Making a holding tank to fit the curved hull space

[WaldPinkler]

I've decided to try make a ply/epoxy holding tank. There's a bunch of questions I need to bounce off others.

I've torn all the old leaking tanks out and rerouting all the wiring and plumbing. It's so glamorous. I'll be getting in there soon for a bit of griding and redoing the bilge paint.

I have the curved hull space forward of where the fuel tanks sit until the mast compression post. Behind that is an equally sized place, not really accessible apart from a sole hatch to access the shower sump. If I remove the rest of the flimsy bulkhead that I already cut then I could slide in there.

The old tank was L-shaped with access from another small cabin sole hatch. I can make this U-shaped and gain an extra 10-15 gallons of holding. Half that hatch allows access to bilge pump and mast wiring.

And my questions....


1 - How to bend the ply to the hull shape? Kerf cuts? Is that strong enough afterwards filled with thickened epoxy and glassed?

2 - How to support the tank? The last wasn't curved, just angled sides and sat against the hull trapping water rusting the stainless as the pics show. Can I make vertical ribs on the tank to allow any water and dirt to continue to the bilge? Same applies with the fuel tanks, the last sat on the ballast slab and collected filth, oil and water underneath with no drainage.

3 - Should I consider utilizing the space forward of the mast post? I can access it now but in the future only by a small cabin sole hatch. Then again, the other area will be boxed in my the fuel tanks.








Excuse those blurry pics. I'm sorting that cable nest and the base of the mast.

[HeywoodJ]

I’m sure you’ll get lots of opinions, but when I built ours I used a foam core. Bent to shape with a heat gun. Then applied skins inside and out. On the last batch of epoxy for the inside skin added graphite to make the inside surface slick/smooth in an attempt to make it easier to clean the tank. Don’t know if that made a difference, but that tank is more than 20 years old, has seen full-time liveaboard use for half that, fits perfectly in the space, and has never emitted any smell in that entire time.

[WaldPinkler]

There's 30mm XPS foam core insulation board at the hardware store here. Kind of thick. Very cheap compared to ply, maybe the same after extra glass reinforcement. Is that was you mean? Sure looks easy to work with.

[rslifkin]

I'd go for foam instead of ply. It'll be easier to bend. I'm currently working on a fiberglass water tank, but I'm making mine integral. So some parts of the tank are just the hull with a couple layers of glass and epoxy added.

XPS foam isn't a good choice though. You'll want something like Divinycell or other structural foam core. That can be bent with heat and is a sturdier core material.

[WaldPinkler]

Where I just ordered my epoxy from has PVC foam board. Damn, I should have looked through the rest of their shop. Looks like another order. It seems ideal compared to ply. 10mm thick.

I'm sure you researched into fibreglass and water. Did you settle on a coating you're happy to drink water from?

[rslifkin]

Originally Posted by WaldPinkler

Where I just ordered my epoxy from has PVC foam board. Damn, I should have looked through the rest of their shop. Looks like another order. It seems ideal compared to ply. 10mm thick.

I'm sure you researched into fibreglass and water. Did you settle on a coating you're happy to drink water from?

I'll be coating the inside with Aquatapoxy A61. Rated for potable water tanks of 50+ gallons. Mine will be about 110 gallons.

[HopCar]

If you still want to use plywood, there is something called bending plywood.
https://www.columbiaforestproducts.c...nding-plywood/

[j.g.evans]

I made a tank by applying a parting agent on the hull and then laid up glass directly on the hull. I then removed this topless tank. The last steps were to use through hulls for hose connections, add an inspection port and apply a top. You could add baffles too.
The good news is if this tank bursts the boat won't sink.

[mitiempo]

The other solution is to use several layers of thinner ply with epoxy in between. Easy to fit a curve this way and then epoxy and glass both inside and outside.

[HopCar]

Originally Posted by j.g.evans

I made a tank by applying a parting agent on the hull and then laid up glass directly on the hull. I then removed this topless tank. The last steps were to use through hulls for hose connections, add an inspection port and apply a top. You could add baffles too.
The good news is if this tank bursts the boat won't sink.

You’ll just wish it would sink.😁

[Cheechako]

There is a ply that is made with wood grain one direction instead of alternating directions. You can readily bend it by hand. Not sure where you get it though, or what it is called. I have a part sheet here. 'Unidirectional ply'?

Found it, here:
https://www.windsorplywood.com/product/bending-plywood/

ooops, I guess Hopcar already found it...

[waterman46]

Build a fiberglass tank right into the hull, an "integral" tank it is called. The bottom and one side of the tank are actually the hull itself. The other surfaces of the tank are plywood with edges shaped to fit the hull contour. Along the upper edges of the plywood you'll add a wooden flange of about 2" width. Now glass mat the inside of this construction right into the hull. Overlap the mat on top of the flange as well, so its all one piece. This flange will be where you'll secure the top of the tank, which can be a flat piece of 5/16" plexiglass, lexan, etc. bolted and sealed well to the flange you made. All connections: fill, pumpout and vent, should be made through the tank top. Obviously the pumpout connection must include a pipe extending to within 1 inch of the tank's lowest point.

Someone may have an idea for a proper inner coating for a fiberglass tank and you might want to research that.

You'll have to re-route any plumbing or wiring that passes through this area of course. BTW the diesel and water tanks in my boat are constructed as integral to the hull. No problems after almost 40 years, and they will never rust or shift!

[waterman46]

Originally Posted by WaldPinkler

Where I just ordered my epoxy from has PVC foam board. Damn, I should have looked through the rest of their shop. Looks like another order. It seems ideal compared to ply. 10mm thick.

I'm sure you researched into fibreglass and water. Did you settle on a coating you're happy to drink water from?

He's building a holding tank. Probably won't be drinking from it. Even in an emergency.

[WaldPinkler]

The tank can't be integral otherwise I can never access that bilge area or the mast post. There is a bunch of space in front of the post but I can't get in there easily and if something went wrong with the floor or beams then it would make it inaccessible.

I want to keep it off the hull to allow any water and dirt to fall to the bilge. Water that came in through the leaky bronze portlights tended to run down behind cupboards then kept the tank sides wet.

[jimbunyard]

You could just use the space itself as a mold, blocking areas you don't want to use as tank with either plywood or foam, and masking it all off with polyethylene sheet secured with double sided masking tape. After forming the lower part, it could be pulled out and finished with the addition of the top and fittings.