holding tank woes

[markxengineerin]

Warning, I tried to clean up as best possible, but this post still contains photos not for faint of heart



Symptoms: my Scad TM-2 external holding tank level sensor suddenly always thinks the tank is full. Strange! Then a head smell in the bilge. Something is amiss. A look under the molded in fiberglass tank shows a telltale stain, and I can see a slow drip when I fill the tank up with water until it pours out the vent. This may only leak under the pressure of a full tank, I'm not sure, but it's a new problem.


I remove the lid to investigate


The connections to the tank look visually ok, as do the seal/flange/screw holes on top.

But the gelcoat inside has a few large cracks


I think my next move is to grind these out?

What is the best way to repair? I'm concerned about trying to glue/paint/otherwise seal a potentially wet/rotted surface, assuming it may not fully dry out, and also with what products to use. There may be wood behind the fiberglass, or maybe a dual wall situation with black water trapped between, because Scad says the tank level sensors can be fooled by waterlogged wood.

What to do about the plastic lid? Replace because it's old? With what? What to use as sealant?

[rslifkin]

I'd grind out the cracked area and find out what's in there. Once you know what's behind the inner liner then you'll be able to determine the best way to reinforce and repair the tank.

[Lepke]

If the cracks have been there for awhile, there's probably rotted wood below. The cracks are probably from not enough support when the tank is near full and the boat is pounding thru waves. If the tank can be removed easily, I'd check for support underneath.

Otherwise grind out, taper the edge for better long term bonding, use a good bonding epoxy to fill and course sand 4" out from the cracks and cover with fg cloth and epoxy to spread the weight bearing.


Jobs like this make me glad I no longer have a yard.

[DirtRdEarl]

You are lucky you can access the area. A simple West System repair. Remove the gelcoat from the repair area. Use multiple layers ofheavy woven mat. Finish with gelcoat on the repair.

[markxengineerin]

The boat does not really pound in the sense that some boats do, so I wonder if the initiation might have been a year ago when I had a plugged vent. I imagine a plugged vent + chemical reactions + temperature change could cause a significant pressure in there.

I went to scratch at the cracks and a screwdriver went right through the wall. It's fiberglass, about .125-.140" thick, and unsupported over a roughly 2'x2' section. When it leaks, the liquid has access to the ENTIRE INACCESSIBLE VOLUME BETWEEN THE LINER AND THE HULL and only gets to the bilge via a tiny drip hole. Needless to say, this is a $%@$ mess to sanitize and clean up. To any boat designers, please do not skimp on tank walls, it will save future owners a lot of anger.

For repair, I will use heavy mat, reinforced especially at the seams, and might even do a thin crossmember because it's the most efficient support to prevent something like this.

Is gelcoat necessary? I talked to West System and they think it's fine without. I would rather avoid that step as I'm trying to get this done asap.

[rslifkin]

I agree with West. No need to gelcoat for a waste tank, raw epoxy interior should be fine.


I also wouldn't use mat, but cloth. It'll be stronger for a given thickness, and not all mat works well with epoxy anyway.

[peghall]

IMO it would be easier and cheaper in the long run to replace it with a top quality plastic tank, 'cuz you won't know till you see a new one what other places on the walls and/or bottom of this tank are just new leaks waiting to happen.

Ronco Plastics Ronco Plastics marine Tanks Ronco Plastics tank drawings is your best source for a tank. They make TOP quality thick-walled (50% thicker than most other mfrs) water and waste tanks for a very reasonable price and have more than 400 shapes and sizes, over 100 of which are non-rectangular, and they install fittings--including diptubes--in the sizes and locations specified by the customer when they make the tank. There are retailers who sell Ronco tanks, but Ronco sells direct for a much lower price...and they're great to work with.

When looking at drawings it's important to know that there is no top or bottom until the fittings go in, and YOU decide where they go. So rotate, flip in any orientation to find the tank that fit your space.


That's my $.02 worth anyway...


--Peggie

[grantmc]

Just what I need with my corn flakes: a scat post. Oh grief.

[GreenWave]

Originally Posted by peghall

IMO it would be easier and cheaper in the long run to replace it with a top quality plastic tank, 'cuz you won't know till you see a new one what other places on the walls and/or bottom of this tank are just new leaks waiting to happen.

Ronco Plastics Ronco Plastics marine Tanks Ronco Plastics tank drawings is your best source for a tank. They make TOP quality thick-walled (50% thicker than most other mfrs) water and waste tanks for a very reasonable price and have more than 400 shapes and sizes, over 100 of which are non-rectangular, and they install fittings--including diptubes--in the sizes and locations specified by the customer when they make the tank. There are retailers who sell Ronco tanks, but Ronco sells direct for a much lower price...and they're great to work with.

When looking at drawings it's important to know that there is no top or bottom until the fittings go in, and YOU decide where they go. So rotate, flip in any orientation to find the tank that fit your space.


That's my $.02 worth anyway...


--Peggie

Was just about to say the same thing - get a tank. I would still seal up this area with fiberglass etc.

[markxengineerin]

The tank in a tank idea would be better if this wasn't already smaller than I would prefer, and if I had more time to design/search/wait for a tank to be made, etc. With several new layers of glass and cross bracing, I'm not very worried about another leak, especially if I keep a better eye on the vent for clogging.

Here's how it looks all sanded. According to West, it is ok to bond to the gel-coat. It would be a lot more work to sand/grind through it to the glass, but if people think that's required, I could..

[GreenWave]

I have to admit I would not have thought about bonding to the gel coat for this application scenario. However, WEST are much better experts than I so...

[rslifkin]

Gelcoat is still polyester resin, so as long as the gelcoat is properly adhered to the underlying layers and properly prepped, I'd expect bonding to it would work fine. I didn't remove the interior gelcoat (just thoroughly cleaned it and roughed it up) when I built my integral water tank. The epoxy has stayed bonded just fine.

[markxengineerin]

Here's the fix, working so far, 3-5 layers glass plus 3/8x1" g10 stringers from McMaster Carr, glassed over. Hope to never see this view again. Thanks for the suggestions