Plywood water/holding tanks - Scantlings

[Pelle]

Hi!
Reading the Gougeon Brothers on boat construction 1985 (has it been revised later??) I'm interested in the possibility of making my own water tanks out of marine plywood + epoxy and tape. Anybody know of where to get the appropriate thicknesses of ply and number of plies with tape/epoxy fillets (for various sizes)?
Anybody have experience with plywood for tanks ??
The tanks will be stand-alone, not integrated in the hull.

Pelle

[Dale R. Hamilton]

Pelle- why would you bother to make plywood water tanks? Odd shape spaces? Fine, use water cells. Their ruberized somptinorother- made and sold by Vettus. Warrentee is 20 years, they come in a variety of sizes, can be hooked together, no taste, on bother and they ain't expensize. Making your own tanks just ain't worth it.

[Dave Wright]

I just can't imagine making plywood tanks for anything because I'd always worry about them not holding up, but........ There was a plywood sewage holding tank in my 1967 Owens Express Cruiser. It was a big rectangular box about 5 feet long, by 20 inches wide, and 10 inches deep, covered with light cloth and polyester resin. There was every indication that it was as old as the boat, and of course I worried about it.

Three years ago I got the gumption to drag it out and take it home from the marina - can't spill that stuff overboard anymore. I could find no external leaks and no soft spots. I chain sawed it up in the back yard, with spray and various particulates flying. It was glassed with polyester resin on the inside too. After hosing it down, I couldn't find any problem areas on the inside either.

So my only experience is that plywood tanks can work. Maybe I'm irrational, but I still wouldn't trust them.

Dave Wright
Seattle, WA

[Charlie J]

made plywood tanks for my trimaran years ago.
A 12 gallon diesel tank and a 22 gallon water tank. Used 1/2 fir marine ply. Filleted the inside and taped those seams then used 5 coats straight epoxy. Glassed the exterior one layer 10 oz cloth/epoxy. Made tops removeable and let all fill, breather, etc openings into those tops.

Did this in 1982, cruised the boat for 3 years, sold it and far as I know she's still using the same tanks. The boat has passed at least 4 insurance surveys with those tanks.

I also used a flexible water tank- 25 gallons. Worked well and was easy to pull out to clean under it.

I'd do it again with out any worries, either way.

[Ross Faneuf]

I made plywood water tanks for Ceol Mor. They are also the settees; it was then only way to get decent capacity tanks because of the configuration of the boat. The original design called for flexible bladder-type tanks, but there wasn't actually room for the tanks as designed if I had set the space up as recommended by the manufacturer to avoid chafing problems.

The tanks are made of 1/2" fir marine ply and FG/epoxy channel sections (where the tanks fit over the floors. I glassed each side of the ply (all work done with System 3 epoxy), with enough epoxy to give a smooth surface. This plastic inner layer is intended to prevent any water migration/saturation, and so far seems to work well. All corner joints have solid (mahogany) gussets, screwed and glued. All joints are taped and filleted. The tanks are partitioned with plywood panels (as above) with limber holes; each tanks has 3 partitions/4 sections. There is a 5" square access hole in the top of each section, sealed with a 1/4" plexiglass panel. The capacities are approcimately 55 gallons and 65 gallons.

So far the tanks have worked exceptionally well. No leakage, no penetration into the ply that I can detect. The water has a slight plasticky taste but is bearable; the only boat tanks I've ever encountered with some taste were some extraordinarily expensive monel tanks, way out of my league.

This was all a lot of work, but very much worth it from my point of view. And they're quite handsome as settees.

I wasn't sure I'd trust plywood tanks either. But I built them very heavily, and and at least I know where to find the builder if something goes wrong. Fact is, tanks are a chancy proposition to some extent no matter what the material or manufacturer. Commericiall produced tanks or built-in tanks in FG boats occasionally fail, and often in interesting and unexpected ways; if I started in on the failures I know of, this would be a really long post (and others can chime in, and maybe will). So you're taking something of a chance with any tank; with build-you-own, you control the chances you take.

The only tanks I've seen that might be more or less eternal were those monel tanks; and even 12 years ago when I built Ceol Mor's the equivalent tanks in monel would have been on the order of $15K. Nobody with that kind of checkbook is asking for hints here...

[ 01-11-2003, 11:41 AM: Message edited by: Ross Faneuf ]

[Mike Keers]

I'll chip in on the positive side. I've made a few ply and poxy tanks over the years--the last time were a holding tank and fresh water tank for my 29 footer, both approximately 25-26 gallons. That was about ten years ago--the boat was sold, and still no problems reported.

As in the post above, the fresh water tank was also the settee seat, and was made from 1/4" and 1/2" ply. Corners were filleted inside, and four or five coats of googe applied--no FG cloth. Also as noted, there was a faint plastic taste for the first several years, but only if the water was left in the tank unused for months.

The holding tank was based on a unique idea I saw in Boatbuilder magazine a few years ago. The tank was built in behind the toilet, using the two 1/2" ply bulkheads that formed the walls of the head for each end of the tank. The hull itself formed the outboard wall. 1/2" ply formed the top, bottom and inboard side. This tank was built a bit heavier and glassed inside, as I was concerned that the hull and bulkheads 'working' under sail might open a seam. The boat sailed many thousands of miles, including to Hawaii under rough conditions, and never leaked a drop.

I'm currently building a 28-foot diesel sedan cruiser, and I'm considering building the tanks into the hull--these would be two 35~ gallon diesel tanks, using the hull bottom (1" thick), hull sides (3/4" thick) and 3/4" ply bulkheads as the other components. The 3/4" cockpit sole will serve as the top of the tanks. Insides will be filleted and taped, and heavily coated, altho not clothed entirely.

I have looked at stock tanks in various configurations--plastic, aluminum, bladder. All have various stories of failures associated with them, and I have some personal experience with plastic water tank failures. I have seen several failures of aluminum diesel tanks as well.

My thinking is, these built-in tanks make the best use of the available space, and will be far heavier construction than anything I can buy or reasonably have made--and if worst comes to worst down the road, I can always drop in bladder or plastic tanks.

Actually, I always planned to build the tanks in, but got lazy once the hull was rolled over, and started shopping for ready-made tanks--after pricing and more thought, I'm leaning towards building them in again--all of the structure is already there, in the form of bulkheads and so forth--at this point, it's just time and googe.

I also plan to build the holding tank in (settee seat most likely, or possibly under the sole in the bilge), and possibly the fresh water tank as well--unless I get lazy and just buy plastic or a bladder for that.

[ 01-11-2003, 01:44 PM: Message edited by: Mike Keers ]

[J. A.Tones]

Pelle - I was in the same position a year or so ago but then came across a real deal on a stainless tank. When I was considering building a tank I asked West Systems for some advice and a chap in the techy dept., who was doing the same thing himself, replied. Damned if I can find the info right now but he gave me a lot of what seemed to be good advise. E-mail the "help" line and get some info direct from them before you go too far. I think its a good idea to build your own 'cause then they fit right and can be any wierd shape you want. In my case there was a big space and a rectangular tank fit fine.
John Tones MV Penta
Sidney BC

[john welsford]

I both use ply tanks and specify ply tanks in my designs, as a general rule I use 3/8 ply up to 10 gals, 1/2in ply up to 30 gals and 3/4 in over that . My own boat will have two 150 gal fuel tanks, two 100 gal water tanks, and a 100 gal heads holding tank. They will all of them be plywood, the internal baffles will be the same 3/4in, the seams including the baffles will be double taped with 15 oz double bias tape inside and out, and the tanks will have three coats of resin. there will be no unsupported panel bigger than 12in long including the baffles and the tank will be supported under and back at 12 in intervals. I use resins supplied by Epiglass ( Akzo Nobel) but understand that WEST do a non tainting one for drinking water, and one suitable for diesel.
The Gougeons do indeed have information on this subject, and used to do a little $2 00 booklet on how to do it.

JohnW

[Dan McCosh]

I built in a 30-gallon holding tank made from 1/2 in. ply, coated inside and out with West, with fileted interior seams. Main reason was to built it in, and use it for a forward berth. That was about 15 years ago, and still going strong. I had the same concerns, but figured any break in the interior lining would show up as a stain before it actually leaked. Actully, epoxy is a lining for many commercial tanks, including plating tanks. The ply is stronger than most plastic-molded tanks. I've seen bladder tanks burst, polyethelene collapse, and aluminium corrode. The berth idea wasn't so hot. The sloshing keeps you awake.

[Barnacle Bob]

I am planning to replace all my alluminum alloy tanks in my 56' cruiser and these will be two 150 gallon water storage tanks,one 150 gallon stainless steel holding tank another, two 400 gallon main diesel tanks. I plan to build the main 400 gallon tanks into the hull of the boat using a Fiberglass Re-enforced Urethane board made by Seikisui here in Japan. The glass provides a grain in this material and I will sandwich three sheets together with the grain alternating 90 degrees in each layer for strength. I'll coat inside and outside with epoxy and a layer of no 8 cloth and tape the corners and joints. This material will not rot nor corrode and should hold up forever. Another supplier of a similar material without the glass grain, but appears to be stiffer is from Last-A-Foam and they have different densities of materials in varying thicknesses for boat building etc. Nice stuff and I will use some of their stuff as well.

[Pelle]

Thanks for all answers!

To sum up what you've written: The tank will need to have baffles and proper support at every foot minimum underneath.
The plywood must be properly protected with a number of epoxy coats on the inside.
Using glass tape and fillets (or solid wood) in the joints to get a powerful bond is vital.

It seems that everyone with personal experience from plywood tanks has not seen any collapse.
I will make some tanks in near future and will be back then with more questions or views.

Thanks!!

Pelle

[Tomcat]

The pressure of keeping water in, is basicaly the same as keeping it out, so it isn't a big deal. I wouldn't relly on coatings alone, but would use light glass on all external surfaces, to ensure a durable surface.

By the way, the Gougeons no longer support his kind of thing. Whether there is any reason to distrust these tanks (I think the fuel ones aren't actualy up to spec from comments they made) they no longer stand behind this use of WEST epoxies. Sounds sort of like a legal issues concern vs a materials concern, but your on your own.