Single vs Twin: It's Baaaaack!

As Marin noted, engine failure in the majority of circumstances happens because of things attached to the basic engine. Many, if not most, of those kinds of things are fixable with a basic spare parts inventory on board, i.e. belts, filters, hoses, impellers, and a bit of knowledge to clear filters, air locks and the like. In today's diesels, catastrophic engine failure is an incredibly rare event. Thus, in my mind, the single v twin arguments has devolved to being mostly a personal preference issue rather than a truly pragmatic one. That said, I have always preferred twins on the quaint notion of having a built-in "get home" system that requires no action from me whatsoever. I have yet to find any other "get home" that convinces me that it is functional and effective for any significant distance in any significant sea conditions, whether it be the Nordhavn folding sail prop driven by a 62HP diesel in a 120,000 lb boat or a hydraulic system driven off a 20KW generator. My personal comfort level is twins with shafts and props fully protected despite the addditional up front and operating costs.

But, hey, that is just me.

THD said:

Thus, in my mind, the single v twin arguments has devolved to being mostly a personal preference issue rather than a truly pragmatic one. That said, I have always preferred twins on the quaint notion of having a built-in "get home" system that requires no action from me whatsoever.

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I totally agree on the personal preference influence on single vs twin opinions. As I have related in the past, we have benefited from the get-home aspect of having two engines four times since buying the boat in 1998. None of these incidents were due to the engines themselves, and one of them was entirely my fault.

But just as great a reason for my preference with twins is I just like running multiple engines. I like being able to manipulate the powerplants to do different things. I like the challenge of learning to master a particular combination of rudder and thrust to achieve a new maneuver. As I've said in other threads, I've only recently "discovered" the wonderful flexibility that power can provide during slow-speed maneuvering in addition to rudder and differential thrust. So I'm having a good time exploring the options that power can bring to the formula.

(I say I "discovered" this.... What I really did was watch enough tugs and fishboats around here and lobsterboats on Prince Edward Island that finally the benefit of using bursts of power finally sunk in.)

So we would never go back to a single engine boat (the GB we chartered was a single). For my wife, she is more confident and relaxed with two engines under the floor, and if one's boating partner enjoys the experience more with two engines than one, than it's well worth the extra fuel and oil and filters.

For me, I would not find operating one engine nearly as interesting as operating two. I'd have three if they would fit.

Another benefit is the ability to secure one if indications are looking odd. My slip neighbor with a 45 PT just replaced his single main because he drove it home on a mildly overheating impeller. Had he had twins, he would have likely secured the motor and came home on the remaining plant.

Also... twins have the benefit of working half as hard and this would add to their longevity and increase the time between load related failures. I've never come home on a single motor, but would if anything looked even slightly odd.

Yes, all our "engine failures" as Mark would term them were actually precautionary engine shutdowns.

Three of them were due to cooling issues and the engine was shut down as soon as the temperature (coolant) indicator began to edge up past normal on the gauge. A previous owner marked all the engine gauges with thin strips of black tape at the normal needle readings so it's easy to tell at a glance if something is beginning to go amiss. So in our case the engine was always shut down long before it reached the overheat stage and we finished the run on one.

That's a nice thing about having two engines----- taking one off-line brings no real penalty so one is inclined to do it-- or at least we are inclined to do it-- the moment something starts looking fishy. Which means the likelihood of causing actual damage to the engine is almost nil.

As opposed to the fellow with just the one engine who begins to experience a problem but might be inclined to run the engine as long as he thinks he can get away with it in an understandable effort to reach a safe haven. The chances of causing damage go up considerably in this situation.

Our fourth engine shutdown to date was caused by my misunderstanding the boat's fuel system and letting an engine get a slug of air during a fuel transfer. So we don't count that one as it was totally my fault. But even in this case having two engines was a benefit because we were in lumpy water at the time with guests who needed to be home by a certain time. So I simply tied off the shaft and we came home on one. We went up the following weekend and bled the fuel system for the other engine in the peace and quiet of our slip as opposed to bouncing around at the south end of the Strait of Georgia with guests hoving around looking at their watches.

More on Twin Manuevering Options?

Marin said:

I like being able to manipulate the powerplants to do different things.

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So this is the dual engine functionality discussion I hoped I would find in this thread. I've got 33 years of single engine experience and will likely charter a twin just for the experience. A better conceptual understanding of twin manuever options would be helpful. Is there more of this in another thread in this forum?

Some claims in this thread's R/M/A discussion seem to depart from guidance in MIL-HDBK-217 which is my limited background for the topic.

From stories heard around the dock:
A 60 ft charter boat was towed back to its slip in my marina with both engines disabled. The boat had run over a log in the Puget Sound and bent both shafts, props, and rudders. There is a chance that a single could run over a log and survive as the prop/rudder is behind/above the keel.

I watched a two prop replacement on a 100+ft yacht at Shearwater after they bent both props running over a log.

I have heard twin engine boaters talking about not replacing a raw water impeller when due on one engine of twins because 'they can not reach the raw water pump' on the engine with the pump on the outboard side, and "anyway they have two engines". A GB 36 twin has limited access to the outboard sides of the engines. I skipped buying a GB 36 twin because I personally could not access the outboard sides of the engines. When traveling in remote areas I want to be able to reach/repair as much of the systems as possible.

Perhaps because twins cost twice as much for routine maintenance, there seems to be a tendency for maintenance to be skipped when it is too difficult.

I have taken my single engine trawler from the Puget Sound to SE Alaska three times, with no system failures, except having a failing starter battery which I replaced in Ketchikan.

I do carry lots of spares, including a spare starter motor. My Lehman 120 single hasn't quit yet. (said with fingers crossed)

All that said, I still have Boat US towing insurance, and would like to have a tender large enough to side tow my trawler.

All this is my opinion from talk around the docks and from personal observation.

Marin said:

So I simply tied off the shaft and we came home on one.

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Marin, just how do you do that, so that the undriven gearbox is not being turned by the prop?

There are engine failures and then there are propulsion system failures. I don't believe that twins are twice as likely to experience engine failure, but I believe that the design compromises incorporated into most twin engine boats places them at higher risk for certain types of failures, specifically prop/strut/shaft/rudder damage which would not as likely occur with boats with keel protected running gear.

If twin manufacturers simply incorporated twin keels to protect the running gear, I think the advantages of a twin would far more greatly outweigh the advantages of a single-engine boat. But that fact that most builders that I'm familiar with do not protect the twin running gear with twin 'bilge keels' renders twins at greater risk of damage due to debris and groundings, in my opinion.

In addition to protecting the running gear, protective bilge keels would also provide passive roll stability which would be a huge improvement to many of our boats' designs.

I'd love to hear Tad Roberts' perspective on the advantages, disadvantages, practicality and costs of protective bilge keels on twin recreational trawlers in the 30-45 foot range. I've often thought that they would provide the needed protection and roll stability on twin boats (like mine!!) that lack both.

I'm 6' 3" and I can get around the outsides of our engines. I won't pretend it's a walk in the park but it's very doable and we have never deferred service or maintenance because it was too difficult to get to the necessary components.

As I've mentioned before, we know or have heard of many, many more instances of single engine boats, including sailboats, experience an engine shutdown or running gear damage and had to come home on the end of a rope-- or in some cases been carried aground--- than twin engine boats that have suffered damage to both sets of running gear or had to shut down both engines. In fact, we have never heard personally at all of a twin engine boat not being able to come home on the other engine when one had to be shut down or after experiencing damage to one set of running gear. And we have never heard personally of a twin engine boat having to shut down or losing power on both engines.

So in our opinion risk of the loss of both propulsion systems on a twin is so remote as to be a non-issue.

BruceK said:

Marin, just how do you do that, so that the undriven gearbox is not being turned by the prop?

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Our boat has convenient tie-off holes in the heavy aluminum extrusions on the inboard engine stringers that hold the primary fuel filters. When we had the boat's original Fram dual-cannister filter system changed to Racor 500s these holes, which are beside the shaft couplers-- were obstructed by the Racors.

So I made a set of strong brackets with shackles that are screwed and glued (5200) to the underside of the heavy cabin floor beams. The shackles are positioned directly above the shaft couplers.

We have a stout line that if we have to shut an engine down I wind around the shaft coupler and secure. The other end of the line is secured to the shackle above the coupler. The line is wound around the coupler in a direction that will oppose the direction the shaft wants to freewheel when the boat is moving forward.

I have said before that, given the reliability of today's diesels, I think the single v. twin has come down to personal preference. That said, on the KK 58, both shafts are protected by keels. The centerline keel sort of "fades" out at about 75% of the boat length and the twin keels start at about 60% of the length. Both shafts, props and rudders are fully protected. The added advantage of this design is that the boat can sit on her own bottom should that become neecessary (or unavoidable). I personally prefer twins and have had twins in 3 different boats. Fortunately, the engines and the engine room on the KK are well designed and access for any maintenance issue on either side of the engine is not an issue.

The added advantage of this design is that the boat can sit on her own bottom should that become neecessary (or unavoidable)

Sounds like a British boat where "taking the ground" is a daily situation.

To my mind every inshore cruisers boat should be built with this in mind.

I love this discussion! When I bought the Bristol, with it's single screw and huge keel, I had been looking for a long time. I've owned twins my whole life, but like most...my boating is a few days off each week and my wife and I must return to reality and go back to work, so we needed the speed and redundancy of a twin. I could not tell the
Chief Pilot we were stuck in Provincetown waiting for parts, so a gas gobbling planing hull with twins filled the bill. When we retire in 4 years, that bill will be filled with the Bristol; slow turning Lugger for fuel economy and range; big keel for protection of the running gear; massive amount of engine room around the diesel to work on it like a gentelman, thus keeping it tip top; the best fuel delivery and polishing sustem I could find; and a HUGE bow thruster up front to make me look like I know how to drive a single. Yes, reduntant engines whether in boats or airplanes are obviously better but for ONLY one reason...to keep moving! If one is not time constrained by a job, or in a hurry to get somewhere then back to the docks, or in horrific weather, (with today's modern wx reporting via i-pad or satellite, you shouldn't be) and you stay within seatow's range with good insurance, then a single will do the job nicely. Robert Beebe went across the Atlantic on a single quite a few times as do hundreds and hundreds of large, single engine commercial freighters every day. I have never had an actual "engine" failure, rather, the hoses, belts, etc. that are attached to them have failed. All easy fixes. Throw out a sea anchor, let the wife maintain the watch, and get down in that big engine room and change the belt! lol My coffee is finished so I'll sign off from my editorial. Thanks Mark for pulling the pin!

Simply being able to maintain control of the boat and be able to move to an anchorage or come home on a remaining engine are my biggest reasons.

With twins... you have decisions that can be made. With a single you have a phone number: 1 (877) 844 5160

Marin I want to see you doing your "walk in the park" and you can take your dog w you. The image would be more fitting that way. On a leash of course.

SomeSailor said:

Simply being able to maintain control of the boat and be able to move to an anchorage or come home on a remaining engine are my biggest reasons.

With twins... you have decisions that can be made. With a single you have a phone number: 1 (877) 844 5160

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Utter nonsense...plenty of situations with even a single and you can keep moving.

I just had a high pressure fuel leak on an external injector feed in the more remote, narrow part of the Intracoastal.

I made it the next 20 miles to a safe haven (where I planned to be overnight anyhow) all the while driving through severe thunderstorms and a tonado watch....safely and without damaging anything....all without anchoring or calling tha number. I also have a VERY inexperienced crew so I had to navigate too.

Maybe some twin drivers need to learn more about dealing with emergencies than thinking 2 is better.

Marin has about the only answer I really accept...he likes driving a twin and that can't be argued with..

Bruce K said:

"Marin, just how do you do that, so that the undriven gearbox is not being turned by the prop?"

Bruce, many transmissions/gear boxes do just fine rotating on a "dead" engine. Our Hurth book says no problem trailing the prop, which we indeed have been forced to do with no overheat resulting. Your manual should specify if it is a problem.

More than a single didn't help these guys!


The Kulluk lost its towline from the Aiviq on Thursday. A second towline was attached for a time, but then early Friday all four engines on the Aiviq failed.
Shell Oil drilling vessel is adrift in Gulf of Alaska | McClatchy

psneeld said:

Utter nonsense...plenty of situations with even a single and you can keep moving.

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Sure. But like we have both said many times, I can shut mine down instantly and still keep moving.

I actually did that yesterday while out just to see what would happen. I took my starboard motor out of gear, and shut it down. The autopilot corrected and I lost about a knot of speed. Just for giggles I went below and checked everything while the wife watched the helm.

The starboard shaft was free wheeling at about half the RPM of the running motor and I checked the oil and came back upstairs and brought it back online. It's a huge degree of redundancy and a wonderful plus in maneuverability, reliability and safety.

sunchaser said:

Bruce K said:

"Marin, just how do you do that, so that the undriven gearbox is not being turned by the prop?"

Bruce, many transmissions/gear boxes do just fine rotating on a "dead" engine. Our Hurth book says no problem trailing the prop, which we indeed have been forced to do with no overheat resulting. Your manual should specify if it is a problem.

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And a simple cross connect of cooling lines can take care of the cutless bearing water supply...might need to add check valves and slightly larger orifice at the feed sources.

According to OC Diver twin owners are less diligent

According to Larry H big twin yacht owners like to run over logs

According to psneeld twin drivers need to learn more about maintenance

According to Mark his single purchase was the smartest decision made in the Bay Area since Alcatraz was closed

Wow, some real here.

sunchaser said:

According to OC Diver twin owners are less diligent

According to Larry H big twin yacht owners like to run over logs

According to psneeld twin drivers need to learn more about maintenance

According to Mark his single purchase was the smartest decision made in the Bay Area since Alcatraz was closed

Wow, some real here.

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Great boaters? You bet!

Plus I said dealing with emergencies...not maintenance... so add reading classes for twin drivers too...

We all understand the concept of redundancy.

Eliminating single points of failure in any piece of equipment has proven itself to decrease outage time. This isn't even arguable, its just a fact.

Excepting for the potential issue of a single protected prop being less likely to sustain damage during a collision, there is zero logical augument that will support the same, or as some would claim increased reliability in a single engine installation vs dual engines.

There are alot of reasons one might prefer a single over a twin engine configuration, but reliability isn't one of them.

ksanders said:

... There are alot of reasons one might prefer a single over a twin engine configuration, but ...

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With singles:

Lower acquisition cost.

Nearly half the engine and running-gear maintenance cost.

With half the engine and drive-train components, half the chance of something breaking/failing.

Shaft and propeller more likely to be better protected.

Engine compartment takes less space and/or is less cramped; accessibility likely to be better.

With two engines:

Something breaking/failing is much less likely to completely disable or shut down boat.

More likely capable of planing speeds.

Greater ease in slow-speed maneuvering.

The feel of adjacent dual throttle controls and reverberation of multiple engines so loved by multi-engine aircraft pilots and fans.

Mark! It's about time you spoke up! And I see there are some fellow pilots in here. I've been flying over the Atlantic and Pacific for a long time and the old days of jets having wet footprints are long gone thankfully. Plus no seatow 8 miles up lol.

markpierce said:

With singles:

Lower acquisition cost.

Nearly half the engine and running-gear maintenance cost.

With half the engine and drive-train components, half the chance of something breaking/failing.

Shaft and propeller more likely to be better protected.

Engine compartment takes less space and/or is less cramped; accessibility likely to be better.

With two engines:

Something breaking/failing is much less likely to completely disable or shut down boat.

More likely capable of planing speeds.

Greater ease in slow-speed maneuvering.

The feel of adjacent dual throttle controls and reverberation of multiple engines so loved by multi-engine aircraft pilots and fans.

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OK, now you've gotten somewhere.

There are things you can do to improve an engines Mean Time Between Failure (MTBF).

To reduce the MTBF you could:

Use a Naturally Aspirated engine
Utilize dry exhaust and keel cooling
Use an engine with no external oil lines
Use an engine with no belts

For me personally a single engine boat utilizing those types of systems would make single engine boating quite palletable.

ksanders said:

To reduce the MTBF you could:

Use a Naturally Aspirated engine
Utilize dry exhaust and keel cooling
Use an engine with no external oil lines
Use an engine with no belts

For me personally a single engine boat utilizing those types of systems would make single engine boating quite palletable.

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Couldn't agree more.

Well that settles it. Singles are better than twins. Discussion over. Everybody can go home now.

A big advantage might be if the noisemaker and engine were identical, or at least identical maker.

A 6-71 for power and a 3-71 noisemaker would allow for a huge spare parts inventory.

Drop a valve and the noisemaker has a new functioning cylinder head, or a replacement exhaust manifold.

Also would reduce the spare parts purchase inventory.

One of the things greatly favoring single engine installations is a reduced Mean Time To Repair (MTTR).

Much lower MTTR is acheived through easier access to the engine for repairs. This makes repairs much faster when a failure occures.