Water getting into under-deck tank

Daoirse · 01 August 2017, 00:23

Hi Folks,

Just bought my first used rib - a Cobra 6.5m. Not sure of exact year but guessing late 90s.

I've had a few teething problems so far, the most significant being that at times, there seems to be water getting into the under-deck tank.

All was OK for a few days while the bilge pump was doing it's job.
Then the Bilge pump failed a couple of times - turns out it was dodgy wiring to the switch.

After heavy rain, there was a few inches of water on the deck. Once I drained the deck, I went for a spin. After a few minutes, I lost power and worked out that there was a lot of water in the fuel. I drained the tank and took over 100L of water out of the tank - plus 40L of Petrol. Same happened a few days later.
So, clearly when there's a lot of water on the deck, it's somehow getting into the tank. There's no sign of fuel coming into the bilge.

I'm not that familiar with Ribs, so I don't know the normal setup. In my boat, the solid stainless pipes come up from the tank a few inches above the deck and the hoses join to them - this is all in the drivers jockey seat. When the deck was flooded, the water seemed to be coming up from the bilge and a couple of inches deep. So, the joints between the hoses and fixed pipes were under water.

Sorry for the long post, but there are a few separate issues contributing to the problem. Ive sorted the bilge pump, but I'm assuming that I can't depend on not having water on the deck in the future.

My guess is that this is probably down to a leak in the tank, possibly around the seal with the hoses , but possibly elsewhere. Other posts about tank leaks talk about fuel getting out rather than water getting in.

I see from other threads that diagnosing / repairing / replacing under-floor tanks is not trivial. Am I right that the first step is a pressure test? Someone mentioned a smoke test is a possibility?

Some side questions:

- Should water be coming up from the bilge - there are three holes for cables to run to the stern.
- Is there any of getting a layout diagram / drawings for the boat - to see where the tank etc is?
- Should cables, lines be running front to back through the bilge?

Thanks in advance

Colum

Jeff · 01 August 2017, 08:09

I would guess that the water is getting into the tank through the sender unit gasket. The connection will most likely be under the seat pod. I don't think Dave fitted an inspection hatch inside the seat base as standard then. You will have to try and remove the seat assembly - it's secured by 4 nuts onto captive studs in each corner. ASAP Supplies have rubber gaskets for the 5 hole senders at good prices. Once done and checked I would fit a screw down inspection hatch in the seat base.
Water on the deck may be getting in through either of the two 4" standpipes that the cables run in and out off. Probably the stern one where the sealant into the deck has failed,,, unless you had so much water on the deck that it over-topped the pipes that are about 4" tall
Good luck
Jeff

Jeff · 01 August 2017, 08:13

Another thought,,,,, was this from when the boat was on the water... If so, it could well be a leaking transom drain bung and it's entering through there. You can soon get a hull full of water then! ,,, or check the elephants trunk for a split . If you have one of those dreadful ball scupper valves instead,,, get rid 😊

Miller · 02 August 2017, 16:06

Second the gasket on the fuel sender...I had someone pressure test my tanks in situ and told me they had massive holes and were letting in water....I ripped the deck off and took the tanks out only to find that it a perished gasket around the sender.....I wasn't a happy bunny!!

Stresspill · 02 August 2017, 17:40

How about a split breather hose? Since the amount of water is so huge it has to be a substantial leak. Or a smaller leak, but constantly under water.

Daoirse · 03 August 2017, 00:36

Hi Folks,

Many thanks for the responses so far.
I'll take some photos to be more clear.
Under the jockey seat, I can see 3 pipes about 4" above the deck. They all come through a panel about 4" x 10". Interestingly, there is no sign of a fuel sender. My guess is that the panel has been added later or modified. That panel is screwed down with 4 or six screws. So, I think if I lift that panel I expect to see the sender blanked off and I hope the source of the problem. My preference is obviously to re-install a fuel sender. Is there anything specific I should be looking for in sourcing a sender?

A bit more detail on the source of the water - once it was pure stupidity on my part - forgot to install the bung in the bilge! After an hour, the deck was full of water. The second time, the bilge pump failed and it rained constantly for 2 days. Both times, under the console, the water was about 2-3" deep. The boat had been fine for a few weeks sitting in the water and hadn't leaked water into the fuel. So this all points to a leak at the top of the tank when the deck is flooded.

A side-issue is that there seems to be no separation between the deck area and the under-deck bilge area. In the area under the jockey seat over the tank , there are three holes from the deck to the bilge. Two of them are being used to run cables and flexible fuel line to the outboard. Am I right to assume that this isn't original and that the deck and bilge should be isolated? If that's the case, then I'm assuming that the area under the jockey seat should be dry. Any reason why I shouldn't try to reverse these modifications and just run a cable conduit over the deck?

One final question, how do I confirm that I've found and fixed the leak? I assume the easiest option is a pressure test. Do you know who I could use to do this? I also assume I'd have to take lots of precautions of emptying the tank and ideally rinse with water ( I have lots of that!)

Many thanks for the input so far. I have another question about another modification that's been done to the stern creating a "well" that temporarily holds water coming in over the gunnel by the engine, but I'll create a seperate post for that.

Colum

Jeff · 03 August 2017, 10:22

Some photos may help,,, the 3 pipes you refer to are usually a larger filler pipe and 2 small - one for supply to the engine and the other a vent. The panel is fitted as standard - it covers a larger hole in the deck over the tank. I assumed these came up inside the console not seat but perhaps yours is a single jockey configuration as opposed to Side by side bench?
The well at the back is a sump and standard and where the pump should be sited,,, it keeps the deck a bit clearer whilst the pump operates.
The better fuel senders IMHO are WEMA,,,,, make sure the sender and gauge are the same US or UK resistance spec. Mercury / Smartcraft are US.
If it is the gasket, which from your issue with leaving the bung out may indicate, it will be pretty obvious. Tear or mounting nuts loose. The sender mount is usually mid way along the tank

GordyP · 03 August 2017, 10:44

If all the obvious things are ok check the welds on the tank, I had issues with a similar age of stainless tank where the welds had hairline cracks.

Daoirse · 08 August 2017, 20:32

Just a quick update - I'm still trying to work out how to share pics:

Big issue is that after taking off the cover , there is still no sign of a fuel sender. A few other points.
- The pipes up from the tank are at the front of the tank - I assume that the feed pipe must be routed backward from the top of the tank to the rear internally?
- While there is no fuel sender visible, it is possible that it's been blanked off and fibreglassed over. There is however no obvious sign of this. Has anybody seen an internal tank with no sender?
- I can't see any obvious sign of any damage to the hoses or the rising pipes.
- The rising pipes are stainless steel, so am I right to assume that that means that the tank itself is stainless?

Will post photos when I can

Thanks,

Colum

Daoirse · 08 August 2017, 23:23

Here are some photos.

This shows the fuel tank under the pilot jockey seat.

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Jeff · 09 August 2017, 10:46

Quote:

Originally Posted by Daoirse

Just a quick update - I'm still trying to work out how to share pics:

Big issue is that after taking off the cover , there is still no sign of a fuel sender. A few other points.
- The pipes up from the tank are at the front of the tank - I assume that the feed pipe must be routed backward from the top of the tank to the rear internally?
- While there is no fuel sender visible, it is possible that it's been blanked off and fibreglassed over. There is however no obvious sign of this. Has anybody seen an internal tank with no sender?
- I can't see any obvious sign of any damage to the hoses or the rising pipes.
- The rising pipes are stainless steel, so am I right to assume that that means that the tank itself is stainless?

Will post photos when I can

Thanks,

Colum

Hi Colum - the pipes at the front of the tank are standard and yes, the pick up pipe should be routed inside to the rear. I cannot see a fuel gauge on your console so it is possible there is no sender ! If not, this simple source of a leak is not there and you are most likely facing a cracked weld :-(
The sender unit would most likely be under the larger round inspection hatch you have open so unless you can get a scope in and have a look around, it does sound like there isn't one. It seems unlikely, but have you checked if rain water is getting in via the fuel filler cap (not sealing properly)
If you have had water getting in, are you also getting fuel out into the hull when the tank is full ? This would drain into the sump or out of the hull bung and be quite obvious.
Good luck
Jeff