When it was first built in 1977 my 50’ foot schooner had a bathtub of sorts built in the aft cabin en-suite bathroom. It was only a short tub with a tapered end, so for anyone over about five feet it was an uncomfortably tight squeeze and for me at six feet it was useless. My first thought was to rip it out and install a shower cubicle that could have been quite large and modern looking with a nice Plexiglas door engraved with some nautical scene. That would have been a relatively easy and inexpensive modification.
However, in the process of taking measurements I realized that if I encroached - just a bit - into the adjacent engine room, I might be able to squeeze an actual full size bathtub into the space, that would serve as a shower tub as well.
Then I really started thinking:
I have a hot-tub at my home where I always enjoy a good hot soak, especially after a day of back aching work on the boat. The therapeutic benefits of a hot soak have been known ever since man first discovered hot springs. If you suffer aches and pains after physical work there is nothing finer than to immerse into a nice hot bath, and if this happens to also have invigorating massaging jets, so much the better.
So what if I could convert a bathtub into a hot-tub on the boat and enjoy the same relaxation? Power boats of Britannia’s size often have full size bathtubs, but on a 50’ foot monohull...not so much.
First I needed to find a bath that would fit, that had to be the type with a sloping back to conform to the shape of the hull. I went to see a local spa/bathtub manufacturer and asked the salesman if they had ever fitted a hot-tub in a boat? His blank expression gave me my answer, but after explaining what I wanted he saw no reason why they couldn't fit jets into one of their regular house baths. He showed me a beautiful dark blue acrylic molding with a sloping back that could be fitted with any number of massaging jets. He explained that there would also have to be a blower motor to shoot water through the jets on demand, and some sort of heater to maintain the water at about 102F degrees, but these items could be mounted anywhere. This idea was intriguing, but starting to sound expensive.
Since I was going to pull the old bath out anyway I decided to do that first, then I could measure accurately and consider which to install, shower or hot-tub.
Like many jobs on boats, removing the old tub turned out to be very much easier said than done. I thought it would just be a simple matter of cutting the bath molding out, but au contraire! It was an unbelievable monolithic structure with two thicknesses of 3/4” inch marine plywood all around, sandwiched between layers of glassfibre and resin, all molded into the side of the hull and bulkheads! It had to be hacked and levered out bit by bit, using a reciprocating saw, a die grinder, a large pry-bar and an even bigger hammer! The whole lot must have weighed something like 150lbs and I imagined the boat coming up three inches on her marks! After three days of struggle and sweat the last piece finally went through the door and the whole area was laid bare down to the floorboards. I wore out five heavy duty saw blades and myself!
Back at the tub manufacturers I discussed possibilities and price, finally deciding to bite-the-bullet and order the bath. I lay in the tub while they marked the position of ten adjustable jets directed on my back, legs and side. To fit the bath I had to “steal” 4“ inches out of the engine room. I did this by making an alcove and installing a nice glass picture of a square rigger with a light behind it and new faucets and a shower.
It certainly caused some strange looks from fellow boaters in our marina, as a friend and I wheeled a full size bathtub with all sorts of pipes sticking out of it down the dock and loaded it on to Britannia’s deck. I joked to one inquisitive fellow that I planned to install it in a big hole on deck and he didn't seem to be too perturbed at all. Evidently I had a reputation for whacky jobs even then.
The bath plonked down exactly where I had planned it and after about a week it looked like it had always been there. I couldn't resist the temptation of see how it felt even with no water.
Britannia has a long keel full volume hull with an amazingly deep 5’ foot bilge stretching 27’ feet from the stern gland to the chain locker that houses the engine and generator and all the other machinery as well. It is more like a long engine room than a bilge. An 11 gallon heater tank sat on a raised platform in this cavity that would never be enough to supply 55 gallons of hot water at 102 degrees Fahrenheit, so I decided to double it by fitting a second heater.
I removed the tank and its platform then built a new base lower down in the bilge, thereby lowering the heater. I then positioned a new heater on top of the old one and piped them together in series. The engine hot water pipes were also connected from the engine to the twin heater tanks. This ensured a continuous flow of very hot engine water pumped through both coils inside the tanks. I also installed a stop valve on the engine pipe to shut off the engine heater circuit, to allow it to initially come to operating temperature more quickly.
The tanks had no water temperature gauge, only a thermostat that shut the power off at 140F, so there was no way of knowing what the temperature actually was. I therefore removed the top drain tap and screwed an automobile capillary engine temperature gauge in its place, that shows the temperature.
Both tanks were then lowered into position using a tackle strapped to the ceiling handrail supports. I then lagged all 
the hot water pipes using split foam tubing all through the boat.
Britannia has two shore-power receptacles supplying two separate distribution panels. These split the load of some of the higher electrical draws like the twin AC units and the washer/dryer, so I installed fresh wiring to the new heater through a second breaker. The reason for wiring the two heater elements separately is that if one fails - as they often can after years of use - I could at least heat water in the other tank from shore-power without running the engine. Flexibility is the name of the game on a boat with so much electrical demand.
After all these shenanigans both units were finally installed, wired and plumbed and it was time to test my armature hydro-electrical engineering theories.
The ambient temperature of the outside water was 45 degrees on the day of the test, effecting the time it took for the engine to achieve maximum temperature. I first switched on both heater elements and the gauge showed a discharge of 23-amps from the single 30-amps shore supply. I closed the engine pipe shut off valve and started the engine to run at 2000 rpm - its normal cruising revs. The thermostats on the heater elements are set at 140F degrees and non-adjustable, so when they reached 140F the electrical side switched itself off. I then opened the shut off valve to allow hot engine water to be pumped through both tank coils and had to remind myself the engine was now having to heat 22 gallons. The whole process took about an hour and I finally had 22 gallons of water at 178F. This is obviously much too hot for use, but would it be enough to fill the bath with piping hot water for a long soak?
I opened the hot water faucet fully, then watched and waited. Scalding hot water crept slowly up the sides of the bath until it slowly began to cool as both tanks depleted their initial hot water. The emersion heater elements had come back on, so I let the cooling hot water continue to run in as it slowly lowered the overall temperature. The engine and immersion heaters must have wondered what was happening to their valiant efforts to keep the water in the tanks at a steady temperature. The bath slowly became full to the maximum level and the water was still too hot at 110 degrees, so I switched the hot tap off and cooled it to 102F with cold water.
I then climbed into the luxuriously warm water and switched the jets pump on to feel a nice massage on my back. I had even fitted a holder for my beer glass, because it would be a disaster to have that tip over from the boat rocking even in a marina. The bath pipes and jets had been insulated with foam when it was made so I was very pleased to find the temperature did not drop much for a half hour soak.
You may be wondering how I could drain such a large capacity of water out of the bath. I had previously installed an automatic the electric shower drain but that wouldwould have taken ages. I simply used the portable anchor/deck wash I had made and hung the outlet hose out of the portlight window and over the side. This emptied the bath in ten minutes.
When we have been out sailing and motor back to our berth the water is usually piping hot, solely through the engine, and I can enjoy a nice soak. I don't anticipate using the tub at anchor and have a big power boat cause a wake and heave 20 gallons of hot water all over the floor.
It has been suggested why I didn't just use the hot tub at home that would have been a lot easier. The easy bit is definitely true, but then how would I have a nice soak when visiting other places on a cruise?
After fitting a new electric toilet and washbasin this bathroom is now reminiscent of some classic Victorian schooner.
A HOT TUB IN A SAILBOAT - WHY NOT?