Engine Preferences

What are pros/cons for single vs twin diesel engines?

Cjsangster said:

What are pros/cons for single vs twin diesel engines?

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This is one where you may want to do a search here on TF. There are a lot of strong opinions on this one. I don't have a strong opinion so I will share some ideas that I've picked up. Here are some advantages of each.

Twins:
- redundancy, so there is a safety factor there if you lose an engine.
- maneuverability
- speed (varies with type of boat etc... but many twins are designed to cruise faster than many singles.

Single:
- less expensive and easier to maintain
- takes up less room in the ER
- generally more fuel efficient
- depending on design, most singles have better prop protection than most twins.

There usually isn't much argument as to the above. The religious debate comes in how you weight each of those factors.

CJ

In the teeny weeny trawler world it is a useful way to increase the hits for an internet site. Thus increasing revenue for the site owner.

In the real world it comes down to what boat do you like. If you like a KK42 it is a moot question unless you include those few with get homes. With DeFevers virtually all have twins, so again a moot question. If you like OAs, again moot as virtually all are twins. Even Nordhavn has singles with get homes or twins. Most charter fishing boats have twins. Most commercial fishing boats have singles.

Or go whole hog and get one of the larger fun vessels with triple or quadruple outboards!

So what boat are you asking about? IMHO, this is the real question.

Most of the worlds fishing vessels operate on a single diesel, and work far offshore with few failures.
The key is maintenance and a good understanding of the equipment, and of course, it’s proper operation.
Unfortunately, the same statistics do not apply to the recreational sector, so twin installations may just double the chances of failure!

sunchaser said:

So what boat are you asking about? IMHO, this is the real question.

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Great advice. I have a single, and that was my preference, but if I had found a design that I liked that had twins I would have been happy to that route and would just concentrate on those advantages.

Greetings,
Mr. Cj. Single or twins? Depends on which and how many of each kind of anchor you have...

..

I've been running twins in different hulls since about 1963. Ships, boats, props and jets. I'm sold, so biased opinion.
Twins usually are smaller engines than a single in the same hull. They use 10-20% more fuel and almost double the maintenance. Except in sport fishers, you don't push twins as hard as a single. But you have much more reliability, especially with the new electronic controlled engines. If you travel to remote places, one set of spares covers both engines. In an emergency or in trouble shooting you can swap parts between engines. You have more options for engine accessories - different pumps, alternators, power take offs. If you learn how to maneuver twins, you don't need a bow thruster. (But I don't need a bow thruster with a single). Fishing I run on one engine unless tuna trolling. I've come back on a single twice (not counting military service) in 55 years, but I ran the most reliable 100% mechanical marine engines made (opinion).

"Most of the worlds fishing vessels operate on a single diesel, and work far offshore with few failures.
The key is maintenance and a good understanding of the equipment, and of course, it’s proper operation."

This is true but most work boats with a single engine do not select an engine from a pickup truck , taxi cab or from a lawn implement.

The go to the heavy duty trucks , earth moving or prime generator style engine sources.

For a White boat a pickup truck engine is just fine , at 200 hours use in a year they will last many decades , till killed , seldom worn out.

For near 24/7/365 operation the heavier duty stuff is a better choice.

Lepke wrote;
“(But I don't need a bow thruster with a single)”

Everything I was going to say would be a bit snarky so I’ll not say.
However it’s obvious to all that thrusters (singles and twins) are beneficial.
I back into my slip and I’ve always been able to do it fine but I’m sure if the wind blew hard enough the bow would get blown downwind. That’s assuming I could get the boat lined up sideways. Of course I’ve gone past and turned around and come back upwind. This would solve the problem. But then I wouldn’t be backed in. I’d probably need a thruster to back in.
But you (Lepke) wouldn’t need a thruster I’m reading. Well I wouldn’t either but I’d go out to the transient float till the wind backed down or go into my slip bow first.
So it depends on what is meant by “need”.
But you can’t do anything a boat with a thruster can do without one.

I have pretty much driven singles all my life, from sailboats, to ski boats, to trawlers. So I'm very comfortable with "the single life" including backing/filling when necessary.

BUT, I have to say, driving a twin with modern electronic engine controls is pretty seductive. It's sort of a "BMW experience."

Experienced this demoing the boat we are now building. Cruising around a marina, just steering with the engines (i.e. going in and out of gear at idle), it was an altogether calm and lovely experience.

Maybe I'm at a point where I don't have anything to prove, I just want to enjoy being on the water.

JustBob
M/V Mahalo - Blog

I do wish I had a jog lever connected to my AP for use in close quarters maneuvering. There are times when backing out of my slip it would be nice. I have to back out to starboard, but my boat likes to back to port. The fairway isn't as wide as the diagonal distance of my boat (anchor to port edge of dinghy mounted on the swim step.

A jog lever would be nice in that I could quickly and easily give hard port rudder and a small shot of power to kick the stern to starboard. It takes way too long to turn the wheel by hand in that situation. A twin would make this a non-issue.

dhays said:

A jog lever would be nice in that I could quickly and easily give hard port rudder and a small shot of power to kick the stern to starboard. It takes way too long to turn the wheel by hand in that situation. A twin would make this a non-issue.

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Coming from past boats that were twins, I was thinking the same thing. A thread occurred a few months ago on this. I did some research on it, and it didn't seem very practical but I have not given up on the idea.

I’m all for twins and thrusters if you can afford them.

But Lepke’s right. Thrusters and twins aren’t needed. I’ve gotten buy for years w/o either. I even made a landing at Ketchikan in 40mph winds. Was kind-of a thumper but no damage. Backing down on an anchor in the wind is a problem at times and would like a thruster for that. Stern would be fine.

Fletcher500 said:

Coming from past boats that were twins, I was thinking the same thing. A thread occurred a few months ago on this. I did some research on it, and it didn't seem very practical but I have not given up on the idea.

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Yeah, I have looked into it a little bit, and I don’t think there is an easy, plug-and-play jog lever for the RayMarine AP system that I have.

I don't know what system you have Dave but Raymarine just added a jog lever for their Evolution APs

Regarding jog levers. If your steering system is hydraulic and the autopilot or wheel sends a "left - right" signal to solenoid valves that then directs the fluid to move the ram(s) or direct the flow from the pump it is probably easy to add a jog lever. Specifically a Non Follow Up jog lever.

A selector switch is installed to select between autopilot / helm and NFU jog lever. Handy as the job sticks are the steering from hard left to hard right may not be any faster. The maximum rudder speed is a function of the steering pump and ram. To get faster rudder response you need a faster pump.

The jog lever is not brand or model specific.

Marine Steering - Jastram Engineering Ltd. - Marine Hydraulic Cylinders, Steering Systems, Digital Ship Controls and Boat Motor Starters - Product Specifications

A good installer shouldn't have too much trouble getting the jog stick up and running.

"Thrusters and twins aren’t needed."

However there are real bad days when anchoring out is far less risky than attempting an internal slip in a marina.

The reputation for reliability in single engine commercial fishing boats and other commercial vessels was earned before Tier 1 engines. All mechanical injector timing. No common rail, no electronically fired injector with a circuit board and a multitude of electronic sensors. No need for a crank sensor, etc. Some commercial boats built in this century are already on their 2nd engine instead of rebuilding. That tells me a lot.
I owned a couple 100' plus tugs with singles, but with a 100% mechanical engine.

dhays said:

I do wish I had a jog lever connected to my AP for use in close quarters maneuvering. There are times when backing out of my slip it would be nice. I have to back out to starboard, but my boat likes to back to port. The fairway isn't as wide as the diagonal distance of my boat (anchor to port edge of dinghy mounted on the swim step.

A jog lever would be nice in that I could quickly and easily give hard port rudder and a small shot of power to kick the stern to starboard. It takes way too long to turn the wheel by hand in that situation. A twin would make this a non-issue.

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Dave:

If I understand your post correctly, you are leaving your rudder at hard over to Starboard and backing against a Port pulling Prop walk. In using your rudder at all in reverse, you are counting on getting enough water against the Port side of the rudder to influence your sternwards direction. I learned on my sailboats that this is futile. Your stern will go where it likes, influenced far more by propwalk than by the rudder. So set your rudder at hard over to Port, then back up. You will find that your "to Port" rate of turning has not increased over being hard over to Starboard AND you now have your rudder set to allow a burst of forward power to straighten out your direction of travel, or even to turn you to port, the direction you actually want to go.

You may have been describing something else altogether, in which case please take no offense for my presumtions.

koliver said:

Dave:



If I understand your post correctly, you are leaving your rudder at hard over to Starboard and backing against a Port pulling Prop walk. In using your rudder at all in reverse, you are counting on getting enough water against the Port side of the rudder to influence your sternwards direction. I learned on my sailboats that this is futile. Your stern will go where it likes, influenced far more by propwalk than by the rudder. So set your rudder at hard over to Port, then back up. You will find that your "to Port" rate of turning has not increased over being hard over to Starboard AND you now have your rudder set to allow a burst of forward power to straighten out your direction of travel, or even to turn you to port, the direction you actually want to go.



You may have been describing something else altogether, in which case please take no offense for my presumtions.

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Keith, you have it about right and your suggestion is pretty much what I do. If the boat gains enough speed in reverse then the rudder will become effective. I don’t go hard over as then the rudder works more as a brake in reverse than a rudder (in reverse at least). What I find is that as my boat is far enough out of the slip that I can start to move the stern to starboard, I often have just enough speed to make the rudder effective. This is the situation where it would be hand to be able to turn the rudder quickly from one position to another.

What I have found is that I do best to leave the rudder in the neutral position as I back up, but then move the rudder to port when I want to pulse forward power to move the stern to starboard. The neutral rudder tends to resist the port propwalk much better than a starboard rudder position.

Of course, the simple solution is to just use the bow and stern thruster that I have to take care of it. However, I prefer to have a technique that isn’t dependent on the thrusters as a matter of course. I’ve been in the situation where my thruster has failed.

My personal opinion is that it depends on where you cruise.
We cruise a mix of sea and canals and whilst it possible to cruise the canals with twins its not desirable for the following reasons. 1, just putting the boat in gear would have you exceeding the speed limit. 2, the engine will not be able to reach efficient working temperature leading to premature wear. 3, Underwater, canals are semi circular in profile and simply coming alongside in twins can risk a prop, rarely does a twin complete a season without a damaged prop. 4, manoeuvrability depends on the skipper, a few are good, many think they're good and some are like a farmer riding a race horse.
I'm biased because I'm one of the old school and can use a single engine/springs and manoeuvre just fine but as age advances it becomes a factor both for myself and the safety of my crew which is why I invented the engine/gear/bow thruster single lever system, everything on one hand, the other hand free for steerage, as I said I'm biased but I've found nothing to beat it for close quarters manoeuvring.
I'm not mercenary, you can buy the pieces and install it yourself, I make no money whatsoever from it.
If you are a 100 yards out at sea you simply go back on course, if you are a foot out going into/through/out of locks you have an expensive bill to pay for your mistake.
As to single engine I prefer to think that electronics and a damp watery atmosphere do not make great bedfellows, sooner or later they will give trouble, the only difference is time.
I would rather buy a rebuilt fully mechanical engine than fit one with electronics, call me a Luddite, I've only broken down once in considerable years regular cruising and that was caused by ? an electrical fuel shut off solenoid.
Of course these are my own personal opinions, now about anchoring ?

Dave, not relying on your thrusters is a smart move. IE, I plan on using both but will also practice in case one or both go down. Previous boat was a 44 with twins but an EC and very light with low draft so she moved like a cork across the water with any wind. I had one engine go down while trying to dock at the yard. The slip was next to the 10M yacht with the crew looking at me of course. It wasn’t pretty, but past practice allowed me to get her in the slip.

Having had the choice between a twin or single KK54 (both on the market at the same time) I’m glad I went with the single. I put in a brand new 6068 which is pretty much bulletproof. One of the biggest failure causes is fuel so unless you run of two tanks and don’t mix the fuel it’s still a big single failure point. I think I’m going to rig up a manifold control that allows me to run off the side takes in addition to the day tanks to help eliminate that issue.

All that said I REALLY wished I had the twin until I put in a stern thruster, with limited trawler experience the single and high windage was rather intimidating.

And yes, lots of practice needed now w/o the thrusters.

Twins Baby!

I've had singles and twins... Again I say; Twins Baby!!

Lepke said:

I've come back on a single twice (not counting military service) in 55 years, but I ran the most reliable 100% mechanical marine engines made (opinion).

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I like a guy with an opinion and isn't afraid to voice it.... nice and loud... just like those 6-71's!

"One of the biggest failure causes is fuel so"

That is why the "day" tank concept works so well.

The fuel can be dewatered , spun, filtered , and finally left standing so the fuel taken out higher up in the tank is usually clean enough.

A high mounted day tank can gravity feed the engine , solving another whole bunch of problems.

Nothing is perfect , but clean fuel can be done , although its easiest with a boat built for the concept.

FF said:

"One of the biggest failure causes is fuel so"

That is why the "day" tank concept works so well.

The fuel can be dewatered , spun, filtered , and finally left standing so the fuel taken out higher up in the tank is usually clean enough.

A high mounted day tank can gravity feed the engine , solving another whole bunch of problems.

Nothing is perfect , but clean fuel can be done , although its easiest with a boat built for the concept.

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Agreed, the KK54 has many of the pieces but the original fuel routing, etc doesn’t give as much redundancy as it could... redoing that is on my list but far down it...

I like our single for a couple reasons. Think about seawater pumps, impellers, belts, zincs.... A twin I assume doesn't mean you have to carry twice as many spares, but it does mean you're having to maintain and keep track of twice as many components. I feel like I've got my hands full just keeping track of one engine.
Secondly, unless it's some of those much larger boats, the twin engine rooms I've seen often have areas of one or the other engine that are hard to get to.
I've really been enjoying our uncluttered ER with the single and lots of space on both sides.

RT Firefly said:

Greetings,
Mr. Cj. Single or twins? Depends on which and how many of each kind of anchor you have...

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You not right...lol