In a reply today someone threw a statistic at me. My first thought was "that can't be right at all". In the modern world we get a lot of stats thrown at us, and it's a really useful skill to be able to look at a stat, and without much effort work out if it is plausible.
I'm not talking about checking it's accurate. Just plausible. Could it be true? Or is it out by several orders of magnitude?
Let's look at the stat from today and work out if it's at all plausible. 1/n
It's actually pretty hard to find the fuel consumption stats for the biggest ships. They are all pretty new, and their Wikipedia pages aren't exactly exuberant in their verbosity. One ship with good data tho is the Emma Maesk. She was the largest container ship when she was built. And her engine is the largest reciprocating engine on the planet.
Wikipedia says she burns about 6500l of fuel an hour. Let's be charitable and say the biggest ships currently (MSR Irina) use twice this.
3/n
So we have a ballpark fuel consumption figure if 13,000l if fuel per hour for one ship.
Ok. Let's look at the cars. As we said earlier. One ship is 87,500,000 cars... That would mean each car is using 0.00015l if fuel per hour. That's litres. For comparison a teaspoon has a volume of 0.005l. or 33.3 times the size. Would be impressive if cars did only burn 0.15ml per hour...
But it's not quite so clear cut. Not every car is moving 24/7. On average it's probably close to 1 hour per day.
4/n
Again we're making an assumption, but it's not implausible. Half an hour each way to work. Maybe an hour or so over the weekend. Obviously some people will do lots, but some people only drive their car once a month.
So 1/24th of the time isn't too bad. Let's call that 5%.
So 0.15l times 24. Cos the ship is driving all day. But the car is only doing so for 1 hour of that. And we get 0.0036l of fuel. 3.6ml. Less than a teaspoon of fuel a day.
So let's tie that back to the original stat.
5/n
For the stat to be true. For the 16 biggest ships to emit the same as all the cars. Then your average car would need to consume less than 1tsp of fuel per day. That's pretty impressive fuel economy... It's completely unrealistic.
So. With basic numbers pulled from Wikipedia and the first page of Google. We can easily see what it's just not plausible that the 16 biggest ships emit the same amount of CO2 as 1.4bn cars.
We're talking out by multiple orders of magnitude. The stat is wrong.
6/6
@quixoticgeek fyi: fuel of big ships is also crude oil, which quite different from something "friendly" as diesel.
@wmd yes. But to within a couple of percent. Burning a ton of bunker oil, and a ton of petrol. Emit the same CO2.
The difference is petrol and diesel are regulated very low sulphur, but even modern bunker fuel is 1% or less.
@quixoticgeek Originally the stat was about sulphur emissions, which may be more accurate, but it got memed into CO2 which is ridiculous. I still see it all the time though. https://www.transportenvironment.org/discover/europes-luxury-cruise-ships-emit-as-much-toxic-sulphur-as-1bn-cars-study/
@TheoEsc yep. I will do a separate thread on the actual stat. This thread was just about how to do a quick sanity check when given a stat. Is it plausible. I used that stat cos it was thrown at me earlier that day.
So the stat in question was "the 16 biggest ships emit more than all the cars in the world". Ok. Let's set out some assumptions. First off. This is talking about CO2 emissions. Now a quick Google search for how many cars are there in the world gives lots of replies of about 1.4 billion.
Ok...
16 ships Vs ~1,400,000,000 cars. Right off this should be enough to instill doubt. 1 ship is equivalent to 87,500,000 cars. That seems... A lot. But big ships are, well, big. So let's look further. 2/n