Beneteau 400 overfilled water tanks

[Oceanride007]

Its possible the Pressure in the tank, popped the worm clamp and the fill pipe is no longer connected.
Hope So!
Reason, Hydrostatic pressure Head can be enormous, if it didn't pop the fill hose it could break the tank, Heres how:
Hydrostatic pressure (top of tank to top of fill pipe) say 2m. Pressure is 2m*9.8*1000= ~20000N/m2. Force=PxA.
Area of your tank say 1m x.5m. Thats 1 metric tons in weight but acting every direction, with the fill hose in and creating back pressure, multiply that, makes no difference if you have vent if its flooding over deck at some time.

Always top up slowly, many fills over the years treating your tanks as a balloon contributes to fatigue and eventual failure.

I hope you lucky.

[gonesail]

we had an electric gauge and made sure we observed it while filling. sorry to hear of your disaster.

[YOLO110]

All great replies guys, Many thanks!

I will be back at the boat in a few days and will have a good look at all.

In any event, I was thinking of fitting new Plastimo bag tanks in the integral original water tank spaces... I have looked the inspection ports and there are lots of little black 'bugs' adhered to the bottom of the tanks... I guess not surprising considering they are 20 years old.

Has anyone any experience with the bag tank options?

Cheers, Peter

[lifeofreilly57]

I helped a friend of mine transport his recently bought Bene 38 back to his home port last spring and on the trip figured out everything that didn't work on it during the transport, including the fuel gauge, but that's another story.
On his 38 there were three water tanks, all had overboard vents, through hulls just below the rails, the water supply tubing was all interconnected at a manifold under the floorboards in the salon. All three tanks were black plastic, most likely HDPE, with inspection ports. Not the best quality tanks I've seen, I wasn't too impressed with the way the hoses were routed or supported, or should I say unsupported. That boat also filled the bilge when we filled the tanks, it also filled the bilge just sitting, we spent the remainder of the trip finding the leaks and then some time in home port tracing the remaining leaks.
One of the vent lines had cracked near the through hull fitting, which was a bugger to get at, good thing the owner is 5'8" and 155lbs, I would never be able to get at it. The original installation put too sharp a bend in the unsupported vent hose, it eventually failed, so it leaked at fill time and when on a starboard tack. Two the the inspection caps leaked, new gaskets cured that.
The plumbing is plastic tubing with compression fittings, similar to what their using in new homes now. This is a good system when installed correctly, it's a bugger when not. Some of the bends were a bit too sharp near the compression fittings and had cracks, once replaced with generous bends the problem went away.
If your leaking when filling check the vent hoses and filler caps. If it leaks when sitting check the lines between the tanks and valve header all the way to the pressure pump. If the pump cycles often with the faucets closed check the fittings on the hoses, that's where they are most likely to fail, of course if a line has chafed the leak can be anywhere.
I can't say that our experience is common, after all it was a 1997 model and the previous owner was maintenance challenged but the plumbing appeared to be original factory. No matter how good the company it only takes one hung over employee on a Monday morning to make your life interesting.
Once straightened out the water system worked fine and didn't fill the bilge.
Now about that fuel gauge.......