Mercury Sterndrive

Bravo to the Sterndrive

If you had to divide trailerboats into just two classes, there are probably a number of approaches you could take. You might choose aluminium or fibreglass, cuddy or centre console, two or four-stroke, pressed or plate, or whatever else is relevant to your particular needs. Yet there is one division that can be applied to almost all categories - outboard or sterndrive.

There's no doubt about the popularity of outboards, yet these days there's no shortage of sterndrives being backed down ramps on weekends either. In fact, the Mercury/Savage prize boat we've talked about for a couple of issues is a sterndrive.

It's powered by the smallest package in the MerCruiser range, the four-cylinder in-line 3.0lt TKS (Turn Key Start) coupled with an Alpha One drive. It's the ideal choice for this particular boat. We visited Mercury and spoke about it and about sterndrives in general.

David Meehan, director of MerCruiser, said: "Over the last few years in particular, sterndrives have really taken off. We used to sell sterndrives by handfuls of tens each year. Now we're selling many, many hundreds per year."

The widespread acceptance of sterndrives definitely indicates that boating enthusiasts will respond when a useful alternative is presented. Sterndrives certainly do have some practical advantages and aesthetic appeals, too. "At boat shows we sometimes get asked if there are any extra maintenance issues with sterndrives, but there really aren't any beyond those that relate to outboards and pretty much all boats in general", he said. If the simple steps in the manual are followed you won't have any problems.

There's a wide range of sterndrives available. The quickest glance through the various brochures, or at the web, certainly shows that there's some interesting engines and equipment available. And Mercury manufactures a great many of these sterndrive packages. Of course, choices narrow somewhat when you have a specific purpose in mind and start to focus on what you really need. Here, we'll give a broad overview of the various products on offer.

All MerCruiser sterndrive engines are four-stroke units. (The MerCruiser/Cummins alliance has diesels.) Apart from the lack of two-strokes, sterndrives are differentiated in the same way as outboards; specifically, in terms of increasing capacity and power. There's also the basic choice of carburetted technology or fuel injection, and of course, V8 engines are available. However, one very big difference that sets current sterndrives apart is some of the manufacturing detail that goes into the engines and accessories.

NEW TECHNOLOGY
These days, manifolds are cast with lost foam technology that creates more uniform wall thickness and avoids thinner areas that can lead to weakness. Also, some sterndrive models have a special layer of ceramic material applied to both the inside and outside of the manifold before it's painted. This is a difficult process and if aftermarket manifolds don't have it, they won't last as long as factory units. Saving a few dollars by purchasing a considerably cheaper manifold will very likely mean that the extra protection offered by the factory approach will be absent.

Aside from very specific treatments like ceramic coatings, the basic painting process has changed a lot over the years, too. Any painter will tell you that preparation is everything and using the right paint is also essential. The process for MerCruiser sterndrives has more than a dozen steps. Zinc chromate primer is used and the various coats are applied using EDP (Electro Deposited Paint) technology in which the paint and the components are given opposite electrical charges. This means that the paint droplets are attracted to the components and find their way into places that would otherwise be difficult to reach. Again, uniform thickness is important.

The paint itself contains special additives that make it flexible. This helps avoid chipping, which could create localized exposure to the underlying metal and create intensified galvanic corrosion at the site of the damage. It's the same story with outboards, too. Exposing small areas of metal to a corrosive environment is actually worse than exposing a larger area, because all of the corrosive potential will be concentrated at that point. In addition to paint, the Bravo range of sterndrives is available with a hard anodized coating which is highly resistant to chipping and corrosion. This is the SeaCore System and it also features enclosed fresh water engine cooling. Also, the patented Xk 360 alloy used in the drives is highly resistant to corrosion.

Although there are no particular disadvantages in owning or maintaining a sterndrive, are there any inherent advantages? We know a number of people who have owned outboards and then switched to a sterndrive and say they'll never go back.

The main advantage is that the mass of a sterndrive sits much lower than that of an outboard and this moves the whole centre of mass or centre of gravity of the boat much lower. Consequently, handling is greatly improved.

Of the boat we're giving away, Meehan says, "It handles like it's on rails." I'm sure I'll agree when I can get away from the keyboard for a day and take it out.

The look of a boat is also a high priority for an owner and sterndrives result in a very neat profile, because there's no outboard hanging off the back. It must be said, however, that some people like being able to see their engines. Personal taste and all that.

One thing that also improves the look and functionality of a sterndrive is a generous swim platform extending out over the drive leg. This feature can also make it easier to step on and off a boat at a jetty. Of course, many outboards have similar platforms. It's just that they're not full width.

BEST OF BOTH WORLDS
Actually, the way both outboards and sterndrives are installed can have a major influence on functionality. Outboards do allow some access to the water directly over the transom at either side of the motor, whereas some sterndrive installations extend across the whole width of the stern and limit access to that section of the boat. Of course, in a full-width installation the top of the engine cover is padded and forms a sunpad which can be excellent if your boating activities include the rest of the family. But as we say, it all depends on the installation.

In the Savage subscription boat, the engine cover doesn't extend the full width of the boat. Instead, it has a lift-up lid arrangement that's padded, but allows some access to the transom on either side, just like an outboard. Additionally, there's a full-width swim platform, so it's the best of both worlds.

Aside from appearance and handling differences, sterndrives make power differently from outboards. For instance, the 3.0lt TKS is rated at 135hp. But for on-the-water performance comparisons a 115hp outboard is chosen.

This is mainly because power is measured at the flywheel (it's actually calculated rather than measured, but that's another story). Any losses that occur in the drivetrain occur after the power figure has been determined. The connecting shaft between engine and propeller driveshaft in an outboard comes straight off the bottom of the engine in the powerhead and mates with the gearing on the propeller shaft.

Torque from a sterndrive has to change direction twice before it reaches the propeller. And even before it passes through the extra set of gears, it also has to pass through a pair of universal joints. All these transfers reduce power. Also, outboards tend to rev faster than sterndrive motors at this level. So, while an outboard might have a lower power rating, it will be spinning faster. Outboards do have a slightly higher top speed and they are a little quicker out of the hole, but not by all that much. Don't forget the handling advantage either.

Another good thing about Mercury sterndrives, in particular, is that they use a V6 gearbox, so they can be fitted with larger propellers than corresponding outboards. This reduces top speed a little, but improves just about everything else like handling, reversing, etc.

So far, we've only been talking about smaller sterndrives that have comparable outboards. The thing is, outboards are only making around 300hp. If you need more power than that you either have to start strapping multiple outboards to your boat or move to an inboard motor working through a sterndrive. Again, multiple outboards isn't a bad look, but if you wanted to keep cleaner lines you'd still go for a sterndrive.

Of course you can't fit more than three or four outboards across the back of a boat so if you still need more, you can fit multiple inboard motors working through dual sterndrives. And that brings us to boat sizes that are probably just a bit beyond our usual considerations, even though they may still be on trailers.

There are a lot of reasons to try a sterndrive boat. If you haven't had one yet you might try one on for size next time you upgrade.