Thundering water tank?

[Jimongee]

This is Jim again…with the 1987 C&C 35 MK III. I purchased it about 6 days ago in Owen Sound, Ontario. I have been sailing it constant since purchasing it and am now making my way to Sturgeon Bay, WI. So this model has 2 fresh water tanks. One on port and one on starboard. I think these or at least one of them were retro fits. Of course I have a water manifold to control which to tank use. Right now I have both tank valves open and I do not know if the water level equilizes.
So here is my question; I filled the starboard tank. Then I started filling the port tank. After a few seconds I head this thundering noise which was clearly the port tank either expanding or? i did not have the fill hose jammed in the fill hole to prevent venting. I stopped filling it immediately until I get the boat to home port in Manitowoc, WI. All of that was *6 days ago. However moments ago as my wife and I were cooking dinner and I had the poorly working pressure pump on I once again heard this thundering metal expansion like a steel drum sound from the right tank. We were doing dishes and running water while the pressure pump was on. At that point I shut it off. The water tanks are aluminum or stainless. If any sailor has a clue about this scary noise ..it would be very appreciated.
Jim n Kathy at dock in Manistique, MI

 

[Davidasailor26]

Do you know if the tanks have vents to equalize pressure with the outside? Usually as the tanks fill or drain the vent will let air in or out. It sounds like you might not have vents or they may be blocked, and the metal is flexing as a result.

 

[Jimongee]

H
thanks Davida sailor! That makes sense. The folks I purchased this vessel from had never ever used the water tanks. As soon as I get it in home port I will trace the vent lines and assure they are clear

 

[RoyS]

Also there should be a manifold somewhere so that you can isolate the tanks and use one at a time. Easy to make out of pvc pipes and valves if you need to.

 

[kloudie1]

Might just be "oil-canning" where a slightly distorted panel of the tank snaps in or out as the weight (level) of the water inside changes..

 

[David in Sandusky]

It happens on our aluminum water tank, usually after filling it.

What happens? After filling, there is very little air in the tank. With the filling cap screwed tight, air intake is very restricted. Pumping water out of the tank creates a powerful vacuum above the surface of the water. This pulls the walls of the tank inward. Eventually, air finds its way into the tank. (Nature abhors a vacuum.) I imagine it finally comes in around the cap, but on further thought, it could also come from the fixture connecting the outflow pipe, or even from someplace downstream. In any case, the inrush of air relaxes the walls of the tank which recover with a banging sound. ("oil canning")

In short, this is a harmless phenomena that is easily prevented by leaving the cap loosely rather than tightly screwed in. Once the water level in the tank leaves enough air space above the surface as we use the water, there is enough air present to prevent the build up of a high vacuum, and the walls are less deformed, while the air to replace pumped water can get in more slowly.

For us, it's just a normal sound on the boat after we fill the tank.If it really bothers you, a small hole in the filling cap would probably prevent formation of a high vacuum as I described. Or simply not tightening the cap!

I speculate that many water systems do not have a vent tube because this could be a place for harmful bacteria or substances to enter your drinking water supply. Unlike a waste tank., potable water does not generate gases to vent, and does not need more oxygen to reduce odor development.

 

[Jimongee]

Hey David n Sandusky! Tank you….oops I meant thank you. Your explanation makes perfect sense. I will replace the water tank cap with a one way vented cap asap. So essence it works exactly like vent on the traps in our home.

 

[RoyS]

Tank needs a vent to admit air to replace the volume of water taken out. Loose fill caps or small drilled holes in the fill cap will greatly restrict water removal. Needs a vent.

 

[Jimongee]

Thanks RoyS. Tomorrow I will see if I can locate the vent and confirm it is clear and open

 

[jviss]

In short, this is a harmless phenomena that is easily prevented by leaving the cap loosely rather than tightly screwed in.

If you had a functioning vent, as water all tanks should, this wouldn't be necessary.