Icon of the Seas fuel efficiency

[AlmondFarmer]

The article detailing Icon of the Seas contains the following paragraph-

“Traditionally, cruise ships are powered by diesel. The advantage LNG has over diesel is it is more energy-efficient and will reduce fuel consumption and emissions.”
 

While possibly true, some of these claims are incorrect or misleading. I will explain how this can be possible.

Diesel fuel has an energy density of 38 MJ/L. 
LNG has an energy density of 22 MJ/L. (Ratio is similar with MJ/kg)

Not all energy of fuel is converted into work.

A modern Diesel engine is 50-55% efficient. 
A modern LNG fueled engine is 45-50% efficient. 

Take two ships, ceteris paribus (all else equal) one modern diesel, one modern LNG powered. Give each an equal volume or weight of fuel of their respective fuels. The LNG ship will use more fuel to travel the same distance as diesel ship. This does not comport with being more efficient or reduced fuel consumption. 
 

If fuel consumption is being measured by amount per passenger, Icon could use less fuel than a diesel ship due to higher occupancy. Or if using the price of the fuels at a given time, it may cost more per mile traveled using diesel than LNG thus making LNG financially more efficient. (Currently using free market prices, this is not the case)

I don’t know what metrics Royal Caribbean uses to measure fuel efficiency so the quoted paragraph can be true, but if so, it is misleading.

What is correct is that a modern LNG engine produces less emissions than a modern Diesel engine. 
Adding shore power or hybrid system will reduce engine usage and thus fuel consumption and emissions of both fuel type engines. 
A new hull design and drag reducing technology will also reduce the amount of power (fuel) required to move the vessel.  
 

For anyone who manages to read this far, I recommend always figuring out what exactly is being compared and how is the comparison being done.

 I once got myself in trouble when somebody asked me if my wife is pretty, “compared to who?” I responded. 

[twangster]

Furthermore it's interesting Bayley references diesel as technically that's not what ships use.  HFO or Heavy Fuel Oil is not the same as diesel fuel.  Ships use HFO.  

[AlmondFarmer]

Heavy fuel oil is much cheaper per unit of energy than diesel fuel but generates much more particulate matter (soot) and other pollutants and also has a lower energy density. 

I have heard, but not confirmed, that Royal ships sometimes use pure diesel in their engines while in port to minimize visible soot as well as at startup then switching to bunker fuel at operating temperature. 

[Poolside]

10 hours ago, AlmondFarmer said:

 I once got myself in trouble when somebody asked me if my wife is pretty, “compared to who?” I responded. 

I think we all know that it has been more than "once". 

[twangster]

10 hours ago, AlmondFarmer said:

I have heard, but not confirmed, that Royal ships sometimes use pure diesel in their engines while in port to minimize visible soot as well as at startup then switching to bunker fuel at operating temperature. 

It is required in some countries.  It's not unusual to see a fuel tanker truck delivering marine diesel on turn around day.  

There are some ports where you can tell they didn't switch fuel sources.