There are many reasons for the success of fiberglass as a boat-building material. It's inexpensive, easy to use in production, and results in a finish that looks good. It's also durable, easy to maintain, and relatively easy to work with and modify. It can be drilled and cut with common power tools, but any modifications have to be done with great care to avoid damaging the boat or causing problems that might not show up until later.

The fiberglass that's used to build most production boats is a "sandwich" composed of fiberglass-reinforced plastic (FRP), the main structural element, and a surface of gelcoat, which provides an attractive finish and protects the FRP from water intrusion. The FRP is relatively flexible, while the gelcoat is hard, brittle and easy to chip away from the underlying material. It's the latter quality that can make modifications tricky unless they're done correctly.

The most common modifications are drilling "through" holes (clearance holes for machine screws or wiring), drilling pilot holes for self-tapping screws, and cutting larger holes for things like electronics or hatches. Here are some common precautions that must be taken and a list of more specific issues for each fiberglass-modifying operation.

Prepare The Area
First and foremost, make sure there are no components like wiring or hoses under the panel that could be damaged when the hole is drilled or cut. Next, cover the area to be modified with a liberal application of masking tape to provide a surface to mark and protect the finish from the power tool, and keep the gelcoat from being chipped during the modifications. In addition to having drills of the proper size, you should also invest in a valuable tool called a countersink, which can be used to chamfer or bevel the edge of a drilled hole.

When you choose to mount something to a fiberglass panel, keep in mind that machine screws used with washers and locking nuts are always preferable to self-tapping or sheet-metal screws, especially if the load placed on them will be tensile (trying to pull the screw out). In cases where the screw will be in shear (the load trying to drag the screw sideways) or there's no access to the back of the panel, self-tapping screws can be used successfully.

Drilling Through Holes
Through holes present the fewest problems because the material around them won't be stressed by having threads formed in it. Holes up to 1/4 inch in diameter can be drilled, starting with a smaller-size pilot hole, followed by increasingly larger drills until the proper diameter is reached. Drilling a large hole in one operation can increase the risk of chipping the gelcoat. When the hole is finished, use a countersink to bevel its edge all the way down through the gelcoat to the FRP (see Figure 1). Holes 3/8 inch in diameter and larger should be made by using a rotary hole saw or speed bore, a type of flat-bladed cutter, either of which causes less damage than a large drill bit. If you haven't used such tools before, practice on some scrap material first.

Drilling Holes For Self-Tapping Screws
Most "self-tapping" screws used in marine applications are actually just common sheet-metal screws, which don't really cut threads for themselves but displace material to form crude threads as they're being driven into the host panel. It's this displacement that causes damage to the gelcoat, especially if the pilot hole for the screw is undersized, leaving excess material to be displaced. The excess material bulges up under the gelcoat, causing it to crack and flake off (see Figure 2 on page 86). The proper pilot- and clearance-hole diameters for some common screw sizes are listed on the table below. Once the hole has been drilled and countersunk, coat the screws with a marine sealant, such as silicone, which will first lubricate their entry, then lock them in place, and seal the hole seal from water intrusion once it has cured.


Cutting Large Holes
In many cases, holes for electronics or hatches won't be round, but square or rectangular in shape. This can present a problem because larger openings have the potential of creating structural weaknesses, and if they have square corners, the results will show up at those corners. Your boat as a structural entity is subjected to enormous stress. When those stresses are transmitted to the panel with the hole in it, they'll concentrate at the corners, and you'll find cracks propagating from those corners in no time at all. The same holds true for any sharp irregularities in the edge of the opening.

The good news is that the potential for these problems is easily avoided. The secret is an oxymoron: round corners. The corners of the opening should have the maximum radius that the accessory being installed will allow, and the entire edge of the opening should be as smooth and regular as you can make it. The radiuses (radii, if you insist) will evenly distribute the stresses to adjacent areas rather than allowing them to concentrate dangerously at the corners. The potential for cracking will be greatly reduced.

The tool of choice for creating large openings is a sabre saw equipped with a fine-toothed, metal-cutting blade. Extra blades may be required because cutting fiberglass will dull them quickly. If a tight-corner radius is required, it may be tighter than the saw's turning radius. (The narrower the blade front-to-back, the tighter the radius you can cut.) If so, you should consider using a rotary hole saw to form the corner contours, then cut out the remainder of the plug with the saw. Don't forget to lay a generous swath of masking tape along the cutting path. This will protect the boat's finish from the saw's shoe and reduce the gelcoat's natural tendency to chip during the cutting operation. You can mark the outline of the hole right on the tape. If you're not using a hole saw to form the corners, drill a starter hole for the sabre-saw blade just inside the cut line. Once the cutting is complete, remove the tape by pulling it toward the center of the hole, and then bevel the hole edge with a fine file or sandpaper, being careful to use only downward strokes to avoid lifting the edge of the gelcoat. 

Sealing The Holes
Any hole that is cut into a cored section of the boat (where balsa, foam or other material is sandwiched between layers of FRP) must have the exposed edges of the core completely sealed by a coating of epoxy or marine-grade sealant before the accessory can be installed. Failure to seal the exposed coring against water intrusion can result in serious damage to your boat. Putting holes in your boat  shouldn't be taken lightly. In fact, professionals will often spend as much time planning their project as they do carrying it out. The old adage, "measure twice, cut once" has never been more relevant.

Pilot and Clearance Hole Sizes

Screw Size    Screw Diameter            Pilot Hole                     Clearance Hole
                                                                  Drill-Bit Size                     Drill-Bit Size
#4                         .112 mils                        3/32 in.                             1/8 in.
#6                         .138 mils                        7/64 in.                           5/32 in.
#8                         .164 mils                        9/64 in.                           3/16 in.
#10                       .190 mils                     11/64 in.                           7/32 in.
#12                       .216 mils                     13/64 in.                             1/4 in.
1/4                        .250 mils        Use of self-tapping screws        9/32 in.
                                                            not recommended       

QUICK TIPS

1.     Make sure there are no components like wiring or hoses under the panel that can be damaged when the hole is drilled or cut.
2.     Start through holes with a smaller-size pilot hole, followed by increasingly larger drills until the proper diameter is reached.
3.     Self-tapping screws  should be coated with a marine sealant once the hole has been drilled and countersunk.
4.     For larger holes, cut round corners using a sabre saw that has a fine-toothed, metal-cutting blade.
5.     Any holes cut into the cored section of the boat must have their exposed edges sealed by a coating of epoxy or marine-grade sealant before an accessory can be installed.