Plastic water tanks bulge when full - breathers are clear - any danger of bursting?

[benzy]

Hi all, wondering if this is a problem or not.

We have a Moody 376 with 250ltr water capacity in 2 x plastic tanks - port and starboard. When filling them to full, the tops noticeably bulge upward with the pressure.

A bit of background (which is long sorry):

The tanks are irregular shaped to fit under the saloon seats, so you might be able to imagine the shape (flat top, flat inboard side, bottom is a slight curve to roughly match the hull - basically the profile is a right angle triangle).

Our setup is as follows:
- Water filler pipe is connected to the starboard tank. It connects at the forward-top-inboard corner of the tank.
- The two tanks are connected by a pipe, which connects at the aft-bottom-inboard corner of the tanks. Water pump offtake is from a T in this pipe.
- Each tank has a single breather pipe. These connect at the forward-top-outboard corner of the tanks. These pipes run forward, behind the woodwork/behind the heads, and up the the anchor well. The anchor well has two small metal “mesh” holes where the breather pipes are able to breath through.
- We also have a manual hand pump at the galley sink which draws from the tanks.

When we fill to “overflowing” the manual hand pump will start discharging water first as that is at a lower water level. If we continue filling faster than this rate of discharge, then, depending on how yhe bost is sitting, water starts coming out of either the deck filler or the breather pipes in the anchor well.

So back to the bulging. When we fill, the tanks fill almost all the way full, and not until the last maybe 30 litres do they start to bulge. The top surface of the tank might bulge upwards in the middle by 10-15mm. I have double checked and both breather pipes are clear and able to “breath” (I pulled them off the tanks and was able to blow through them).

Some people I’ve asked say this is a problem and we are in danger of bursting the tanks. Some people seem to think it’s not a problem.

I found another thread on the Catalina forum where people report a similar issue and seem to think it’s just a fact of life (https://www.catalina36.org/forum/tec...en-water-tanks)

So I’m wondering if anyone else has plastic tanks that will bulge when full?

Any thoughts?
Cheers