Single Or Twin Engines- Which Is Best?

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

21 comments on “Single Or Twin Engines- Which Is Best?

  1. mike marsh

    I’m an old man, and always wanted my own boat.
    I already know I have no right getting a 40’boat.

    The two boats I’m looking at are a 41′ egg harbor twin diesel, and a 36′ single screw trawler.
    and my question:
    What is the difference between the Egg engine and the trawler engine.

    mike m

  2. Peter Swanson

    Not to be flip, Mike, but the Egg Harbor has twice as many of them. There is probably quite a bit of difference between the boats, in fact I’m sure of it, but the engines or engine will pretty much work the same way. The real question is what kind of boating will you be doing, what can you afford and which boat will be the most fun for you. Because, you know, you really do have a right to get out on the water and enjoy life.

  3. Robert Williams

    I’m glad I found this article. I’ve been all about “sails”, for most of my days and engines were merely a necessity for getting around when you had to use them. Since an injury that will hinder me for the rest of my life, though, I have to switch to power boats and I’m leaning towards the Trawler. I have argued my conclusions (for my personal belief that a single diesel, properly matched with boat and drive and a bow thruster is all I need) with many people, as I have been learning about my new quarry. This piece gives me a bit more assurance and I think I’ll go forward with this issue settled (at least, for me). Thanks. Now I just have to muddle through all the different makes and models of diesels to find which is best and I’ll be all set. Should be a hoot. Happy Sailing, everyone…

  4. Marshall

    This is such a great article. Being a boat owner (Fu Hwa 34′ trawler) and a former journalist, it is well written and pertinant. Having a single screw, I always have anxiety attacks when leaving and coming into port, with such tight confines that our marina has. A thruster is definitely a plus, but I use it only when I have to. My confidence level is improving, but there’s nothing better than practicing getting into and out of your slip. Happy motoring.

  5. Barrie Marfleet

    Hello Bill: this was a great article and very timely for us as we are looking for a trawler and I have flip-flopped between looking at single engine or twins. Now I going for single for sure.
    Here is my question: In my research, some boat owners tell you what the fuel consumption is per hour. What I have discovered is a wide swing. some owners say 1.5 gph right up to 5.5 gph or higher. How do I determine which boat with whatever engine is going to be the most economical to operate. Is there a chart that shows speed and gph vs the size and weight of boat out with this information?



  6. Tom K.

    I would go with the single engine,
    I started on a 21′ double ended single screw boat when I was 10. My dad the tug captain made sure I knew how to handle the boat before I was allowed to take it out. After 20 years in the Coast Guard I teach boat owners how to maneuver around the docks and it make no difference how many engines you have. If your taught how your boat responds forward and astern with wind and tide you’ll have the tools to make it with less problems. once out on the water twins only burn more fuel and maintenance dollars.
    Keep in mind if you have one engine you have more engine room and it is my belief that you will make sure it is properly maintained.

  7. Gregg Dearth

    I’m surprised no one made a comparison to aircraft. Look at the number of single engine aircraft compared to multi engine varieties. If a pilot can rely on a single engine to take him safely up and down, surely a captain can rely on a single engine boat. Given fuel prices these days, I think the argument for a single versus twins is pretty much settled.

  8. Peter Swanson

    That has something to do with the fact that when a twin engine aircraft loses an engine, it creates some problems unique to an aircraft. Think swimming with one arm.

  9. Duane willoughby

    We want a 34-35 ft with a single or twin diesel – my wife sometimes thinks she wants extra doers did we are also looking at mainships or Californians – but more important is the efficient use of the engine – how do you choose between an 85-135hp Lehman or Perkins or cummins that goes 7/8knts versus a 210-250 cummins or cat that you could cruise at 8knts but bring it up to 15knts if needed or wanted

  10. David Howe

    Good dialogue! I am retired Navy and own a 36′ steel shrimper that was fitted with a single 1990 210-hp Cat 3208 NA to haul nets. I converted her for diving and light salvage — no nets, but much gear on board. I usually run at 1500 rpm, burning about 2 gph for 7 knots, and goose her to 2500 rpm for a few minutes every day or two to clear her throat. I have had twin-engine boats up to 60 feet and much prefer a single. One that runs reliably is better than two that do not. Maneuverability has never been a problem, even in high winds, but you have to learn your boat. The extra space that a single engine allows in the engine room is a factor in maintenance and comfort. I am too old to enjoy boat yoga while changing oil and filters. Over the past 15 years the engine has run more than 8,000 hours without problems. With proper maintenance I expect to get at least 30,000 hours, despite the occasional comment that the 3208 is a “throwaway” engine because it lacks cylinder liners. I wish Cat still made that engine. I recommend carrying a spare for every critical part that cannot be jury-rigged at sea — a complete seawater pump, oil pump, transmission cooler, high-pressure oil lines, belts, alternator, etc. — anf get a shop manual for the engine! Buying a full complement of spares is a lot cheaper than buying, feeding and servicing a whole other engine that you do not need.

  11. R Welch

    I’m convinced – a single engine will be my route !! It seems to me a that boat loaded with critical spare parts is only as good as the guy putting them on. I believe a basic course in Diesel Engine repair is most appropriate. Where are there such schools ?

  12. C Evans

    The single engine does make sense to me. I am looking at an older trawler built by
    a well known builder that initially installed diesel twins but after the first five or so built,
    changed to single screw . The question now is do I buy the twin screw at a discounted “right” price that I am comfortable with. How would the future resale be affected?
    Can one engine run at a time to save on fuel? I expect the drag of the second prop would
    outweigh any benefit. Being from the sailing side this is a different game. Is anyone
    able to guess a % of extra fuel used with a second screw?
    Purchasing a new model would be wonderful, but if not two screws the dream may end.
    Any thoughts? Great site by the way.

  13. Mark

    Great article and excellent points. So one question; does this apply to boats with outboard motors as well?

    In-short, I’ve seen a lot of trawler style boats with two smaller outboard engines VS. one single larger outboard engine. Any thoughts on the same topic but regarding outboards?

  14. Samuel Tunis

    As a “landlubber” in the midwest, I watched such things on TV and wondering why anyone would ever use a dual-engine configuration. Watching shows like Dexter and seeing his boat having a dual-engine setup just confused me, from a vanilla physics standpoint.

    I’m happy to see that the results are (almost exclusively), “Speed, maybe” and “Maneuverability, maybe” with no other really valid reasons helps to reinforce what I pretty much already knew just based on a simple understanding of physics. The additional practical reasons to not run multiple engines just makes for icing on that well thought-out cake.

  15. Lisa cook

    I am currently sitting on our expensive chartered 60 foot boat in the British Virgin Islands, and within the first 2 hours of our week vacation, we lost a propeller. So now we are functioning with a single engine. This has dramatically changed the course of our captain as to where we do, or were we do not go. All I can say is, having 2 engines has saved us. And for anyone traveling from Florida to the Bahamas, it is my understanding double engines can be life saving.

  16. Mike Dunn

    As someone who spent 7 years traveling single handed through the various river systems of North & South America in a 26 ft. boat (Zeno’s Arrow), as well as having spent 13 years working on Expedition ships in the Antarctic and elsewhere I can honestly say I would not be here to write this had I not as standard practice had two identical engines with lots of spares. The one single engine boat I did own (44 ft.)was lost off Panama in 1995 partly due to a shaft failure. One vs two? Application, application, application.

  17. Jack Vermeulen

    Here’s another example where two engines make a difference. This is for a gas engine – which was not discussed, yet perhaps a more common installation than diesel for smaller boats.

    My prior boat was a 28ft. The single engine exhaust manifold riser plugged up while running at 3500RPM. It literally melted the rubber coupling from the exhaust to the outdrive setup. Seawater (in semi-rough seas) surged out of the gap now exposed. I couldn’t restart the engine since that would only add more. My son-in-law towed me back, but otherwise it would have been a tow call.

    That was actually the second time an engine has overheated, first time seagrass plugged the raw water intake. I had a small outboard engine that saved me that time. That was right at Deception Pass, WA, a sort of scary place for this to happen.

    Current 37ft boat with twin gas, had a transmission heat up past 210F (oil pump failure). That set off the ECU warning system. I had no idea there was such a monitoring system. No problem though. As soon as I saw the flashing warning light I turned off the port engine. Only later did I learn the problem’s source.

    Each of my engines is fed from an independent fuel tank. So if somehow one had a fuel issue, the other may not. I realize it also could, but that’s another leap. There are many stories of fuel issues with diesels and systems to help.

    So although I agree with the single engine thoughts, the one comment about amateur use should have explored more. There are differences from commercial use. We use our engines relatively hard and not in open seas. You can’t check the exhaust manifold without pulling them – not a trivial job (another article).

    Although I like to go slow – (8-10) is more relaxing, I also run at 26knots (wife command) which is around 3500 rpm. That’s a totally different environment. Yes props are more exposed, although even that varies as to exposure. I have prop pockets and props are slightly above the keel.

    I like to think of the 2nd engine as a working “spare” vs having a spare for every part 🙂

  18. Larry

    I am investigating the possibility of buying my first boat which a 28 to 30 foot Cruiser is being considered. The general intention is to stay in around the Great Lakes and Georgian Bay and smaller Lakes in Ontario. I certainly appreciate reading all of the details regarding the general maintenance, fuel economy and the redundancy factor. I have friend that owns a Monterey 27.6 Cruiser with twin 4.3’s. I have only been on their boat three different times and the general stability of the boat while walking around and sleeping at night is good. Generally cost is a big factor for me and the initial capital cost and subsequent maintenance costs are driving me toward a single engine however could someone tell me if a single engine is equally as stable (side to side movement) vs twin. Or does the beam provide the lions share of the stability on the boat? Thanks for the anticipated assistance


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  20. Mike Whalen

    Please be advised.
    One may purchase twin screw drives for a single engine boat.
    With twin screws come twin tranys.
    This application provides for simple maneuverability and,
    greater fuel economy.

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