Boat Outboards 2013By Michael Vatalaro
Published: April/May 2013
The past decade has seen an amazing array of changes and improvements in outboard engines. If you haven't shopped for one lately, you're in for a nice surprise.
Carburetor Becomes A Dirty Word
Before we delve much further into the differences between modern four-strokes, it's important to understand two things: why you can't buy a new carbureted two-stroke anymore, and why the original four-stroke engines generally weighed 20 percent more than a two-stroke of similar power. Traditional two-stroke engines have intake and exhaust ports on opposite sides of their cylinders; these are "opened" and "closed" by the edges of the piston (skirt) as it travels up and down. Really, they're blocked or unblocked, as the piston passes by the openings and, importantly, there are points in the process when both ports are open at the same time. That means that during the down stroke, when the fuel/air mixture is being drawn into the cylinder from one side, some of it passes unburned right over the top of the piston head, and out the exhaust port on the opposite side (illustration A below). These are, in emissions parlance, "unburned hydrocarbons," a fancy way of saying wasted gas and oil. With outboards being cooled by water drawn from the lake or ocean, and the exhaust gases mixing with that cooling water on its way out, this wasted gas and oil ends up in the water. The Environmental Protection Agency (EPA) put an end to this type of contamination with regulations that took effect in 2004, and became more stringent in 2007.
©2013 Mirto Art Studio, www.mirtoart.com
The spark plug fires on every upstroke in a two-stroke engine. you can see how the gas (green arrows) can pass right over the piston head from the right-hand side of the cylinder and out the exhaust on the left side during the down-stroke.
Four-strokes, on the other hand, use valves on the top of the cylinder — as many as four valves per cylinder, depending on the design, to introduce the air, and release the exhaust from the cylinder. Critically, the four up-and-down motions of the piston (or strokes) separate the intake and exhaust processes, so the fuel/air mix can't escape (see illustration B below). The result is far fewer unburned hydrocarbons. Engines that meet the 2013 standard produce on average 91 percent fewer emissions than old two-strokes. Making all those valves open and close at precisely the right time requires a whole complement of extra parts — camshafts, lifters, rockers arms, and so on. The result was also around 50 to 100 more pounds in parts in a midsize four-stroke compared with the simpler two-stroke.
©2013 Mirto Art Studio, www.mirtoart.com
Four steps are required for every power-stroke (firing of the spark plug).
That's where things stood for a long time. Four-strokes were inherently cleaner and heavier, and two-strokes were lighter but dirtier. But Honda, Mercury, and Yamaha have all spent the past few years developing new four-strokes that perform and weigh comparably to two-strokes. And Evinrude, Mercury, and Yamaha have all developed two-strokes that are as clean running as a four-stroke. Each has pulled off a neat engineering trick.
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The Repower Predicament
So which outboard is right for you?
Repowering right means sorting through the many options now available to find the one that best matches your boat, the way you use it, and your brand preferences. You'll want to consider the following:
- Boat design. Your engine choices will be limited by the weight your boat can carry, the horsepower it can handle, the space you have on the transom, the fiberglass work necessary to mount the new engines, and the structure at the stern.
- Duty cycle. Stratified combustion will save fuel if you spend most of your time trolling; mid-range punch will be better to get you through a choppy inlet or out to your offshore fishing grounds.
- Maintenance schedules. In general, DI two-strokes will have longer intervals between scheduled services because they lack mechanically controlled drive trains that need adjustment every 500 to 1,000 hours.
- Electrical demand. If you want to keep those fancy electronics running all the time, make sure to check not only the rated amperage output, but also output at lower rpm, where it could be half of what it is at cruising speeds.
For a full breakdown on each of these considerations, see "What Should You Consider When Planning A Repower With New Outboards".
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