets of the outboard.

• Propeller thrust during normal operation is horizontal, as opposed to the straight shaft inboard system, which typically operates at a 12- to 14-degree downward angle. The horizontal thrust results in additional propulsion efficiency. Maneuverability is excellent, as prop thrust is directed to where the engine is steered.

• Hull trim is easily adjusted with built-in hydraulic trim systems. By adjusting the engine trim up or down, you change the propeller thrust angle from horizontal, thereby either depressing the stern, which raises the bow when tilting the engine up, or raising the stern, which lowers the bow when tilting the engine down.

• Combine the trim factors with the ability to run at a fairly high percentage of full power, and the outboard will usually have a speed advantage over the same horsepower inboard engine.

• When installed properly, outboards have the ability to kick up if they hit a submerged object, potentially decreasing the damage to the lower unit.


• Outboard engines are, for the most part, louder than conventional inboard engines. The close-fitting engine hood does not allow for additional sound-deadening insulation to be installed.

• They are constantly exposed to the elements, as the entire assembly is mounted on the transom.

• They tend to be more expensive than inboards of similar horsepower.

• Large outboards can be hard to steer, as the entire unit must turn. Engine and propeller torque can have a significant effect on the steering effort, even with hydraulic systems.

• It is impractical to perform routine inspections and monitor systems while under way, as would be important on a long trip.

• Proper flushing of the cooling system is critical for engine longevity.

• Outboard engine parts are designed specifically for that application. There are few interchangeable or adaptable components, therefore price and availability can be an issue in certain areas.


In general, outboards offer excellent acceleration and higher top end than inboards, but poorer tracking. They don’t maneuver at speed as well, but their low-speed maneuverability is better.

Codega says outboards offer mass-produced reliability in a complete bolt-on package, eliminating much of the human error and inconsistencies that can occur during installation.

Outboard boats typically are fitted with transom cutouts and, as such, must be fitted with a proper transom well to help eliminate water taken on in following seas. Peters says some people are concerned about operating in rough water because of the lower transom height. However, boats can be fitted with brackets to mount the engines farther aft and eliminate the transom cutout. This also will move the boat’s center of gravity farther aft, which has its drawbacks.

“Moving the engines farther aft increases the speed needed to get the hull up on plane, thus reducing midrange performance,” says Sorensen. “Bracket-mounted outboards also submerge more readily, since a wave coming from astern will immerse the engine before lifting the boat.”

Peters says he is increasingly building larger boats with higher horsepower and multiple-outboard configurations to meet client demands. However, he says that for boats larger than 35 feet, outboard power pushes the envelope for performance. Also, beyond that length there is a limited market and questionable resale value.

Peters says clients are leaning toward 4-stroke outboards for noise reduction and fuel efficiency. Ellis also touts 4-stroke technology for its efficient, vibration-free operation.

Codega feels that old thinking about outboards is becoming blurred, due to newer technology in outboards. “We are dealing with the availability of higher horsepower outboard engines with increased reliability and durability,” he says.

Still, some experts say that since the engine isn’t accessible for inspection while under way, there will be fewer indicators of pending mechanical failure than with an inboard or sterndrive.


Also known as an inboard/outboard or I/O system, the sterndrive is an interesting combination of an inboard straight shaft and an outboard propulsion system. As such, it has inherited both the strengths and weaknesses of each system.

The concept behind the sterndrive configuration is to locate a conventional inboard engine as close to the transom as possible. By placing the engine aft, the center of gravity is moved aft, which can improve performance and planing characteristics. But that all comes with a price and typically creates access issues for service and maintenance. Also, steering the sterndrive is accomplished through articulation between components of the outdrive, which increases the complexity of the system.


• Sterndrives give a boat a clean appearance, with no powerhead extending above the deck.

• Most are quiet in operation, as the cooling water and exhaust exit through the prop.

• Sterndrives offer a degree of familiarity in operation and maintenance, as they are based on conventional automotive style engines combined with the maneuverability of an outboard engine package.

• Hydraulically actuated cylinders give the ability to trim up or down, as with an outboard, which makes the system convenient is shallow water. A sterndrive also is fairly easy to trailer, as the drive unit can be tilted up.

• It’s relatively easy to tilt the drive up to free prop entanglements.

• With no powerhead above the transom, the sterndrive is favored by some anglers.

• The aft engine placement provides for a roomier interior layout than with conventional inboards.

• They provide good low speed and close-quarter maneuverability.


• The engine is mounted very close to the transom, limiting access for routine service and maintenance.

• With the engine mounted extremely low in the bilge and at the aftermost portion of the boat, oily bilge water collects around the engine oil pan. As many sterndrive engines use stamped steel oil pans, corrosion and oil leaks can become a serious issue, especially with an older boat.

• The exhaust system is a combination of complex castings and hoses. Because the engine sits very low in the hull and so far aft, the risk of back flushing the engine exhaust with sea water remains a concern. Many boat designs prevent the installation of exhaust risers that are high enough to prevent this from occurring.

• The power transmission from the engine is redirected through the sterndrive by a series of three shafts, connected by two sets of bevel gears. The shafting, when viewed from the side, forms a “Z” to lower the prop into clean water below the transom. Each change of shaft direction complicates the power transmission and reduces final power output.

• The sterndrive pivots both laterally and vertically. To accomplish this, the assembly has four steel bearings mated to the aluminum housing. These are weak spots that must be routinely lubricated with lithium grease. Unfortunately, the grease fittings are submerged, requiring the boat to be hauled. Failing to address proper maintenance will allow the bearings to wear and corrode, causing loose and sloppy steering at the very least, and early component replacement.

• Bellows hoses that maintain the watertight integrity of the sterndrive tend to wear at the points they contact each other and the sides of the drive. When barnacles are allowed to grow on them, the hard growth can begin to cut into the bellows very quickly, as the bellows flex with trim and steering. This requires constant monitoring that only can be accomplished with the boat out of the water.


Practical sterndrive applications are in the 20- to 35-foot range. Most of the experts agree that 40 feet is the maximum length with current sterndrive technology. Gerr says displacement also must be considered. He says sterndrive power picks up where outboards start to leave off, around 250 hp.

Peters says that in vessels smaller than 35 feet, sterndrives are easier to work with from a manufacturing and installation standpoint when compared to traditional inboards. And they typically provide better performance than straight shaft inboard installations in the 35- to 40-foot range.

With small planing hulls, he says the sterndrive places weight in the vessel where it is needed. Peters doesn’t recommend sterndrives over 400 hp, as factory packages aren’t available and that could create warranty issues. He adds that diesel packages work well.

Most manufacturers have switched to cast aluminum oil pans, which eliminate the previously mentioned corrosion issue. But be certain to carefully inspect older sterndrive boats for steel pans. Ellis says saltwater corrosion remains an issue with sterndrives.

My opinion, in addition to that of many industry professionals, is that although manufacturers have made progress with respect to corrosion control, it still remains a problem. A sterndrive unit cannot be tilted high enough to remove it from the water when not in use, and there are no provisions to flush the cooling system while the boat is in the water.

Saltwater sterndrive boats should be stored out of the water if possible. It is certainly true that with proper grounding, impressed current systems, proper zinc installation and vigilant monitoring, most corrosion issues can be kept in check. But remember, there is a high price to pay for neglecting any of this.

Volvo IPS

Volvo’s new Inboard Propulsion System is a complete package: steering wheel, controls, instrumentation, engine and drive unit. The engine is basically what you would expect to see in an inboard boat, with the propulsion unit resembling the lower unit from either a sterndrive or outboard. The big departure from traditional systems is that the drives are installed through the bottom of the hull, not the transom, and the twin props face forward. The system is designed for planing hulls between 35 and 50 feet, with maximum design speed from 25 to 45 knots.


• The propulsion unit eliminates the numerous through-hull penetrations normally associated with inboard installations.

• Forward-facing propellers operate in undisturbed water and provide horizontal thrust angles.

• Counter-rotating props on each drive cancel out rotational losses.

• When the drives are rotated, the entire propeller backwash and full thrust are turned in the desired direction.

• Steering is electronically actuated and progressive.

• The propulsion unit installation absorbs the propulsive and steering forces, enabling the engine to be soft-mounted to reduce vibrations.

• Exhaust is expelled under water, reducing noise and fumes on board.

• There are no bellows, tilt/trim or steering joints to contend with.


• IPS is available only as a twin-engine configuration.

• The drive system is available only as a complete package and isn’t compatible with other manufacturers’ engines.

• As with any new technology, there will be a learning curve for field technicians troubleshooting the system.

• Impact damage and groundings may prove costly, as the entire lower unit is designed to break away upon impact.

• Service and replacement parts availability may be limited in less-traveled locations.


After testing the Volvo IPS, its advantages become clear. Performance and handling is much better than conventional inboard or sterndrive propulsion setups. The system appears to be well designed and constructed. Disadvantages seem minor in comparison to the benefits for boats within the design range — twin-engine vessels from 37 to 50 feet designed to run between 25 and 45 knots.

Once the hull is configured during the manufacturing process, installation of the system is straightforward. Conventional through-hull penetrations for shaft log, shaft strut mounting, rudder post, seawater intake, and cooling water and exhaust discharge are eliminated.