We used a chalk-line to mark the center down the length of the hull and marked the dimensions as set forth in the plans with a permanent marker. What lay before us, was a giant rectangle of blue Styrofoam, 20' long and 8' wide and 6" thick, with marks all over it. We screwed all the pieces together using 5 1/2" deck screws and got busy putting the assembly on sawhorses in order to cut the rough shape of the hull. We initially started cutting the Styrofoam with a fine tooth hand saw. This makes a giant mess and required a lot of clean-up after the fact. This changed after we adopted the hot wire foam cutter idea.
Once the rough shape was cut, we lined the garage floor with painters plastic (which I highly recommend so that you don't glue the hull to the floor) and took everything apart. Now, for vacuum bagging...
The difficult part about the vacuum bagging process is that you need several people to do a variety of different jobs. Here's how I delegated the jobs:
- Epoxy mixer
- Epoxy spreader
- Styrofoam stacker
- another person insuring no gaps between joints,
- and the last person screwing the pieces onto each other.
We used some of the 2:1 epoxy provided by Universal Hovercraft (which didn't last very long, by the way) and later on in the project started using 5:1 marine epoxy from a local company called Fiberglass Hawaii. Last thing on epoxy: An accurate delivery system is crucial; there should be no time wasted on measuring this stuff! I recommend the WES delivery system. There's no thinking involved. It automatically does the work and delivers 5 parts of resin with one pump and then 1 part of hardener with another pump. Easy Peasy! Once the epoxy had been spread and all the pieces were in place, the race was on to set up the vacuum bag system.
We used some pretty thick painters plastic (5 mil) as our boundaries so that nothing would rip seeing as we are a bunch of clumsy asses in a confined area. Also, good quality duck tape or Gorilla tape is essential. This ensures minimal leaks between the plastic and the garage floor. The clock officially starts as soon as you mix that first cup of epoxy, so make sure that everything is in place to drag over the entire assembly. And for Pete's sake, do a couple practice runs before you even think about mixing epoxy. Ensure that everyone is familiar with exactly what they will be doing if you're bringing in some outside help. Your goal should be to have everything epoxied and the vacuum bag set up within 25 minutes of having mixed that first cup of epoxy. We worked with some intense background music in order to motivate us!
Here's how the specifics go, as far as the vacuum bagging is concerned. You will need: a layer of plastic for the garage floor; a layer of plastic to go on top of the hull; a layer of blankets; a garden hose and vacuum hose; a scale and finally, one last layer of plastic on top of it all, the edges of which you will tape to the garage floor.
On the garage floor, you should start with a layer of painters plastic. Then, you should have the actual body; or hull, or whatever you're going to bag. Then, you'll have another piece of plastic on top of the hull in order to protect the next few things from the epoxy. On top of that sheet of plastic goes the blankets. We used 3 blankets that were about 8' x 8'. This ensured plenty of coverage over the entire hull. The purpose of the blanket is to protect the last outer layer of plastic from being sucked up by the vacuum hose. For uniform vacuum pressure throughout the bag, we actually used a cheap garden hose (about 40') and drilled a bunch of 1/2" holes in it. These holes prevent you from having one vacuum point that will probably squeeze really hard and make the epoxy too thin at that particular point and it wouldn't bond correctly at other points. We ran the hose around the perimeter and then started a little spiral toward the center after we made it all the way around the first time. After laying the hose down, scrunch the blanket up over the hose without losing too much coverage from the edges of the hull. The blanket provides a barrier so that the plastic won't get sucked into the hose and rip.
The vacuum bag is almost complete at this point. The last step is the most important. Lay the top-most layer of plastic on and ensure that it conforms to the shape of the hull. Make sure to account for the actual contour of the hull so that when the vacuum is turned on the plastic will hug the Styrofoam and not stretch over that stair-step shape between the edge of the hull and the floor. You'll also need to cut a hole in that top-most layer in order to connect the modified garden hose to your vacuum hose. Tape the hole shut around the hose once it's all connected so that it won't leak any air. Cut the hole before-hand so that there is no time wasted while the epoxy is curing. I put a small scale under the plastic somewhere near the edge of the hull so that we could see it without stepping on the craft. I recommend finding a cheap one that is approximately 1' x 1'. This makes it easy to see how many pounds per square foot of pressure you have once the vacuum is on. At this point, tape the top-most layer to the garage floor. Every inch of plastic has to be taped down so that there are no leaks whatsoever. Lastly, turn the vacuum on and check for leaks. I recommend having beer on hand at this point because it will take a little while for the epoxy to cure and after all that, I sure needed one.
So, hopefully, you didn't go cross-eyed in that long-winded explanation of how to vacuum-bag. Seeing as how the hull is the attach-point for everything, it's crucial that it is constructed correctly. Some of the later steps in the process can be fixed with minimal time and effort, but this one is not easily fixable and if done incorrectly can be very costly, both time and cost-wise.
This was our first big hurdle! It took several nights of planning and another additional night to actually execute this. Take the time to actually plan this out. Don't get in a rush. This is a big step.
Additionally, here are a few tips that I learned along the way:
- Using a fine tooth hand-saw to cut the rough shape of the hull is not a bad idea. However, a hot-wire saves a lot of time and patience (and arm fatigue) if you choose to go that route. Go to Hot Wire Foam Cutter Info on the web to get some advice. This guy seriously knows what he's talking about. Try practicing on some scrap before you start cutting the real deal though.
- Vacuum bagging ensures even adhesion of the epoxy along the flat surfaces of the Styrofoam. There is not a more efficient way to do this business. However painful and time-consuming it is, make sure you do it when it is called for. Universal Hovercraft actually got this one right.
- Epoxy does not stick to painters plastic after it has cured.
- Use gloves and work in a well-ventilated area when handling epoxy. Floor or standing fans help a lot here. Epoxy stinks and sticks to everything!
- Having enough people is crucial. Make sure you don't try to Superman some of these steps solo. You might find yourself stuck and having wasted a lot of time and money. However, if it comes down to having too many chiefs and not enough Indians, know when to tell people to stand by and watch!