Question about Fresh Water Tank and Bilge

[SteveNewport]

Hi Folks - we have a 1991 41c.

In short: I fill my fresh water tank, then it seems lose the water, but I can't find water anywhere that it could have leaked. I don't leave the power on, so the pumps are not pumping it. I am trying to access the water tank, but it is under the engine room floor and appears to be permanently sealed. Looking for advice.

Here are more details:

I wanted to see if anyone can answer some questions about the Fresh Water tank and how it is mounted, accessing it, if anyone has ever had challenges with their Hatteras fiberglass water tank?

I am having some strange symptoms when filling the tank. I put a ton of water in it last weekend and then it was dry a week later. It is a 100 gallon tank and if there was a leak, I would expect to find a ton of water in the bilges, which I don't see - but maybe the bilge pumps pumped water overboard?

The water tank is located between the engines, under the engine room floor down the center line of the boat (see photo)

There is a piece of 3/4" plywood over the tank - I don't see any mounting screws on the plywood? does anyone know if this plywood can be removed to inspect the water tank? How were these water tanks installed/mounted? Can you access them?

It is a fiberglass tank. Has anyone had problems with the tank leaking? it doesn't seem likely, but wondering if others know?

(Note, the center of the bilge under the engine room floor (where the water tank is located) appears to be a "dry sump" that does not appear to have drains to the bilge? Our house batteries are in this same "dry sump", but the batteries are inside dry boxes... am I seeing this correctly? is this center area of the bilge a dry sump?)

When searching for the missing water, I found water inside the "dry sump", forward of the water tank - and outside of the battery boxes, if this makes sense? there do not appear to be any drains that would allow this water to get to a bilge pump)

I am trying to get the water level gauge off the top of the water tank to physically measure the water level in the tank (the gauge only shows 1/8 tank, not matter how far I fill it... so I think it is stuck) - the water level gauge threads are not cooperating. I have PB Blaster soaking them.

I appreciate any suggestions or ideas.

Cheers
Steve

[SKYCHENEY]

The tank is fiberglass as are all Hatteras tanks.

[SteveNewport]

yes, I know they are fiberglass, thank you. Do you have any other ideas on my situation? thank you!!

[SKYCHENEY]

Some have been compromised from being over pressured when filling but all I have seen with this issue split open on the top of the tank.

[rwaldrop13]

OK, shooting in the dark here because my water tank problem was both similar and different. I had reached the point of replacing the fill and vent lines, and the tank cap/gage (I was going to install a KUS freshwater gage and sender). Does your boat have an accumulator to pressurize the water system? If so, it may be causing back-pressure in the water system and forcing water out the vent. It may be time to chip at that paint in the square in the plywood around the tank-top fittings and see if it lifts off.

[bostonhatteras]

Sky is correct on the 'bursting' of the tank due to overfilling. If the boat is in freezing conditions the tank could have frozen with water in it and cracked the sides. Use heat to get the gauge off, propane played on the thread interface will do it. The plate on the top of the tank isn't what you want to play with, its threaded into the, then green, fiberglass and the machine screws won't want to come out so don't try. 100 gallons is a lot to lose but the keel is hollow albeit foamed but could take a bunch. once you get the gauge out drop a light into the tank and an inspection camera to look around. If nothing is found, rig up a plug for overflow and filler and a fitting arrangement for gauge hole that you can pump air into. add a pressure gauge and a way to hold the air in and test it like a plumbing system would be for permit inspection, just only use like 5-8 psi. wait and see if tank leaks. As for the factory overflow line, its grossly undersized and seems designed to damage tanks. I eliminated mine and plumbed the overflow to the bilge

[bostonhatteras]

The gauge is likely toast. the metal used for the float rods probably has deteriorated completely at this point. new brass units are available on this site from the online parts catalog. Whats the garden hose in the photo doing?

[Pascal]

A few years ago my boat washer tried to help and full up my water tanks… full blast. Ended up pressurizing the transom tank which cracked. I repair one accessible crack with glass but I still have a leak somewhere, only when the tanks are over 3/4 full. Annoying but not a priority item now.

So yes your tank could have a leak but the water would end up in the bilge and you d hear the pump cycling and see the light coming on and off.

Problem is that hatteras used thin copper vent lines, at least on the earlier boats, so they can’t handle the volume of water that can be used to fill.

[bostonhatteras]

A few years ago my boat washer tried to help and full up my water tanks… full blast. Ended up pressurizing the transom tank which cracked. I repair one accessible crack with glass but I still have a leak somewhere, only when the tanks are over 3/4 full. Annoying but not a priority item now.

So yes your tank could have a leak but the water would end up in the bilge and you d hear the pump cycling and see the light coming on and off.

Problem is that hatteras used thin copper vent lines, at least on the earlier boats, so they can’t handle the volume of water that can be used to fill.

I just sent a little welded aluminum hull to the welder to fix someone's 'favor'

[jim rosenthal]

A few years ago, I changed the water tank level gauge. I had to take the old one out with an angle grinder; the gauge was corroded to the fitting it was in and no amount of twist with a wrench or big ChannelLocks would budge it. Cutting it up to get it out was safer. When you install the new one, make sure to grease the threads.