Dale's hot tub in a swimming dock. Heated with a rocket.

A hot tub is mostly a recreational device. We don't have a recreation section. There's a rocket powered water heater, so I've put it here. Near the bottom, I explain how to do this with a FREE TUB.

Every pond and lake needs a swimming dock. Ponds in Canada get cold. It's quite common to see cottages by lakes and ponds that have a hot tub either by the beach or more commonly, up a bunch of steps to the cottage. People swim until they get cold, then they dash up the steps to roast in the tub. Lots of walking and often the parents get separated from the kids. Many cottages have a limited water supply and a small electrical system. Tubs by the cottage, often tax both systems.

The hot tub in a swimming dock, solves the above problems. I searched Google Images for a swim dock with a hot tub and found nothing. Hot tubs are incredibly heavy. A dock capable of supporting one would be expensive to build and prone to tipping with so much weight above the water line.

The simple solution is to have a big hole in the decking that fits the tub. As the tub is filled, the tub sinks into the pond. Eventually the rim comes to rest on the decking. Plan it so that only a few hundred pounds of tub are supported by the dock. This will ensure that the dock becomes very stable when used for diving and horseplay. I've used a diving board on a lightly built swim dock. As you walk out on the board, the dock lowers to that side. Jumping force is absorbed by further lifting of the wrong side of the dock. The resulting cannonballs are quite lackluster.

This sort of tub would not require chemical treatment. It is filled with pond water that is regularly swapped out. If the pond is managed to have water that is clean for swimming, the tub will be good. Change the water. A battery powered pump moves water to and from the tub. It's pretty much a sideways movement of water since the tub sinks while being filled and rises when emptied. This won't need much power. A hand pump could do it.

HEATING --- We use a recycled hot water tank with the effluent from a rocket stove run through the center. No copper coils or other malarkey. We're not drinking this water. Direct heat transfer from flue to water. The tank is open to the atmosphere and thus, could never explode. Water is pumped into the bottom portion of the tank either manually or with a DC pump. A separate little pipe runs from the top of the tank, to beneath the decking. This gives a secondary blow out spot if bathers somehow managed to plug the outflow. The overflow near the top of the tank, flows down an insulated pipe to an aluminum diffuser at the bottom of the hot tub. Rather than flowing out at one spot, which could burn somebody, the hot water flows out of hundreds of little holes. A child restricting fence could keep kids away from the tub and heating system. A splash guard could protect the feed tube from water coming from the diving board area.
SOLAR HEATING --- A well insulated tub with a clear plastic cover, will heat quite a bit in the sun. Late afternoon is the most likely time for me to go swimming. Both tub and pond water are likely to reach their highest temperature after 4 pm in summer. Fill it in the morning.
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USING A FREE TUB --- THOUSANDS OF HOT TUBS ARE DISPOSED OF EVERY YEAR --- I have been paid to get rid of tubs. Last year, I asked my tenant, Randy to keep his eyes open for a good tub. He found a free one that has insulation similar to the one in the photo. These tubs are retired when something goes wrong. Usually burnt out motors and pumps are the problem. Sometimes owners got sick of the chemicals and the huge electrical and water bills. Many tubs come available when houses are sold or they are demolished. I have never seen one that was thrown out because the fiberglass had failed. All I want is an insulated vessel that will hold water, not a jet massage.

PREPARING THE TUB FOR SUBMERSION --- We don't want the insulation to get wet. The whole exterior, insulated portion of the tub needs to be wrapped with EPDM pond liner and sealed really well with marine grade silicone caulking or other super duper sealant. Before this happens, any heavy metal bits such as motors and pumps should be removed. All sharp bits should be ground smooth and any damaged portions of the foam should be repaired with a bottle of suitable foam. Foam could also be built up in certain areas to create a better surface for the liner to rest on. I would use a few layers of plastic tarp material as added insurance against puncture. The rubber would then rest against the tarp. Water pressure will hold it in place just as it does when leaky boats are covered with rubber. All of the holes into the tub where jets used to work, must be covered with good fiberglass patches. We don't want water in there. The portion of the tub where the liner is glued to the fiberglass will never be submerged in the pond. It will sit a foot or so above. The Insulation and air trapped by the liner, gives the tub buoyancy. The rim must meet the deck far enough above pond level so that the dock receives some of the weight.

My best pond site sees very little wind, so not much risk of being blown ashore. Oars and an anchor could be used to position the platform wherever it is wanted or a swimmer could push it along. A rope strung across the pond might be handy. Use a hitching post to tie off in the deepest spot.

I want to have a shore mounted dock. The swim dock rope should lead to it for night use and tie up. If a house boat is ever added, the dock could be tied there.

Legionella and other water quality issues. Regular swapping of the water should do it. This sort of tub will experience huge temperature swings. Problems generally occur when water is held at near body temperature for extended periods. If the heater is run in the 160 F range, most things will be killed. Since we're using pond water, there will be many microscopic life forms present. In tubs that use a poison, the most resilient species take over. There are many soaps that won't hurt the pond. Bathers will be allowed to use these in the tub prior to draining. I will experiment with a biochar filter and with adding wood ash to the water.

I've been thinking quite a bit about water heating lately. It's been cold outside. Here's a crude plan to heat a greenhouse tub. --- https://permies.com/t/31352/rocket-stoves/Dale-Oil-Tank-Hot-Tub

This is about a tub molded into a RMH. --- https://permies.com/t/32394/rocket-stoves/RMH-bathtub-Tank

My free tub has insulation similar to this one.

I'm interested to see what happens here. I'm a huge fan of sitting in warm water. . .

Julia Winter wrote:I'm interested to see what happens here. I'm a huge fan of sitting in warm water. . .

Yes, so am I. The main idea is quite simple. The form that the deck takes could go in many directions.

Just a swimming deck with the tub and a garbage can of firewood.

A big swim deck with a little cottage on it.

An attachment for the back of a houseboat. Un-seaworthy vessels come up very cheaply. It can be very expensive to repair an old wooden hull. Pond liner does the trick at about $1 per sq. ft. If a free one comes up, I'm in.

Hot, soapy showers could be part of all options.

A diving board could be part of all.

The photos show a neat unit that could set you back $35,000