Single Or Twin Engines- Which Is Best?

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There are certain subjects that bring out spirited opinions in everyone. In trawler circles, a dead ringer for dockside debate concerns the argument of one engine versus two engines. There are claims that can be made to support either point of view.

Having said that, I now contend that much of the emotional and gut-feel intuition you have about this subject may be suspect-or at least open for analysis. You see, I intended this article to come together in a "Point...Counterpoint" format, showing the arguments on both sides of this popular Holy Place topic.

I discussed the subject with professionals in the marine engine industry, asking some pointed and open questions. By identifying the valid pros and cons, I thought we could all get a little more informed about the real issues, and the benefits of single engine and twin engines. The different opinions could be listed and shared with you, and then we could all choose up sides. Boy, was I in for a surprise...

The opinions of these marine diesel experts were decidedly one-sided-in favor of a single engine installation. This is simply amazing, considering the many thousands of boats built with twin diesels. However, these people know their business, and their experience/opinions are a direct result of many years in the marine industry- serving commercial, military, fishing, and pleasure boat applications.

There is apparently a lot of vaporous thinking out there concerning reliability, maneuverability, safety, economy, and applicability when discussing a single engine versus twin engines.

Obviously the final decision is up to you, the boat owner. But just listen to what these guys have to say...


Did you know nearly one hundred percent of the commercial fishing and workboat vessels (of all sizes) are single screw/one engine boats? Scallop draggers, lobster boats, charter fishing boats, longliners, crabbers, seiners, whatever- these vessels are all powered by a single diesel engine. (And that engine usually has keel cooling and a dry exhaust.)

Bob Tokarczyk, Marine Sales Manager at Bell Power Systems (the Northeast distributor for John Deere Power) summed it up by stating, "From an engine representative point of view, engines aren't troublesome. If they are maintained, they basically will give the dependability and durability expected.

"If you look at the commercial side, you'll find almost all commercial vessels are a single engine vessel-you only see multi-engined vessels when there are rigorous situations, such as ferry boats, where there are exceptional demands placed on the application."

All of these engine manufacturer representatives felt the same-the reliability of the engine (and drive components) is not in question. Commercial vessels go out in much worse conditions than we do, and they put thousands of continuous hours on their engines each year.

Bill Naugle, Senior Application Engineer at Caterpillar, had another persuasive argument. "A fuel-related problem that kills one engine will probably kill both engines, so reliability isn't equated to redundancy. The redundancy reason just isn't valid.

"Any modern diesel is pretty reliable, and things just don't fly apart anymore. But if anything in the system fails, you're done-so maintenance is very important. Professional fishboat operators probably maintain their equipment a lot better than pleasure boat owners, which is a factor to consider.

"Pleasure boat owners don't run their engines hard (high rpm and horsepower load) in the first place, so the engines are not being stressed. Anybody who buys a trawler is not a go-fast guy. These people probably are going cruising, so they are more worried about noise and fuel consumption. Many are ex-sailors. These people just don't abuse machinery-so things last forever."

Bill Hirt, Application Engineer at Cummins Marine, adds that manufacturers build engines for different horsepower ratings, which allows the designers to insure reliability at lower load ratings. "Typically, we put a derated-type engine into trawlers, which may be the same basic engine as a higher horsepower version, but set up for less horsepower demands. And there is less stress in these lower horsepower applications."

Obviously it is important to have all the necessary spares on board your boat, so if there is a problem it can be resolved safely and timely. Relying on the second engine to get you home because you don't carry a spare waterpump impeller isn't the mark of proper seamanship.


A boat with twin engines is easier to maneuver, no doubt about it. So if you are going into places where you are worried about high winds, small tight spaces, then the two engine setup may be the answer. But even this seemingly obvious conclusion didn't phase the experts.

Al Kozel, Vice President of Marine Sales at Detroit Diesel, said it best. "I think a lot of pleasure boat owners are uncomfortable docking a boat with one engine. Having twin engines makes a big difference in maneuverability. But with a properlysized bow or stern thruster, a single engine vessel can turn circles in its own length-so that takes care of the maneuverability question."

Again, they draw our attention to commercial boats, where operators work 15/18 hours a day, 5/7 days a week. They have no problems maneuvering around a 40'/50' boat with a single engine. They know the thrusts, and they know how to get into a spot, taking advantage of whatever conditions exist at the time.

The general feeling is that maneuverability is a thinking person's game. If you think about what you are doing, knowing you have a certain amount of side thrust coming off your prop, knowing where the winds are and so on, you can utilize all these factors to successfully maneuver your trawler. The commercial people think about the conditions and vessel characteristics, and make them work to their advantage.

By having two engines, we have the piece of mind that we can achieve something that really could be done with a single screw-but maneuvering a single screw requires more thinking.

Not one of these engine professionals felt maneuverability was enough reason to consider two engines. Get a thruster...or practice.


The experts agree this is a question of perception. Many people feel redundancy is safety, but it gets back to the factual versus perceived value of two engines. If you really sit down and pick it apart, there are not many factual reasons for two engines, and you might even find reasons that make two engines less desirable for safety.

Kurt Hoehne of Alaska Diesel Electric (makers of Lugger diesel engines) points out some lessobvious safety considerations. "Remember, as many failures occur due to damage to the propeller or shaft as to the engine itself breaking down. A single prop is usually protected by the keel, and therefore is less likely to be damaged."

The props, struts, and shafts of twin engines are very much exposed to lines, nets, trash, logs, or other potentially damaging objects. A single propeller in an aperture is protected by the keel. This is a point to consider if you cruise in areas with floating debris.

Kurt continues, "And while the second engine is a good backup, other systems are routinely installed. A 'get home' engine is very popular, either on its own drive system or connected to the main shaft. If it's connected to the main shaft, it becomes useless if the main shaft and/or propeller are damaged. Some get home systems are installed with saildrive units and folding props, so they're there when needed, but have only minimal drag the rest of the time.

"Some stern thrusters are an appealing alternative. These transom-mounted units can be lowered and rotated 90 degrees to act as get home power. This eliminates drag, yet helps maneuverability, and provides a safety margin."

But Bill Naugle of Caterpillar doesn't see the need for exotic alternatives. "All the fishboats run around with one engine. And these professional fishing boats don't have alternative powerplants or get home systems. If you are on a coast somewhere, go talk to the fishing people-people who know how to make the engine room a very reliable place for all machinery. They go out all the time with a single screw vessel."


The initial cost of twin engines is usually higher, as is the cost of operating them.

Bob Tokarczyk said it best. "With two engines, you have twice the worries, twice the maintenance, twice the instrumentation, twice the number of filter changes you have to think about-as well as everything else that needs to be done with two engines. Overall, the costs are far greater."

Bill Naugle adds, "Maintenance and cost are pretty much doubled. If you have a specific horsepower requirement for a given hull, it doesn't really matter whether you power with two small diesels or one bigger diesel-the fuel consumed is pretty much the same (the smaller engines probably use a bit more). But all the other costs are doubled."


Now that we have looked at reliability, maneuverability, safety, and economy, how does all this apply to your boat and cruising agenda? What else do you need to think about? Here are their comments...

Brian Smith of American Diesel, "It is important to size the engine correctly. American Diesel sticks with the naturally aspirated diesel design, and stays away from turbocharging for the majority of these applications."

Kurt Hoehne of Alaska Diesel, "For the long distance cruiser like a trawler, both single or twin engines should be sized and geared to provide the speed you are looking for. Getting this formula correct is critical."

Bill Naugle of Caterpillar, "Having two small, hightech diesels to produce the horsepower of a big single diesel may be an option, but there is a question of overall life of the engine. The larger engine is going to live much longer than the smaller engines. If you are in a situation where life is at risk (as is the case offshore)-this is a consideration."

Bob Tokarczyk of Bell Power, "Buy a good reliable engine, an engine that is rated properly. Don't go out and buy the smallest, highest horsepower engine-it just won't be as reliable or as durable. Buy an engine that is designed for running longer periods of time, not just for high performance."

Al Kozel of Detroit Diesel, "There is absolutely nothing wrong with a single diesel engine. And there is no real justification for twins for most applications. If one engine can do the job, then there really is no point in having two."

Bill Hirt of Cummins Marine, "If I were a diehard sportfisherman, you better believe I would want two big engines-but for a trawler application, I probably would just have one engine."


This will probably generate more discussion than answers-but that's fine. It is important to scrape away at the perceptions, and look at the real issues and tradeoffs.

Your boat should be equipped to suit your own desires and requirements. If your speed requirements can't be satisfied by a single engine installation, for instance, then that is the end of the debate for you.

The same is probably true if you feel redundancy is best, no matter what else is said on the subject. No doubt about it, two engines are found in thousands of boats. If you don't have a thruster, twin engines will definitely give you the edge in maneuverability.

However you come out in the dockside argument, take a hard look at your own rationale, and see if these experts have put anything in a different light for you. If you are considering a new boat with an engine option, try both single and twin engine versions before making a decision.

And let the great debate continue...


I would like to thank the following individuals for their comments and experiencebased opinions. I got their names directly from the engine manufacturers, so be clear these people know what they are talking about!

Kurt Hoehne
Alaska Diesel Electric
Seattle, WA

Bill Naugle,
Senior Application Engineer
Caterpillar Inc.
Mossville, IL

Brian Smith
American Diesel Corporation
Kilmarnock, VA

Al Kozel,
Vice President of Marine Sales
Detroit Diesel Corporation
Detroit, MI

Bill Hirt,
Application Engineer
Cummins Marine
Charleston, SC

Bob Tokarczyk,
Marine Sales Manager
Bell Power Systems (Distributors for John
Deere Power Systems)
Essex, CT