First came the oar, then sail, then steam, but for the modern small-boat owner who wants to be sure of getting there, the answer has long been gasoline. The only trouble with gas is that it is both dangerous to handle and expensive to buy. For this reason the small-boat man has long looked with envy at his big brother, the diesel user. Both gas and diesel engines work on the same internal combustion principle, in which fuel is exploded in a cylinder to push a piston which creates the power, but diesels do it far more simply, safely and with cheap fuel. So why aren't diesels powering all boats?
For the most part, because they are just too big. Industrial diesels are often three stories high, locomotive diesels run up to 16 large cylinders and large marine diesels come in 20-ton economy sizes. In the 1930s Mercedes-Benz tucked a compact diesel under the hood of a production passenger car. Diesel-power Mercedes taxis began roaming New York's streets in 1959, and diesels now account for 14 per cent of Mercedes automobile sales. But it took 30 years for diesels to catch on in the small-boat field. Now boat man Carl Kiekhaefer, who makes his own Mercury gas engines and is not inclined to altruistic philanthropy, is beginning to import Mercedes-Benz diesels to link with his Merc-Cruiser outdrive for powering small boats. Moreover Britain's Perkins Engine company has just developed the first outdrive specifically designed for diesel use and a new souped-up diesel engine aimed at the European small boat market where high fuel costs make the diesel especially attractive. Ruston Rover has put out a marine conversion of their famed Land-Rover engine, and Sweden's Volvo-Penta, the forerunner of diesel outdrives, is offering a wide range of diesels from 2 to 155 hp. "For trouble-free boating use diesel," advises Volvo Director Harold Wiklund, but he believes it will be many years before diesels truly conquer the market for boats under 30 feet.
The diesel's drawbacks, like its advantages, are an integral part of the compression-injection system first patented by Rudolf Diesel in 1892. Where a gas engine sucks in a mixture of fuel and air, and then ignites it with an electric spark, the diesel takes in only clean air, and therefore needs no carburetor for premixing. Because it is so inflammable, the gas engine mixture can be compressed to only one sixth of its original volume, which limits the amount of combustible material and so limits the power per stroke. But the diesel can squeeze its pure air to [1/14] of its original volume, and by so doing heat it to a higher temperature (800°) than the burning point of the fuel. At top compression, an injector exerting even higher pressure sprays fuel into this seething furnace which mixes with the hot air and automatically ignites, eliminating the need for spark plugs and the whole electrical system that goes with them. The one fussy thing about a diesel is the fuel injection. For each cylinder, fuel injectors must measure out an' accurate dose of fuel and deliver it in an even, fine mist at a precise moment. Injectors are therefore precision instruments and they object to dirt. As the piston is forced down by the explosion, a valve opens, allowing the hot gases to escape. Later in the descent the piston uncovers a fresh air intake which scavenges the remaining exhaust and fills the cylinder with pure air for the next stroke. In four-cycle diesels the piston makes an extra stroke just to force out any loitering fumes, because exhaust is hot and would expand the incoming fresh air to prevent a full supply. Diesel engines can be souped up by blowing the fresh air in under pressure (supercharging) to give the engine more power. Two common complaints against diesel—smell and noise—are the result of improper care. Since diesels seldom complain, they rarely get the attention accorded their gas-powered cousins, hence dirty injectors or the wrong grade of fuel make their presence noisily and noisomely known.
Because of its relative simplicity, its reliability, its fuel economy, its thermal efficiency and its cool exhaust, the diesel is obviously a boatman's dream come true. The trouble is that all these virtues are dependent on high compression, and high compression needs a stout cylinder and a big housing to contain it. Commercial fishermen, who tend to be austere and thoughtful investors, have long chosen diesel power, but the small pleasure boat field has so far given it mostly lip service. The small-boat builder seldom builds a hull strong enough to support a diesel installation. The small-boat owner is seldon willing to pay the diesel's high original cost. Using only two-thirds as much fuel as a gasoline engine, and with the fuel costing only half as much, the diesel is only one third as expensive to feed as the gas engine. Ranged against the heavy fuel appetites of equivalent gas engines, diesel pays off after a mere 600 hours of running—but it costs twice as much to buy. Right now a diesel installation still requires a substantial foundation for support and to absorb vibration. Fuel is not readily available, nor is it of uniform quality. So Sunday drivers, hot rodders and many other small-boat owners will go with gas for some time. But for those who want to pay the price (small in contrast to savings in maintenance) the age of the diesel is at hand.