There’s a rich history behind stern drives and there are a million and one stern drive boats on the water, yet there are just two big players in today’s market: MerCruiser and Volvo Penta. In a few unusual cases you might find an Ilmor here and there in serious high performance power boats, and even more rarely you may see a diesel stern drive from Yanmar. But for the most part, one of the two big suppliers is going to provide the engine and drive for whatever runabout, day cruiser, or sports cruiser you may be looking at. In many cases the boatbuilder will only supply one brand or the other, so you won’t have any say in the matter. But in some other cases, the choice between a MerCruiser and a Volvo Penta will be left up to you.
What is a Stern Drive?
Before we look at each of these big players, let’s make sure everyone’s on the same page and understands exactly what a stern drive is and how it works. A stern drive, sometimes also called an I/O, or Inboard/Outboard, is essentially a combination between inboard engine and an outboard drive unit. The engine itself is oriented like an automotive engine, and the rear of the engine mates with a drive unit much like the lower unit of an outboard, which can tilt. These two parts join at a hole in the transom of the boat, where a rubberized part called the “boot” seals out the water but flexes to allow the drive unit to move and turn. Thus, the boater gets to enjoy the benefits of inboard power (an automotive-like engine that’s easy to work on and has optimal weight distribution), while also gaining certain advantages of outboard engines (the ability to trim the drive as needed for optimizing performance, the ability to tilt it up for mooring, enhanced handling with the articulating drive, and reduced draft).
Of course, no marine propulsion system is perfect. The biggest down-sides to having a stern drive usually boil down to reliability, and maintenance costs. Due to the additional moving parts there are more potential failure points than there are on either outboard or inboard boats. And the boot we mentioned earlier is a particularly problematic part that commonly needs to be replaced every three to five years.
Although stern drive technology has remained more or less the same for many years, in the recent past there’s been at least one major development: the creation of Volvo Penta Forward Drive.
Forward Drive units face forwards, instead of facing aft. This reduces trim range and eliminates the ability to trim the drive up when moored. However, it also moves the propellers forward about 27 inches, so they remain under the boat and away from the transom. This is ideal for watersports, especially wake surfing, where it’s possible to fall into the water very close to the transom of the boat.
MerCruiser Stern Drives
The low-end range of stern drives offered by Mercruiser has shrunk a bit in recent years. Although MerCruiser still offers a 3.0L 135 horsepower in-line four package via Mercury Remanufacturing, it no longer offers these smaller block engines with new stern drives. The increase in price due to EPA requirements for a catalytic converter, along with the increasing popularity of outboards on smaller boats, has eliminated much of the market for these diminutive stern drives through the past decade.
Today, MerCruiser’s smallest offering is a 4.5L V-6, which comes in 200 and 250 horsepower models. Next in the line are a pair of 6.2L V-8s, which put out 300 and 350 horsepower. Finally, MerCruiser has a 8.2L V-8 that generates 380 horses in its regular incarnation, and 430 horsepower in the H.O. (high output) version. For anything bigger, you'd have to go to Mercury Racing. While they offer some more powerful packages, they’re quite specialized, far more rare, and are intended mostly just for high-performance boats as opposed to your average recreational craft.
In addition to its gasoline power packages MerCruiser has a line of diesel engines which can be rigged with their stern drives including a 2.0L in-line-four (115, 130, 150, and 170 HP), a 3.0L V-6 (150, 230, and 270 HP), and a TDI 4.2L V-8 (335 and 370 HP). These diesels are less commonly seen in pleasure boats then their gasoline counterparts, but can be a good choice in certain circumstances.
All of MerCruiser’s power choices mate up with Alpha One, Bravo One, Bravo Two, or Bravo Three outdrives. Each have slightly different ratings and applications, but the most significant thing a boat-buyer needs to know is that Bravo Three drives have twin contra-rotating propellers, while the others have a single propeller. While not always available for smaller, less expensive power packages, having the twin props provides distinct and significant performance advantages. It enhances steering and maneuverability quite a bit (especially at slow speeds where many single-prop stern drives tend to wander and require constant steering adjustments), assists in getting on plane more quickly, and reduces bowrise.
Volvo Penta Stern Drives
Volvo-Penta more or less invented the production stern-drive with the Aquamatic, introduced in 1959. But like Mercury, due to price spikes the company trimmed back its offerings in 2011 and eliminated their series of four-cylinder 3.0L engines which competed directly with—and were based on the same GM block as — Mercruiser's “entry level” offerings. The new requirements for catalytic converters on engines of this size boosted cost by thousands of dollars, and rather than fight the current, Volvo simply eliminated the 3.0 from its line. It’s interesting to note that this price spike has also boosted the popularity of small outboard-powered bowriders, which traditionally were powered by stern drives.
Today, the Volvo-Penta range starts with 4.3L V-6 gasoline engines, in 200, 240, and 280 horsepower increments. Volvo also has a 5.3L V-8 line, which is offered in 300 and 350 horsepower. For the largest stern drive power needs they also offer a 6.2L V-8, in 380 and 430 HP models.
Volvo also has a diesel line, with their D3 (2.4L), D4 (3.7L), and D6 (5.5L) diesels. These are less common in America than in some other markets, and are available in 140, 170, 200, 220, 225, 260, 301, 330, 370, and 400 horsepower increments.
Like MerCruiser Volvo also offers both single and dual propeller models. SX, SP, and 100 drives feature single aluminum props, while DP, DPS, DPH, DPR, and Forward Drive drives have aluminum, stainless-steel, or Nibral dual propellers.
Comparing MerCruiser and Volvo Penta Stern Drives
Fuel economy for the MerCruisers and Volvo Pentas are comparable across their lines; going back through years of boat reviews and performance tests shows that neither brand holds a big advantage in this regard. What about reliability and consumer satisfaction? Both companies have received numerous NMMA CSI (Customer Satisfaction Index) awards through the years, which requires achieving 90-percent or higher customer satisfaction ratings.
One area in which MerCruiser may have a slight advantage, depending on where in the nation you live, is in the availability and cost of parts and service. There are MerCruiser dealers and MerCruiser mechanics virtually everywhere you find boats, but in some areas finding a Volvo Penta service location can be a bit tougher, and the parts can be a bit pricey.
On the flip side of the coin, Volvo Penta drives have a well-deserved reputation for shifting more smoothly than MerCruisers, and rough shifting is among the top complaints of stern drive owners. Volvo Penta also has a more straightforward warranty program, offering complete coverage for two years (or maximum engine hours) and covering major components for an additional three years beyond that. Some MerCruiser models get only one year, some get as much as three, and some only apply when the engine is bought new as part of a boat-motor-outdrive package.
So: which one takes the cake? Both, and neither. In this highly competitive market, if either builder turned out a clearly inferior product it surely wouldn’t last for long. As to which one you should choose, beyond the differences mentioned above it’ll depend to a large degree on what brand of boat you choose, where you live, and the relationship you have with your local dealer. Remember, in many cases there isn’t even a choice to be made because the boatbuilder may only install one brand or the other. Only one thing is for sure: if you’re buying a new boat with stern drive power, it’s a good bet you’ll have either a MerCruiser or a Volvo Penta under the engine-box.
For more information about the different power choices you have with modern boats beyond just stern drives, be sure to read Marine Engines and Power Systems: The Basics Behind What Powers Your Powerboat.
Editor's Note: This article was originally published in January 2013 and updated in November 2018.