Found a great help site and thought I would pass some pointers that have helped me.

Drilling and Repairing Fiberglass

A few steps that should be followed


Fiberglass is still the preferred material for boats. It is light and it is durable-- two important components for boat building. While many of the new models you will read about in these pages are made up of composites and use new design technologies, it is still a fiberglass boat that comes off the assembly line. Chances are good that you are going to have to drill a hole or two in this time-tested material when installing a new GPS or make a minor repair after a day when docking the boat didn't go as planned. It is doable, despite the initial fears of every fiberglass first-timer. After all, boats aren't supposed to have holes in them.

But drilling a hole goes with the territory. Literally.

(1) Before you do anything, check to see what is behind where the drill is going to go. Eiden says there are too many stories from backyards where someone has drilled through the fiberglass into a fuel line, hydraulic line or electrical cables.

(2) If you are installing load-bearing hardware, it is essential you use a backing plate. This ensures a strong holding capacity. If your boat is a newer model, you'll find the fiberglass is actually thinner as a result of the composites used in the materials. This fact alone will require the use of a backing plate even if the hardware isn't going to be stressed. If there isn't enough room for a backing plate you are going to have to rethink where that load-bearing hardware should be placed.

(3) Boats vibrate. As a result, thru-bolting is the preferred technique (a bolt is placed through the fiberglass and into a wooden or metal backing plate and secured with a nut). Consider using nylon locking nuts with washers for optimal fastening on backing plates.

(4) Know the size of the hole you need to make in the fiberglass. Your new component probably comes with bolts or screws for installation. Installation instructions often provide the hole size required for mounting. Use a drill bit one size smaller to drill a "pilot hole". It is usually a good idea to make a small indentation in the spot where you want the hole before any drilling is done. This prevents the drill from wandering. Go slow and let the drill do the work. When complete, use the next size drill bit until you have made a hole into which the bolt/screw is going to fit.

(5) Use a countersink bit to make an indentation in the hole. This allows the bolt/screw to be level on the surface while also eliminating stress on the gelcoat that can result in surface cracks.

(6) If the hole you need to make is larger than half and inch, forget using the drill altogether and instead use a round or hole saw.

(7) If you need to make a hole perpendicular to the surface (usually used when installing load bearing hardware like blocks or hatch cover hinges) consider using a drill guide alignment tool to ensure the hole is properly drilled.

(8) Be sure to bed all fasteners above the waterline with a marine grade silicone sealant. Below the waterline, bed all fittings with a polysulfide sealant such as 3M's Marine 101 (and if the bond is to last as long as you own the boat, 3M's 5200 is the preferred choice).