I recently started dreaming about building a bathtub out of wood. This is due to myself being a rather tall guy and hate the short bathtubs. (Though I scarcely have time to go for a soak, the one time a year I do, I want it large enough haha…)
Having no experience in boatbuilding or bathtubs for that matter I’d like to see if there is someone here who can give me some advice.
Questions I have are:
1. Is this a totally stupid idea?
I’ve seen it being done but very rare to find in a common house.
2. What would is the best option.
Cedar?
Mahogany?
Something else exotic?
3. How should it be sealed?
Epoxy ?
Marine grade polyurethane?
Both inside and outside?
How many coats?
4. I would think titebond 3 isn’t a bad glue to use for this, but maybe rather epoxy?
Any advice or past experiences that can be shared is greatly appreciated.
Thanks.
Replies
I can't speak to any of the rest of it, but I can say you definitely need something more suited to the purpose than titebond 3. At least the titebond 3 I have says it isn't to be used in continuous submersion, nor under the waterline (sorry it seems it won't let me upload a photo). It seems to me this would be both of those.
I also notice it says not for load bearing applications and a bathtub full of water would be a fairly significant load, especially if its extra large as you've indicated.
Thanks for that. I definitely did not read the fine print thoroughly.
Id be looking for some resources on Japanese soaking baths. I had the pleasure of staying at an inn in Kyoto that had a full cedar bathroom. The tub was very deep but not super long, which was really nice actually. I’m 6’1”fyi.
I would use marine grade epoxy rather than Titebond III. I would also use only clear wood, as it will be much more difficult to successfully and permanently seal knots. This would rule out Eastern red cedar. Also no sapwood, as it is not at all durable.
A surface finish alone, such as polyurethane, is prone to sudden and catastrophic failure. You need to use a penetrating sealer first, to prevent water incursion if (or when) the surface finish fails. Abatron makes a two part finish that soaks into the wood and then hardens/consolidates it. Make sure that the two types of finish are compatible.
The boat world is likely to have experience/opinions about this as well.
Also, thermally modified stuff is pretty readily available these days. I think that stuff get can away with just a sealer.
I recently built a plywood boat and I would recommend a similar approach to the tub. All the wood is sealed with 3 coats of epoxy (many options, I used MAS) and there is a layer of fiberglass on all the wear surfaces. This is fully waterproof and durable. I would use thickened epoxy for gluing everything together. Basically you are building a boat and filling it with water instead of putting it in the water.
clcboats.com has a bunch of information on building wooden boats and may be a good resource for this project
I built one once for a client, an "ofuro" it's called but I cheated. It was oval shaped about 3 feet wide and about 6 feet long. It was redwood and for outdoor use. I found some redwood recycled tank wood so most of the coopering was already done, at least the almost straight sides, i just had to make new clean surfaces, it must been a huge tank originally because in six feet it was a very slight curve. I had to modify for the curved ends. The parts were doweled together and the dowel holes for the most part were already there. I used resorcinal glue but honestly I always wondered if that was necessary. There was no glue or adhesive when it was a tank. I had a tinker make and install the metal straps. The redwood was old growth and perfect and would be almost impossible to find today. Mine had been a water tank but I've found old wine vat wood and that sometimes becomes available. I don't know about a bathtub, the wood has been " plonked" I call it, it has absorbed and been stained by the wine ,quite beautiful actually but has also acquired a permanent musky wine aroma as might you if you bath in it!
The proper wood for an ofuro is hidoki which is to say japanese cypress. Cedar very clear might be easier to obtain than redwood of the quality required. Cypress - eastern or western, should be available. I can get an endless supply of western cypress and free for the taking where I live but its green. I have a stash of French oak that is from wine barrels, also plonked , and oak is a common wood material used in barrels. Anyway, coopering is how liquids have been stored in wooden vessels for about forever ,I don't see any reason to try and reinvent the wheel. The tub I made was some decades ago but apparently it's still in use.
The coopering is one method I considered and it seems to be a good approach, yet, I'd like to stick with a more modern approach.
Thanks for your response
I would have to imagine that it would tub would be made much in the same way as a cedar strip canoe. There are lots of good resources on how to make a canoe too!
A strip canoe is designed to resist water pressure from the outside. The water pressure tightens the joints bteween the strips. Filling one with water on dry land is a bad idea.
Most modern cedar strip canoes are covered with fiberglass and resin on both the inside and outside. They are not a wooden boat, but a wood reinforced fiberglass boat. The construction process results in something very similar to a modern bathtub.
A canoe is designed to canoe. A 100 pound boat with 2 200 pound humans displaces 500 pounds of water, mostly down where the curves of the hull are strongest. A 12' canoe with a 3' beam has a volume approaching 350 gallons or almost 3000 pounds of water. That's on the inside, not the direction of stress a canoe is designed to resist. One of the older Grummans might just survive.
Don't fill a canoe with water unless it is in the water.
Most wooden boats have an external layer of cloth saturated with resin as a protective coating. That does not make them fiberglass boats, they are considered wooden boats. The exception is the strip plank construction that requires caulking and need to soak water to make the hull watertight. There is not much in common between a boat and a tub, I guess you can seal the drain and put an outboard motor on a tub and call it a boat but you can hardly take a wooden boat and make it a tub, the water is on the wrong side. Protecting a bath tub and making it watertight will depend largely on the construction technique. A canoe is only suitable for embarking once in the water, it would be very risky to have my 200 pounds step in a canoe when on the hard. The bathtub however will need to be stiff and wood movement considered in the design and build so seasonal movements do not create gaps. My go to finish would then be a clear epoxy primer followed by several coats of polyurethane varnish. Mahogany would be my material of choice.
Never done this but sticking my oar (sic) in to support the boat building approach.
No, a bathtub is not a boat, but it does not have to be light either. It would be easy to make more than robust enough. The fibreglass and epoxy coating would ensure watertightness.
I have three concerns -
1. Drain holes - need to be designed with great care.
2. Smoothness - a lot of epoxy needed and a lot of sanding.
3. Heat - epoxy softens with heat - how does it cope with 40c water? What about if 60c water was used?