The future of inboard engines?

From Ocean Navigator #115
July/August 2001
Auxiliary engines for voyaging sailboats have certainly evolved in the past few decades: from the nearly ubiquitous in-line four-cylinder gasoline powerplant to the high-rpm, turbocharged marine diesel of today. What if we look ahead? Will the reign of the internal-combustion diesel continue indefinitely? Or are there other technologies that could replace it?

It wasn’t more than a few decades ago that the auxiliary engine on most sailboats with inboard engines was powered by gasoline. Long before turbochargers, electric fuel pumps, and electronic controls were widely available, these gas engines had good old-fashioned carburetors, mechanical fuel lift pumps and simple points and condenser ignition systems. Owners of these gas engines, like the old Atomic 4s, were familiar with the ritual of first running a blower to clear the engine space of explosive gasoline fumes.

Diesel engines, of course, had the advantage of a far less inflammable and generally less expensive fuel. But marine diesels had the reputation of being too large and heavy to put into the average voyaging boat. That perception changed dramatically in the last two decades as diesel manufacturers made lighter, smaller diesels. Soon, of course, gasoline engines become the rarity.

With the transition from gasoline to diesel complete, what could be the next step in powerplants for the voyaging sailboat? The likely bet is that we will see more powerful and more efficient diesels. But what about other technologies? Could we see the replacement of the internal combustion engine?

What about external combustion engines?

Probably the best-known and most widely used external-combustion engine before the advent of the gasoline internal-combustion engine is the steam engine. Early in the development of automobiles, steam engines were a serious rival to gasoline powerplants. A Stanley Bros. steam-powered car set a land speed record of 126 mph at Ormond Beach, Fla., in 1906. It wasn’t until 1910 that a gas-powered car bettered that mark by three mph. Steam engines fell out of favor, however, and, at this point, don’t seem a viable alternative to the marine diesel.

Another type of external combustion engine is the Stirling engine. In a simple version of this engine, invented in September 1816 by Robert Stirling, a Scottish minister, a cylinder filled with helium gas is heated at one end and cooled at the other. Inside the cylinder is a loosely fitted piston called a displacer. Plumbed into this main cylinder is a smaller cylinder that has a tightly fitted power piston. Externally applied heat at one end of the cylinder causes the gas to warm and expand. As it expands, it pushes on the power piston, forcing it down its cylinder. The power piston is linked to a crankshaft. As the power piston moves in the cylinder, the linkage turns the crankshaft. Meanwhile, the hot gas in the main cylinder is moved by the displacer to the cold side of the main cylinder. At the cold end, the gas cools and contracts. This decrease in volume and, hence, pressure, draws the power piston back up the power cylinder, further turning the crankshaft. The displacer, which is driven off the crankshaft, moves the gas back to the hot end where it can once again be warmed as the engine begins another cycle. And because there is no explosive burning of fuel as there is in a gasoline or diesel engine, Stirling engines are reportedly much more quiet then their internal-combustion cousins. This description of the workings of a Stirling engine is simplified to aid understanding of this unfamiliar machine. There are many possible Stirling engine configurations.

So Stirling engines will turn a crankshaft, but why would we want to turn our backs on our trusty diesels? One of the main selling points for Stirling engines is efficiency. A diesel engine has to mix fuel with air and burn it at very high temperature inside a cylinder. To do this as efficiently as possible is challenging enough at fixed rpm. However, a diesel must do this within a range of rpm settings. The external-combustion Stirling engine, on the other hand, burns its fuel only to supply heat to the hot side of the main cylinder. The burner is optimized to produce heat at one fuel flow rate.

One company working on perfecting a Stirling engine design is the Quiet Revolution Motor Co. (, based in Spring City, Tenn. QRMC originally planned to produce airplane engines for use in general aviation. However, lately the company, which reportedly has one Stirling patent and several more in the works, has focused on distributed power generation as a promising market for Stirling engine technology. Stand-alone generators running Stirling engines to produce electricity should be a excellent application for Stirling technology. According to QRMC president Philip Hodge, another promising field for Stirling engines is marine applications. This is because the greater the heat differential between the hot and cold sides of the Stirling cylinder, the more power you can get out of the engine. And in marine applications, you have an excellent coolant available in sea water. “It’s harder to get heat out than to put it in,” said Hodge. “Sea water represents an unlimited heat sink for shedding heat.” What will it take to get QMRC’s Stirling engines in production? According to QRMC president Hodge, it’s a matter of finishing development work, something QRMC claims it can do with a little more than a $1 million of investment capital. “It will happen at the speed of investor money,” said Hodge with a chuckle.

The Swedish Navy has already realized the advantages of quiet Stirling engines for stealthy submarines. The Gotland class of Swedish subs are powered by Stirling engines.

A Stirling engine on a boat could be used as a propulsion engine but it can also be employed as a genset to produce electricity. A New Zealand company named Whisper Tech has developed a Stirling engine-based genset called the WhisperGen PPS16 DC ( This unit can burn diesel fuel, kerosene, liquid petroleum gas, compressed natural gas and other fuels. It produces 750 watts at 12 volts. The unit also uses waste heat from the burner to make hot water. The WhisperGen is sea water cooled and weighs 198 pounds. Not the most impressive power-to-weight ratio (a 30-pound gasoline-powered generator will produce 1,000 watts), but the Stirling does so quietly and efficiently. According to David Moriarty, managing director of Whisper Tech, LTD., in Chistchurch, New Zealand, the company is currently focusing on the European market. In an e-mail message, Moriarty states: “We now have about 100 systems out in boats and we expect to produce about 600 over the next 12 months.” Moriarty plans to offer the WhisperGen in the U.S. market by the end of this year.

Looking further forward, (past the coming Stirling engine era!) there is yet another technology that could be the ultimate replacement for the diesel engine in a voyaging sailboat. Without any moving parts, fuel cells are not engines in the usual mechanical sense. Fuel cells can, however, produce electricity to drive electric motors that can do all manner of work on a voyaging sailboat, including, of course, turning a propeller for propulsion.

A fuel cell works by combining hydrogen with oxygen and produces two things every voyaging sailor craves: electricity and water. And a fuel cell does this with no moving parts and producing no pollution (other than some waste heat). The limitations on fuel cells include the expense of the cells, the difficulty of finding a hydrogen-equipped filling station, and the unwieldy nature of hauling hydrogen fuel around. Automobile companies are working on solving these problems in order to produce fuel cell-equipped cars. For example, they are perfecting a process, called re-forming, that, often using a multi-step process, can tickle out hydrogen gas from standard gasoline or diesel fuel. Daimler Chrysler even promises to market fuel cell-powered cars by 2004. Should the automobile industry succeed in producing an inexpensive fuel cell, voyaging sailors will probably be the next group to adopt it.

By Ocean Navigator