Programs Blog

A How To: Not run out of water while crossing the Pacific

October 04, 2023
Me and our lovely water maker in the engineering space!

Author:

Ship’s Log

Date: 10/4/2023

Time: 1900

Location: 16°46.606’ S   179°19.626’ E

Weather: Wind out of the WxNW, Force 4, calm seas, stratocumulus clouds
You may be wondering; how do 30 people sail for weeks on end in the pacific without running out of water? Well, I’ve got an answer. In short, there is a machine on board that pushes water with so much force through membranes with such tiny pores that it can extract even the small sodium chloride molecules (NaCl) from sea water, producing fresh water that we drink and cook and clean with. 

This system, called a reverse osmosis water maker, is used to turn fresh water into sea water in ships all over the world as well as some coastal cities with limited access to fresh water. Here, I will explain the water maker system on board our ship, the Robert C. Seamans. Our water maker is locating in a room off the engine room below sea level. There, you find a valve in the bottom on the ship where salt water is taken up and used in science, saltwater showers, and the water maker. As a safety measure to prevent flooding, the engineers have access to a lever that closes the valve, shutting down the related systems. Because this intake is located below the water level, there is high pressure outside of the boat pushing the salt water up into the lower pressure pipes and through the sea strainer. The sea strainer (see photo) filters out large things like fish and seaweed. The S306 trip actually caught a fish through this strainer! 

From there, the salt water travels to the media filter pump, a centrifugal pump that sucks it in through the center and pumps it out the sides. Our engineering room conveniently has a “how things work” book that explains these pumps with great illustrations, here is the one explaining a centrifugal pump, essentially a propeller, flinging water outwards. From the media filter pump, the water travels to the media filter. Water enters the bottom of the media filter and is pushed through layers of different size sand grains which filters out sediment and small creatures like zooplankton and phytoplankton. After the sea strainer and media filter, the sea water is ready for the water maker. 

The next stop is the low pressure pump. Another centrifugal pump that creates pressures of around 70 psi and sends the water first through a 20-micron filter and then through a 5-micron filter. The next pump is the coolest of them all. It creates pressures of around 800 psi! This high-pressure pump is called a piston pump and works with the help of a motor that pushes a piston in and out. With these high pressures, water is pumped through the final membrane. It enters a long tube with a coiled filter membrane in the center. The pressure then pushes water molecules through this membrane in a process that prevents salts from passing through, both because of their size and charge. The result is drinkable water at the center of this coil that is used throughout the ship. A byproduct of this process is extra salty water called brine that is discarded overboard.

Our water makers produce around 70 gallons of fresh water per hour and 420 gallons of brine per hour, about six gallons of brine for every gallon of fresh water. The system uses around 3 to 4 gallons of fuel four 18 hours of water production.  Water system maintenance is equally complex. In order to keep the filters operational, fresh water is pumped backwards through the system after every use, killing any critters that may be growing in the salty environment and flushing out the filtrate. Additionally, the water makers are turned off when the ship is in highly populated waters with sediments that may clog them. 

I hope you have enjoyed this delve into the water making system that keeps us all alive while at sea. We sure appreciate it!

SHOUTOUTS TO THE FAM

Alice, thank you so much for all your kindness these past few days. I really don’t know what we would have done without you. Missing the lovely blue house with amazing views. Hope you get some good painting in.

DOTTY: You may be reading this from Boston! What a curveball 😉 We all miss you dearly and shout “DOTTY” instead of “three” at every muster call. Ur so strong girlie <3 We all wish you a safe and relaxing recovery.