Jun 11

Jun 11 Are Sterndrives Dead? What's Next for the Revolutionary Propulsion System

With boat sales at all time highs and outboard sales through the roof you have to wonder when the party ends. Every party comes to and end and the boat party definitely will unfortunately. That said, when it comes to the marine market, is the future of sterndrive power in peril? Let’s take a look at the sterndrive market, where the technology is working now and where it is heading.

When it comes to luxury boats I believe there is no substitute for inboard or sterndrive technology. The quiet operation, smooth torque delivery and incredible array of power available are just a few of the benefits, along with the more sleek aesthetic. Boaters that are shopping in the luxury segment from small single engine luxury boats to serious luxury sport yachts are definitely considering the look of the boat and outboards don’t scream luxury. I know Mercury is trying to change that with the incredible Verado V12 but I’m not sure that works for more traditional luxury boaters. Design is an underappreciated aspect of sterndrives in general.

Some boats are designed for sterndrives, the style and design is critical. Tucked in golf shirts and loafers only.

You could argue how much room you save with the outboards but on a three million dollar luxury boat, nobody cares, they just don’t want to see or hear those engines. There is plenty of room in larger boats, and the ability to have an unencumbered swim platform is nice too for many boaters. For pure aesthetics and luxury appeal, sterndrives or inboards win.

When it comes to performance the obvious advantage of outboards is the lighter weight, moved further aft and the ability to adjust the engine height on the fly with a jack plate. Another part is the gear reduction suffers less power loss in an outboard. But, that is only true to a point.

Tuff Marine builds some of the fastest V hulls on the water and when they want to showcase a demo boat, they often run a single sterndrive with big power for the biggest number possible.

With the emergence of the V8 outboard platform form Mercury Racing, power to weight is incredible, yet even a 450R can’t hold a candle to the power and torque of a naturally aspirated Mercury Racing 860, a 9.0 liter DOHC with 4 valve cylinder heads. Even the 565 Mercury Racing power package is going to be able to head to head on the same boat, depending on how it’s setup.

Bang for the buck, that’s a serious amount of power in a relatively compact package. With Mercury Racing M6 drives you have a peerless durable power and torque. Narrow beam, no problem.

Power is power and with sterndrives you can get an ungodly amount of power these days. From custom engine builders, achieving over a 1,000 HP was once only for race engines, now you can get warrantied engines with over 1,000 HP.

Mercury Racing’s dual calibration 1350/1550 turbo engine is an absolute monster, although rarely seen as a single, you can’t match that power with less than three 450Rs and then you’re losing all the advantages an outboard had. For large high performance boats, cats and big V hulls alike, big twin sterndrives is unbeatable. The problem with outboards beyond triples is the drag and weight become an issue. You will see 5 and even 6 outboards on some center consoles but they aren’t performing that great when it comes to speed and power ratio. Cost is a factor as well if you’re looking at twins versus quads.

Mercury Racing drive are designed to handle up to 1550 HP reliably.

Another market for sterndrives that are driving sales for Volvo and MerCruiser is the forward drive technology. Volvo was a pioneer here, introducing the forward facing propellers similar to their IPS drive system for larger vessels. Mercury Marine followed suit with the Mercruiser Bravo 4S. Both are targeting the family tow boat market.

The forward drive design opens a new market for sterndrives, tow boats. Qualities of a sterndrive with the advantages of an inboard. Volvo Penta has the forward drive, Mercruiser has the Bravo 4S.

As opposed to a V drive or direct drive inboard, the forward drive and 4S offer many of the benefits of a sterndrive with some of the advantages of an inboard. Moving the propellor ahead, away from swimmers is safer. With that you get the ability to trim the drive while it’s fully submerged with counter rotating propellers for incredible thrust and control. For that use case they are quite good and that is a significant market now.

I would say the tow boat, surf boat, luxury bowrider and larger luxury boats are really the key markets holding up the sterndrive sales. Obviously sales have fallen since outboards really took off about 10 years ago where the fishing and performance segments continue to dominate. But there is one area in the future that really might help sterndrives and it is mild hybrid technology.

In previous articles I have mentioned the idea of mild hybrid systems for use in more marine applications. Mild hybrids have been fully adopted in automotive as they are an affordable and efficient stop gap between full electric and more complicated series hybrid systems. In a mild hybrid, the electric motor takes over as the starter generator, with a 48 volt system, offering electric assist at low speed and acceleration.

The compact 3.0L inline 6 mild hybrid AMG Mercedes engine. 429 HP, more torque, super efficient and light. (photo credit: Mercedes Benz PR)

For boats, it makes tons of sense. You could take a common 300 or 350 HP sterndrive V8 package and immediately add 20% more torque with reduced emissions. At low speed in harbors, marinas and waterways you could run on pure electric power, only using the gas engine when underway. For sterndrives of all kinds a mild hybrid really changes the conversation for efficient and quiet operation.

Mild hybrids are not sophisticated and there is no charging necessary. This is the biggest burdens for full electric adoption for marine use; limited charging, limited range and huge cost disadvantage. A mild hybrid system addresses all of those objections.

There is a tiny industry of entrepreneurs that are trying to solve the electric power solution for boats and they make all of the mistakes you would expect. We are seeing hydrofoils with electric outboards, an obvious non starter. And you’re seeing electric inboard designs that use hull designs that are woefully inefficient to showcase any of the advantages and electric motor can offer. I covered the reasons hydrofoils don’t work well in powerboats and how bad these designs are in another article. The electric boat effort is mostly comical at this point.

GM is a major marine engine supplier. Expect automotive technology like mild hybrids to trickle down. A GM 6.2 Marine V8 seen here.

If you are a larger OE manufacturer, like Mercury Marine, Volvo Penta and even smaller players like Ilmor the electric option isn’t viable yet and won’t be for a very long time. But, the utilization of mild hybrid systems on traditional 4 cylinder, V6 or V8 sterndrive / inboard package makes complete sense. With very little R&D and proven technology from leading suppliers on the market, the sterndrive system might become the leader in efficiency, power and cost of operation.