Once the design for the wave energy converter had been finalized, the team decided to start with building the buoy. During all of the frantic redesigning and changes from the WEC 003 to 004, the one thing that never changed was the size of the buoy. For that reason, the purchase order for the special marine-grade foam had been placed much earlier than any other supplies, and so the eight 5-gallon buckets were already sitting in the student workshop waiting to be mixed.
Tasked with designing the massive float, I spent a great deal of effort trying to keep it simple and easy to build. The objective was to maximize buoyancy and durability while minimizing cost and fabrication time. Ultimately, I proposed to build a cylindrical, solid foam buoy 5 feet in diameter and coated with epoxy resin and yellow color additive. Had the buoy been designed to be hollow, that same 40 gallons of liquid foam could have made a much larger, more buoyant buoy for the same price. Alternatively, we could have built the same size buoy but used less foam and saved some cash by leaving the center hollow. Besides the costs associated with testing the WEC in the ocean, the foam was the single most expensive purchase for the entire project, costing more than $1,000 before shipping.
The company U.S. Composites sells several different densities of foam. Depending on the application, you may want a low-density foam to fill large spaces or a high-density foam for strong structural objects. The lighter density foams, such as the 2 lb/ft^3, have the largest expansion ratios of all the foams but are the weakest and most brittle. Being fragile and easily damaged, low density foams are typically used to fill plastic barrels or forms that already have a strong exterior shell. There are stronger foams with higher densities, 8 lbs/ ft^3, that are used to make structural flotation devices and can withstand large forces smashing into them; however, these are heavier and have a smaller expansion ratio than the other foams. Since all the foams are sold by the weight, these high-density foams will expand less and therefore are much more expensive if you need to fill a large volume.
Foam Test Batch
The team ended up using the 4 lb/ft^3 density because we wanted to fill 75 cubic feet and needed to make a buoy that was tough enough to take a beating in the ocean waves. While not the weakest, the 4 lb/ft^3 foam will still break and crumble when put in tension. This is important because the heavy spar had to be mounted to the underside of the buoy in a way that would not pull on the buoy and cause it to break apart.
This was also why we built a solid buoy. A hollow buoy would have needed to be reinforced, like concrete with rebar, and would have added to the complexity and cost of building. To prevent this, two steel plates were used to sandwich the foam on top and bottom and keep it in a constant state of compression. This allowed for the spar to be mounted to the bottom steel plate, while the vertical load from the spar would be transferred to the top plate through eight lengths of 1/2″-13 all-thread. This way, the buoy was always in compression and the spar had a flat rigid connection point with the buoy (more details of the spar in the next article).
Since no one had any experience using this two-part marine-grade expanding foam, we decided to conduct a small experiment before attempting to build the full-sized buoy. The total cost of the foam was over $1000, and it had taken two weeks to build the cylindrical wooden mold, so we had to get this right on the first try. The final buoy was engineered to have a total volume of 75 cubic feet and provide enough buoyancy to float the steel spar, all the hydraulics and electronics, and the heave plate, while still being able to bob up and down in ocean waves. It was critical that we get the mixing ratio correct to ensure the buoy would not sink.
For this small-scale foam test we built a one-cubic-foot box from scrap pieces of 1/4″ plywood and wood screws. According to the manufacturer, this closed-cell urethane foam would expand 15 times its original liquid volume in less that five minutes and harden in 15 minutes. Mixing was simple. It was a 1:1 ratio of Part A to Part B. The manufacturer recommended that a drill and paint mixer attachment be used to mix the two parts for 25 seconds, then you would have another 20 seconds to pour the mixture into the mold before the expansion process took off. For the test, we simply used a wooden stick for mixing, but later, for the larger batch mixing the drill was really handy.
The results from the test were impressive. Once the two parts were mixed together, the yellow foam instantly began to expand and produce a considerable amount of heat and CO2. Our math was a bit off, and the excess foam spilled out from the top of the mold and even caused the walls to burst open from the built-up pressure inside. This was particularly interesting: when confined to a closed space, the foam expanded outward, pushing against the walls of the wood box instead of pushing upwards. Seeing this, we decided to add three ratchet straps around our larger mold to reinforce the side walls and prevent excessive warping.
After allowing the foam cube to cure for 24 hours, we removed the wood screws and tore open the plywood box. The foam was hard and completely cool to the touch. Getting the block of foam out of the mold was easy. For the most part the foam did not stick to the wood and the sheets of plastic peeled off the surface of the foam and exposed a smooth, flat surface. Best of all, the bottom of the block cured evenly and there were no signs of voids or pockets.
Building the Mold
The process for building the full-sized buoy was very similar to the test, only much bigger. The top and bottom of the mold were cut out of 3/4″ plywood and for the walls we wrapped two sheets of 1/8″ plywood to make a four-foot-tall cylinder with a five-foot diameter. Before pouring the foam, PVC pipes were inserted in the center of the buoy to create hollow cavities for the all-thread. The spacing and position of these PVC pipes was critical since this was how the steel plates would be bolted together to sandwich the foam and secure the spar to the bottom of the buoy.
Just like the test cube, sheets of plastic were used to line the inside of the mold. Once complete, the mold was wheeled outside using a pallet jack.
Pouring the Foam
Due to the larger volume, we assumed that it would be alright to mix two gallons of Part A with two gallons of Part B to get batches that would expand to 60 gallons. For out first pour it was great, but the following pours resulted in uneven expansion and the formation of large cracks at the surface. In between pours, the foam was allowed to set and cool for 90 minutes, but due to the increased volume-to-surface-area ratio more time was needed to fully cool. Because of this oversight, the core of the buoy remained extremely warm while the top surface cooled quickly and developed a thick crust.
Being a highly exothermic process the temperature inside the mold continued to increase and subsequent pours reacted even faster (the expansion rate is extremely temperature-dependent), causing the surface to harden even before the foam underneath was done expanding. This is when the cracks formed, as pocket of gas ruptured through the newly formed crust and the foam below the surface continued to pushed upward.
Shaping the Buoy
We waited five days to allow the foam to cure and cool before removing the wooden mold. During this time the buoy continued to off-gas so we kept it outside for several nights before moving it back into the student shop. After unwrapping the side walls it was obvious that the large batch pours resulted in thick, uneven layers. This would have been a big concern if we had not already planned to squish the buoy in between two steel plates, so while not ideal it would certainly work. More disappointing was the final size of the buoy. Due to mixing errors we did not get the maximum expansion possible, resulting in a buoy that was 20% smaller than expected.
During the mixing process it was very important to keep the two solutions completely separate from each other and carefully clean the inside of the mixing containers after each use. Trying to be efficient, we marked the inside of clean five gallon buckets to indicate the two gallon and four gallon levels. Next, we added Part B into the bucket and filled it to the first mark. Then we added Part A — it was less viscous and easier to pour than Part B up to the second mark and we quickly mixed the two together with the paint mixer and drill. Unfortunately, we did not realize that the expansion process started instantly once the two mixtures combined.
I had assumed the reaction was delayed during the 20 seconds of mixing. Once we began adding the second mixture, the liquid began to slowly rise and expand but was undetectable since we were still adding more fluid to the mix. As a result we were not adding the entire two gallons of Part A as intended. A better method would have been to measure out each mixture into two different buckets and then pour those two buckets into a third mixing bucket.
With the side walls and top of the mold removed, the buoy continued to slowly expand and off-gas for the next two days. What started as small cracks on the surface slowly began to grow into large cracks as the buoy made horrible snapping and cracking sounds.
Once there were no more sounds coming from the mound of foam, we took an electric chain saw and began to shape and sculpt the mass. A bevel was added to the bottom but was made smaller than initially planned to conserve on foam. The chamfer was added partly to assist with the hydrodynamic behavior of the buoy as it bobbed up and down in the waves, but more importantly helped to mask a large hole in the bottom. I think it makes the buoy look really cool too.
After sanding and smoothing the entire surface, two coats of a marine epoxy resin were painted onto the surface. A bright yellow pigment from Tap Plastics was added to the resin to give the buoy a nice, bright, identifiable color. The resin was easy to apply with paint brushes and rollers and actually covered much more surface area than expected. We only used one gallon of the marine-grade epoxy resin Side-A in conjunction with the Side-B Medium 109 hardener. Side-B hardener 109 was used since it would cure clear and allowed for 4-5 hours before drying. We initially tried the fast-drying 102 hardener but this resulted in a really pink-orange buoy and dried a little too quickly for our use.
Installing the Hardware
With the top and bottom fully cured it was finally time to install the all-thread and the steel plates. We had to use a mallet to hammer some of the all-thread through the PVC pipes since the heat from the expanding foam caused the PVC pipe to heat up and bow a bit. With the all-thread installed through the buoy it was then possible to hang the 170lb and 130lb steel plates that had been cut out using a CNC plasma cutter. The last step was to bolt down the all-thread and compress the plates together against the foam buoy. Eyebolts and two 5,000lb hoist rings were also added to allow for the buoy and assembly to be lifted with the shop crane. The entire process tool about 4-5 weeks to complete and already the shop was beginning to fill up with steel beams and hydraulic parts. This was going to be a big project.
In the next article I’ll be writing about the 20ft long spar that was mounted under the buoy and how it was designed and built to be modular. After the 10th and final article is published I’ll be posting more detailed instructions about the build, but in the meantime please let me know if you have any questions about the buoy build process in the comments below. Thanks!