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Water Tank overflow / air vent?

I am about to install a new water tank in my narrow boat and I--m unsure about the best approach to arranging the overflow/air vent. Both the original and new tanks are around 100gals.
The tank is under the floor of the forward cockpit and sits flat on the bottom of the boat and is 500mm high, so the top of the tank as about level with the boats water line. The original tank was GRP and although the top was reinforced with external ribs any amount of upward pressure on the top of the tank put unacceptable load on the top of the tank and therefore we were never able to push water up an overflow by any more than a couple of inches. Consequently, we have lived with never filling the tank to the top. The overflow/air vent terminated inside the boat and simply acted as a vent.
The forward well is not self draining; there--s a wooden floor above the tank supported on removable steel floor bearers. Any water that does get into the forward well drains though a pipe running the length of the boat and into the engine well.
The new tank that is on order is polypropylene and I--m told by the makers (Henderson--s Plastics) that the tank will be tested to a pressure of 8ft of water head and that pushing water up an overflow of up to about two feet above the top of the tank should not be a problem. I hope they--re right?
I really need the tank to overflow to the outside of the boat, probably through a skin fitting at least a foot or so above the water line; my concern is how I prevent any possibility of contamination of the fresh water tank by ingress of canal water, and or creepy crawlies?
I--m considering some kind of non-return valve in the overflow pipe and providing a separate air vent, above the overflow, inside the boat.
I would appreciate your thoughts on the matter; do you have a better solution? Do you know of any suitable devices that would fit the requirements?
Many thanks for your time.

Asked by: Peter Bone  | 8.42pm, Tuesday 4 May

Readers say:

Bring the filler up to about gunwale height (maybe through the cant between your front well and front deck if you can)
Have the breather / overflow an inch or two lower than the filler. On a previous boat it terminated in a "U" bend with a gauze cover draining over the front deck, thus neatly keeping creepy crawlies out.

Ray Butler  | 6.44PM, Wednesday 5 May

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