Even the hardest–core sailor realizes it's time to fire up the motor when the winds drop and there's still some distance to go to the slip or the anchorage.

Twenty years ago there were few choices, and they were mostly bad. But there's a reason for that: Internal combustion engines used to power sailboats arguably have the toughest job of any marine engine.

Look aft, and you'll see a saildrive installation under this Arion 29 sailboat.

Consider the proposition: sit in the bilge for hours, days or even months at a time, silent and unused. Then fire up on demand, work hard for a short time, and go back to another long nap. That's a tall order for an engine and its fuel system, and it typically resulted in cranky engines and even crankier sailors.

Today's sailor still faces a tough situation with marine engines but stands a much better chance of getting the propulsion he or she needs, when it's needed. Here's a look at what you can expect if you're buying a new inboard–powered sailboat or repowering a classic.

POWER OPTIONS

There are two ways to power a sailboat with an inboard engine: the traditional straight shaft and prop, and the saildrive. The shaft and prop setup is identical to the one on a conventional inboard powerboat. The saildrive, meanwhile, is an inboard engine with what looks like the lower unit of an outboard engine extending down through the hull.

A straight–shaft arrangement is common on conventional cruising boats, where there is often an extensive keel through which the shaft passes as it leads to the propeller. It's sometimes also installed on boats without full–length keels, using shaft struts for support.

A saildrive installed aboard a Grampian 28 sailboat.

Saildrives are typically installed on racing boats, since they have lower drag. They also may have a slight efficiency advantage because they have a vertical leg extending straight down from the bottom of the boat, which ends in a 90–degree turn aft, and therefore the propeller thrust is transmitted parallel to the direction the boat is traveling. By contrast, in a conventional inboard shaft installation, the shaft exits the bottom of the boat at a downward angle, which means thrust is not parallel to travel; the more the angle, the greater the wasted thrust.

As with all things boat–related, there are advantages and disadvantages to each. Straight–shaft installations employ widely known technology and components because they're so common, for example. And a diver can do some lower–unit maintenance. But, they have higher drag and a less–efficient thrust angle. Other minuses include the fact that alignment is critical to control sound and vibration, and they initially involve higher installation and labor costs. In addition, straight shafts require through–hulls for cooling water.

Saildrives, meanwhile, have lower drag, no alignment issues, and less–expensive installation costs. They don't require through–hulls for cooling water, nor stuffing boxes or shaft seals. But, installation does involve a big hole in the bottom of the boat and often a single, rubber O–ring to seal the opening. And transmission maintenance usually entails a haul out.

A close-up look of a saildrive with a folding prop.

 

BRAND OPTIONS

Whichever setup works for your needs, you'll likely choose an engine by either Volvo Penta or Yanmar, since they make up about 80 percent of the business worldwide. In the United States, Yanmar has a significantly greater slice of the pie, a fact both companies agree on, though neither would provide an exact accounting.

For recreational sailboats from 25 feet to 50 feet, inboard auxiliaries run from about 9 horsepower to more than 110 horsepower. In an average year, 1,900 to 2,200 inboard auxiliaries are sold, according to Douglas Rose, product manager for Volvo Penta's Americas office in Chesapeake, Virginia. That figure pales in comparison to an average year for I/O engines, of which perhaps 75,000 to 80,000 are sold, and for outboards, of which nearly 100,000 are sold, he says.

Volvo Penta became well known in the United States after it introduced the first stern drive, called the Aquamatic, in 1959. Today its sailboat diesel engines range from 11.8 horsepower to 174 horsepower and are offered in both straight–shaft and saildrive configurations, on sailboats from C&C (saildrive only), Tartan, some of the Sabre line, as well as a number of custom designs. Interestingly, Rose says the company sells more saildrives in Europe than in North America. "The European market has taken to it more readily than the American market," he explains. He speculates that the reluctance on this side of the pond may be due to some early bad experiences with units sold by some companies. Rose notes that these early saildrives were often nothing more than a gas outboard powerhead built on top of an outboard bottom end, and they suffered from disastrous reliability and corrosion problems.

Yanmar has been making industrial and other diesels since 1912. Its Marine Group, with North American headquarters in Georgia, makes everything from 9-horsepower to 900-horsepower engines. The company has been in the North American market for about 30 years and dominates the marketplace here for both repowers and new boats. According to Tom Watson, Yanmar's sales and marketing manager, repowering over the last 10 years "was probably 20 percent to 30 percent of the business." He notes that much of the work involved replacing venerable old gasoline engines, like the Atomic Four, with newer, more reliable and safer diesel engines.

A lightweight electric motor installed on a bracket.The company offers both straight–shaft and saildrive installations, but "North America is primarily a shaft market," Watson says. Some performance–oriented sailboats, such as C&C Yachts, are delivered only with saildrives, but the majority of sailboats here have straight–shaft installations. You can find Yanmar engines on Hunter, Catalina, Beneteau and Jeanneau boats as well as many custom designs.

PROP OPTIONS

Regardless of your installation setup, you'll also need to select a propeller: either a fixed–pitch one or a feathering prop.

Many cruising sailboats have basic, fixed–pitch propellers. These provide good thrust under engine power but impose drag under sail, usually a loss of one–half to 1 knot on the top end. Fixed–pitch props also usually perform well in reverse, important in close–quarters handling.

A second type of propeller employs either a "feathering" or a folding mechanism to reduce drag when not under power. Feathering props turn the blades edge–on to the water stream, reducing drag by as much as 80 percent. A folding prop does just what it sounds like, folds the blades back into a streamlined, compact package that slips easily through the water.

Feathering/folding props are usually more expensive than their fixed–pitch brethren, due to the mechanical complexity involved. Choosing the best propeller for your boat also becomes somewhat more difficult when you consider that performance characteristics differ from one company's design to another. However, Rose says Volvo Penta has noticed more people choosing lower–drag propellers in recent years. As a result, Volvo Penta now offers them in two-, three- and four–blade designs. They were designed, according to Rose, "with reverse thrust and low speed as the first priority, followed by low noise and vibration, then low cavitation, and finally high forward thrust." Rose says high forward thrust "is fairly easy to come by in the design process, but not so the other performance characteristics."

YANMAR MARINEAn example of a saildrive from Yanmar.

EMISSIONS RULES

Interestingly, while the bigger recreational diesel engines are quickly being dominated by electronically controlled, common–rail systems, most sailboat engines are still mechanical. The reasons for both situations are the same: the timing and applicability of new emissions regulations. Smaller sailboat engines are treated as off-highway engines, which means they need only comply with the less-stringent regulations governing those uses, while the larger, high-horsepower engines are required to meet tougher standards.

In the United States, the Environmental Protection Agency (EPA) began regulating truck emissions for carbon monoxide and other pollutants. The pollution limits were gradually extended to off–road applications, including marine engines, during the first part of this decade. The limits have been phased in over time and by engine cylinder displacement, so that not all engines are on the same schedule.

Most pleasurecraft engines currently must meet the so–called Tier 2 emissions limits. Tier 3 goes into effect in January 2009. Volvo Penta recently announced that its smallest engines, the D1 and D2 diesels, are its first to meet the new Tier 3 standards.

For bigger marine diesels, electronic controls and common–rail fuel systems are the most practical way to meet the ever–tightening emissions limits. According to Rose, "we can meet Tier 3 with mechanical controls with these smaller engines, but the bigger engines will probably require some sort of exhaust gas treatment." This may include some type of catalytic converters or particulate traps, perhaps even both.

At the Tier 3 emissions levels, modern marine diesels will be causing significantly less pollution than just 10 years ago – a reduction that may be as large as 70 to 80 percent from pre–regulation days. There is a Tier 4 on the books but no timetable set for compliance. Some in the industry joke, half–heartedly, that with Tier 4, "the engine has to take in smog and exhaust clean air."

Volvo Penta recently switched to an electronic, common–rail system for its largest sailboat diesels and has added a partial CANBus information capability for its smaller, mechanical diesels. This data, compatible with NMEA 2000 protocols, will enable use of some of the more advanced gauge packages Volvo Penta has and can integrate a great deal of operational information, with the exception of fuel flow.

Yanmar's Watson says his company is in a similar position in terms of mechanical versus electronic diesels right now for the sailboat market. Watson believes the future regulatory schemes limiting diesel pollution will determine how fast electronic controls are adopted, a position shared by most in the industry.

Whether mechanical or electronic, today's sailboat auxiliary engines are far more reliable than those of just 10 years ago. They provide high torque and thrust levels at low speed around the dock, unlike their gasoline predecessors. They no longer stink or emit large blue plumes upon startup, and they continue the historical reliability of the simple diesel engine. Today a buyer has only to consider the size of the engine and type of propeller, for the most part. It's a big improvement from when sailors began their days following the wind.

Tom Tripp is a freelance writer specializing in technology and marine science whose work has appeared in publications such as Northeast Boating and Chesapeake Bay Magazine. In addition to contributing features on new boats and technology, Tom writes a blog here on Mad Mariner.