Engine Performance


#1

Please excuse the ignorance of a decky. I’m way out of my depth here. This discussion is purely hypothetical and not based on any real project.

Let’s say you had a boat you wanted to power with 400hp worth of diesel engine(s). What I would like to know is what the effect on performance would be if you powered a given hull with 1-400hp engine, or 4-100hp engines. Is 1-400hp engine more fuel-efficient than 4-100hp engines? If the 1-400hp engine could not achieve theoretical hull speed for a given hull, would 4-100hp engines make any difference? There are other more obvious concerns like having 4 main engines to take care of instead of 1. What I’m interested in is how it would affect fuel consumption and the speed of the vessel.

All of this is assuming appropriately sized shafts, props, etc. for the engines in question. Thanks for humoring me and excusing my engineering ignorance!


#2

[QUOTE=PaddyWest2012;168359]Please excuse the ignorance of a decky. I’m way out of my depth here. This discussion is purely hypothetical and not based on any real project.

Let’s say you had a boat you wanted to power with 400hp worth of diesel engine(s). What I would like to know is what the effect on performance would be if you powered a given hull with 1-400hp engine, or 4-100hp engines. Is 1-400hp engine more fuel-efficient than 4-100hp engines? If the 1-400hp engine could not achieve theoretical hull speed for a given hull, would 4-100hp engines make any difference? There are other more obvious concerns like having 4 main engines to take care of instead of 1. What I’m interested in is how it would affect fuel consumption and the speed of the vessel.

All of this is assuming appropriately sized shafts, props, etc. for the engines in question. Thanks for humoring me and excusing my engineering ignorance![/QUOTE]

im not an engineer, but

a single engine single screw 4000 hp boat will out pull a 4000 hp multiengine multiscrew boat every time with less fuel consumption. one large propeller with deeper immersion is more efficient than two or more smaller propellers.

That is why the vast majority of ships are single engine single screw.

In theory, a father / son engine arrangement on a single shaft might be more efficent under different load conditions.

However, a controllable pitch propeller is a better approach to maximize power and efficiency under different load conditions.


#3

Makes you wonder why all the great liners were quadruple screw.


#4

The passenger liner issue was maximum speed in a semi-displacement hull, not fuel efficiency. Fuel was very cheap back then. Also, the draft would have been too great and the speed too slow with one propeller large enough to absorb 250,000hp.


#5

probably reached the limit of single engine and propeller size for their era.


#6

for speed you want high horsepower driving small diameter high rpm props

for efficiency you want a slow turning prop with hp enough to maintain hull speed

for bollard pull you need excess horsepower and slow turning large diameter props

theke


#7

[QUOTE=tugsailor;168360]im not an engineer, but

a single engine single screw 4000 hp boat will out pull a 4000 hp multiengine multiscrew boat every time with less fuel consumption. one large propeller with deeper immersion is more efficient than two or more smaller propellers.

That is why the vast majority of ships are single engine single screw.

In theory, a father / son engine arrangement on a single shaft might be more efficent under different load conditions.

However, a controllable pitch propeller is a better approach to maximize power and efficiency under different load conditions.[/QUOTE]

another thing to consider is the redundancy…multi screw vessel lose one you can still limp home…

also, there is the maneuverability aspect of multi screw vessels…


#8

On a diesel-electric vessel, more engines are always better. That why it’s easier to run the engines on economic speed.


#9

[QUOTE=Kraken;168376]On a diesel-electric vessel, more engines are always better. That why it’s easier to run the engines on economic speed.[/QUOTE]

Could you explain that a little more? I’m not quite sure I fully understand. By more engines do you mean more screws, or do you mean multiple small diesel generators feeding into one large electric propulsion motor? Why is more engines more efficient in this case?


#10

To the OP the most efficient set-up is going to be a large, slow-turning single prop. For engine power it’s going to be a big direct drive (no reduction gears) low-speed diesel on heavy fuel oil. If you need more maneuverability, add a (low-cost) bow thruster and throw a ship handling text in the book shelf. That’d be the cheap way to make ocean passages.

As far as the multi-engine liners, to transmit that much power to a single prop, to be efficent it would be too large for the draft.

One issue is that big marine engines are expensive, small mass produced engines are cheap. If you want to go fast on the cheap a couple big gas-powered car engines is the way to fly.


#11

The original question was about maximum effective power and efficiency.

It’s exactly as Kennebec Captain stated: one slow speed (about 100 rpm) direct reversible engine turning one large propeller.

For a lot of reasons: safety and environmental protection being foremost, virtually all tugs are now twin screw with one engine each shaft.

I do not think that the USCG allows any single screw tug to tow an oil barge.

The few single screw tugs that I still see in service are doing harbor work, or towing logs, wood chips, salt, cement, gravel, riprap, and other low value, low environmental risk cargoes. I don’t think that most customers will accept a single screw tug.

What(and when) was the last U.S. single screw tug built?

Triple and quad screw tugs are built for services where shallow draft is critical. Four screws in tunnels keep the draft shallow and provide the most push for a driven draft at the expense of fuel efficiency and maneuverability (in tunnels).

Twin screw provides maneuverability and redundancy at an acceptable loss of fuel efficiency for most applications.

Diesel electric is the most effective approach for DP applications. Obviously, it would make sense to have several gen sets and only run as many as needed to produce the necessary power for the actual load conditions.

So far, controllable pitch has not caught on for tugs in the U.S. due to its high initial cost.

With the exception of Western Towboat (and Hyak), z-drive has not caught on for linehaul tugs. Z-Drive is reserved mostly for ship escort and ship docking.

Actually, I think Western Towboat is blazing a smart and innovative trail into Z-drives for linehaul tug boats that will be increasingly imitated. They probably have less initial and everyday cost efficiency, but that is offset by increased maneuverability and capability.


#12

[QUOTE=PaddyWest2012;168392]Could you explain that a little more? I’m not quite sure I fully understand. By more engines do you mean more screws, or do you mean multiple small diesel generators feeding into one large electric propulsion motor? Why is more engines more efficient in this case?[/QUOTE]

I mean multiple diesel generator sets. It must be sized so that one generator set could cover base load on 70-80% of engine output. Moreover, you just scale it that you start generator sets after the need for power.

With battery banks getting cheaper we could eventually cover peak load with power from the battery bank and have engine generator sets constantly running on economical speed. And then you could also use pure LNG engines (they do not like big fluctuations on power demand).

http://www.norwegianelectric.com/download.aspx?object_id=38EF583AB62B4C3F87EFE0BD75E979CE.pdf


#13

One of the advantages of a diesel-electric propulsion system is that the main generators, which are typically larger and have lower specific fuel consumption than typical auxiliary diesel engines, can be used to produce power for all onboard consumers. Of course, you also have more losses in the drivetrain (generator, converter, propulsion motor).


#14

From an operator’s point of view diesel-electric is nice because it give a wide choice of speeds to achieve various objectives. While a low-speed direct-reversible diesel is very efficient, even waste exhaust heat is captured and used, they are very limited as far as range of speeds, especially the older ones. At lower engine speeds the low-speeds suffer from incomplete combustion. Basically in my case, at sea for long periods the acceptable range is between about 85 and 95 rpm which is between 17 kts and 19 kts. If the schedule requires lower speeds one common tactic is to shut down and drift.


#15

[QUOTE=Tups;168412]One of the advantages of a diesel-electric propulsion system is that the main generators, which are typically larger and have lower specific fuel consumption than typical auxiliary diesel engines, can be used to produce power for all onboard consumers. Of course, you also have more losses in the drivetrain (generator, converter, propulsion motor).[/QUOTE]

On many older diesel/turbo-electric plants, the propulsion generators are on a separate system from the “hotel” load. . . the electrical losses do have an effect on plant efficiency. During WWII, vessels like the T2 were electric because it took less work to wind generators and motors than it did to hob gears. . .


#16

Could you really call an ocean liner a semi-displacement hull? As fast as the fastest ocean liners were, they might have achieved close to theoretical hull speed, but they didn’t surpass it.


#17

The same reason the Queen Mary had reduced turbines and the Normandie was turbine-electric. The British were good at casting and the french were good at making wire. They each played to their strengths.