What I Did On My Summer Vacation

It’s Tuesday, and I’m back, as promised. My absence these past few days has been due to the family summer vacation. So as I sat down today to get out my first blog after vacation, all I could think of was that dreaded assignment we all got on the first day of school each Fall: “Please write an essay about what you did on your Summer vacation.” I really hated writing essays — especially on that topic. But I guess my teachers knew what they were doing, since that is exactly what I’m doing today. However, as you might guess, my essay will have a “water” twist to it.
We wound up taking a cruise to Ensenada, Mexico. We had been on two similar cruises in the past 15 years or so, and enjoyed them greatly, so we decided to try it again. Only this time, we didn’t bother to get off the ship at all — after a couple of day trips into Ensenada, we had no great desire to visit this city again. For one thing, you “can’t drink the water.”
Which got me to thinking, how does a cruise ship deal with its water needs? With thousands of passengers and crew, and all the same types of water demands that we see in any city — except landscaping, of course — each ship must have a fully self-contained water department. And for that matter, a power department, a sewer department, and a trash department, too. These needs go beyond the departments we would initially identify as necessary for a cruise ship’s operations: ship propulsion, navigation, dining, entertaining, hotel, gambling, shopping, photography, general maintenance, and other departments. But how long could a cruise ship operate without its water department? Clearly, although it’s invisible to guests on the ship, the water department is essential.
Sounds like an interesting job, by the way!
Most cruise ships actually obtain a lot of their water while in port — although I hope we didn’t take on any water in Ensenada! But how much water might they need? The ship I was on had about 2000 passengers, plus about 1000 crew members, so about 300 people on board. Average domestic water usage — without landscaping — is around 100 gallons per person per day. This probably is pretty close to the demand on a ship, even though most of my fellow passengers only used water for ice in their assorted “adult beverages.” (And a few of my fellow passengers really went through a LOT of ice!)
But everyone still used toilets and took showers and washed their hands. They may not have done any laundry, but several ship’s crew members are assigned full time to laundry detail, washing dining room linens and towels and bedding, among other things. The dining rooms also must go through a lot of water, washing and preparing a LOT of food.
And in a way, the ship also does some “landscaping” type water use, hosing down outdoor decks, and filling pools and water slides. In short, I would put per capita water consumption on a cruise ship at something pretty close to our normal household consumption of 100 gpd. That means one day on the ship would require 300,000 gallons of water. And a three-day cruise like the one I was on would require about 1 million gallons of water.
One million gallons of water will need a pretty large home, about 134,000 cubic feet. If a single tank was used to hold this supply, its dimensions might be something like 20-25 feet in diameter, and half the length of the ship! That might be possible. But what if we consider even larger ships, and even longer cruises. Pretty soon, taking enough water on-board in port just becomes impossible.
Fortunately, cruise ships have two things going for them in the area of water. First, they are surrounded by it on their voyages. Yeah, it’s kind of salty, and not fit for human consumption, but there is a lot of it. Second, they have excess energy on board, which can be used to purify the water.
Where does this “excess” energy come from? Their power plants! Most of these vessels use petroleum-based fuels like diesel for propulsion and electric power generation. Burning such fuels generates a lot of heat, and that heat needs to be taken away from the engines somehow. So, using equipment that is similar to the radiator in your car, the engines are cooled with water. That hot water is used to generate steam for several purposes on-board, including space heating. But that hot water can also be used to distill sea water, converting it into pure water. The product of such treatment is actually too pure, and so a part of the routine activity in a cruise ship’s water department is adding stuff to the water!
So, on my Summer vacation, I though about water. I think I need another vacation!
Actually, I will be “off line” the next two days, as I will be attending — not teaching — a training class. Even teachers need to learn stuff from time to time.

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