I stopped by my local marine store the other day to say Hi! to the folks there. They know I’m a boat project writer and hooked me up with a customer who had just come into the store. He had an older bass boat whose builder had gone out of business. He was looking for a replacement windshield for the boat, seems he was tired of bugs in his teeth when at speed. The only replacements he could find were heat-formed Plexiglas, too curved for his dash. We talked about his needs versus his skill level and came up with several options for him to pursue.
While this article pertains to this replacement windshield, the techniques apply to any on-board plastic project.
There are two kinds of plastic windshield material in common use, acrylics and polycarbonates. These are more commonly known as Plexiglas and Lexan. Both work well. Plexiglas is a little less expensive and doesn’t scratch as easily as the more expensive but stronger Lexan. For most boat owners doing their own work, Plexiglas is a better choice as it is easier to get and easier to work with.
Either material is sold in a wide variety of thicknesses, colors and sizes. Both can be expensive if you purchase full sheets. Better to find a supplier who will cut to size or offers pre-cut sizes. Many advertise on EBay. Local plastic suppliers are another option; many will cut material to a rough size for you. They’ll even custom fabricate an item for you but that can be extremely expensive.
For this project, I recommended clear Plexiglas, ¼” thick.
The windshield in question was a three sided deal with a flat front panel and two side wings that mounted on the front of the dashboard. The first proposal we discussed was to simply cut the front panel and side wings from flat plastic and fasten them together along the seams.
One method of doing this would be to use a length of piano hinge at each seam. One leg of the hinge would be bolted to the front panel ad the other leg to a side wing. The advantage of this design is that fabrication is extremely simple: cut to size and bolt together. The angle between the side wings and the front panel would be infinity variable, making installation simple.
A similar solution would be to use a narrow strip of aluminum, bent to the required angle, between the side wings and front panel. It would then be bolted together. The angle between the wings and front panel would depend on the angle of the strip so it would need to be a little more precise.
Either of these two options would work fine. The downside is that they both would look a lot like a WW-II fighter plane canopies, not the sleekest of designs.
The third option is the one that would look the best but be harder to fabricate. The side wings and front panel are cut as a single piece and then heated and bent to the right angles. As I said, it is a little harder to fabricate but the techniques are easily mastered.
Any of the three options will require several of the following techniques to produce an acceptable job.
Plexiglas can be cut to size using table saws, band saws or hand held saber saws. Table saws should use carbide blades and all blades should be fairly fine toothed. The trick is to go slow so you don’t overheat the plastic. If you do, it will melt and reform behind the blade. Cut slightly off the line to allow a little room to clean up the edge. Plexiglas comes with protective paper on both sides; leave it on until the last possible moment to reduce the possibility of scratching or damaging the surface.
No matter how fine a blade or smooth a cut you made with the saw, the edge will need finishing. If left unfinished, the raw edges will eventually form micro-cracks that will develop into full-blown cracks.
Start by sanding the cut edge. Start with 80 grit and sand out the saw marks. Go to progressively finer grits, sanding out the previous grit’s marks. End up with at least 600 grit wet or dry. This edge should look smooth but matt. You are not done yet. The final polishing should be done with a buffing compound, like Tripoli Compound. If you don’t have that, use toothpaste! Buff and polish the edge until it is as smooth and shiny as the surface of the Plexiglas.
Drilling and Fastening Plexiglas
You need to be extremely careful in drilling holes in Plexiglas; one wrong move could ruin your expensive piece of plastic. The first thing to note is that fastener holes will need to be drilled oversize. Plexiglas expands and contracts with changes in temperature and too tight a fastener hole will crack the Plexiglas. A 5/16” hole for a ¼” fastener is about right.
You also have to be careful with the drill itself. A normal drill tends to dig in, grab and crack the Plexiglas. They make special drills for use with Plexiglas but you can accomplish the same thing by dulling the drill’s cutting with a pass on a grinder. I have also used normal drills in battery power drills at an extremely slow speed with success.
Never use countersunk fasteners in Plexiglas. The countersunk head wedges in the plastic as it expands and contract and will soon develop cracks around the hole. Pan head screws and washers are the best way to go,
Plexiglas can be heated and bent to shape. Aircraft bubble canopies are an extreme example. For our design a simple straight bend is required. The easiest way to do this is to use a Plastrip Heater. These are made for bending plastic and cost around $40. They are placed on top of the area to be bent and plugged in. They reach about 280 degrees Fahrenheit and can be used on plastic up to ¼” thick. It will take at least 30 minutes to heat the plastic to the point you can safely bend it.
Once heated the plastic side wing can be bent to the required angle. Be sure to make a template or gauge to show exactly how far to bend the plastic, this is the tricky part. By-the-way, you will need to remove the protective paper coverings before bending the plastic.
These are some basic techniques used when working with plastics. You may not need a windshield but you could apply these ideas to any number of other boat projects that will use Plexiglas or Lexan. Good Luck!