Single vs Twin: It's Baaaaack!

[Marin]

Then of course their is the hugh maintenence cost savings of the single over the twin.

This is a significant misperception. We know people with GB36 singles and when they have told us their annual engine maintenance costs they are not much different than ours. In fact, unless there is a significant repair project--- replacing engine mounts, exhaust systems, etc.---- the difference in service/maintenance cost between our acquaintances with single GB36s and our boat in a typical year amounts to an engine's worth of extra oil (12 quarts in the case of an FL120), an additional oil filter, and additional fuel filter elements. So a couple hundred dollars at best.

 

[funangler]

This is a significant misperception. We know people with GB36 singles and when they have told us their annual engine maintenance costs they are not much different than ours. In fact, unless there is a significant repair project--- replacing engine mounts, exhaust systems, etc.---- the difference in service/maintenance cost between our acquaintances with single GB36s and our boat in a typical year amounts to an engine's worth of extra oil (12 quarts in the case of an FL120), an additional oil filter, and additional fuel filter elements. So a couple hundred dollars at best.

I agree 100% It only gets expensive when you need to rebuild two transmission, two props or two engines. The moral of the story is survey older boats very carefully and expect possible repairs. Better to buy boats with good histories or new!

 

[Northern Spy]

I think i may have forgotten to define what stoichiometric is.
Stoichiometric is the ratio of one reactent to another reactant in a chemical reaction. The optimum ratio of fuel to air is called the stochiometric ratio and for gas its 14.7:1 and diesel it is 14.5:1.Any other ratio inside the cylinder will result in less than optimum results.

Ummmm no. Your stochiometry is right, but that is not how a disel burns. The efficiency of a diesel comes from that fact that it burns lean as it a diffusion flame pattern, and also because it is is not mixed as it enters the combustion chamber. It is slow to burn, with excess oxygen. This is the main reason diesels have more torque as well, as it is a burn rather than an explosion.

 

[bfloyd4445]

This is a significant misperception. We know people with GB36 singles and when they have told us their annual engine maintenance costs they are not much different than ours. In fact, unless there is a significant repair project--- replacing engine mounts, exhaust systems, etc.---- the difference in service/maintenance cost between our acquaintances with single GB36s and our boat in a typical year amounts to an engine's worth of extra oil (12 quarts in the case of an FL120), an additional oil filter, and additional fuel filter elements. So a couple hundred dollars at best.

oh gee golly, My error, huge is not appropiate. I should have stated twice as much as a single. What comes to mind are repairs like oil coolers heat exchangers exhaust manafolds etc but the fact of the matter is that modern marine engines are pretty darn forgiving and can really take a beating before repairs are needed. We know this must be fact because teins are very popular and we have already established they are less likely to be fully maintained than single engine boats..............
Marin, your absolutely right

 

[Marin]

It's true that if one owns a twin for a long period of time there will be twice the expense of bigger ticket items like oil and transmission heat exchangers, water pumps or at least their impellers, perhaps fuel lift pumps, having alternators overhauled, and so forth.

So in that respect the statement "twice the cost of maintaining a single" can be a significant number at times. But components like these are not crapping out and needing replacement on any sort of frequent basis.

So from an annual perspective, the cost difference between operating one engine or two is pretty insignificant as I described earlier.

How insignificant over time will depend greatly on the age and condition of the boat in question, as well as how it was operated and maintained by previous owners, and how it is operated and maintained by you.

 

[RickB]

In your example Rick it looks like there has been no account for piston friction, bearing friction, gear friction ect. It would seem to me "mechanical work energy" would be output energy .... the power to turn the propeller shaft. Surely they need to separate the mechanical energy of frictional parts and what's left to power the boat .. or whatever.

The portion labeled "mechanical work energy" includes internal friction, pumping losses and all the other mechanical losses.

There is much more to the subject than heat losses.

 

[RickB]

Any other ratio inside the cylinder will result in less than optimum results.

What utter nonsense. Stoichiometric means that there is sufficient fuel to consume all the oxygen. No fuel is wasted and no excess oxygen is "wasted" just to carry heat away.

You are stuck in a spark ignition engine loop. The discussion is about diesels.

 

[bfloyd4445]

The captain of the cruise ship Norwegian Star told me 70% of the ship's power went toward propulsion while 30% went toward "life support" such as lighting, heating/cooling, cooking, sewage processing, water making, etcetera.

I think his numbers are incorrect. Life support would have to include energy used to run the main engines since without them there is no life support. If he is referring to creature comforts his 30% may be correct. 70% for engine systems friction drag losses and actual prop
HP may even be in the ball park. Interesting thought. thanks

 

[bfloyd4445]

What utter nonsense. Stoichiometric means that there is sufficient fuel to consume all the oxygen. No fuel is wasted and no excess oxygen is "wasted" just to carry heat away.

You are stuck in a spark ignition engine loop. The discussion is about diesels.

Rick, I thought I stated it was the optimum ratio of the two reactants fuel and air? Isnt that the same thing you said?

 

[Nomad Willy]

I started a new thread on mixture and would like some input as there's much that I don't understand.

 

[RickB]

Isnt that the same thing you said?

No.

A diesel engine only injects the amount of fuel required to produce the power it is tasked to deliver.

At idle or low power there is very little fuel injected and the huge majority of the cylinder volume never sees fuel at all. Diesel engines do not have a homogenous mixture or burn like a spark ignition (SI) engine.

The plume of injected fuel begins to burn at the points where the fuel droplets are small enough to heat the quickest and have enough oxygen available ... that condition described the outer edges of the plume. The center is almost free of oxygen and is relatively cold. The volume outside the plume has no fuel so there is nothing to burn.

With regard to smoke at high loads, it results from an excess of fuel for the amount of oxygen available to burn it. Semantically, you could say it is an over-rich mixture but since there is not a homogenous mixture of fuel and air across which a flame front moves but is relatively narrow plume of fuel with distinct areas of combustion surrounding or surrounded by areas where the fuel is "baked" into little bits of carbon called soot, it does not smoke from a "rich mixture." That is an unfortunate bit of shorthand for a very complex set of combustion chamber conditions.

Since every kid grew up knowing everything about gasoline engines, when they grew up and bought a diesel they try to explain or understand diesels from the same point of view and it doesn't work that way.

Getting back to your comments about diesel fuel being a cylinder lubricant, if the injector plume ever reaches the piston or cylinder wall it is a major problem. It should never ever happen. If unburned fuel remains to coat the cylinder wall or piston crown it is a major problem. All the fuel should be burned without contact with the cylinder walls or piston.

You guys need to just forget this "mixture" thing when you are talking about diesels. It just confuses things for the members who are contributing more myth and nonsense to the thread.

I don't want to come across like Marin in the Boeing ignorance posts but I am beginning to understand why he wrote what he did.

 

[RickB]

I think his numbers are incorrect. Life support would have to include energy used to run the main engines since without them there is no life support. If he is referring to creature comforts his 30% may be correct. 70% for engine systems friction drag losses and actual prop
HP may even be in the ball park. Interesting thought. thanks

Oh Lard tunderin' Jayzuz Jarge! Talk about the blind leading the blind!

"engine systems friction drag losses"

Please take this conversation offline or something, it makes me queasy.

 

[Peter B]

Never mind Rick. Just have a quick coffee and a lie down, and all will be well. Some of us are indeed learning new things. They say you learn a new thing every day if you open your ming to it. I found that explanation re how diesel burns, as compared to a sparked ignition with petrol, (our word for gas - but you knew that), very interesting. I knew there was quite a lot of difference, but not that much...explains a lot.

 

[SomeSailor]

So from an annual perspective, the cost difference between operating one engine or two is pretty insignificant as I described earlier.

And that is more than offset by the pleasure of operating twins over singles. The maneuverability, peace of mind and reduced load on the engines is significant to justify them in my mind. Plus resale will hold up better as some point as well.

 

[psneeld]

Where are Mythbusters when you need them????

 

[Art]

Where are Mythbusters when you need them????

At a pub slurping up libations with the Ghost Busters!

 

[Moonstruck]

Boats-B-Us! And, We-B-Boats! [/FONT][/COLOR]

Happy Marine Design Daze! - Art

I think Art is not sleeping on his boat. He's sleeping with his boat.

 

[Art]

I think Art is not sleeping on his boat. He's sleeping with his boat.

No, No, No - I sleep IN my boat ... now that's sexy!

 

[bfloyd4445]

No.

A diesel engine only injects the amount of fuel required to produce the power it is tasked to deliver.

At idle or low power there is very little fuel injected and the huge majority of the cylinder volume never sees fuel at all. Diesel engines do not have a homogenous mixture or burn like a spark ignition (SI) engine.

The plume of injected fuel begins to burn at the points where the fuel droplets are small enough to heat the quickest and have enough oxygen available ... that condition described the outer edges of the plume. The center is almost free of oxygen and is relatively cold. The volume outside the plume has no fuel so there is nothing to burn.

With regard to smoke at high loads, it results from an excess of fuel for the amount of oxygen available to burn it. Semantically, you could say it is an over-rich mixture but since there is not a homogenous mixture of fuel and air across which a flame front moves but is relatively narrow plume of fuel with distinct areas of combustion surrounding or surrounded by areas where the fuel is "baked" into little bits of carbon called soot, it does not smoke from a "rich mixture." That is an unfortunate bit of shorthand for a very complex set of combustion chamber conditions.

Since every kid grew up knowing everything about gasoline engines, when they grew up and bought a diesel they try to explain or understand diesels from the same point of view and it doesn't work that way.

Getting back to your comments about diesel fuel being a cylinder lubricant, if the injector plume ever reaches the piston or cylinder wall it is a major problem. It should never ever happen. If unburned fuel remains to coat the cylinder wall or piston crown it is a major problem. All the fuel should be burned without contact with the cylinder walls or piston.

You guys need to just forget this "mixture" thing when you are talking about diesels. It just confuses things for the members who are contributing more myth and nonsense to the thread.

I don't want to come across like Marin in the Boeing ignorance posts but I am beginning to understand why he wrote what he did.

Did I say cylinder lubricant? I think I said that diesel fuel is not only a fuel but acts as a lubricant inside the engine. Dosen't the fuel act as a lubricant for the injectors and some valve train components? If this were not so people would not have experianced engine failures due to the decrease in the lubricity of the reformulated diesel fuel a few years ago. Thats when i began to use an additive and have never stopped. Modern fuel has additives to increase the lubricity in some qareas by the mixing of bio D.
Now correct me if I'm wrong, I'm learning, well attempting to anyway. A diesel engine is really just a pump and each stroke it will result in the intake of the same cylinder volume. Since each stroke will result in the same intake volume how exactly does the throttle work?
Hey Rick, thanks for taking thr time to give us a detailed explanation it helps jog my memory to better understand whats going on

 

[Marin]

Older jerk injection pumps like the CAV/Minemec/Simms pump on the FL120 use the fuel they are pumping to lube the plungers in their bores and also the moving parts in the injectors. I don't know the role of fuel lubricity in more modern injection systems.

 

[RickB]

Did I say cylinder lubricant? I think I said that diesel fuel is not only a fuel but acts as a lubricant inside the engine.

Yes you did, post # 744:

"If the mixture is lean the result is less hp and more engine wear due to lack of lubrication."

Dosen't the fuel act as a lubricant for the injectors and some valve train components?

Only in the injection system. Why do you think it lubricates "some" of the valve train?

Now correct me if I'm wrong, I'm learning, well attempting to anyway. A diesel engine is really just a pump and each stroke it will result in the intake of the same cylinder volume. Since each stroke will result in the same intake volume how exactly does the throttle work?

Keep learning but stop posting nonsense as if you knew what it meant or was true.

The "throttle" on a mechanically injected diesel (with a very few exceptions) is merely a lever connected to a another lever that applies or removes spring tension from a component of the governor that converts centrifugal force into mechanical motion that in turn controls the amount of fuel injected.

On an electronic diesel, it changes a setpoint in the feedback loop that measures and adjusts rpm.

 

[Nomad Willy]

Rick wrote;

"The "throttle" on a mechanically injected diesel (with a very few exceptions) is merely a lever connected to a another lever that applies or removes spring tension from a component of the governor that converts centrifugal force into mechanical motion that in turn controls the amount of fuel injected."

So it's a fuel Limiter? Limiting the amount of fuel that can be injected at a given rpm? And the spring limits the fuel even down to very low loads including idle where the load is only the friction of the engine? The governor seems to be the secret of the system.

 

[bfloyd4445]

i made a mistake then. I thank you for pointing that out. Sometimes i get in a hurry and i just write without properly reveiwing my work.

<<If the mixture is lean the result is less hp and more engine wear due to lack of lubrication.">>

Rick, big time error thank you sir.

<<On an electronic diesel, it changes a setpoint in the feedback loop that measures and adjusts rpm.>>
Go on Rick I am listening. I understand the old tractor diesel controls but not the new systems. I assume you are referring to a charactoristic cure that is programmed in the cpu which is used to control fuel volume in relation to engine rpm?

 

[bfloyd4445]

<<Keep learning but stop posting nonsense as if you knew what it meant or was true. >>
....maybe someday i will know everything. You my friend have also have made nonsensical typos during this exchange which have been pointed out during this conversation. as stated I am appreciative of your corrections

 

[RickB]

So it's a fuel Limiter? Limiting the amount of fuel that can be injected at a given rpm? And the spring limits the fuel even down to very low loads including idle where the load is only the friction of the engine? The governor seems to be the secret of the system.

Be careful using the term "limiter" as that has a very specific meaning in terms of engine control. For instance a "start fuel limiter" controls the amount of fuel delivered during start to avoid smoke and an overspeed. Your mechanical governor is at "full throttle" when the engine is at rest and when it is started.

Look up flyball governor, there are some outstanding descriptions of how they work. Unless you have an electronic engine, you have a good old fashioned flyball whirling around at the end of your fuel injection pump.

And yes, the governor is the heart and brains of the outfit.

 

[bfloyd4445]

That's still an irrelevant statistic because when the one engine fails in a single you're through boating for the rest of the day other than as a barge on the end of a line. And while the odds of the one engine in your boat failing are exactly the same as the odds of either of the two engines in a twin failing, the odds of you being left adrift and hoping that a tow arrives in time are almost infinitely greater than the odds of a fellow with a twin engine boat who experiences a failure of one of his engines being left adrift and hoping that a tow arrives.

So the real question is, do you want to be left adrift when an engine has to be shut down or do you want to keep going when an engine has to be shut down? That's the only question worth asking as far as I'm concerned.

Marin, you are forgetting that there are DeFevers out there with electric get home motors..........would those technically be twins?

 

[bfloyd4445]

Be careful using the term "limiter" as that has a very specific meaning in terms of engine control. For instance a "start fuel limiter" controls the amount of fuel delivered during start to avoid smoke and an overspeed. Your mechanical governor is at "full throttle" when the engine is at rest and when it is started.

Look up flyball governor, there are some outstanding descriptions of how they work. Unless you have an electronic engine, you have a good old fashioned flyball whirling around at the end of your fuel injection pump.

And yes, the governor is the heart and brains of the outfit.

Hey Rick, what do you think of the four piston eight cylinder direct injection engines that will soon be powering helicopters?..some kind of configuration like that. anyway the piston fires both ways acting like two cylinders. I read of this design a while back

 

[Northern Spy]

Hey Rick, what do you think of the four piston eight cylinder direct injection engines that will soon be powering helicopters?..some kind of configuration like that. anyway the piston fires both ways acting like two cylinders. I read of this design a while back

I'll have to look that up. Sounds interesting. I used to work on 8 cylinder, 16 piston diesel engines. Rick probably did too, if I recall his background correctly.

 

[markpierce]

Didn't ALCO diesel engines of WWII era (used in both submarines and railroad locomotives) fire both (up and down) ways? Those were rather tall engines.

 

[Marin]

The opposed pistons in a single cylinder is something Fairbanks-Morse perfected prior to WWII as it was the diesel that was used in a lot of our submarines and later in F-M railroad locomotives. I'd not heard of a single piston moving back and forth in a cylinder with a combustion chamber at each end, however. How does the piston connect to something to transmit the power it generates to something that turns?