How to clean a boat water tank

With plans to enter the Pacific Ocean on the horizon, we decided to clean our fresh water tank on the boat. Neither my husband, Simon, nor I knew how to clean a boat water tank but we knew that we’d eventually need fresh clean water coming from it!

The first step we took was to locate access to the tank.

We found a plastic see-through cover below our floorboards in the saloon. Simon tried to twist it open by hand and when that didn’t work, he used our friendly persuader (a hammer) to get the cover to move. After a few failed attempts we decided to call, Oyster, the manufacturer of our boat, to ask for some instructions.

During the phone call Simon was instructed that a tool was required to open the caps. Yes – the Oyster engineer told him about caps and not a cap! Little did we know that there were two access caps to our 1000 litre fresh water tank.

Once the caps were opened, the tanks could be emptied, cleaned by hand, disinfected and then clean water flushed through the system. (Keep reading the full article below and/or watch the video to see how we tackled this cleaning job!)

How to Clean A Boat Water Tank Video

Simon and I started the job at 10am and 10 hours later at 8pm we finally finished.

Who would have thought that cleaning a fresh water tank would take so long?!

After getting direction from Oyster, our next task was to find the special tool that opened the cap. Thinking back to the week I spent in Gran Canaria (pre Atlantic Crossing) doing an inventory, I remembered a round metal ‘thingy’. I couldn’t put it on our inventory list because I didn’t know what it was for!

First I sent Simon to our captain’s cabin to locate the tool. No luck. Eventually, an hour later, he found it below our daughter’s bunk bed.

We then spent another couple hours getting the caps off.

As usual, it was not as simple as putting the tool on and using it. We had to remove a board and unbelievably that took an hour!

Simon unscrewed the board but it wouldn’t budge. We called Oyster again asking if it was okay to use force to get the board up. Fortunately we got our answer quickly – the board that needed to be removed was inserted before the rigging was set up. The rigging therefore was holding quite a bit of pressure on the board. Once Simon knew it was okay to pry the board up, he spent a while doing so.

With the use of the friendly persuader on the tool, the cap began to unscrew.

Once we got the cap off I instantly noticed a putrid smell. Furthermore, I could see little black dots along the side of the walls. Simon took a paper towel and wiped the walls to discover brown gunk.

Throughout the last couple years we’ve always bleached, chlorinated and disinfected our water but never opened the tanks to see what state they were in. To date we’ve only ever used our fresh water for washing dishes, showers and laundry. We also would boil food with the water but that would have killed anything in it anyway.

Clean a boat water tank

Had we known that the tank was so filthy we would have cleaned it sooner.

I suppose it’s one of those things where you just hope that it’s okay. Knowing that we’ll need to depend on good fresh water in the future, the priority increased.

As a side note, whenever we put water in our tank, we use pre-filters before it enters the boat. The water goes through to water filters. It then sits in our tank and we also have another filter before it comes out our drinking tap. So…by the time we drink our water (if we did) it’s been filtered a few times.

That being said, if the tank is slimy no amount of filtering will allow water to be drinkable!

Clean a boat water tank

Simon spent hours cleaning the tank. He used a big huge bristly brush, sponges and washcloths. The first time he washed the tank with vinegar. Simon brought a hose in through the window and washed the walls down. After the vinegar wash, he used bleach and then washed that through. Simon then filled up the tanks, added a Water Tank and System flush solution (purchased at West Marine).

The solution was left to sit in the tank for a set duration. Simon then opened all the taps so the solution would run through the whole system.

The tank was emptied and another spray down and spot check was instigated.

Clean a boat water tank

And then finally, we filled up our tanks, changed our water filter and tried a glass of water!

The result was positive. The water coming from our fresh water tank had no odor and tasted like bottled Checklists for Sailors
Checklists for Sailors

You won’t need a checklist telling you that you need to clean out your water tank. A bad taste or smell will let you know that the job needs to be done. There are however, other sailboat maintenance items that need to be done routinely.

If you’re a new boat owner and haven’t purchased by guide titled, ‘Checklists for Sailors – Passage Planning, Sailboat Maintenance, Cleaning, Medical and more,’ you might want to take a look at it. I offer both a digital and paperback version on my online shop and you can also find the book on your local Amazon website.

When Simon and I started out, we didn’t know how to do much of anything. Over the past three years we’ve learned loads and whatever we do learn we’re working hard to pass it on to you.