Inspiration
This project would not have been possible without my mechanical engineering classmates and friends Kelly MacDonald, Tom Wiesenberg, and Maddie Albert. We're always bouncing ideas off of each other, using the skills we've developed to theory craft and decide if an idea is silly, feasible, or maybe a little bit of both. This project is quite silly; it is purely for our amusement and relaxation. The question of feasibility was the driving motivation here - are we as engineers capable of completing such a difficult project? We dwelled on this question for many weeks after long hours at our internships, having meetings, researching, and drawing up prototypes in CAD. Evidently, we decided that within the 7 days after our internships ended and classes began, we just might be able to pull this off.
Execution
The basic construction of the hot tub consists of two intermediary bulk containers (IBCs) and nine pallets. IBCs are generally used to store food grade liquid and are commonly found on websites such as Facebook Marketplace. We had found other DIY hot tub enthusiasts using a single IBC for a simple 2-3 person hot tub, but we wanted to have as many of our friends join us as possible. The two IBC design allows for six people to comfortably sit inside the hot tub, with extra seating around the outer walls. In order to support this load and constrain the two IBCs, we chose pallets as we could get them for free from Formlabs, Kelly's place of work. We decided against directly coupling the two IBCs and ripping out the middle wall due to the increased chance of water leakage which we wanted to avoid at all costs.
Plumbing and heating posed major challenges through out this project. Many hours were spent researching viable heating solutions. To appease our skeptical stakeholders, open flame and high voltage electric heaters were not considered. We decided to use a propane heater as the flame was enclosed and the system had enough power to heat up the tub in a reasonable time according to heat transfer calculations. The specific heater we purchased came with a 12v pump which was an added bonus. Implementing the plumbing involved many hours in the Home Depot PVC section. Finding fittings that matched the heater and pump interface and the PVC pipes was unexpectedly difficult. The layout of the tubing was such that water would exit the heater and enter each tub by the bottom through a bulkhead after splitting through a T connector. Water in the tubs would then cycle into four more bulk heads, two per tub. Each tub had two outputs for redundancy in case one was blocked. After rejoining, the water would flow through a filter, then the pump, and then back into the heater to close the loop.
The final construction of the tub utilized a skin layer of decent scrap wood that was stained with stain-sealer from Home Depot. This layer hid the pallets and made for a comfortable outer bench. LEDs were installed on the outside of the tubs facing in through their translucent plastic. The final touch were a pair of bubble mats, originally intended for bath tubs, that brought our creation from two hot buckets to two classy hot tubs. Underwater benches were tested and ultimately dropped for sake of simplicity and lack of space.
Final Thoughts
This project was incredibly fun to design, build, and use but it was an even better learning experience. We were able to come directly from our internships and put our new skills and abilities to practice without the safety of net senior engineers. I learned wood working skills through disassembling and reassembling pallets to fit our dimensions, project management skills as I worked with my team and their needs, and stakeholder needs/assessment skills through our fundraising and working with non-engineering friends. In terms of cost, we aimed to spend $1000, received $600 in funding, and the total final cost was around $1200 for over 100 hours of work.
