Decision: A single or two engines

I've had both - three Riviera sport-fishers with twins and now a 53ft Westcoaster semi displacement with a big single and a big hydraulic bow-thruster.
The single with bow-thruster is much easier to handle in close quarters than the twins and has much lower maintenance costs. I would never want to go back to twins.
With a bit of practice, you can get pretty damn good handing a big boat on your own especially if you have an aft station.

Boat handling can be learned or fixed with thrusters.

That was never a concern of mine.

Being stranded some place I don't want to be is. The cost difference can add up quick and be a heck of an inconvenience. As posted before, it doesn't have to be throwing a crank that does it either....it can be something simple, yet not easy to fix or get a part for.

Barling1 said:

I"m considering on purchasing a Mainship 350/390 and whether it should be a single or two engines. The single comes with a bow thruster. Two engines would be better for docking and may be safer, but I would loose the economy of a single. How much more would it cost to run two engines for economy? How much more to maintain two engines? What are the real trade offs? Thanks.

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For a true apples to apples comparison, we need to know if the MS350s you are comparing have the same total hp, or does the twin version have twice the hp?
IF the same total hp, your maintenance costs should be similar, since your total amt of oil, size of filters, cost of impellers, etc should be near twice for the bigger single than for each of the small twins. If using 2 of the same engine, then double those costs. That won't be a deal maker, though, as your frequency of purchasing those items will be very low. Most do the servicing annually, so take your annual cost and compare it to things like slip fees, haulout costs, fuel consumption and maintenance becomes insignificant.
For manouvering, redundancy in case of a failure, get home safety, twins easily justify any small extra cost.
Running twins at trawler speed will cost no more than a single, as it is not the number of props in the water that defines the cost, it is the size of the stern wave. Keep that wave down and your economy survives. Let it grow and economy is lost.
Not many equipped with twins run them at the speed they would make with a single. Are you that kind of person? OTOH, getting there a little sooner is worth something too.

My first big boat was twins in 1961. And all my personal boats since were twins except for a couple sailboats. In all that time I came in on one engine twice. Neither time was caused by an engine. I have twins because I like the handling. Having a spare is gravy.
You do have twice the engine/transmission/propulsion train maintenance, but in proper installs, twins can be as economic in fuel as singles.

After operating single engines (lawn mowers, outboards, sailboats, motor boat, automobiles, and once an airplane), the engines never failed to "bring me home" over the last 60 years of their operation (knock on wood).

(My Dad's B-17 bomber was shot down over Germany even though it had FOUR engines.)

BruceK said:

The standard here, in more recent boats, is twins, often plus bowthruster, OR single plus thrusters both ends. And some twins, with thrusters both ends,sometimes with integrating Yacht Controller.

Problem is, I think, anecdotally, thrusters fail more engines,and if they do, an operator used to manouvering/docking with thrusters has to work without. I`ve seen it happen on our marina, and suspect it`s a greater problem than engine failure needing redundancy. And it`s likely to happen just when you don`t want. like if you need the thrusters for extended periods and they overheat or drain their batts. However that redundancy can be mighty helpful for manouvering.

And that`s why I prefer twins. But read up on the old threads, no lack of them.
The hairy chested Swinging Ds will tell you a single no thrusters is all you need, but I disagree, some help is no bad thing.

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On the third day of boat ownership one of our engines had to be shut down due to a fractured injector fuel supply pipe. Of course it was getting dark and the wind was howling. So, yes, it was a fuel problem but not the kind of problem one can mitigate with diligence. I am, accordingly, a fan of twins. Then, there was our recent experience in the Ottawa locks when a drive coupler fractured on one transmission. Our second engine saved the day. Yes, many, many folks go decades with singles and never have a problem, so some say. We do have a bow thruster but I have learned to dock the boat without using it except for convenient but not mecessary tweaks.

I bought a Mainship pilot 34 with single 370hp Yanmar. easily planes at 15kts. I looked at twins as well but chose the single (with thruster) due to the excellent engine room space (even for storage), much reduced costs, reduced weight, reduced noise, and my broker showed me the stats that the single engine was more popular and sold for more for this particular boat.
The trawler you seek may be more popular in the twin version - get some comps and see.

PS we love the boat. 250 gal of fuel at 8 kts trawl speed lasts a long time. I average 3.5 gph running a mix of trawl and planing speeds.

I have been around boats most of my life. I will only own boats with twins anymore. Problems happen and they happen more often on the water. Wires get corroded, batteries fail, bad fuel, mechanicals fail, etc, etc, etc.

A boat is not like your car, where you will pull over to the side and call AAA to come and tow you home. Add that to the fact on a boat, weather may or may not be with you and having a means of making headway is going to mean the difference between life and death. Sure you can call Boat US or Vessel Assist. The last person here in the PNW that I know called Vessel Assist was told that it would bat least 5 hours before they could even start heading their way.

With twins, 90% of the time, I can get myself home.

twin or single

twins says it all...twice the maintenance costs and fuel costs and half the range...the access to the engine is a BIG factor.

twins vs.single

I have owned several single engine aircraft is my lifetime. Engine failure is not an option. Proper maintenance, on the ground or dock, is the key for keeping the propeller or screw turning...just sayin'. KNOW YOUR BOAT.

Some of it depends on your need for speed. If you are wanting/needing planing speed then twins. If not the single is fine for most everyone. IMHO, everything else is secondary or justification. I know, as I am king of justification. The admiral will tell you so.

I agree with most posts in part. For your choice of boats (mid 30" Mainship) a single engine makes much much more sense. Single will have a better protected prop and rudder., where as the twins are fully exposed. Also, your dinghy can be rigged as a spare pusher to get home.
Look in the engine space and image getting to everything in there to fix/maintain those systems all of which require fixing periodically. If you can't make the fix then you will hire someone to do the fix and they work by the hour. Soo if it is difficult to do a repair because of space the repair will take longer = more costly. Also look to the generator's location for service as well.

Good Luck

I was a sailor all my life, but in 2010 I bought a 1980 46' Cheoy Lee trawler with twin Ford Lehman 120 HP diesels. I considered installing bow thrusters because I was intimidated by its size. But I could spin that boat in its own length. In my cruise down the entire Eastern Seaboard, and staying in marinas every night, I never hit a dock.

I burned about five gallons an hour.

One data point - On the way home from buying my boat (it was a multi-stage trip) the stbd engine coolant circulation pump suffered a seal failure. After a few minutes diagnosing the resulting high temp alarm we were back on our way to Pensacola, (Gulf Breeze) still a day and a half away. We were able to cruise at an easy 5.5 knots without pushing the port engine at all. We were never dead in the water. Had we been further along in the trip around New Orleans or anywhere further west with barge traffic, it could have been much more interesting if running a single engine. In addition we picked up an errant crab trap in the ICW channel (my fault due to miscommunication with the lookout/Admiral) around SC or Georgia. This was again, no big problem as we were able stop the affected engine and still maneuver to a safe location, anchor, and dive on the fouled prop.

Don't misunderstand. I am a twin engine guy and my boat was as well with skeg protected props. I'm all about access to systems. The Mainship that was mentioned is unbelievably cramped with twins almost to the point of inaccessibility hence recommended a single with keel and access over twins. I am pro twins especially if accessible and in our case protected running gear

Joe Pica said:

Don't misunderstand. I am a twin engine guy and my boat was as well with skeg protected props. I'm all about access to systems. The Mainship that was mentioned is unbelievably cramped with twins almost to the point of inaccessibility hence recommended a single with keel and access over twins. I am pro twins especially if accessible and in our case protected running gear

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I agree. I was lucky with my 46' Cheoy Lee. I had walk-in headroom and access 360 degrees to both engines. With twins, bow thrusters and stern thrusters make no sense to me. I would never buy any trawler with poor engine access.

I have had the privilege of running both the twin (Yanmars) and single (Cat) with thruster versions of that very boat. Without question I'd go with the single. With the thruster it is an easy boat to dock. The engine room is MUCH more ergonomic with the single. The only reason to go twins in my opinion is if your require speed.

I recently sold a Selene 53 with a single main engine, and I have another boat (Bruckmann 47) on order. In the Selene I had a single diesel engine (Cummins,I think model was 8089) and a 50hp Yanmar "Get-Home". In four years, I never ran the Yanmar, except 10 minutes every few months to keep it in shape. In four years and over 1,500 engine hours if cruising (live-aboard cruising 2-3 months each summer), mostly in the Bahamas, the Cummins proved perfectly reliable. Indeed, the only problems I had with any engine was the Northern Lights generator engine, and these problems were due to sensor failures, which caused the engine to shut down.

I must admit that I am a stickler for preventive maintenance; this is essential when cruising at length is Bahamas, where getting spare parts is difficult, expensive and slow. I replaced impellers before failure and even replaced entire raw water pumps whenever their seals began to weep water. I was careful about replacing internal zincs and had engine serviced by a certified Cummins mechanic every year. I change oil every 150 hours or so, even if still in the Bahamas. If your maintenance strategy is to fix things only when they break, a single engine may be a bigger risk for you than for me.


I have ordered my new boat with a single engine, and the new boat is smaller, so no room for a get-home engine. I did this because this style of boat is currently only available new with twins jets or pods. (Actually, Sabre's little brother, Back Cove offers single.) Keep in mind that the water in Bahamas is generally pretty shallow, and a single generally draws more than the same boat with twins (because, as you move the props outboard, they move slightly upward, following to the cross section of the hull near the stern). So why a single for shallow waters? The Bruckmann has a full keel! So the prop is very protected. The Selene also had a full keel. I only touched bottom a few times in the four years I owned the boat, and always kind of gently, but I never felt any anxiety about turning my props into tulips. Grounding was my first concern in the single-vs.-twins decision because grounding is the most likely problem where we cruise, and that's why I chose a single engine. Your situation and priorities may be different.

By the way, I agree with everyone's advice that twins and a single burn about the same amount of fuel, for a given boat at a given speed.

Also, in a new boat, a single is generally cheaper than twins, but that is partly because the twins are often the "high-power" option, having greater total horsepower and offering greater speed, at a higher cost. I don't know if this is true for the Mainship models you are considering.

Note: On the Selene, I DID have a STERN thruster as well as a bow thruster. I usually looked like a docking genius to people who didn't know the joys of a stern thruster. I will have both bow and stern thrusters on the new boat, too.

I didn't catch whether you are looking at a new boat or used. If new, maybe you can buy a stern thruster with the money you save by going with a single engine. That's what I'd do.

Single screw vs. twin screw

Barling1 said:

I"m considering on purchasing a Mainship 350/390 and whether it should be a single or two engines. The single comes with a bow thruster. Two engines would be better for docking and may be safer, but I would loose the economy of a single. How much more would it cost to run two engines for economy? How much more to maintain two engines? What are the real trade offs? Thanks.

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Not saying not to buy twins, but as the buyer's agent that we used said, twin engines and twin heads can double the cost of maintenance.



As for the Mainship 350/390, that's what I started out looking for -- until I read a review of a Mainship 34T, when they first came out in 2004. After boarding a 390 and a 34T, we chose the 34T. Why? In large part because I didn't want to enter the engine compartment through the salon. On a 34T, you enter from the cockpit. The other significant deciding factor for me was wanting to enter the salon from the cockpit through double doors/French doors rather than a sliding door. The vast majority of sliding doors on a boat are a pain in the a**. My wife couldn't understand my feelings about that until I invited her to open the sliding door on 390 we compared against the 34T. She had to use both hands and muscle to simply open the door, thanks to rollers that seem prone to go bad.


FYI, not saying we'll never own a boat with twins, but we chose a single with a bow AND STERN THRUSTER. A stern -- known to some as a "marriage saver" is far less expensive to maintain than a diesel or gas motor! I included some of my rational in my latest blog on CruisingTheICW.blog.


FYI, we love our Mainship! Since Marlow bought out Mainship, I wish they would come out with a 43' aft cabin based on the Mainship or Sabre 43'. I love the big Marlows, but have no desire to maintain any boat 49' or larger.

The critique of twins beginning overpowered depends upon the boat. The Great Harbour displacement only Trawlers are designed with twins (54 or 75 hp) depending whether 37s or 47s., in huge engine spaces permitting easy and complete access to all systems. They enjoy a proven commercial hull design very similar to tugs/tows that affords the skeg protected props and rudders as well as a draft of 2"10 in salt water and 3' in fresh.

Lots of good advice here from experienced salts. I'm turning a single screw and in a nutshell, I believe in preventative and scheduled maintenance. In my experience, a trip down below to an inviting, well lit and user friendly ER means more time spent giving that single all the love it needs. I can get to all serviceable points without drawing blood, cramping up or calling 911! If, when crawling into, over, wedging in, limboing and doing the Yoga workout you brushed off at the gym sounds like fun, then add hot turbos and other extremity searing components, rolling seas and Murphy's law... These factors are, IMHO, one of the major contributing factors in ER neglect vis a vis, poor maintenance. If I had the where with all to move up to something that had the ample, comfortable ER that offered the same access, I would possibly go twin screw but not necessarily. ymmv

I Prefer Boat with a Good Keel and a Big Rudder!

I have read this forum for a long time and didn't feel I had much to contribute, but this thread, in my opinion, misses a very important point, so I joined the forum to make the following comment:

I have always single handed my boats. Having had both fairly large (46') and small (down to 10') sailboats, a medium sized sport fishing boat (36') and now an Albin 43 trawler, I believe that the primary handling factor at slow speeds is the size of rudder(s) working in concert with a good keel. The sailboat had a single engine and was a dream to handle - it's rudder was about the size of a barn door. Docking, even in cross winds and currents was quite easy. The sport fishing boat was sometimes challenging - it had twin engines and a fairly deep keel, but dinky rudders. When docking, the primary directional control was differential power, as the rudders at slow speed were almost useless. Cross winds and cross currents were always a challenge. The Albin trawler has fairly large rudders (and a fairly long and deep keel), which are every effective even at slow speeds, so docking is quite easy using differential power and rudder to counter crosswinds/currents. I have never operated a boat with bow or stern thrusters, so cannot comment on them. When I first acquire a larger boat, I practice "docking" out in the bay, using a couple of half-filled milk jugs to maneuver around.

A lot of good conversation, and some pretty good points made. I don’t know that I can add much to the conversation - other to weigh in as an actual owner of the boat you are looking for. My boat has twin Yanmars. It was the best boat on the market here on the west coast after 18 long months of crawling through all kinds. For my use, I would have been just as happy with a single. (The two MS 350/390 with singles I looked at were in rough condition compared to mine)

After reading everyone’s comments, you’ll need to weigh your needs and priorities. Here are a few of my thoughts.

Speed - twins. Mine tops around 16 kt. (17+ on that perfect, flat calm day). Do you have a need to run that fast? Most folks with singles that I know like to cruise at 8 kt. Mine will do that efficiently and at a quiet low RPM. However, my compromise for speed/efficiency is 11/12 knots and makes a big difference here on west coast where harbors are long distances apart. That speed is inefficient and pushing it for the single. The singles are really not good planing boats, whereas my twin will.

Maintenance costs - single. No doubt. I disagree with a previous statement that a single is equal in maintenance costs because it is twice the HP as the twins. Do your own checking, but you’ll NOT find that filters, impellers, oil volume is twice as much on a Cat 3116/3126 vs. a Yanmar 4 cyl. Rather a twin has twice as many parts, and more labor to perform the maintenance. Getting to items on the outboard side of the twins is a b!tch! The port raw water impeller is impossible. The whole pump must be removed which adds several hours of labor. That said, in the big picture of your monthly total coast to own, including slip fees, it is not a huge increase.

The redundancy/safety argument - advantage twins. However, I fully agree that with proper maintenance and a few spares, you shouldn’t lose sleep running either boat offshore. My boat has traveled from the central California coast, all the way around the tip of Baja, and into the Sea of Cortez. This done at efficient trawler speeds of around 7.5 kt.

Yes, there are advantages to the newer 34T or the 400. At that point it becomes a budget issue. You are probably looking at the 350/390 because they can be had in the upper $80k to low $100 - $110k.

Bottom line, as I said - for my use, I could have been happy with either. You just will pay a bit more to have that speed capability and redundancy in your back pocket.

Good luck!!

Phyrcooler, the boat with twin Yanmars I ran a couple times was based out of Oxnard. Granted that was about 12+ years ago. Sound plausible it's the same boat?

No need for twins for good speed. My 38' with single 450 tops out at 30kts. I cruise either at 7.7kt or 19-21kts. Engine has been rock solid reliable, over ten years in the boat and 2600hrs logged with zero failures. And engine had 1200hrs on it when I put it in the boat.

/https://tinyurl.com/yaq3gnch copy to your browser and view if it doesn't open.

This link is a 360 pic of the walkin engine room housing twins on an GH N-37. It is a spacious engine room with great access to all systems and both sides of the twin natural non (turbo'd) with twins (burn only 2 gph at 7 knts.) in just a 37' boat. Super simple impeller change out. The rudders are huge and fish tails so very responsive. Great low speed handling. Draw back is this is an 8 mph (knt) boat and won't go faster.

Full disclosure: My N-37 was sold a year ago so this is not a sales ad for my boat just a post to illustrate that different boat designs can defy usual rules of thumb.

Ski in NC said:

No need for twins for good speed. My 38' with single 450 tops out at 30kts. I cruise either at 7.7kt or 19-21kts. Engine has been rock solid reliable, over ten years in the boat and 2600hrs logged with zero failures. And engine had 1200hrs on it when I put it in the boat.

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That's nice Ski, but neither your boat or the Great Harbour are relevant to the OP and the specific boat in question here. You see a lot of singles up in New England, like the Wesmacs, with 1000 horsepower singles. I totally get that. Not an option on the old MS 350/390's.

The vast majority of "trawlers" have twin engines and a flybridge. Your choice will be limited if desiring the opposite.

We are trawler shopping (long term 5 years or so out) and looking at a single for the economy and keel protection. Our waters have shallows with bars that like to move around.

I can comment from being a Mainship 400, single, and a lot of time behind both single and twin engines boats and planes (seeing how some one mentioned planes)

To put the airplane thing to bed, a twin engine plane IS more expensive to operate than it's cousin single, but not double and for the most part only goes 10 to 20% faster..... BUT if one fails, it's a minor inconvenience, not a major emergency. Out of some 20 planes that Ive had over the years, I've had 4 engine failures... all in singles. So, I like twins.

As for the boats, Ive had a number of both singles and twins and my current Mainship is a single. Love the boat, but would ABSOLUTELY have a twin next time.

The old "singles are fool proof and super reliable" just is a bunch of poppycock. There's NO way you can guarantee your single just won't up and croak leaving your stranded to the elements. I've had that happen a number of times over the years, and often some little unpredictable thing that you just can't prevent against.

So the issues regarding an engine failure are significant with a single, and minor with the twin. And yes, twins probably have a double chance of a failure.

I'm convince that if operated at hull speed, the difference in operating cost of the single vs twin is imperceptible.

And the twin you have the option of going fast. And there's a number of reasons for doing so. Just to improve the ride in many cases, especially and irritating quartering fallowing sea just a tad over your single cruise speed. HUGE advantage for the twin. Another is to beat a closing time after unexpected delays, or get thru two areas of low tide without waiting another day.

And the twin wins, hands down, on maneuverability. For all you die hard "can to anything in a single" crowd, show me how to back into a slip without finger docket with a stiff crosswind. Just ain't gonna be pretty.

The maintenance won't be double in the twin. Might be close but there's a scale of economy when doing 2 vs. 1. The oil change on the twin does not take twice as long as the single. And you might get a break on ordering parts in quantity, but a moot point for the most part.

And, I could argue just as strong for a single.

But today, I like twins.